Construction Materials in civil engineering

CE 212Engineering Materials

2009-2010

2

Introduction

The structure of materials can be described on dimensional scales1. The molecular level2. Materials structural level3. The engineering level

3

1. The molecular levelo Smallest scale (atoms, molecules or aggregation

of molecules)o Realm of materials scienceo Particle sizes: 10-7 – 10-3 mmo Examples: crystal structure of metals, cellulose

molecules in timber, calcium silicate hydrates in hardened cement paste, variety of polymers

4

o Atomic models used for description of the forms of physical structure (regular or disordered)

o Chemical and physical factors determine material properties o Chemical composition and/or the rate of chemical

reactions determine material properties such as porosity, strength, durability, etc.

o Mathematical and geometrical models are employed to deduce the way materials behave

5

2. Materials structural level

– Up in size from the molecular level– Material considered as a composite of different

phases Phase IIPhase I

• Concrete • Asphalt

particles such as aggregates distributed in a matrix such as hydrated cement or bitumen

Examples:• Cells in timber • Grains in metals• Concrete• Asphalt • Fiber composites• Masonry - regular composition

Entities within the material structure

Deliberate mixing of disparate parts

6

o Particle sizes : 5x10-3 mm (wood cell) - 225 mm (brick length)

o Individual phases of the material can be recognized independently

o More general information can be derived from examination of the individual phases of the material (Multiphase models allow prediction of material behavior)

7

b) State and properties: Structure of material is affected from chemical and physical states of phases. Behavior of material is affected by properties of phases

Three aspects must be considered while formulating the models;

a) Geometry: Particulate or disperse phase scattered or arranged within the matrix or continuous phase model considers shape and size distribution and concentration

c) Interfacial effects: Existence of interfaces between phases may introduce additional modes of behavior. (strength: failure of material being controlled by band strength at an interface)

8

3. The engineering levelo Total material is consideredo Material is considered as continuous and homogenous o Average properties for the whole volume of material

bodyo This level is recognized by construction practitioners

9

Technical information on materials used in practice comes from tests on specimens of the total material

Strength and failure tests provide technical informationDeformation tests used in practice Durability tests

Representative cell: minimum volume of the material that represents the entire material systemDimensions for cells: 10-3 mm for metals – 100 mm for concrete –1000 mm for masonryIsotropic material: properties same for all directions – unit cell is a cubeAnisotropic material: properties change with dimensions – unit cell is a parallel pipe

10

CONCRETE

11

CONCRETEA composite of mineral particles (aggregates) distributed in a matrix of hardened cement paste (mixture of powder cement and water at the beginning)Versatile, comparatively cheap and energy efficient

Great importance for all types of construction throughout the worldConcrete is fresh and plastic at the beginning (throughout some time after mixing of constituent materials)Final properties of the hardened state of concrete have been gained slowly through timeProperties change with time50-60 % of ultimate strength is developed in 7 days, 80-85 % in 28 daysIncreases in strength have been found in 30 year old concrete

12

History of concrete is very old

Mixtures of lime, sand and gravels have been found in Eastern Europe, in Egypt and in Ancient Greek and Roman timesThis dates from about 5000 BC

History of concrete

Similar materials still known as pozzolona

Romans; first concrete with a hydraulic cement (lime + volcanic ash from near Pozzuoli)

Active silica and alumina in ash reacts chemically with lime

13

Roman structures;

Foundations and columns of aqueducts

Fig. 1: www.wonderquest.com/fountain-octopus-enzyme.htmFig. 2: http://www.artchive.com/artchive/r/roman/roman_colosseum.jpgFig. 3: www.dolceroma.it/images/common/dove/pantheon.jpg

In arches of the Colosseum

In the dome of the Pantheon

14

In 1756, John Smeaton

Mixture of burnt clay bearing limestone & Italian pozzolanafor producing a suitable hydraulic cement to be used in construction of EddystoneLighthouse

Picture ref: http://www.scienceandsociety.co.uk/results.asp?image=10307921Extra info: http://en.wikipedia.org/wiki/Eddystone_Lighthouse

15

In 1790, James ParkerPatented “Roman cement” from calcareous clay burnt in a kiln and ground to a powder

In 1824, Joseph AspdinPatented “portland cement” an artificial mixture of lime and clay bearing materials used in repairs of Thames Tunnel in 1828

In 1890s improvement in kiln technology reduced the cost of Portland cement production. Then widespread production and use started worldwide

16

CONSTITUENT MATERIALS OF CONCRETE

Portland CementsRaw materials; Clay and calcareous stonesSilica from clay + lime from calcareous stone(SiO2) (shale) (CaO) (Chalk or limestone)

Al2O3, Fe2O3, MgO, K2O also exist in clay

17

Chalk + clay reduced to 75μm or less and mixedBlend fed into upper end of inclined long (up to 250m), 6m diameter rotating kiln which is heated to 1500°C at lower end

Cement manufacturing process; simple but involves high temperatures

20°C 250°C 650°C 950°C 1250°C 1500°CDrying Preheating

decomposition of clay minerals

CalciningCaCO3 CaO + CO2

Burning or clinkeringcombination of oxides toproduce calcium silicates,calcium aluminates and calcium aluminoferrites

Raw materials

Clinker

Fuel + air

20°C 250°C 650°C 950°C 1250°C 1500°CDrying Preheating

decomposition of clay minerals

CalciningCaCO3 CaO + CO2

Burning or clinkeringcombination of oxides toproduce calcium silicates,calcium aluminates and calcium aluminoferrites

Raw materials

Clinker

Fuel + air

Fig. The processes taking place in a Portland cement kiln in the wet process

18

At 600°C, CaCO3 in chalk decomposes to give quicklime (CaO) and gaseous CO2

Form as a result of these reactions

Fusion reactions start at 1200°CCalcium silicates, 2CaOSiO2 or 3CaOSiO2Calcium aluminates, 3CaOAl2O3Other oxides act as a flux

Clinker particles (a few mm) emerge from kilnAfter cooling, 3-4% gypsum (CaSO42H2O) is added to clinkerMixture is ground to powder (2-80μm size), (300m2/kg specific surface)

www.cement.org/tech/cct_port_cem_prod_tech.asp

19Diagrammatic representation of the wet process of manufacture of cement

20Diagrammatic representation of the dry process of manufacture of cement

21

Principle oxides in cementCaO (lime):CSiO2 (silica): SAl2O3 (alumina): AFe2O3 (iron oxide): F

Composition;

Four main compounds (phases) formed in fusion process:Tricalcium silicate: 3CaO.SiO2 (C3S)Dicalcium silicate: 2CaO.SiO2 (C2S)Tricalcium aluminate: 3CaOAl2O3 (C3A)Tetracalcium aluminoferrite: 4CaOAl2O3Fe2O3 (C4AF)

22

Each cement grain consists of an intimate mixture of these compounds. Direct chemical analysis is not possible to determinethe amounts. Instead BOGUE formulas are used that were calculated from the results of oxide analysis.

SFASCSC 85.243.172.660.707.4 % 3 −−−−= 3SOS =

SCSSC 32 754.087.2 % −=FAAC 69.165.2 % 3 −=

FAFC 04.3 % 4 =

If A/F > 0.64

SFASCSC 85.286.248.460.707.43 −−−−=SCSSC 32 754.087.2 −=

FAAFCFC 70.11.242 +=+

If A/F ≤ 0.64

23

Compositions of Portland Cements

Oxides (% by wt) Range CaO 60-67 SiO2 17-25 Al2O3 3-8 Fe2O3 0.5 – 6.0

Na2O + K2O 0.2 – 1.3 MgO 0.1 – 4.0

Free CaO 0 – 2 SO3 1 – 3

Principle oxides: CaO & SiO2 ∼ 3 to 1by wt.

Principle oxides: C3S & C2S ∼ 75 – 80 % by wt.

The approximate range of oxide composition that can be expected for Portland Cements

24

Composition of cement depends on quality and proportions of raw materials (limestone and clay)Relatively small variations in oxide composition result in considerable changes in compound composition

Properties of compound cement constituents

MediumHighLowLowHighC4AF

Very slowVery highLowLowFlashC3A

HighLowHighMediumSlowC2S

MediumMediumHighHighMediumC3S

FinalEarly

Sulfate resistanc

e

Heat of hydration

Cementing valueRate ofreaction

25

Typical Portland Cements

Oxides (% by wt) A B C D

CaO 66 67 64 64

SiO2 21 21 22 23

Al2O3 7 5 7 4

Fe2O3 3 3 4 5

Free CaO 1 1 1 1

SO3 2 2 2 2

Potential compound composition (% by wt)

C3S 48 65 31 42

C2S 24 11 40 34

C3A 13 8 12 2

C4AF 9 9 12 15

Typical or average P.C

High-Early Strength P.C

Low Heat P.C

Sulphate Resisting P.C

26

Türk Çimento StandardlarıStandard No İsim Notasyon TS 3441 Portland Çimentosu Klinkeri TS 19 Portland Çimentosu PÇ 32.5

PÇ 42.5 PÇ 52.5

TS 3646 Erken Dayanımı Yüksek Çimento EYÇ 52.5 TS 21 Beyaz Portland Çimentosu BPÇ 32.5

BPÇ 42.5 TS 10157 Sülfatlara Dayanıklı Çimento SDÇ 32.5 TS 10158 Katkılı Çimento KÇ 32.5 TS 26 Traslı Çimento TÇ 32.5 TS 20 Yüksek Fırın Cüruflu Çimento CÇ 32.5

CÇ 42.5 TS 809 Süper Sülfat Çimentosu SSÇ 32.5 TS 640 Uçucu Küllü Çimento UKÇ 32.5 TS 22 Harç Çimentosu HÇ 16 TS 12139 Portland Cüruflu Çimento PCÇ/A

PCÇ/B TS 12140 Portland Kalkerli Çimento PLÇ/A

PLÇ/B TS 12141 Portland Silika Füme Çimento PSFC TS 12142 Kompoze Çimento KZÇ/A

KZÇ/B TS 12143 Portland Kompoze Çimento PKÇ/A

PKÇ/B TS 12144 Puzolanik Çimento PZÇ/A

PZÇ/B TS 23 Çimento Numune Alma Metodları TS 24 Çimentoların Fiziki ve Mekanik Deney

Metodları

TS 687 Çimentonun Kimyevi Analiz Metodlar

27

ENV 197-1’e Göre Çimento İçinde Bulunabilecek Malzemeler(Materials in cement)

Malzeme Kısaltma Sınırlamalar PÇ Klinkeri K C3S+C2S ≥ % 66.7

CaO/SiO2 ≥ 2.0 MgO ≤ % 5

Granüle Yüksek Fırın Cürufu

S Camsı faz miktarı ≥ % 66.7 CaO + SiO2 + MgO ≥ % 66.7 (CaO + MgO)/SiO2 > 1.0

Doğal Puzolan P Reaktif SiO2 ≥ % 25 Endüstriyel Puzolan Q Silisli Uçucu Kül V KK ≤ % 5

Reaktif CaO ≤ % 5 Reaktif SiO2 ≥ % 25

Kireçli Uçucu Kül W % 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm

% 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm

Pişirilmiş ŞeyL T σ28 ≥ 25N/mm2

Hacim Genl. < 10mm Kalker L CaCO2 ≥ % 75

Kil miktarı ≤ 1.2 g / 100g Organik Madde Miktarı ≤ % 0.2

Silis Dumanı D Amorf SiO2 ≥ % 85 KK ≤ % 4 Özgül yüzey (BET) ≥ 15m2/g

Minör İlave Bileşen F Kalsiyum Sülfat Katkılar

28

TS EN 197-1 Çimento Tipleri ve Kompozisyonları (Types of cement and compositions)

Çimento Tipi

Adı Notasyon K S D P Q V W T L MİB

I Portland Çimentosu I 95-100 - - - - - - - - 0-5 II/A-S 80-94 6-20 - - - - - - - 0-5 Portland Cüruf Çimentosu II/B-S 65-79 21-35 - - - - - - - 0-5

Portland Silis Dumanı Ç II/A-D 90-94 - 6-10 - - - - - - 0-5 II/A-P 80-94 - - 6-20 - - - - - 0-5 II/B-P 65-79 - - 21-35 - - - - - 0-5 II/A-Q 80-94 - - - 6-20 - - - - 0-5 Portland Puzolan Çimentosu

II/B-Q 65-79 - - - 21-35 - - - - 0-5 II/A-V 80-94 - - - - 6-20 - - - 0-5 II/B-V 65-79 - - - - 21-35 - - - 0-5 II/A-W 80-94 - - - - - 6-20 - - 0-5 Portland Uçucu Kül Çimentosu

II/B-W 65-79 - - - - - 21-35 - - 0-5 II/A-T 80-94 - - - - - - 6-20 0-5 Portland Pişirilmiş Şeyl

Çimentosu II/B-T 65-79 - - - - - - 21-35 0-5 II/A-L 80-94 - - - - - - - 6-20 0-5

Portland Kireçtaşı Çimentosu II/B-L 65-79 - - - - - - - 21-35 0-5 II/A-M 80-94 6-20 0-5

II

Portland Kompoze Çimento II/B-M 65-79 21-35 0-5 III/A 35-64 36-65 - - - - - - - 0-5 III/B 20-34 66-80 - - - - - - - 0-5 III Yüksek Fırın Çimetosu III/C 5-19 81-95 - - - - - - - 0-5 IV/A 65-89 - 11-35 - - - 0-5

IV Puzolanik Çimento IV/B 45-64 - 36-55 - - - 0-5 V/A 40-64 18-30 - 18-30 - - - 0-5

V Kompoze Çimento V/B 20-39 31-50 - 31-50 - - - 0-5

29

EN 197-1 The 27 products in the family of common cements

30

Table cont.

31

Özellik Çimento Tipi Dayanım Sınıfı Sınır Kızdırma Kaybı (%) CEM I

CEM III Bütün Sınıflar ≤ 5

Çözünmeyen Kalıntı (%) CEM I CEM III

Bütün Sınıflar ≤ 5

CEM I CEM II

32.5, 32.5R, 42.5 ≤ 3.5

CEM IV CEM V

42.5R, 52.5, 52.5R

SO3 (%)

CEM III* Bütün Sınıflar

≤ 4.0

Cl (%) Bütün tiplert Bütün Sınıflar ≤ 0.10 * CEM III/C %4.5’e kadar SO3 içerebilir. t CEM III % 0.1’in üstünde Cl- içerebilir. Bu durumda Cl- miktarı belirtilir.

TS EN 197-1 Çimentolarında Aranan Kimyasal Koşullar

32

EN 197-1 Chemical Requirements given as characteristic values

33

TS EN 197-1 Dayanım Sınıfları

Dayanım Sınıfı Basınç Dayanımı Sınırları (N/mm2) 2G 7G 28G 32.5 - ≥ 16 ≥ 32.5, ≤ 52.5 32.5R ≥ 10 - ≥ 32.5, ≤ 52.5 42.5 ≥ 10 - ≥ 42.5, ≤ 62.5 42.5R ≥ 20 - ≥ 42.5, ≤ 62.5 52.5 ≥ 20 - ≥ 52.5 52.5R ≥ 30 - ≥ 52.5 EN 197-1 Mechanical requirements given as characteristics values

34

Hydration(Cement + Water) paste initially fluid mixtureFluidity or consistency remains constant for an initial period after mixing

Final set (max 10 hours after mixing): Mix is completely stiff, hardening and strength gain starts

Initial set (2-4 hours after mixing): Mix starts to stiffer, fluidity is lost at a faster rate

Rate of strength gain is fast for the first 1-2 days . It continues with a decreasing rate in time

35

Rat

e of

hea

t out

put

(J/k

g/se

c)

A

Dormant period

B

C

0.1 1.0 10 100

Time after mixing (hours, log scale)

Rat

e of

hea

t out

put

(J/k

g/se

c)

A

Dormant period

B

C

0.1 1.0 10 100

Time after mixing (hours, log scale)

Hydration reactions are exothermic A: High but very short peak lasts only a few minutes

Dormant period: cement is inactive (2-3 hours)B: Broad peak after final setC: Sharp peak (seldom) after one or two days

36

Heat of HydrationR

ate

of H

eat

Evo

lutio

n

Time

Stage I Rapid Heat Evolution (<15 mins)

Stage II Dormant Period Important for transportation (2-4 hrs)

Stage III Accelerating Stage Begins with initial set (4-8 hrs)

Stage IV Deceleration Stage No longer workable (12-24 hrs)

Stage V Steady State

I

II

III IV

V

nucleationdissolution

hydrolysisC3S reacts

diffusion control

Initial set

Final set

37

VicatVicat apparatusapparatus

Typical Setting Times for Portland Cements

(Gebhardt 1995 and PCA 1996).

38

Vicat Needle Penetration Evolution

39

32

__

32

__

3 3263 HSCACHHSCAC →++

Retarding is provided by addition of gypsum which reacts with C3A to form calcium sulphoaluminate (ettringite)

Hydration compounds involve all four main compounds simultaneously. Processes are extremely complex and not fully understood. Simplified description consider the chemical reactions of each of the compounds individually

1) Initial peak “A” is due to: Rehydration of calcium sulphate hemihydrate

2

____235.02 HSCHHSC →+ (H = H2O)

2) Initial reaction of aluminate phases

633 6 AHCHAC →+

Very rapid reaction of C3A with water results in a flash set in a few minutes

Calcium sulfoaluminate(Ettringite)

Relatively slow reaction. Gypsum added (3-4%) used up to after first 24 hours after mixing. Then C3A hydration reaction taken over and ettringitetransform into monosulphate form ( ). This occurs as peak “C”in cements with C3A>12% 163 HSACC

40

3) C4AF reacts similarly over same time scalesReaction products are similar to C3A products

(has little effect on the overall cement behavior)

4) C3S and C2S react to form bulk of hydrated material after these initial reactions are completed. They are responsible for most of properties of hardened cement

CHHSCHSC 362 3233 +→+Most of the main peak “B” is due to this reaction

CHHSCHSC +→+ 322 342

(Reaction of C3S - faster)

(Reaction of C3S - slower)

41

FLY ASH(cured for 5 years)

42

FLY ASH(cured for 5 years)

43

aggregate

Transition zone Bulk cement paste

Fracture surface of 24 hour old cement paste, showing C-S-H and ettringite

44Typical hydration product development in Portland cement paste

Calcium silicate hydrate (C-S-H) is responsible for strength and other properties

After 1 day CSH dominates,thus; Ca(OH)2 production enhanced

45

Development of strength of pure compounds from Portland cement

46

Increasing temperature accelerates reactions of hydration. Reactions stop completely below -10ºC

Fresh cement and water Initial set

Two or three days old paste

Mature paste

47

1) After mixing fresh cement particles dispersed throughout mix water as single grains or small flocs. Spacing depends on w/c

Microstructural development during HYDRATION of cement

2) During dormant period, ettringite is formed at cement surface as sharp needles or rods.

3) At the end of dormant period, ettringite from adjacent cement particles has began to interfere and C-S-H with a spicular crumpled-foil form has started to appear. Solid layers of foil are a few molecules thick

4) During subsequent hydration, a dense continuous gel of C-S-H is formed between particles, resulting in increasing strength. Also large hexagonal crystals of CH are formed. Some larger pores remain unfilled between grains, and fresh unhydrated cement is left in center of grains.

48

Model – Hydration of cement paste

49

Pore size distribution in 28-day-old hydrated cement paste

50

Structure of hardened cement paste

Residue of unhydrated cement, at center of original grainsHydrates, mainly calcium silicates (C-S-H); also calcium aluminates, sulphoaluminates and ferritesCrystals of calcium hydroxide (calcite)Unfilled spaces between cement grains, called capillary pores

C-S-H occupy about 75 % of volume of HCPC-S-H govern mechanical propertiesC-S-H structure: from poorly crystalline fibers to crumpled sheet-like network of

colloidal scaleExtremely high specific surface: 100-700 m2/g (~ 103 times higher than cement particles)Spaces between C-S-H particles: gel pores: ~ 0.5-5nm ~ 27 % of C-S-H weight

Note: Don’t confuse gel pores with capillary pores (on the average about 2 orders of magnitude larger)

51

Strength of hardened cement paste

Strength arise from Van der Waals forces between hydrate layers

Quantitative estimate of unhydrated cement, hydrated gel and capillary pores was done by Powers in 1950s.

Important futures of his model are:1) Hydration takes place at constant volume

Vcem+water = Vunh.cem+gel+cap.pores

2) Same gel is produced at all stages of hydration regardless of type of cement and water/cement ratioConstants are; a) Chemically combined water: ~23% by wt of cementb) Relative density of gel solids = 2.43c) Relative density of gel+pores = 1.76

52

Gel occupies (including the pores) a space about 1.8 times that of unhydrated cement For too small a space, hydration stops when products grow to fill this space (complete hydration never occurs)For too large a space, 100% hydration doesn’t fill the space (capillary pore)For hcp in water: at w/c = 0.38 → 100% hydration fills space completely and no capillaries form

53

(a) (b)

Composition of hydrated cement paste at the final stage of hydration after prolonged storage a) in water, b) sealed

54

For sealed hcp, self-desiccation occurs at low w/c because of insufficiency of water, hydration stops before it is affected by lack of space for gel. Break-even point for w/cis 0.44.The curves show the final stage (100% hydration) which is rarely achieved. Therefore, hcp contains less cement gel and more unhydrated cement and capillaries than those shown in the figuresUnhydrated cement, not detrimental to strength, results in self-healing property.

55

Water in hardened cement paste

56

Water vapour: in partially filled larger voids

Capillary water:: in capillary pores; bulk water free from attractive forces of solid surfaces. In voids > 50nm (large capilleries)It is free water, and removal does not cause shrinkage In voids < 50nm (small capillaries)capillary tension forces dominate and removal of water may result in shrinkage

Adsorbed water: On solid surfaces under influence of surface attractive forces up to 5 molecular layers (~ thickness of 1.3 nm) Lost on drying to 30% RH and this contributes mainly to shrinkage

Interlayer water: In gel pores < 2.6nm under influence of two surfaces very strongly held. Lost on drying at elevated temperatures and/or to 10% of RH. Causes in considerable shrinkage (Van der Waals forces pull solid surfaces closer together)

Chemically combined water: combined with fresh cement in hydration reactions. Not lost on drying. Heating to very high temperatures evolves this water through decomposition of paste.

57

ADMIXTURES

Chemicals added immediately before or during mixingSignificantly change fresh, early age or hardened properties to advantage Used in small quantities (1-2 % by wt of cement)

PlasticizersWorkability aids; ↑ fluidity or workability of concrete at same w/cWater reducers; ↓ w/c and thus ↑ strength and durability at same workability

58

1) Normal plasticizers; based on lignosulphonates or hydroxycarboxylic acids

2) Superplasticizers; modified lignosulphonates or based on sulphonated melamine or naphtalene formaldehydes– Great increases in workability (flowing concrete) (segregation

occurs if used with high doses of normal plasticizers or high water contents)

– Great decreases in w/c (down to 0.2 at normal fluidity) and thus very large increases in strength (high-strength or high-performance concrete)

59

Plasticizers adsorbe on cement particle surfaces, giving slight negative charges to the surface and thus particles repel each other, breaking up any flocs and causing a better dispersion and wetting of particlesThis results in increased fluidity and slight increase in strength at same w/c ratioPlasticizers may cause retardation of setting time and also may entrain 1-2% air into concrete

60

Accelerators

Increased rate of hardening and enhanced early strength May allow early removal of formwork

May reduce curing time for concrete placed in cold weatherCaCl2 is a popular accelerator. It may cause increased creep and shrinkageProhibited in R.C and P.S.C due to corrosion of steel in presence of chloride ions

61

Typical effects of calcium chloride admixture on (a) setting times, and (b) early strength of concrete

62

Retarders

Delay setting timeCounteracts accelerating effect of hot weather (especially for long transportation distances)Avoids cold joints and discontinuities by controlling setting in large poursSucrase and citric acid and calcium lignosulphonateare examples

63

Air entraining agents

Organic materials which entrain controlled amount of microscopic(less than 0.1mm) bubbles into cement paste of concreteBubbles preserve stability during mixing, transporting, placing,compaction and setting and hardening

Note; entrained air and entrapped air are different

Air entrainment is done for providing freeze-thaw resistance to concreteIn winter time, water in capillary pores expands on freezing resulting in disruptive internal stresses. Successive cycles of freezing and thawing may lead to progressive deterioration. Entrained air, uniformly dispersed in hcp with a spacing factor of not more than 0.2mm, provide a reservoir for water to expandEntrained air volumes of 4-7% by vol. of concrete is required to provide effective protection.

64

Secondary effectsIncrease in workability due to lubricating affect of small air bubbles About 6% decrease in strength for each 1% of air. However, improvement in workability may allow to partially offset the loss in strength by reducing water content and thus w/c ratios

Organic substances reduce surface tension of water and bubbles form during mixingLong chain molecules have hydrophilic and hydrophobic endsThey align themselves radially on surface of air bubble with hydrophilic ends in water and hydrophobic ends in air. Thus they provide air stability

Air bubble

hydrophobic

hydrophilic

65

Cement replacement materials (CRM)Mineral additives that partially replace portland cementCould be by-products from other industries (Economically advantageous)They enhance concrete properties in a variety of ways

Pozzolanic behavior

A pozzolanic material is one which contains active silica (SiO2) and is not cementitious in itself but will, in a finely divided form and in presence ofmoisture, chemically react with calcium hydroxide at ordinary temperaturesto form cementitious compounds

Pozzolanic reaction (Secondary reaction)

S + CH + H → C - S - H

66

1. Fly ash (pulverized fuel ash); ash from pulverized coal used to fire power stations

2. Ground granulated blast furnace slag (ggbs); slag from scum formed in iron smelting in a blast furnace, ground to a powder

3. Condensed silica fume; sometimes called microsilica; very fine particles of silica condensed from waste gases given off in production of silicon metal

4. Natural pozzolans; some volcanic ashes

5. Calcined clay and shale; clay and shale minerals heat treated

Dictionary definitions:Pulverize; to reduce to dust or powder, as by pounding or grindingSmelt ; to melt or fuse (ores) in order to separate the metallic constituents. Husk; the dry external covering of certain fruits or seeds

Types of Cement Replacement Materials

6. Rice husk ash; ash from controlled burning of rice husks after rice grains have been separated

67

Typical composition and properties of cement replacement materials

Fly ash Oxide Low lime High lime

Ggbs Silica fume PC

SiO2 48 40 36 97 20 Al2O3 27 18 9 2 5 Fe2O3 9 8 1 0.1 4 MgO 2 4 11 0.1 1 CaO 3 20 40 - 64 Na2O 1 - - - 0.2 K2O 4 - - - 0.5 Specific gravity, (gr/cm3) 2.1 2.9 2.2 3.15

Particle size (μm) 10-150 3-100 0.01-0.5 0.5-100

Specific Surface (m2/kg) 350 400 15000 350

68

High lime fly ash and ground granulated blast furnace slag are not true pozzolanas. They have certain self-cementing due to high CaOcontent. They may be used at high substitution rates (up to 90%)Low lime fly ash is used at most 40% replacement

Silica fume is used at most 25% replacement (needs superplasticizerto maintain workability)Particles of artificial pozzolanas are smooth surfaced and spherical (Thus they improve workability)

69

Age (days)

Com

pres

sive

Str

engt

h (N

/mm

2 )

70% P.C + 30% F.A

100% P.C

w/c or w/(c+fa) = 0.47

Age (days)

Com

pres

sive

Str

engt

h (N

/mm

2 )

70% P.C + 30% F.A

100% P.C

w/c or w/(c+fa) = 0.47

Pozzolanic reaction and then early strength development is slowWith silica fume, delay is much less due to high surface area and active silica contentAt later ages concretes with cement replacement materials exceed strength of Portland cement only concretesSlower pozzolanic reaction reduces porosity Pozzolanic reaction enhances transition zone between aggregate and cement paste

70

AggregatesDisadvantages of hardened cement paste (hcp); 1. Dimensional instability (high creep and shrinkage)2. High costRemedy to disadvantages; Put aggregates into cement paste → Produce concreteAggregates occupy about 70-80% of total concrete volume

71

Objective;Use as much aggregate as possibleUse largest possible aggregate sizeUse a continuous grading of particle sizes from sand to coarse stones

Thus;Void content of aggregate mixture Amount of hcp required Minimized

Coarse agg.

Fine agg.

72

Concrete composite models

a) Two-phase model for describing deformation behaviorCoarse aggregate dispersed in mortar matrixCoarse and fine aggregate dispersed in hcp matrix

b) Three-phase model for considering cracking and strengthAggregates + hcp + transition or interfacial zone (∼ 50 μm)(cracking and failure starts from interfacial zone, the weakest phase)

73

Types of aggregates

1. According to origin;a) Natural aggregates from natural sand and gravel deposits and

crushed rocks b) Specifically manufactured aggregates such as fly ash pellets,

granulated blast furnace slag

2. According to size;a) Fine aggregate; Particle size from 0 to 4 mm

Ex; natural sand and crushed sand b) Coarse aggregate; Particle size from 4 to 16 or 32 mm

Ex; gravel, crushed limestone

1. According to origin2. Accroding to size3. According to density or specific gravity

Dictionary definition:Pellet; small, rounded or spherical body

74

3. According to density or specific gravity;

a) Normal density aggregates; natural aggregates, Examples; gravels, igneous rocks (basalt, granite), sedimentary rocks (limestone, sandstone)• Mineral composition is not that important • Specific gravities; 2.55 – 2.75 gr/cm3

• Concrete density; 2250 - 2450 kg/m3

• Gravels from deposits in river valleys or shallow coastal watersare directly used after washing and grading, particles are round

• Bulk rock sources (granite, basalt, limestone) require crushing giving angular and sharp particles

Dictionary definitions:Igneous; produced under conditions involving intense heat, as rocks of volcanic origin or rocks crystallized from molten magmaSediment; mineral or organic matter deposited by water, air, or ice.

75

c) Heavyweight aggregates; minerals like barytes to barium sulphate ore and steel shots • To produce high density concrete (3500 to 4500 kg/m3)

(For nuclear radiation shielding)

b) Lightweight aggregates; pumice (a naturally occurring volcanic rock),artificial lightweight aggregates (sintered fly ash, expanded clay or shale, foamed slag)

• To produce lower density concretes (less than 2000 kg/m3)advantages; reduced self-weight, better thermal insulation

• Reduced specific gravity (less than 2.0 gr/cm3) due to voids in particles

• Reduced strength of concrete due to increased porosity

Dictionary definition:Sintering; to form a coherent mass by heating without melting. Barytes; a white or colorless mineral (BaSO4); the main source of bariumOre; a mineral or natural product serving as a source of some nonmetallic substance

76

Grading or particle size distribution (Why do we need that?)

Properties of aggregates

Overall objective– To calculate suitable grading for good workability and stability(continuous grading → low void content)

How much sand?How much crushed

stone?

77

Grading or particle size distribution (Sieve analysis)– Aggregate samples dried, weighed and passed through a stack of

the sieves• Sieve sizes in mm (0.25, 0.50, 1, 2, 4, 8, 16, 31.5)

– Weight of aggregate retained on each sieve measured and converted to percentage retained and then to cumulative

– Then plotted against the sieve size to obtain grading curve

Sieve

SieveShaker

0.25 0.5 1 2 4 8 16 31.50

102030405060708090

100

78

Grading curvesStandards for aggregate define limits inside which the grading curves for coarse and fine aggregate must fall

0.25 0.5 1 2 4 8 16 31.50

102030405060708090

100

0.25 0.5 1 2 4 8 16 31.50

102030405060708090

100

79

1. Limits for Grading (with square opening) Sieve

size 0-8 mm 0-16 mm 0-31.5 mm 0-63 mm (mm) A8 B8 C8 A16 B16 C16 A32 B32 C32 A63 B63 C63 63 100 100 100 31.5 100 100 100 61 80 90 16 100 100 100 62 80 89 46 64 80 8 100 100 100 60 76 88 38 62 77 30 50 70 4 61 74 85 36 56 74 23 47 65 19 38 59 2 36 57 70 21 42 62 14 37 53 11 30 49 1 21 42 57 12 32 49 8 28 42 6 24 39 0.25 5 11 21 3 8 18 2 8 15 2 7 14 2. Limits for Quality (max. %)

Property Fine Aggregate Coarse Aggregate Deleterious Substances

1. Clay lumps 2. Soft particles 3. Coal and Lignites 4. Mud and clay

1.00 -

1.00 3.00

0.25 5.00 1.00 0.50

Sulphate Soundness

1. With Na2SO4 2. With MgSO3

15.00 22.00

18.00 27.00

Abrasion 1. Los Angeles 2. Impact

50 45

Freeze and Thaw (DIN 4226) - 4.00 Organic Impurities: The aggregate may give yellow or lighter color in a 3% solution of, NaOH, but not dark colors

Table. Outline of TS 706 Limits for Concrete Aggregates

80

Example problemDetermine mix proportions of sand and crushed stone such that fineness moduluof mixture will be 4.30.

Sieve size (mm)

Material Passed (%)

0.25 0.50 1 2 4 8 16 31.5

Sand (%) 18 23 28 48 60 90 100 100 Crushed stone (%) 0 0 0 0 5 40 60 100

Fineness modulus; Sum of the cumulative percentages retained on the sieves of the standard series

Fineness modulus ↑ coarser material

81

Fineness Modulus = ------------------------------------------------

For sand = ---------------------------------------

For crushed stone = -----------------------------------

82

Mix proportions by vol: a (for sand), b (for crushed stone)

Using law of simple mixtures; m1a + m2b = mm

a + b = 13.33 a + 5.95 b = 4.30

a = 0.63b = 0.37

Sieve size (mm)Material Passed (%)

0.25 0.50 1 2 4 8 16 31.5

Sand (0.63) 11.3 14.5 17.6 30.2 37.8 56.7 63 63 Crushed stone (0.37) 0 0 0 0 1.9 14.8 22.2 37 Mixture 11 15 18 30 40 72 85 100

83

Limiting grading curves

0.25 0.5 1 2 4 8 16 31.50

102030405060708090

100

84

Grading curves of sand and crushed stone

0102030405060708090

100

0.25 0.5 1 2 4 8 16 31.5

85

Grading curve of the mixture (sand+crushed s.)

0102030405060708090

100

0.25 0.5 1 2 4 8 16 31.5

86

In case of more than two aggregate fractions

1=++ cbam

iiii PcPbPaP =++ 321

mjjjj PcPbPaP =++ 321

Can be extended to as many equation as numberof size fractions which provides full conformity to the desired grading curve

Ideal (desired) grading curvesTS 500 grading curve

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

maxmax

420Dd

Dd

P iii

Fuller parabola

i

ii D

dP 100=

where;Pi = % passing from ith sievedi = opening size of ith sieveDmax= Max particle size (sieve size through which 100% of aggregate passes)

87

Normal weight aggregates contain pores (typically 1-2 % by volume)Particles can absorb and hold water

Completely (oven dry)All pores empty

Air dryPores partially filled

Saturated surface dryAll pores full but no excess water

Saturated or wetexcess water

Field conditions Possible onlyin lab. conditions Field conditions

Absorb some of mix water in freshconcrete

Absorb some of mix water in freshconcrete

No absorbtionand no addition

Add to mix waterin fresh concrete

Aggregate Moisture Conditions

OTHER PROPERTIES OF AGGREGATES1) Porosity and absorbtion

88

Amount of water available for cement hydration, i.e. non-absorbed or free water is of prime importanceTherefore, to ensure the required free water/cement ratio, it isnecessary to allow for the aggregate moisture conditionWhen calculating the amount of mix water;

If aggregate is drier than SSD, extra water must be added If it is wetter, then less mix water is required

2) Elastic properties and strengthElastic properties of aggregates have major influence on elasticproperties of concreteStrength of normal weight aggregates are higher than hcp and do not have major influence on strength of normal strength concreteIn high-strength concrete (greater than 70-80 MPa), strength of aggregates and effect of transition zone between aggregate and hcpbecome seriously important

89

3) Surface characteristics Surface texture have greater influence on the flexural strength than on the compressive strength of the concrete (rougher texture results in a better adhesion)Surface cleanliness is also important for adhesion (surface should be kept clear of the materials such as mud, clay etc.)Better adhesion » stronger interface between aggregate and hcpStronger interface zone » higher mechanical performance

90

Fresh state / early age properties of concrete

Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structureOperations: batching, mixing, transporting, placing, compacting, surface finishing

Treatment (curing) of in-placed concrete 6-10 hours after casting (placing) and during first few days of hardening is important

Fresh state properties enormously affect hardened state properties

91

Main properties of fresh concrete during mixing, transporting, placing and compacting

• Fluidity or consistency: capability of being handled and of flowing into formwork and around any reinforcement, with assistance of compacting equipment

• Compactability: air entrapped during mixing and handling should be easily removed by compaction equipment, such as poker vibrators

• Stability or cohesiveness: fresh concrete should remain homogenous and uniform. No segregation of cement paste from aggregates (especially coarse ones)

Fluidity & compactability known as workabilityHigher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability

92

Compaction of concrete

Finishing of concrete

93

Fill concrete into frustum of a steel cone in three layers

Hand tap concreteIn each layer

Lift cone upDefine slump as downwardMovement of the concrete

Workability measurement methods1. Slump test2. Mini-slump test3. Compacting factor test4. Vebe test5. Flow table test1. Slump test - simplest and crudest test (standardized in ASTM C 143

and EN 12350-2)

94Fig; http://myphliputil.pearsoncmg.com/media/nccer_carpentry_2/module03/fg03_00900.gif

Lift cone up

Define slump as downwardmovement of the concrete

Fig; http://www.arche.psu.edu/thinshells/module%20III/concrete_material_files/image002.gif

95

TrueValid slumpmeasurement0-175 mm

ShearMixes havingtendency to segregate –repeat test

CollapseSlumps greater than175 mm - self-levelingconcrete

Consistency grade Slump (mm) Recommended method of compaction

Stiff, K1 0 - 60 Mechanical compaction like vibration

Plastic, K2 60 – 130 Mechanical or hand compaction (rodding, tampering)

Flowing, K3 130 – 200 Hand compaction or no compaction

Self compacting, K4 ≥ 200 No compaction

96

2. Mini-slump test

• Used for workability testing of cement pastes

• Mini slump cone is a small version of slump cone

• The cone is placed in the center of a piece of glass, paste is cast into cone and then the cone is lifted to measure the average spread of paste.

w/b : 0.2 sp:%2

w/b : 0.22 sp:%2

97

3. Compacting factor test(to distinguish between low slump mixes)

Upper hopper

Lower hopper

Cylinder

approx. 1m

1. Concrete is placed in an upper hopper

2. Dropped into a lower hopper to bring it to a standard state and then allowed to fall into a standard cylinder.

3. The cylinder and concrete weighed (partially compacted weight)

5. The concrete is fully compacted, extra concrete added and then conrete and cylinder weighed again (fully compacted weight)

weight of partially compact concreteCompacting factor = weight of fully compact concrete

98

4. Vebe test

Vebe time is defined as the time taken to complete covering of the underside of the disc with concretecontainer

1. A slump test is performed in a container 2. A clear perspex disc, free to move

vertically, is lowered onto the concrete surface 3. Vibration at a standard rate is applied

99

5. Flow table test(to differentiate between high workability mixes)

1. A conical mould is used to produce a sample of concrete in the centre of a 700 mm square board, hinged along one edge

2. The free edge of the board is lifted against the stop and dropped 15 times

Flow = final diameter of the concrete(mean of two measurements at right angles)

100

Some degree of correlation between the results exist, however the correlation is quite broad since each tests measures the response to different conditions

Correlations between compacting factor, Vebe time and slump

101

A) Behavior of fresh concrete after placing and compacting

From placing to final set, concrete is in a plastic, semi-fluid stateHeavier particles (aggregates) have tendency to move down(SEGREGATION)Mix water has a tendency to move up (BLEEDING)

1. Segregation and Bleeding

EARLY AGE PROPERTIES OF CONCRETE

102

A layer of water (~ 2 % or more of total depth of concrete) accumulates on surface, later this water evaporates or re-absorbed into concrete

BLEEDINGWater-rich pockets

Surfacelaitance

Mix water Cement and

aggregates

Surface laitance; water rich concrete layer hydrating to a weak structure (not good for floor slabs that need to have hard wearing surface)Water-rich pockets; upward migrating water can be trapped under coarser aggregate particles causing loss of strength and local weakening in transition zone

Other effects of bleeding;

103

2. Plastic settlement

Horizontal reinforcing bars may put restraintto overall settlement of concrete

Then plastic settlement cracking can occur

Vertical cracks form along line of the bars, penetrating from surface to bars

Cracks

Reinforcingbars

Plan

Section

104

On an unprotected surface, bleed water evaporates.

If rate of evaporation > rate of bleeding, then surface dries (water content reduces on surface) and plastic shrinkage (drying shrinkage in fresh concrete) will occurRestraint of walls of concrete causes tensile strains in near surface regionFresh concrete has almost zero tensile strength, thus, plastic shrinkage cracking results cracking is in fairly regular “crazing” form

3. Plastic shrinkage

Plastic shrinkage cracking will be increased by greater evaporation rates of the surface water which occurs, i.e. with higher concrete or ambient temperatures, or if the concrete is exposed to wind

105

4) Methods of reducing segregation and bleed and their effects

CAUSES OF BLEEDİNG

Poorly graded aggregate with a lack of fine material with particle size < 300µm

High workability mixes

REMEDIES

1. İncrease sand content2. Air entrain concrete as substitute for fine materials

Provide high workability with superplasticizers rather than

high water contents

Use very fine materials such as silica fume

106

REMEDIES for PLASTIC SETTLEMENT or PLASTIC

SHRINKAGE CRACKS

Revibrate surface region, particularly in large

flat slabs

Apply good curing that stops moisture loss fromsurface as soon as after placing is possible and forfirst few days of hardening

107

B) Curing

Curing methodsSpraying or ponding surface of concrete with waterProtecting exposed surfaces from wind and sun by windbreaks and sunshadesCovering surfaces with wet hessian and/or polythene sheetsApplying a curing membrane, a spray-applied resin seal, to the exposed surface to prevent moisture loss

Curing; proctection of concrete from moisture loss from as soon after placing as possible, and for the first few days of hardening

108

i) Effect of curing temperature

Hydration reactions between cement and water are temperature-dependent and rate of reaction increases with curing temperatureAt early ages rate of strength gain increases with curing temperature (higher temperatures increases rate of reaction, thus more C-S-H and gel is produced at earlier times, achieving a higher gel/space ratio and thus higher strength)At later ages, higher strength are obtained from concrete cured at lower temperatures

(C-S-H gel is more rapidly produced at higher temperature and is lessuniform and hence weaker than produced at lower temperatures)

Standard curing temperature is 22 ± 1 º CHydration proceeds below 0 º C, stop completely at -10 º C

109

ii) Maturity

Cement hydration depends on both time and temperature

10)(T . +Σ= tMaturity → shows correlation with strength

T= -10 ºC is datum lineAt T= -10 ºC, hydration reactions stop, no maturity developedt (hours), T (ºC )Useful in estimating strength of concrete in a structure from strength of laboratory samples cured at different temperatures

110

DEFORMATION OF CONCRETE

Causes of deformation of concrete1) Environmental effects

e.g. moisture movement and heat2) Applied stresses

e.g. short and long term

111

The response of concrete to a compressive stress applied in a drying environment

Load application

Load removal

Elasticstrain

Creepstrain

Elasticrecovery

Creeprecovery

Shrinkagestrain

Stra

in (co

ntra

ction)

t1 t2 Time

112

I → before t1→ net contraction in volume known as shrinkage due todrying, t1→ stress is applied and held constant, t2→ stress is removed(without stress it follows dotted extension beyond t1,differencebetween solid and dotted curves shows effect of loading)

II → on loading → immediate strain response (proportional to stress for low stress level)

III → Compressive strain increases at a decreasing rate, this increase,after allowing for shrinkage represents creep strain

IV → Upon unloading, immediate strain recovery is less than immediate strain on loading.

V → Time-dependent creep recovery

113

1) Drying shrinkage

Loss of capillary water, adsorbed water and interlayer water results in a net volumetric contraction called shrinkageShrinkage is expressed as a linear strain through determination of length change

114

Maximum shrinkage occurs on first drying, considerable part of this is irreversibleThere is a continuous but small swelling of the hcp on continuous immersion in waterAs the strength of the hcp increases → less shrinkage and swellingLow w/c; high degree of hydration; age - decreases porosity

- increases strength- decreases shrinkage

wetting

dryingwetting

wettingdrying

drying reversibleshrinkage

firstdrying

swelling

shrinkage

Initial dryingshrinkage

Time

Swelling with continuous immersion

115

Mechanisms of shrinkage and swelling

1) Capillary tension

Free water surfaces in capillary pores will be in surface tension and upon drying due to drop in ambient vapor pressure free surface becomes more concave and surface tension increases

r R2T )ln(

0 θρ=

PP

Kelvin’s equation

P0: vapor pressure over a plane surfaceT : surface tension of liquidR : gas constant θ : absolute temperatureρ : density of the liquidr : the radius of curvature

d

p0, (r = ∞) p, r

p1, r = d/2

116

Tension within water near meniscus = 2T/r which must be balanced by compressive stresses in surrounding solid.

If evaporation ↑ compressive stresses ↑ shrinkage ↑

Exposing hcp to a steadily decreasing vapor pressure, pores gradually empty starting with the widest first

Higher w/c pastes will shrink mored

p0, (r = ∞) p, r

p1, r = d/2

117

2) Surface tension or surface energy

Surface of both solid and liquid materials is in a state of tensionTo increase surface area, work should be done against this tension forceSurface tension forces induce compressive stresses=2T/r which are important in hcp whose average particle size is very smallAdsorption of water molecules onto the surface of the particles,reduces surface energy and reduces balancing internal compressive stresses leading to an overall volume increase, i.e. swelling

118

3) Disjoining pressure

Water inside gel pores is under influence of surface forces 5 molecules (1.3nm) thick adsorbed water forms on solid surface at saturation which is under pressure from the surface attractive forces. In regions narrower than twice this thickness (~ 2.6nm) interlayer water will be in hindered adsorption resulting in development of swelling or disjoining pressure which is balanced by tension in interparticle bondOn drying, thickness of adsorbed water layer drops and reduces disjoining pressure. This results in an overall shrinkage

Capillary water

Disjoining pressure in region of hinderedadsorptionCapillary

tension

Adsorbed waterlayer

119

4) Movement of interlayer water

Interlayer water has an intimate contact with solid surfaces , to move interlayer water, high energy is neededMovement of interlayer water likely results in significantly higher shrinkage than movement of equal amount of free or adsorbed water

120

Suggested shrinkage mechanismsRelative humidity (%) Source 0 10 20 30 40 50 60 70 80 90 100

<-------------------------------- disjoining pressure -------------------------> Powers (1965) <------------ capillary tension -------->

Ishai (1965) <------surface energy-------> < ------------- capillary tension -----------> Feldman and Sereda (1970) <---interlayer water---> < --capillary tension and surface energy ->

Witmann (1968) <----surface energy---> < -------- disjoining pressure -------------->

Opinion is divided on the relative importance of the above mechanisms and their relative contribution to the total shrinkage

121

1) Drying shrinkage of concrete

Aggregate → dimensionally stableAggregates put restraint to shrinkage deformation of hcp in

concrete

Degree of restraint depends on; aggregate volume concentrationmodulus of elasticity of aggregate

In hcp; unhydrated cement grains also act as a restraint

Drying shrinkage of concrete < drying shrinkage of cement paste

a) Effect of mix constituents and proportions

122

Influence of aggregate content in concrete on the ratio of the shrinkage of concrete to that of neat cement paste

Range for normal concrete

Aggregate content (% by volume)

Shrink

age

ratio

Normal concretes have a shrinkage of 10-30 % of that of neat paste

123

Normal-weight concreteLightweight concrete

Elastic modulus of aggregate (kN/mm2)

Shrink

age

afte

r 1

year

of

drying

(micro

stra

in)

Normal density aggregates have higher stiffness(higher E – modulus)They give more restraint to concreteLightweight aggregate concretes tend to have higher shrinkage

124

Combined effects of aggregate volume ratio and stiffness

npc g

shsh)1( −= εε

concrete ofstrain shrinkage =shcε

hcp ofstrain shrinkage =shpε

content volumeaggregate =g

hcp of ratio poissons =pμaggregates of ratio poissons =aμ

hcp of elasticity of modulus =pEaggregates of elasticity of modulus =aE

7.12.1) 2-(1 2 1

) 1( 3

p

p −=++

−=

a

pa E

En

μμ

μ

125

b) Effect of specimen geometry

Size and shape of concrete specimen influence rate of drying anddegree of restraint from the core, e.g. a member with a large surface area to volume ratio will dry and shrink more rapidlyRestraint from central core of a concrete element which has higher moisture content than the surface puts the surface into tension.Thus, under these tensile stresses, surface cracking may occurC3A and sulphate content of cement affect the shrinkage, also, alkali content and fineness has a significant effect

126

2) Autogenous shrinkage

Continued hydration with an adequate supply leads to slight swelling of cement pasteConversely, with no moisture movement to or from the cement paste, self desiccation leads to removal of water from the capillary pores and autogenous shrinkageIts magnitude is an order of magnitude less than that of drying shrinkageMore pronounced in concretes with low water-to-cement ratios

127

CO2 + H2O → H2CO3

H2CO3 + Ca (OH)2 → CaCO3 + 2H2O

H2O is → weight of paste ↑released shrinkage accompanies,

strength ↑and permeability ↓

Max carbonation at 25-50 % RHIf saturated, H2CO3 can not penetrate concreteIf dry H2CO3 can not form

3) Carbonation shrinkage

Carbonation shrinkage is not a result of loss of water, its cause is chemical

Explanation;Ca(OH)2 is dissolved from stressed region resulting in shrinkage CaCO3 crystallizes in pores reducing permeability and increasing strength

128

Thermal ExpansionCement paste and concrete expand on heating

Thermal expansion of cement pasteCoefficient of thermal expansion of hcp=10 -20 x 10-6 / ºCThe value depends on moisture contentDisturbance of equilibrium between water vapor, free water, freely adsorbed water,water in areas of hindered adsorption and forces between layers of gel solids willdetermine the behavior of cement paste upon being heated.

Thermal expansion coefficient is needed in two main situations;

1. To calculate stresses due to thermal gradients arising from heat of hydration

2. To calculate overall dimensional changes in structures

129

Thermal expansion of concrete

Coefficient of thermal expansion of most rocks = 6 - 10 x 10-6 / ºCTherefore, coefficient of thermal expansion of concrete is less than that of hcpSince aggregate occupies 70- 80 % of concrete volume, effect of humidity is very much reduced, therefore, we assume a constant coefficient of thermal expansion for concrete. This value depends on concrete mix proportions, cement paste content and aggregate type.At temperatures higher than ~ 60 ºC, differential stresses set up by different thermal expansion coefficients of paste and aggregate can lead to internal microcracking

130

Stress-strain behavior

Elasticity; hcp has a near linear compressive stress-strain relationship, modulus of elasticity can be determined from stress-strain data

3c )p-(1 gp EE =

paste of elasticity of modulus =pE 0P when elasticity of modulus c ==gE

porosity apillary cpc =

(represents the modulus of elasticity of the gel)

131

Effect of w/c and age on the elastic modulus of hcpEl

astic

mod

ulus

, (k

N/m

m2 )

10

20

30

0.20 0.30 0.40 0.50 0.60water/cement ratio by weight

37142860

Age(days)

132

Models for concrete behaviorConcrete is a composite multiphase materialElastic behavior depends on elastic properties, relative proportions and geometrical arrangement of; a) unhydrated cement, b) cement gel, c) water, d) fine aggregate, e) coarse aggregate

The model for the concrete behavior requires the following1. The property values for the phases

a. The elastic modulus of the aggregate, Eab. The elastic modulus of the hcp, Epc. The volume concentration of the aggregate, g.

2. A suitable geometrical arrangement of the phases

Several models (unit cubes) are proposed to predict average behavior Assumption for model analysis;1) Applied stress remains uniaxial and compressive throughout the model2) Effects of lateral continuity between layers can be ignored3) Any local bond failure or cracking does not contribute to deformation

133

Hansen’s models;

g

Matrix (hcp) and distributed phase (aggregates) arrangedparallel with direction of loading. Phases undergo same strain

Known as; Parallel-phase modelEqual strain model

Strain compatibility;εc = εa = εp

Equilibrium;Total force = Σ force on each phaseExpressed in terms of stresses and area

σ.1= σa.Va + σp.(1-Va)

Model IMatrix Aggregate

134

Constitutive relations;both phases and concrete are elastic

cc E εσ =c aa E εσ =a pp E εσ =p

Substituting into equilibrium equation)1(E E E ppaacc aa VV −+= εεε

From compatibility equation)1(EEE pac aa VV −+=

135

Matrix (hcp) and distributed phase (aggregates)arranged in series with direction of loading Phases are subjected to same stress

Known as; Series - phase modelEqual stress model

Equilibrium;σc = σ a = σ p

Strains;Total displacement = Σ displacements in each of the phasesexpressed in terms of strainεc = ε a.Va + ε p. (1-Va)

Model II

g

σ

Substituting from constitutive and equilibrium equations and rearranging gives

p

a

a

a

c EV

EV

E)1(1 −

+=

136

Counto’s model;

Aggregate set within hcp complying with volume requirements

Greater resemblance to concreteCombination of Hansen’s two models

)1()1(1

apaa

a

p

a

c VEVEV

EV

E −++

−=

137

Allpaste

Allaggregate

Ec Ea

ModelA

C

B

0 0.5 1.0

Volume concentration of aggregate, Va (g)

Ep

Models A & B give upper and lower boundaries to concrete modulus of elasticity. Model C gives intermediatevalues.

The effect of volume concentration of aggregate on elastic modulus of concrete calculated from simple two-phase models

138

1 5 10

1

2

3

4

5

Model A

C

B

Ea/Ep

E c/E

pPrediction of elastic modulus of concrete (Ec) from the modulus of the cementpaste (Ep) and the aggregate (Ea) for 50 % volume concentration of aggregate

139

Observed stress-strain behavior of concrete

Cementpaste

Concrete

Aggregate

1000 300020000

10

20

30

40

Strain (μs)

Stre

ss,

N/m

2

Stress-strain relationships for cement paste, aggregates and concrete “typical behavior of hcp, aggregate and concrete”

Stress-strain behavior of both aggregates and cement paste is substantially linear almost up to maximum

Composite concrete with intermediate stiffness is markedly non-linear

140

Strain

Stre

ss

Working stress

B

C

A

x0

First cycleSecond cycle

Stress-strain relationships for cement paste, aggregates and concrete “typical short term behavior of concrete”

Unloading/loading cycles show substantial ,but diminishing hysteresis loops.Explanation lies in contribution of microcracking to overall concrete strainsTransition zone is a region of weakness and ever before loading some microcracks occur in this zone. Bleeding, drying and thermal shrinkage determine the number and width of these cracks.As stress level increases, these cracks increase in number, length and width, causing progressively increasing non-linear behavior

141

Elastic modulus of concrete

C

AB

Strain

Stre

ss

Different definitions for elastic modulus,-A: slope of tangent to the curve at any point

(tangent modulus)-B: initial tangent modulus-C: slope of the line between the origin and

a point on the curve (Secant modulus)

According to the testing method,- static modulus of elasticity- dynamic modulus of elasticity

142

n: fundamental resonant frequencyl: length of specimenρ: density of concrete

ρ224 lnEd =

Dynamic modulus of elasticity approximates to the initial tangent modulus (line B).It is higher than secant modulus

• Static modulus of elasticity: secant modulus is calculated from readings of strain at a stress at 40% of ultimate strength. Cylindricalor prismatic specimens are used and loaded longitudinally with a static load

• Dynamic modulus of elasticity: dynamic test is applied to a prismatic specimen and dynamic elastic modulus is calculated as

143

Modulus of elasticity increases with age and decreasing w/c. Thus, increasing compressive strength of concrete results in increased modulus of elasticity

Proposed relations for mean moduli;

cud fE 16.031+=

cuc fE 2.020 +=

(GPa) E ),(MPafcu

Ed= dynamic modulus of elasticityEc= static modulus of elasticityfcu= ultimate compressive strength (28 day strength)

144

for water-saturated cement paste, 0.25 ≤ μ ≤ 0.30 on drying it reduces to 0.2μ is largely is dependent of w/c, age and strength

Anson proposed the relation;n

apc V )1( −= μμfor μp = 0.22, n = 0.42

Poisson’s ratio

μc = poisson ratio of concreteμp = poission ratio of cement pasteVa = volume of aggregates

145

Creep

Magnitude of creep strains is as great or greater than elastic strains on loading. Therefore, they have a significant influence on structural behaviorCreep and shrinkage are interdependent, creep is higher while the concrete is simultaneously drying

146

εsh

(a)

Stra

in

Free shrinkage (no stress)

εbc

(b) Basic creep (stress, no loss of moisture)

(c)Time

Total strainεcr

εdc (drying creep)

εsh

εbc

Total creep stress and drying

εsh: free shrinkage of unloaded concrete in drying condition

εbc: basic creep under loading with no drying (sealed concrete)

εdc: drying creep under loading and drying condition

Total creep strain;εcr= εbc + εdc

Total strain;εtot= εsh + εcr

= εsh + εbc+ εdc

147

Factors effecting creep1. Moisture content before loading: completely dried concrete has very small

creep2. Level of applied stress: creep increases almost linearly with applied stress

up to stress/strength of 0.40 to 0.603. Concrete strength: increasing strength decreases creep4. Temperature: increasing temperature increases creep significantly5. Aggregate volume concentration: aggregate is inert in creep. Hence,

creep of concrete is less than that of cement paste

6. Elastic modulus of aggregate: increasing modulus of aggregate decreases creep of concrete

Neville proposed a relationship:εcr= εcrp (1-Va)n n = 1.7 – 2.1

148

Mechanisms of creep1) Moisture diffusion: applied stress changes internal stresses and upsets the

thermodynamic equilibrium in hcp. Moisture then moves from smaller to larger pores, resulting a. Pressure drop in capillary poresb. Adsorbed water gradually moving from zones of hindered adsorptionc. Interlayer water diffusing slowly out of the gel pores

2) Structural adjustment: stress concentrations in hcp may cause consolidation due toa. Viscous flow with adjacent particles sliding past each otherb. Local bond breakage followed by reconnection nearby

3) Microcracking: defects and cracks existing in concrete before loading propagate and form new cracks and this contributes to creep strains

4) Delayed elastic strain: active creeping component (water in capillary or gel pores) act parallel with elastically deforming component (unhydrated cement particles, gel particles, calcium hydroxide crystalls) Load will be transferred to inert material which then deforms elastically

149

Overall Outline

• Introduction• Concrete• Bituminous materials• Masonry • Polymers and polymer composites• Cement-based fiber composites• Metals• Timber

150

Chapter Outline

• CONCRETE– Cement– Admixtures– Aggregates– Strength and failure of concrete– Durability of concrete– Statistical quality control in the production of

concrete– Property composition relations for concrete and

concrete mix design

151

STRENGTH AND FAİLURE OF CONCRETE1) Strength tests

a) Compressive strength testb) Tensile tests

i. Direct tension testii. Flexural testiii. Splitting tensile test

2) Factors influencing strengtha) Transition zoneb) Water to cement ratio

i. Prediction of concrete strength c) Effect of aged) Effect of humiditye) Effect of aggregate properties, size and volume

concentration3) Cracking and fracture in concrete

152

STRENGTH AND FAILURE OF CONCRETE

Strength of concrete is most important because structural elements must carry imposed loads safelyIn loading test: max stress = strength of specimen

Also, strength is related to: elastic modulus, durability (permeability)Different types of loading in structures result in different modes of failureRelevant strengths: Compressive, tensile, torsional (shear), fatigue, impact, strength under multiaxial loading

Under compressive loading, at max stress, test specimen is still whole (with extensive internal cracking). Complete breakdown subsequently occurs at higher strains and lower stress

http://www.ame-geolab.com/CRIB%20MATERIAL%20PHOTOGRAPHS%20testing%20013.jpg

153

1. Strength tests

a) Compressive strength test: Cubes (100x100x100 mm or 150x150x150 mm)Cylinder: (D= 100mm, H= 200mm or D=150mm, H= 300mm)Steel moulds are used, concrete compacted top surface smoothed

SteelPlatens

1. Cubes or cylinder are placed between steel platens of loading machine

- Cubes are loaded on two parallel smooth steel moulded surfaces- Cylinders are loaded on top and bottom surfaces. Top surface is a trowelled surface.

Therefore it must be capped either with plaster paste or with molten sulfur

2. Load is increased to its ultimate value in a few minutes

154

Cracking patterns during testing of concrete specimens in compression

Cube (H/D=1)

Cylinder (H/D=2)

Cracking pattern of cubes and cylinders show that CUBES are not in uniaxial loadingThis is due to end restraint because of friction between loading platens and concrete surface. This induces lateral tensile stress in both platen and concrete due to Poisson effect. Concrete is in a triaxial stress state with consequent higher strength compared to uniaxial stress caseIN CYLINDERS, there is a centre portion not affected from end restraint. Thus, this central zone is in uniaxial stress state. Therefore, cylinder strength ≅ 0.80 cube strength

Cracking at approx. 45° to axis near ends

Cracking parallel to loads away from ends

155

Height/diameter ratio1 2 3 40

0.8

1.0

1.2

1.4

1.6

1.8

General relationship between height/diameter ration and compressive strength of a concrete cylinder, (Gonnerman, 1925)

Slenderness ratio (h/d) affects strengthAvoid h/d < 1 for test specimensh/d = 1 for cubes, h/d = 2 for cylinders

Height

diameterRelative

st

r.

156

b. Tensile tests

i. Direct tension testResults are not very dependable (due to eccentric loading and failure at grips) not very much used

P

PCross-sectional Area A

fd = P/A

Tensile behavior of concrete can be evaluated by;i. Direct tension testii. Flexural testiii. Splitting tensile test

157

ii) Flexural test

L

P

fb

End area = bxd

-

+

Stress distribution

Modulus of rupture = 2bdPLfb =

Prism specimen(100x100x400mm or 150x150x600mm)

Load is applied at third pointsFailure occurs when flexural tensile crack at bottom of beam propagatesupwards throughout beamModulus of rupture > direct tensile strength

158

iii) Splitting tensile test

compressiontension

Stress distribution

ddlPf s π

2=

Cylinder specimen (φ150mm, height 300mm)Placed on its side in a compression testing machine and loaded across its verticaldiameterStress distribution on a plane of vertical diameter is a near uniform tensionFailure occurs by split or crack along this plane

For the same compressive strength:Modulus of rupture > cylinder splitting > direct tension

159

10 20 30 40 50 60 700Compressive strength (N/mm2)

Modulus of rupture

Cylinder splitting

Direct tension

Ten

sile s

tren

gth

(N/m

m2 )

2

4

6

8

Typical relationships between tensile and compressive strengths of concrete

160







161

Aggregate

hcp

Transition zonearound aggregate particles

Use 3-phase model when considering strength of concrete

Transition zone (~50μm wide) is the weakest phase, cracking and failureinitiate in this zoneDue to drying shrinkage, cracks are present before loadingAs loading increases in compression or tension, cracks in this zone start propagating into hcp, resulting in paths through concrete

Typical crack path through normal strength concrete

2. FACTORS INFLUENCING STRENGTH

a) Transition Zone

162

b) Water/cement ratio:Strength of concrete depends on;

Strength of hcpStrength of aggregatesStrength of transition zone

Strength of hcp is governed byPorosity (indirectly depending on w/c)degree of hydration

Strength of transition zone is dependent on;w/c

163

Abrams; 1/

2c W C

kfk

=when w/c (by wt)k1 & k2 empirical constants depending onage, curing regime, type of cement, entrained air, test method, aggregate typeand size

Graf; 2( / )cc

cG

ffK W C

=where fcc = compressive strength of cementKG = empirical constant related to testing condition KG = 4-8, average = 6

i) Prediction of Concrete Strength

Various relationships are given by several researchers

164

Feret;

2)(vwc

cKf Fc ++=

when c, w, v → volumes of cement, water, and airvoids in 1cm3 of concrete

KF = empirical constant depending on age, type andamount of cement

KF = 80 – 300 MPafor 7 days strength KF = 150 MPafor 28 days strength KF = 180 MPa

Bolomey;

`)( kvW

CKf Bc −+

=

where c, w → weights of cement and waterv → volume of air voids

KB = empirical constant depending on age, type and amount of cement

KB = 7 – 35 MPafor 7 days strength KB = 15 MPafor 28 days strength KB = 19 MPak`=0.5

165

Mix design methods use one of these formulas or curves or tables based on these equations to estimate w/c required for a given strengthTo achieve a homogenous, cohesive concrete without significant segregation w/c < 10.4 < w/c < 0.7 → produce concretes of normal to medium strength (20-50 MPa)w/c ≈ 0.20 – 0.30 → high or very high strength concretes are produced (70 - 150 MPa)

By using superplasticizer to achieve adequate workabilityBy incorporating silica fume at 5-10 % cement replacement to improve properties of transition zone while also increasing strength of cement paste to a limited degreeBy selecting aggregate having high inherent strength and good bondcharacteristics

With this approach, at w/c ≤ 0.26, compressive strengths > 130 MPa are achieved and placed with conventional mixing, transporting and compaction methods, but with extreme care given to high standards of site practice and supervision and quality control

166

c) Effect of ageHydration reaction continues in time with decreasing rateThus even after years, in presence of moisture, there will be some strength increase

d) Effect of humidityCuring in water results in higher strength compared to air curing Moisture all through the life of concrete provides higher strengths

167

e) Effect of aggregate properties, size and volume concentration

Aggregate strength becomes important in high strength concretes. WHY??

Increasing volumetric proportion of aggregate in the mix, at a constant w/c, produce a relatively small increase in concrete strength. There is a maximum limit to aggregate content (~ 80) for practical concretes

Larger maximum aggregate size reduces strength at lower w/c. In high strength concretes, Dmax is limited to 8 or 16 mm. WHY???

With some carbonate and siliceous aggregates, structure and chemistry of the transition zone is influenced by aggregate mineralogy and surface conditions. Increased surface roughness improves bonding due to mechanical interlocking. Thus concretes with crushed rock aggregates have about 15 % higher strengths than concretes with smooth gravel aggregates

168

3. CRACKING AND FRACTURE IN CONCRETE

Stage 4

Stage 3

Stage 2

Stage 1

Stre

ss (% u

ltim

ate)

30

50

75

100

Strain

Stress – strain behavior of concrete under compressive loading: a) from Glucklich (1965); b) From Newman (1966).

Transverse

Longitudinal

Volumetric

169







170

Stage 1: Below 30% of ultimate load transition zone cracks remain stable, stress-strain curve is approximately linear

Stage 2: Between 30-50% of ultimate load. Cracks increase in length, width and number. However, they still remain stable, non-linearity is observed.

Stage 3: Above 50% of ultimate load, cracks start to spread into matrix, towards 75% of ultimate load, cracks become unstable and curve further deviates from linearity

Stage 4: Unstable crack growth and propagation is frequent, leading to high strains. Transverse strains start increasing faster than longitudinal strains resulting in an overall increase in volume

171

Strain

Stre

ss

MortarConcrete

hcpCoarse aggregate

Typical stress-strain characteristics of aggregate, hardened cement paste, mortar and concrete under compressive loading

172

Chapter Outline



concrete mix design

173

Durability of concreteOutline• Transport mechanisms through concrete• Flow processes• Degradation of concrete

1) Durability against freezing and thawing2) Durability against chemical action3) Durability against very high temperatures

• Durability of steel in concrete– Principles of corrosion of steel in concrete– Two stages of corrosion damage

• Carbonation induced corrosion• Chloride induced corrosion

174

Durability of concrete

Durability: ability of a material to remain serviceable for at least the required lifetime of the structure

Concrete is not inherently of high durabilityDegradation of concrete arises from;

Environment to which concrete is exposedInternal causes within concrete

Most important factor is the rate at which moisture, air or other aggressive agents can penetrate the concrete

175

Transport mechanisms through concrete

Porous impermeable material Porous permeable material

High porosity, low permeability Low porosity, high permeability

Hcp and concrete contains pores of varying types and sizes. Rate of flow (permeability) depends on not only porosity but the degree of continuity of pores andtheir size

176

Flow process depend on degree of saturation of hcp or concrete

At very low humidities: moisture is in vapor state and adsorbed on dry surfaces of the paste

with increasing humidity; adsorption is completed. Flow is taking place through thepore as a direct vapor movement due to pressure gradient

Vapour flow Liquid flow Adsorbed phase

177

At humidity sufficient for water condensation:vapor flow is enclosed through a shorter path

At increasing humidity: condensed water zone extend and flow is augmented by transfer in adsorbed layer

178

At increasing humidity: liquid flow under pressuregradient in incompletely saturated state

Finally at high humidities: liquid flow under pressure gradient in completely saturated state

179

Flow processes

Movement of a fluid under a pressure differential – i.e. permeationMovement of ions, atoms or molecules under a concentration gradient, i.e. diffusionCapillary attraction of a liquid into empty or partially empty pores, i.e. sorption

180

Flow processes

1) Flow or movement of a fluid under a pressure differential

xhKux ∂

∂−=Darcy’s law

For flow in x direction, ux = mean flow velocity

xh

∂∂

= rate of increase in pressure head in x-direction

K = coefficient of permeability (m/sec)

181

2) Movement of ions, atoms or molecules under concentration gradient, process of diffusion governed by Fick’s law

xcDP

∂∂

−=

For flow in x direction, P = transfer rate of substance per unit area normal to x-direction

gradiention concentrat=∂∂xc

D=diffusivity (m2/sec)

182

3) Adsorption and absorption of a liquid into empty or partially empty pores by capillary attraction

2/1Stx =

where, x = depth of penetrationS= sorptivity (mm/sec1/2)t = time (sec)

183

Exposure zonePrimary transport mechanisms

Atmospheric Gas diffusionWater vapour diffusion

Splash

Tidal

Submerged

Water vapour diffusion ionic diffusion

Water vapour diffusionWater absorption ionic diffusion

Ionic diffusionWater permeability

Primary transport mechanisms in the various exposure zones of a concrete offshore structure

184

Coef

ficien

t of

per

mea

bilit

y

10-12

10-10

10-8

10-6

0 10 20 30

Age (days)

The effect of hydration on the permeability of cement paste (w/c =0.7)

Capillary porosity (%)

Coef

ficien

t of

per

mea

bilit

y (x

10-1

3 m/s

ec)

0 10 20 30 40

1

2

3

4

5

67

8

910

The relationship between permeability and capillary porosity of hardened cement paste

Permeability; measure of the ability of a material to transmit fluids

185

0.3 0.5 0.7Water/cement ratio

Perm

eabi

lity

(x 1

0-13

m/s

ec)

2

4

6

8

10

The relationship between permeability and water/cement ratio of mature cement paste (% 93 hydrated)

186

Degradation of Concrete1) Durability against freezing and thawing

(important in cold climates)

Damage occurs due to;freezing and thawing cycles of water in capillary pores and entrapped air voids of the cement pastewater in the pores of aggregates may also freeze and affect the durabilityof concrete against frost actionwater in the gel pores is adsorbed on CSH surfaces and does not freezeuntil temperature drops to about -78 ºC. After capillary water has frozen, it posses a lower thermodynamic energy than still-liquid gel water, whichtherefore tends to migrate to supplement capillary water and thus increasedisruption

Water – cement ratio is a controlling factor of durability of concrete against freezing and thawing cycles because its magnitude determines the amount and size of the capillary pores in the cement paste. For this reason, water-cement ratio is limited in specifications for durability of concrete against frost action

187

Max. values of w/c for durability against frost action

Climatic condition Severe Mild

In air In water In air In water Types of structures

Fresh Salty Fresh Salty Thin elements 0.49 0.44 0.40 0.53 0.49 0.40 Medium size elements 0.53 0.49 0.44 - 0.53 0.44 Exterior faces of mass concrete (large elements)

0.58 0.48 0.44 - 0.53 0.44

188

Air-entrainment into concrete increases the durability of concrete against freezing and thawing. Optimum percentage of air-entrainment is about 4-6 % by volume. Air entrainment is achieved through some chemical admixtures and thus use of air-entraining admixtures has been a rule for concretes subjected tosevere climatic actionFreezing of the water in the pores of the aggregates may also damage concrete via damaging aggregate particles.Durability of aggregate against frost action depends upon its pore characteristics and saturation degree. Indirect or direct tests of freezing and thawing are used to measure durability of aggregates

189

The effect of air entrainment and water/cement ratio on the frost resistance of concrete moist-cured for 28 days (US Bureau of Reclamation, 1955)

Air entrained

Not air entrained

Water/cement ratio0.4 0.5 0.6 0.7 0.8

1000

2000

3000

4000

Num

ber o

f fre

eze-

thaw

cyc

les

for

25%

wei

ght l

oss

190

Therefore, to make concrete durable1) Cement paste should be made durable

(↓ w/c, air-entrain, pozzolanic materials)2) Use durable aggregates3) Apply good curing to concrete

Concrete should be tested for durability against frost actionPrismatic specimens are subjected to 300 cycles of freezing at -17 ºC and thawing at + 4 ºC, a cycle completed in 4 hours. Specimens are tested non-destructively from time to time by measuring dynamic modulus of elasticity through resonance frequency testing. Decrease in Ed with respect to control concrete indicates the degree of durability against frost action

191

Freeze – thaw cabin

192

Concrete in industrial buildingsConcrete containing contaminated aggregatesConcrete in contact with sulphate soils or sulphate ground watermay be subjected to the harmful effects of some chemical compounds

2) Durability against chemical action

Degree of action of certain chemicals in concreteDegree of Action Chemical Weak Strong Very Strong

pH value 6.5 – 5.5 5.5 – 4.5 < 4.5 CO2 in water, mg/lt 15 – 30 30 – 60 > 60 NH4 in water, mg/lt 15 – 30 30 – 60 > 60 Mg+2 in water, mg/lt 100 – 300 300 – 1500 > 1500 SO4

-2 in water, mg/lt 200 – 600 600 – 3000 > 3000 SO4

-2 in soils, mg/kg 2000 - 5000 > 5000 -

193

Sulphates can react with hydrated aluminate phases in hardened cement paste to produce ettringite through reaction;

3 18 3 32. . 2CH 2S 12H C A3CSHC ACS H + + + →

OHCaSOOCaOAlOHSOOHCaOHCaSOOCaOAl 24322322432 3233122)(2183 →+++

This is an expansive reaction, causing disruption

NaOHOHCaSOOHOHCaSONa 222)( 242242 +→++

224224 )(22)( OHMgOHCaSOOHOHCaMgSO +→++

Reactions of other forms of sulphates can occur with CH in hcp forming gypsumagain with an increase in volume;

GypsumSeverity of attack depends on the type of sulphateDamage of attack by MgSO4 > Damage of attack by Na2SO4 > Damage of attack by CaSO4

194

Deterioration due to sulphates decreases with;

⇓ C3A contentHigher cement content and lower water-cement ratio. WHY??Cement replacement materials decreases permeability and improvesresistance to sulphates

195

To resist the weak action;Concrete properly designed and well consolidated and curedw/c should be less than 0.60

To resist strong action;Concrete should be properly designed and well consolidatedw/c should be less than 0.50Lateral dimension of the structural elements should be enlarged to ensure some protective coverSpecial cements & cement replacement materials should be used

To resist very strong action; In addition to all above, protect the concrete by applying a protective cover

196

3) Durability against very high temperatures

Concrete maybe subjected to very high temperatures above 1000ºC in the case of;

Fire of buildingsPavements of air fieldFurnaces or chimneys of industrial buildings

Rate of degradation depends on;Maximum temperature Concrete constituents Size of element

197

For the cement paste

a) at 105ºC, capillary and gel water has been lost and shrinkage occurs

b) at 250-300ºC, aluminous and ferrous constituents of cement loose crystal water

c) at 400-700ºC, siliceous constituents loose crystal water and shrink meanwhile Ca(OH)2 looses its water and converts to CaO. After cooling, if it is wetted, it again converts to Ca(OH)2 and expand. These may result in severe cracking in hcp.

198

For the aggregates

a) Aggregates expand very mildly with increasing temperatureb) At 550ºC, some siliceous aggregates may expand excessively

due to a change in crystal structurec) At 900ºC, limestone aggregates may be calcined (separated

from CO2)d) Basalt, blast furnace slag, crushed firebrick usually don’t

change their volume extensively up to and well above 1000ºC

199

To produce a concrete durable against medium temperatures (500-600ºC)A proper design must be achieved Use limestone aggregates

To produce a concrete durable against higher temperatures (900-1100ºC)A refractory concrete should be designedStabilizing agent (shamotte earth, powdered silica, etc.) should be used in place of some fine aggregateUse basalt, blast furnace slag or crushed firebrick

On the other hand, aluminous cement is very profitable for refractory concrete

200

Durability of steel in concreteSteel exist in concrete for reinforcing the concrete to compensate for weakness of concrete under tensile and shear stressesSound concrete provides excellent protective medium for steel.If protection is broken, steel is left vulnerable to corrosion. Corrosion products, being expansive, caused cracking and/or spallingof concrete, exposing steel to more rapid corrosion

http://cce.oregonstate.edu/structural/images/corrosion_1.JPG

201

Principles of corrosion of steel in concrete

Metal

Oxygenatedwater

O2Fe2O3.H2O

Fe(OH)2

Fe++ 2OH-

1/2O2

H2O

Cathodicarea

Anodicarea

Fe2e-

Spacial arrangement of corrosion reactions of iron in moist air or oxygenated water

Anode reaction;2Fe → 2Fe++ +4e-

Cathode reaction;4e + O2 + 2H2O → 4OH -at some distance from surface2Fe++ + 4OH -→ 2Fe (OH)2 (Ferrous hydroxide (black rust))Followed by4Fe(OH)2 + O2 → 2Fe2O3H2O + H2O (Ferric hydroxide (red rust))

202

Electrolyte is the pore water in contact with steelNormally highly alkaline (pH = 12-13) due to Ca(OH)2 from the cement hydration and the small amounts of Na2O and K2O in cementIn such a solution, anodic reaction gives out Fe3O4 instead of Fe++. Fe3O4 is deposited at metal surface as tightly adherent thin film and stops any further corrosion (Steel is said to be passive)

Concrete passivation may be broken by;A loss of alkalinity by carbonationChloride ions

203

Different forms of damage from steel corrosion

cracking spalling

lamination corner effects

204

Two stages of corrosion damage

Level of corrosion tocause concrete cracking

t0 t1age

Corr

osio

n

t0 = time for depassivating agents to reach steel and initiate corrosion t1 = time for corrosion to reach critical levels, sufficient to crack concrete

205

Carbonation induced corrosion

Carbonation first occurs on the surface and progresses inwardsReaction is a diffusion controlled process

2/1 tkx = k = constantx = carbonation deptht = time

High quality, well cured concrete is subjected to limited carbonation(only 20 – 30mm after years)

206

Chloride induced corrosion

Common sources of corrosionCalcium chloride (an accelerating admixture)Contamination in aggregatesSea water for coastal and marine structuresRoad deicing salts (on bridge decks)

First two enter into concrete during mixing and steel may never be passivated (t0=0)Last two have to penetrate the concrete cover sufficiently to depassivate the steel (t0=finTransport mechanisms are permeability, diffusivity, sorptivity

207

If protection against corrosion can not be guranteed, then Use corrosion inhibiting admixture (calcium nitrate)Use corrosion resistant stainless steel bars or epoxy coated bars Cover concrete surface by a protectorApply cathodic protection

208

Chapter Outline



concrete mix design

209

Statistical quality control in the production of concreteOUTLİNE

• TS 500• TS EN 206-1• Conformity criterion

210

Statistical quality control in the production of concrete

Concrete quality will show variation due to factors such as;variations in concrete making materials, production and measuring methods and human,Compressive strength of concrete, which is an excellent measure of not only the mechanical strength, but also some other properties of concrete, is taken as a basis for investigating the quality of the concrete

When a concrete structure is designed, the designer will first decide about the compressive strength of the concrete. This strength shall be selected among the values given in the standards of that country, taking into account

Level of importance of structureMaterialsEquipmentProduction facilities

TS 500: Requirements for design and construction of reinforced concrete structures → revised on Feb., 2000, April 2002

TS 11222: Ready - Mixed concrete → Revised on Feb., 2001

TS EN 206-1: Concrete, Ready - Mixed concrete → obligatory standardis adopted to accord with EU countries (March, 2004)

Problem; Different conformity conditions and rules are defined by abovestandards

(abrogated after acceptance of TS EN 206 – 1)

212

TS 500 (2000) (Design principles for reinforced concrete buildings)Presents concrete design strengths (fcd) as;C14, C16, C20, C25, C30, C35, C40, C45, C50

C14 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).

TS500 specifies that these strength values are lower limits withconfidence degree of 90%.

That is, only 10% of the specimens taken from the produced concrete may have compressive strengths below these design strengths

TS 500 strength requirements

213

fcafcd

t σ

Considering test variation in conrete strength obeys normal probabilistic distributionlaw, the standard deviation (σ) for the concrete production plant should be known for this purpose

Calculation of aim strength (given by TS 500 (1984))(not included in TS 500 (2000))


214

TS 500 (1984)

Confidence limits in normal distribution

Confidence parameter

(t)

Degree of risk (r)

Degree of reliability

(1-r) 0.00 0.50 0.50 0.67 0.25 0.75 1.00 0.16 0.84 1.28 0.10 0.90 1.65 0.05 0.95 1.96 0.025 0.975 2.33 0.01 0.99 3.00 0.001 0.999

Risk is P (fc ≤ fcd)Reliability is P (fc > fcd)

Then required average strength (aimed or targeted strength) to be used in mix designcalculations is given for 90% confidence as:fca = fcd + 1.28 σ

fcd = design strengthfca = aim strength


215

If standard deviation (σ) is not known, then aimed strength is obtained by increasing the design strength by Δf as given

for BS14 & BS16 → Δf = 4MPafor BS18 - BS30 → Δf = 6MPafor BS35 - BS50 → Δf = 8MPa

Ex; for a design strength (fcd) of BS25, strength to be aimed in mix design is calculated as: fca = 25+6 = 31MPa


216

TS 500 specifies to take3 test specimen for 1 unit (1 unit = 100m3 of concrete or 450m2 of area )

3 specimens → 1 group3 groups → 1 party P (G1, G2, G3)

Compressive strengths of these specimens at 28 days after standard curing are evaluated statistically and arithmetical mean (fcm) is determined for the sample space.

Acceptance or rejection of concrete is made on two basis:Mean value (fcm) and minimum value (fcmin) are determined

for acceptance fcm ≥ fcd + 1.0 (MPa) (mean value of each party)fcmin ≥ fcd – 3.0 (MPa) (Minimum arithmetical mean calculated from

groups of each party)


217

TS EN 206-1 strength requirements

TS EN 206-1Concrete design strengths C8-C100 (16 concrete classes are defined)Strengths are given as C20/25

C 20 / 25 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).

28 day compressive strengths of concrete cube specimens in MPa (a=15cm).

Lightweight concrete design strengths LC8 – LC80 (14 concrete classes are defined)

LC 8 / 9 28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).

28 day compressive strengths of concrete cube specimens in MPa (a=15cm).

218

Minimum rate of sampling for assessing conformityMinimum rate of sampling

Subsequent to first 50m3 of production Production

First 50m3 of production Concrete with production

control certification Concrete without production control certification

Initial (until at least 35 test results are obtained)

3 samples 1/200 m3 or 2/production week

Continuous b (when at least 35 results are available)

1/400 m3 or 1/production week

1/150 m3 or

1/production day

a Sampling shall be distributed throughout the production and should not be more than 1 sample within each 25 m3 b Where the standard deviation of the last 15 test results exceeds 1,37σ, the sampling rate shall be increased to that required for initial production for the next 35 test results

a

219

Conformity criteria for compressive strength

Criterion 1 Criterion 2 Production Number “n” of test results for compressive strength in the group Mean of “n” results

(fcm) N/mm2 Any individual test result (fci) N/mm2

Initial 3 ≥ fck + 4 ≥ fck - 4 Continuous 15 ≥ fck + 1.48 σ ≥ fck - 4

TS EN 206-1

220

TS EN 206-1 performance requirements

Two definitons are given for concrete;1) Designed concrete; concrete for which the required properties

and additional characteristics are specified to the producer who is responsible for providing a concrete conforming to the required properties and additional characteristics

2) Prescribed concrete; concrete for which the composition of the concrete and the constituent materials to be used are specified to the producer who is responsible for providing a concrete with the specified composition

221

TS 500 vs. TS EN 206-1

TS EN 206-1 is more specific by means of durability regulations than TS 500. Limit values and conformity criterions are given for properties other than strength

The number of samples to be tested is smaller in TS EN 206-1. This can be a disadvantage.

Ref; Selçuk Türkel, Kamile Tosun, The evaluation of TS EN 206-1, TS 500 and TS 11222 standards from the view point of concrete

222

Chapter Outline



concrete mix design

223

Property composition relations for concrete mix designOUTLINE

• Workability• Concrete mix design calculations

a) Preliminary designb) Trial batch production and measurement

224

Property composition relations

Required propertiesWorkabilityCompressive strengthDeformation (elastic, creep, shrinkage, thermal)PermeabilityDurability against freezing & thawing, chemical attack, fire

225

THE MIX DESIGN PROCESS

226

Workability

Empirical equation that relate the mix water to workability

(10 )W mα= −

where, W = amount of mixing water, dm3/m3

α = coefficient depending on consistency & type of aggregate m = fineness modulus of the aggregate mixture

αave

474336-40Fluid403731-33Plastic373328-30Stiff

Sea sand & crushed stone

Natural sand & crushed stone

Natural sand & gravel

Consistency grade

αave

474336-40Fluid403731-33Plastic373328-30Stiff

Sea sand & crushed stone

Natural sand & crushed stone

Natural sand & gravel

Consistency grade

227

Concrete mix design calculations

a) Preliminary designGeometrical compatibility

3 31 1000a c w

A C Wa c w v v m dmδ δ δ

+ + + = + + + = =

228

2)(vwc

cKf Fc ++=

Bolomey

Feret

Graf

Abrams CWc kkf /

2

1=

2)/( CWKff

G

ccc =

`)( kvW

CKf Bc −+

=

Strength requirement;

(taken by weight)

(taken by volume)

(taken by weight)

(taken by weight)

229

Constraint on air content;

Assumption for volume of entrapped air voids∼2-3 % for stiff consistency∼1-2 % for plastic consistency∼0-1 % for fluid consistencySolve for C, A, W, V where C+A+W = Δth

230

b) Trial batch production and measurement

Preliminary design → first approximation to actual valuesTrial batch produced based on proportions obtained from preliminary design. Only water requirement is adjusted till required workability is obtainedThis is done by adding mix water incrementally to the mixture of dry ingredients (cement + aggregates) in the mixer

Workability is tested by slump test throughout incremental addition of mix water. Unit weight of the trial batch is measured (Δm). Strength specimen (cylinders or cubes) are cast from the trial batch with desired workability. Specimens are tested at 28 days to check whether strength requirement is satisfied or not. If not mix proportions are altered.

231

SummaryDetermine w, c, a, v considering given requirementsCalculate theoretical weight (Δth)Cast trial batchMeasure actual unit weight (Δac)Calculate actual proportions using Δth, Δac

232

Example (Concrete mix design)Given: Portland cement (fcc = 41MPa, δc=3,15kg/m3)Aggregate : mixture of sand & sandy gravel

Sieve size (mm) 0.25 0.50 1 2 4 8 16 δ Sand 8 23 45 60 100 100 100 2.60 Sandy - gravel 1 3 7 12 20 55 100 2.65

Desired properties:Fresh concrete: plastic consistencyHardened concrete: BS16

Restrictions:For the aggregate mixture, m=4.07Assume v=1%Cmin=300kg/m3

233

Overall Outline


234

Chapter OutlineBITUMINOUS MATERIALS

– Introduction– Sources of bitumen– Chemistry and Molecular structure– Types of bitumen– Aggregates– Strength and Failure

• Modes of breakdown• Evaluation of road condition

– Viscosity of bitumen– Factors affecting deformation of bituminous mixes– Property composition relations for bituminous mixes

235

BITUMINOUS MATERIALS

Bituminous materials include all materials consisting of aggregate bound with either bitumen or tar. Mineral dust called “filler” is also used. Bituminous materials are used in highway engineering to construct flexible pavements.

Factors determining the type of bituminous mixturesBitumen contentBitumen gradeAggregate gradingAggregate size

picture: www.saocl.com

236

SOURCES of BITUMEN

1) Natural deposits (types of deposit range from almost pure bitumen to bitumen-impregnated rocks and bituminuous sands with only a few per cent bitumen)

• Rock asphalt (Porous limestone or sandstone impregnated with bitumen with 10% content, “Val de Travers”, Switzerland, “Tar sands” of North America)

• Lake asphalt (Bitumen lake with mineral matter dispersed throughout the bitumen, Trinidad lake (55 % bitumen, 35 % mineral matter, 10 % organic matter)

2) Refinery bitumen

237

2) Refinery bitumen: residual material left after the fractional distillation of crude oil

Petrol Kerosene Diesel oil Lubricating oil Bitumen

Cutback bitumen

Penetration gradebitumen

Fluxing/blending

Crude petroleum

Products of distillation

Fig. Preparation of refinery bitumens

238

Chemistry and molecular structure

Bitumen is a complex colloidal system of hydrocarbons which is soluble in trichloroethyleneConstituents of bitumen areCarbenes; fraction insoluble in carbon tetrachlorideAsphaltenes; fraction insoluble in heptaneMaltenes; fraction soluble in heptane

239

Types of bitumen

1) Penetration grade bitumen: refinery bitumen with a range of viscosities Penetration test classifies the bitumen according to hardness. For road bitumen penetration grades is from 15 to 450.Softening point, on the other hand, specifies the viscosity

2) Oxidized bitumens: air is introduced into bitumen under pressureOxygen(from air) reacts with some compounds to result in more asphaltene with higher molecular weight. Thus, hardness of bitumen is increased with reduced ductility and temperature dependence. Mostly used for industrial applications such as roofing and pipe coatings

240

3) Cutbacks: penetration grade bitumens for which the viscosity is temporarily reduced by dilation in a volatile oil. After application, oilevaporates and bitumen turns to its former viscosity

Curing time definesSlow curingMedium curingRapid curing

Cutbacks

241

4) Emulsions: two-phase system made up of two immiscible liquids, bitumen being dispersed as fine globules in water. Emulsifier provides dispersal of bitumen globules. Emulsifier in a hydrocarbon chain with cationic and anionic functional group. Hydrocarbon chain has affinity for bitumen where as ionic part has affinity for water. Each droplet carries a like charge, depending on charge of ionic part of emulsifier. Cationic emulsions are positively charged. Anionic emulsions are negatively charged. Cationic emulsions aid adhesion of bitumen on negatively charged aggregate surfaces.

Attraction of particleto substrate

WaterPhase(continuous Phase)

Bitumendroplet(dispersed phase)

Repulsion betweenparticles(like charges repel)

Surfaceactive agent

Opposite chargesattractAdhesion

of bitumento aggregatesurface

242

Aggregates

Aggregates make up about 92 % of bituminous materialsCoarse aggregates → retained on 2.36mm sieve, Fine aggregates → passes 2.36mm sieve but is retained on 75µm sieve. Filler → passes 75µm sieve

Open textured aggregate mixtures: Grading is continuous and provides a dense packing of particles. Strength and resistance to deformation are largely determined by aggregate grading with bitumen acting as adhesiveDense graded aggregate mixtures: Aggregate grading is still important but properties are determined largely by the matrix of fines and bitumen

Aggregate particles must have sufficient strength for surfacing materials, they must be resistant to abrasion and polishing. Shape and surface texture are also important.

243

Strength and Failure

Purpose of the road is to distribute applied load from traffic to a level which the underlying subgrade can bear

There are two forms of failure:Road surface may deteriorate through breakdown of surface material so that skidding resistance dropsRoad structure deteriorates – gradual and develops with the continued application of wheel loads

244

Modes of breakdown

Bituminousmaterial

Granularmaterial

Subgradesoil

rutdepth

Permanent deformation

Nearside wheeltrack

Case I

movingwheelload

Asphaltlayer

Granularlayer

Subgrade

εz , critical vertical subgrade strains

εt , critical tensile strain

fatigue crack

Fatigue cracking and critical strains

Case II

245

Two modes of breakdown

Case I - Permanent deformation occurs in wheel tracks. Rutting is associated with deformations in all layers of pavement. It islinked to loss of support from subgrade soil.Case II - Cracking appears along wheel tracks, cracking is caused by tensile strain developed in bound layers as each wheelload passes. It is a fatigue failure.

246

Evaluation of road condition

If failure can be defined, life of a road can be determined provided that loading can be assessed and performance of the materials evaluated. Complete collapse of roads are not encountered. Therefore, failure of roads must be identified in terms of serviceability and/or repairability.

It is accepted that if cracking is visible at surface, road is regarded as being at critical condition or as having failed.For no visible cracking, if rut depth reaches 20mm, road is regarded as having failed

247

Viscosity of bitumenViscosity of a liquid is the property that retards flow so that when a force is applied to the liquid, the higher the viscosity, the slower will be the movement of the liquid. The viscosity of bitumen is dependent upon both its chemical composition and its structure.

There are two most common measures of viscosity

Softening point: Temperature at which a bitumen reaches a specified level of viscosity

Standard tar viscometer: (used to measure the viscosity of tars) time taken for 50ml of the tar to run out of a cup through a standard orifice

224

248

Another test to evaluate viscosityPenetration test: commonly applied to bitumen for material

characterization. The test measures the hardness of bitumen which is related to viscosity. It measures depth to which a needle penetrates a sample of bitumen under a load of 100 gr over a period of 5 sec at a temperature of 25ºC

Bitumen is viscoelastic; therefore, the penetration will depend on the elastic deformation and viscosityBitumens are thermoplastic materials so that they soften as the temperature rises but become hard again when the temperature drops. Susceptibility of bitumen to temperature is determined from the penetration value and softening point temperature

Viscoelasticity describes materials that exhibit both viscous and elastic characteristics when undergoing plastic deformation

249

Factors affecting deformation of bituminous mixes1) Bituminous viscosity: when a stress is applied to a bituminous material,

both the aggregate particles and the bitumen will be subjected to the stress. Aggregate particles, being hard and stiff, while undergo negligible strain, whereas the bitumen, being soft, will undergo considerable strain. Deformation is associated with movement in the bitumen and the extent of the movement will depend on its viscosity.

2) Aggregate: bituminous mixtures utilizing a continuously graded –aggregate rely mainly on aggregate particle interlock for their resistance to deformation. Thus, grading and particle shape of aggregate are major factors governing deformation. Characteristics of fine aggregate are important in gap-graded materials. WHY??

3) Temperature: permanent strain increases with temperature due to the reduction in viscosity and stiffness of bitumen.

250

Property composition relations for bituminous mixes

McKesson – Frickstad (California) formula

P = 0.015a + 0.03b+ 0.17cwhere, a = % of aggregate retained on No 10 (2mm) sieve

b = % of aggregate remaining between No 10 (2mm) and No 200 (0.075mm) sieves

c = % of aggregate passing No 200 (0.075mm) sieveP = % of bitumen by weight of the bituminous mixture

In this equation, amount of bitumen is related to the surface area of the aggregate groups.

251

In lab., some tests have been done to determine the optimum bitumen content

Compression testStability test (Marshall test, Hubbard-Field test, Hveem test)Triaxial test

252

Required properties are 1) minimum stability of 225kg (2207N), 2)a maximum flow of 0.5cm, 3) air voids (3-5%) and 4) degree of saturation of aggregate voids with bitumen (75-85%).

Test is repeated with mixes of different bitumen contents and bitumen content fulfilling all requirements is determined as OPTIMUM BITUMEN CONTENT

Marshall test (most common)

ObjectiveTo determine an optimum binder content from a consideration of mix strength (stability), mix density, mix deformability (flow)

Cylindrical samples of 10cm diameter and 6.25cm height specimen is placed between the crescent shaped testing heads of testing machine and the force is applied from the side surfaces, subjecting the sample to an internal shearing towards the stress free plane surfaces. Loading speed is 5cm/min. Resultant maximum load and the corresponding deformations are measured

Asphaltsample

253

5 6 7 8Bitumen Content (%)

Unitweight

Airvoids

Marshallstability

Flow

In compacted bituminous mixture

in water)(weight - air)in (weight airin weight

=d

∑+=

i iagg

iagg

bit

bitm PP

G

)()(

100

δδ

100xG

dGV

m

m −=

Existing unit weight

Theoretical max, unit weight

Air void content

Analysis of mix design data from the Marshall test

254

Example

Properties and mix proportions of materials used for a bituminous mixture are as follows

Following data are obtained from a lab. specimen made using this mix:weight in air = 111.95 grweight in water = 61.16 gr

Material Density (δ) Mix proportions (P) Asphalt 1.04 kg/dm3 10 % by wt Crushed stone 2.70 kg/dm3 90 % by wt

255

Calculate unit weight of the lab. specimenCalculate the theoretical maximum unit wt. of the mixCalculate volume percentage of voids in the lab. specimenThis mix is placed on the road and consolidated under a roller. After consolidation a core sample is taken and the unit wt. of this core sample is determined to be 2.13kg/dm3. The specified degree of compaction for this pavement being at least 95% of the compaction of a laboratory specimen, indicate if this bituminous pavement is acceptable?

256

Solution:a) d =

b) Gm =

c) v =

d) specified unit weight =

257

Overall Outline


258

Brickwork and Blockwork(Masonry)

Masonry;One of the oldest building materialUsed by mankind for more than 6000 yearsAncient civilizations of Middle East, Greeks and Romans used masonryMany of mudbrick work has been lost. However, stone structures such as Egyptian pyramids, Greek temples and many structures from fired clay bricks have survived for thousands of yearsThe Romans used both fired clay bricks and hydraulic mortar

259

These are four main techniques for achieving stable masonry

a) Dry stone walls: Irregularly shaped laminar pieces are placed by hand in an interlocking mass

b) Ashlar: Medium to large blocks are made to a few sizes and assembled to a basic grid pattern either without or with mortar having very thin joints

c) Normal brickwork: Small to medium units of different sizes are assembled to a basic grid pattern and mortar is used as a packing material

d) Random rubble walls: Irregularly shaped and sized pieces are bonded together with adherent mortar

260

(a) (b)

(c) (d)

261

MaterialsSand: particles with sizes from about 10mm diameter down to 75μm

diameter obtained from riverbeds, sea beaches, older deposits from alluvial and glacial action. Most common sands are based on silica (SiO2)

Sand should be free of clay particlesMortar sand should have particles smaller than 5mm, Good range of particle size distribution is also needed (for good packing)Naturally occurring sands should be sieved and washedVery flaky and vary absorbent sand particles are not desired

262

Aggregates: Natural aggregates, sintered fly ash pellets, expanded clay and foamed slag

Binders: Binds mixtures of sands, aggregates, fillers to make mortar for masonry

Organic plasticizers: For improving plasticity or workability of mortars. These

plasticizers also entrain air as small bubblesLatexadditives: Synthetic copolymer plastics may be produced in the form

of a “latex”, a finely divided dispersion of the plastic in water usually stabilized by a surfactant such as a synthetic detergent. Solid content is ~ 50% of dispersion. They increase adhesion of mortar to all substrate. They increase tensile strength and durability

263

Mortar

Mortars are composed of cementing material, sand and water. There are three types depending on type of binder:

1) Cement mortar 2) Mixed mortar 3) Lime mortar

The sand to be used in mortar must be free of mud, clay, organic matter. It must have a good grading in size range of 0 - 4mm.

The composition of the mortar is chosen in such a way to obtain a good workability. For this purpose, all the voids of the aggregate must be filled with the paste of the cementing material and all the sand particles must be surrounded by a film of this paste. Existence of smaller particles in the sand is advantageous because; they fill the voids between larger particles and reduce the need for cementing material

If the water content of the paste is high, then the strength of the mortar will be lowered, if the cementing material is in excessive, the mortar is said to be too rich, and it tends to crack due to shrinkage. Taking all these factors into account, certain mortar compositions have been accepted as a result of many years of experience.

264

Types and Mix Proportions of Masonry Mortars (TS 2848)

Volume Proportions of Mix Class of Mortar

Type No

Type of Mortar Sand Cement Masonry

cement Lime putty

PowderLime

Minimum compressive strength (MPa)

A - Cement 3 1 - - - 15 1 Cement 4 1 - - - 2 Mixed 4 1 ½ - - 3 Mixed 4 1 - - ½ B 4 Mixed 4 1 - - 1

15

1 Mixed 7-9 1 2 - - 2 Cement 5 1 - - - C 3 Mixed 5 1 - 1 -

5

1 Mixed 6-8 1 - 2 - 2 Mixed 6-8 1 - - 3 D 3 Mixed 2-3 - 1 - -

2

E - Lime 3 - - 1 - 0.5

The last column shows the expected minimum compressive strength of the mortar at the age of 28 days

265

Natural stones;Rocks from magnetic, volcanic, sedimentary or metamorphic origin make up the natural building units for masonry. Magnetic and volcanic origin stones, being crystalline, give good quality materials. Examples are: granite, diorite, rhyolite, basalt, andesite, trechite, etc. However, care should be taken for mica, feldspar and other nondurable minerals contained in these rocks

Sedimentary rocks are not crystalline but layered. They may contain voids and fossils. Examples are: conglomerate, breccia, sandstone, shale, limestone, chert. Sandstones and limestones make good stones for masonry.

Metamorphic rocks are those who experienced a change in their structure. Good quality stones are gneiss, quartzite and marble. However, slate and shale are of poor quality.

241

266

Properties of Natural Stones

Type of Stone Comp. Str. (MPa) Unit wt. (kg/dm3)

Water absorption (%) by wt.

Basalt 90 - 165 3.35 0.5 – 2.0 Granite 75 - 145 2.65 0.6 Andesite, Diorite 55 - 75 2.56 0.2 Dense limestone 55 - 102 2.70 0.2 – 2.0 Travertene 32 - 97 2.5 – 2.60 0.4 – 2.4

267

Fired Clay Bricks

Bricks are made by forming the unit from moist clay by pressing,extrusion or casting followed by drying and firing (burning) to a temperature usually in the range of 850-1300ºCDuring firing process complex chemical changes occur and clay particles are bonded together by sintering (transfer of ions between particles at points where they touch) or by partial melting to a glass. Most important properties of bricks are compressive strength, size tolerances and unit weight. A low unit weight is desired toreduce the dead load and to provide heat and sound insulation

268

Definitions Dimension (mm) Tolerances for dimensions (mm)

Normal brick (NT) 190x190x50 50 ± 2 Modular brick (MT) 190x90x85 85 ± 3, 90 ± 3 Brick block (BT) 190 ± 3 or 190 - 2

Comp. Str. Type of Brick Unit wt (max) (kg/dm3) Mean

(MPa) Min (MPa)

λ

Cmhkcal

Absorption max (%)

Solid Brick (DOT/20) Solid Brick (DOT/12) Solid Brick (DOT/8)

1.80 1.80 1.80

20 12 8

16 9.5 6.5

0.75 -

Brick with vertical holes (DDT/15) Brick with vertical holes (DDT/10) Brick with vertical holes (DDT/6)

1.20 1.20 1.20

15 10 6

12 8 4.5

0.34 -

Brick with horizontal holes (YDT/3.6) Brick with horizontal holes (YDT/2.4)

0.8 0.50

3.6 2.4

3 2 0.32 -

Solid clinker brick (DOK/30) Clinker brick with holes (DEK/30)

1.80 1.80

30 30

25 25 0.75 8

Requirements for Factory made Bricks (TS 705)

269

Concrete Units

Used in building partition in houses since 1920sConcrete unit is poured into a mould and vibrated and demoulded after setting. It is a labor demanding and slow processConcrete is filled into a mould (die) and a dynamic presshead compacts the concrete into the die. Then, green concrete is ejected on to a conveyer system and taken away to cure either in air or after in streamThus solid and hollow bricks and blocks are produced either in dense concrete or as a porous open structure by using gap-graded aggregates and not compacting fullyBlock sizes are 200x200x40 (mm), 150x200x400 (mm)They contain usually two large vertical holes. TS 406 classifies them into strength levels of 2.5, 5, 7.5, 10 and 15 MPa. Maximum unit weight is not to exceed 1.60 kg/dm3

270

Masonry constructionBasic method of construction; units are laid one on top of another in such a way that they form an interlocking mass in at least the two horizontal dimensionsMortar needs to be firmly dry for dense low absorption units. While high absorption units need a sloppy wet mortarWalls and columns are built by laying out a plan at foundation level and masonry rises up layer by layerThe foundation layers are horizontal It is essential to maintain the verticalityThickness of mortar joints must be kept constantJoint color and shape influence the appearance

271

Reinforced and post-stressed masonry forms

(a)

(a)Bed joint reinforced(b)Reinforced pocket walls

(b)

Reinforced pocket wall

Pockets formed as built, shuttered then filled with concrete to bond in the reinforcement

272

(c)Grouted cavity(d)Quetta bond

Bars either bonded in with mortar as the brickwork is rasied or grouted in with concrete in lifts

Vertical or horizontal or both rebar reinforcement

Ties at 5/m2

Concrete grouted cavityWidth = 2 x cover +bar

(c) (d)

273

Stretcher or half bond

Soldier course

Quarter bond

Half brick bonds

Bond (interlocking) between units is achieved asStretcher bond or half bondThird quarter bondSoldier course

Bond patterns

274

Typical block/brick cavity wall

275

Flat shear ties at 5/m2

Mortar filledcollar joint < 25 mm

Collar-jointed brick wall

276

Header

CloserStretchera) English b) Flemish garden wall

c) Heading d) Rat-trap

Bonded wall types

277

Structural behavior

Unreinforced masonry is; good at resisting compression forces, moderate to bad at resisting shear forces very poor when subjected to direct tension

However, reinforced masonry is good also at resisting tension forces

Any masonry under compressive stress also resists bending since the compressive prestress in the wall must be overcome before any tensile strain can occur

Most of small masonry structures are still designed using experience-based design rules. Strength of masonry elements are predicted from strength and/or other characteristics of materials used in masonry construction

Then check is made against worst loading conditions obtained from past data. A factor of safety is applied to allow for statistical uncertainty in the material characteristics and loads. Relatively high (3-5) safety factors are used due to high variability and brittle failure mode

278

Forces on walls

Lateral(flexural)

Shear(in plane)

Compressive

Shear(normal)

Tensile

279

Generated by tensionstresses in the bedjoints

Disturbing force e.g. due to wind load

Restoring force

Generated by massacting at centre of gravity

Gravity mechanism Flexural resistanceCrack

Mechanisms for resisting bending forces in masonry cantilevers

280

Compressive loadingMasonry is most effective under compressive loadIf a load or force is put on a wall at a point, it would logically spread or toward from the point of application in a stretcher bonded wall since each unit is supported by the two units below it.

Such a compressive force causes elastic shortening (strain) of the masonry. As a result of Poisson’s ratio effects, a tension strain and hence a stress is generated normal to the applied stress.

281

Stability: slender structures

For masonry wall and columns, if ratio of height to thickness (h/d = slenderness ratio) is small, then the strength will depend largely on the strength of the constituent materials. Other factors that affect the strength of masonry are • Degree of workmanship• Thickness of joints• Type of arrangement of bricks

190 9010

9010

190

211 brick length column

282

Some empirical formulas for masonry design for h/d ≤ 10

Brocker formula

23max unitmortar σσσ =

e

dh

mortarunit

++

⎟⎠⎞

⎜⎝⎛ +

=)(316

104

max

σσ

σ

Graf formulaσ : in kgf/cm2

e : + 10 for good workmanship0 for medium workmanship

- 5 for poor workmanship

283

Comp. strength of stone (MPa) Type of wall Type of mortar 20 30 50 80 120

Walls with irregularly shaped natural stones

Lime Mixed Cement

0.2 0.2 0.3

0.2 0.3 0.5

0.3 0.5 0.6

0.4 0.7 1.0

0.6 0.9 1.2

Walls with natural Stones with horizontal joints

Lime Mixed Cement

0.3 0.5 0.6

0.4 0.7 1.0

0.6 0.9 1.2

0.8 1.2 1.6

1.0 1.6 2.2

Walls with stones with more than 3-plane faces

Lime Mixed Cement

0.4 0.7 1.0

0.6 0.9 1.2

0.8 1.2 1.6

1.0 1.6 2.2

1.6 2.2 3.0

Walls with matural stone blocks

Lime Mixed Cement

0.8 1.2 1.6

1.0 1.6 2.2

1.6 2.2 3.0

2.2 3.0 4.0

3.0 4.0 5.0

Comp. strength of bricks (MPa) 2.5 5 7.5-10 15 25 35

Brick Walls

Lime Mixed Cement

0.3 0.5 0.6

0.4 0.7 1.0

0.6 0.9 1.2

0.8 1.2 1.6

1.0 1.6 2.2

_ 2.2 3.0

Table a - Allowable Stresses for Masonry (DIN 1053)

Allowable stresses in this table are acceptable for h/d ≤ 10. If 10 < h/d ≤ 20, reduced allowable stresses for slender walls should be used(Table b). Slender walls with h/d > 20 are not permitted for masonry.

284

Linear interpolation for intermediate values permitted

Allowable stress of unslender wall (MPa) h/d 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.6 2.2 3.0 4.0 5.0

12 - - 0.3 0.4 0.5 0.6 0.6 0.7 0.8 1.1 1.5 2.0 3.0 4.0 14 - - - 0.3 0.3 0.4 0.4 0.5 0.6 0.8 1.0 1.4 2.2 3.0 16 - - - - - 0.3 0.3 0.3 0.4 0.6 0.7 1.0 1.4 2.2 18 - - - - - - - - 0.3 0.4 0.5 0.7 1.0 1.4 20 - - - - - - - - - - 0.3 0.5 0.7 1.0

Table b - Reduced allowable stresses for slender masonry, in N/mm2

Allowable stressesfor slenderwalls h/d > 10

285

Movements and elastic moduli of masonry materials

Masonry component material

Thermal expansion coefficient α (per ºC x 10-6)

Reversible moisture movement (%)

Irreversible moisture movement (%)

Modulus of elasticity E (N/mm2)

Granite 8 - 10 - - 20 – 60 Limestone 3 - 4 0.01 - 10 – 80 Marble 4 - 6 - - 35 Sandstone 7 – 12 0.07 - 3 – 80 Slate 9 – 11 - - 10 – 35 Mortar 10 – 13 0.02 – 0.06 - 0.04 – 0.1 20 – 35 Dense concrete brick/blockwork

6 - 12

0.02 – 0.04

- 0.02 – 0.06

10 – 25

LWAC blockwork 8 - 12 0.03 – 0.06 - 0.02 – 0.06 4 – 16 AAC blockwork 8 0.02 – 0.03 - 0.05 – 0.09 3 – 8 Calcil brickwork 8 – 14 0.01 – 0.05 - 0.01 – 0.04 14 – 18 Clay brickwork 5 - 8 0.02 0.02 – 0.10 4 - 26

286

Example;A masonry wall of 190 mm thickness (1 brick length) and 3.04 m height will be built of solid bricks DOT/12 according to TS705 and mixed mortar C-3 according to TS2348. Determine the maximum load that could be permitted on a 1m length of this wall. Solution;

101619304

dh ratio sSlendernes >===

from Table a σall= 0.9 MPa (for σbrick = 7.5 – 10 for mixed mortar)

from Table b σall= 0.3 MPa (for σall = 0.9 MPa for unslender wall and h/d=16)

Mixed load on 1m length of wall

mkNmmNAP all /57)119.0(/103.0 226max =×××=⋅= σ

287

Overall Outline


288

Polymers and polymer compositesPolymeric materialsproduced by combining a large number of small molecular units (monomers) by polymerization to form long chain molecules

1) Thermoplastic polymersLong chain molecules held together by weak Van der Waals forces cycle of softening by heating and hardening by cooling can be repeated almost indefinitely. However, with each cycle, the material tends to become more brittle2) Thermosetting polymersEpoxies and phenolics are principle examples cross linking exists between chains3) Foamed polymersTwo-phase system of gas dispersed in solid polymer, produced by adding a blowing agent to molten resin. Gas is released and causes polymer to expand. Small gas cells increase the volume of resin many times. They can be either thermoplastic or thermosetting. 4) ElastomersLong-chain polymer molecules made of coiled and twisted chains. Flexible material that undergoes very large deformations. Volcanization provides rigidity and hardness.

289

Fibers for polymer compositesLoad transfer from matrix to fibers results in high-strength, high-modulus material. Glass, carbon, boron fibers are examples of amorphous and crystalline fibers used in polymeric matrices.

1) Glass fibersManufactured by drawing molten glassfrom an electric furnace through platinum bushings at high speed. Filaments cool from liquid state at about 1200ºC to room temperature in 10-5 seconds.

Winding

Sizing process

Platinum bushing

Strands

204filaments

Moltenglass

290

Four types of glass are used for fibers- E-glass of low alkali content (most used one in composites in

construction industry)- A-glass of high alkali content (previously used in aircraft

industry)- Z-glass developed for reinforcing cement-based materials

because of its high resistance to alkali attack - S2-glass fiber used for extra high strength and high modulus

applications in aerospace

291

2) Carbon fibersPreviously produced from pitch obtained by destructive distillation of coalSynthetic fiber polyacrylonitrile is spun and concurrently stretched so that molecular chains are aligned parallel to the fiber axis. Fiber is then heated under tension to 250ºC in oxygen environment where it gains strength. Carbonization starts when polymer is heated in inert atmosphere. Higher stiffness is obtained through greater heat energy given to carbon filament

3) Aramid fibers (Tradename - Kevlar)Most successful commercial organic fiber developed by DuPont Comp. Poly-parabenzamide fibers E=130GPa2 forms of Kevlar fiber

Kevlar 29: high strength and intermediate modulusKevlar 49: high modulus and same strength as Kevlar 29 (preferred for high-performance composite materials)

292

Mechanical properties

Polymer propertiesMechanical properties are highly dependent on network of molecular units and on the lengths of cross-link chains. Curing process determines the length of cross-linked chains. Composites are heat cured to maximize mechanical properties

Mechanical properties of thermoplastics arise from properties ofmonomer units and high molecular weight

293

Mechanical properties of common thermosetting and thermoplastic polymers

Specific weight

Ultimate tensile strength (GPa)

Modulus of elasticity in tension, GPa

Coefficient of linear expansion 10-6/ºC

Thermosetting Polyester 1.28 45 – 90 2.5 – 4.0 100 – 110 Epoxy 1.30 90 – 110 3.0 – 7.0 45 – 65 Phenolic 1.35 - 1.75 45 – 59 5.5 – 8.3 30 – 45 Thermoplastics Polyvinylchloride (PVC)

1.37 58.0 2.4 – 2.8 50

Acrylonitrile butadiene styrene (ABS)

1.05 17 – 62 0.69 – 2.82 60 – 130

Nylon 1.13 – 1.15 48 – 83 1.03 – 2.76 80 – 150 Polyethylene (high-density)

0.96 30 - 35 1.10 – 1.30 120

294

Fiber propertiesComposite materials have high specific strength and high specific stiffnessachieved by the use of low-density fibers with high strength and modulusvalues

Specific weight

Ultimate tensile strength (GPa)

Modulus of elasticity in tension, (GPa)

Carbon fiber Type I 1.92 2.00 345 Type II 1.75 2.41 241 Type III 1.70 2.21 200 E – Glass 2.55 2.40 72.4 S2 – Glass fiber 2.47 4.6 88.0 Kevlar fibers 29 1.44 2.65 64 49 1.45 2.65 127

295

Polymer composites

Mechanical properties of polymers can be greatly enhanced by incorporating fillers and/or fibers into resin formulations

For structural applications, such composites should 1) Consist of two or more phases2) Be manufactured by combining separate phases such that

dispersion of one material in the other achieves optimum properties of the resulting material

3) Have enhanced properties composed with those of individual components

296

In fiber-reinforced polymer materials, primary phase (the fiber) uses plastic flow of secondary phase (the polymer) to transfer load to fiber. This results in a high-strength, high-modulus composite.High strength and high modulus properties of fibers are associated with very fine fibers with diameters of 7-15μm. Fibers are usually brittle.Polymers may be ductile or brittle and generally have low strength and stiffnessBy combining two components a bulk material is produced with a strength and stiffness dependent on fiber volume fraction and fiber orientation.Interface between fiber and matrix plays a major role in physical and mechanical properties of composite materialLoad is transferred from fiber to fiber through the interface and the matrix

297

Assumptions made for the analysis of composite materials

The matrix and the fiber behave as elastic materialsBond between fiber and matrix is perfect and consequently there will be no strain discontinuity across the interface Material adjacent to fiber has the same properties as the material in bulk formFibers are arranged in a regular or repeating array

Properties of interface region are very important for fracture toughness of the materialWeak interface → low strength and stiffness → high resistance to fracture (ductile)Strong interface → high strength and stiffness → low fracture toughness (very brittle)

298

Elastic properties of continuous unidirectional laminae

1

E11, σ11

E22, σ22

2

3

Basic laminate

299

1) Longitudinal stiffness

The orthotropic layer has three mutually perpendicular plane of property symmetry, characterized elastically by four independent elastic constants.

E11= modulus of elasticity along fiber directionE22= modulus of elasticity in the transverse directionν12 = Poisson’s ratio i.e. strains produced in direction 2 when specimen is

loaded in direction 1.G12= longitudinal shear modulusν21 = Poisson’s ratio, i.e. obtained from E11 ν21 = E22 ν12

If line of action of a tensile or compressive force is applied parallel to fibers of a unidirectional lamina, εm = εf provided bond is perfectAs both fiber and matrix behave elastically thenσf = Ef εf and σm = Em εm where εf = εm

300

As Ef > Em, stress in fiber must be greater than stress in matrixThus fibers carry major part of applied loadComposite load Pc = Pm + Pfσc Ac = σm Am + σf Af

σc = σm Vm + σf Vfwhere A = area of the phaseV = volume fraction of the phaseVc = volume of composite = 1for perfect bondεc = εm = εfFrom above equationEc εc = Em εc Vm = Ef εc VfEc = Em Vm = Ef VfEc = E11 = Em (1-Vf) + EfVf (law of mixtures)

1σ11

2

3

1σ11

2

3

301

2) Transverse stiffness

Applied load transverse to fibers acts equally on fiber and matrix; therefore,σf = σmεf = σ22/Ef and εm = σ22/Emε22 = Vf εf + Vm εm orε22 = Vf σ22/Ef + Vm σ22/Em (substitute σ22= E22ε22)E22 = Ef Em / [ Ef (1-Vf) + EmVf ]to take into account of Poisson contraction effectsE22 = Em`Ef / [ Ef (1-Vf) + Em`Vf ]where Em`= Em / (1-νm

2)

1

σ22

2

3

1

σ22

2

3

302

Elastic properties of short-fiber composite materials

As aspect ratio (l/d) decreases, effect of fiber length becomes more significant When a composite containing uniaxially aligned discontinuous fibers is stressed in tension parallel to the fiber direction there is a portion at the end of each finite fiber length and in surrounding matrix where stress and strain fields are modified by discontinuityEfficiency of the fiber to stiffen and to reinforce the matrix decreases as the fiber length decreases

303

Diagrammatic representation of the deformation field around a discontinuous fiber embedded in a matrix

Uniform load

Deformationof matrix

Shear stress at interfaceTensile stress in fiber

Fiber

ResinUniform load

lc/2 lc/2

σf,max

The critical transfer length over which fiber stress is decreasedfrom maximum to zero at the end of fiber is referred to as half the critical length of fiber

304

To achieve the maximum fiber stress, fiber length must be equal to or greater than the critical value lcReinforcing efficiency of short fibers is less than that for long fibers. Orientation of short fibers in a lamina is random and therefore lamina is assumed to be isotropic on a macro scale.Rule of mixture for long-parallel fiber case is modified by inclusion of a fiber orientation distribution factor n

Ec = E11 = ηEf Vf + Em Vmη = 0.375 for a randomly oriented fiber array

= 1.0 for unidirectional laminae when tested parallel to fiber= 0 for unidirectional laminae when tested perpendicular to fiber= 0.5 for a bidirectional fiber array

305

Failure of the fiber composite

σfu

σ

σfσcu

σc

σmuσ′mσm

matrix

composite

fiber

ε εfu ε

σfu

σmu

σ′m

0 Vf

Fiber contro

lled

σ cu= σ fu V f+

σ′ m (1-V f)

σcu = σmu (1-Vf)

matrix controlled

σ

1VminVcr

mfu

mmucrV

σσσσ

′−′−

=mmufu

mmuVσσσ

σσ′−+

′−=min

306

- If failure is due to breaking of fibers (fiber controlled, Vf > Vmin, then σcu > σmu) σcu = σ′m Vm + σfu Vf = σ′m (1-Vf) + σfu Vf

- If failure is due to matrix flow (matrix controlled, Vf << Vmin, then σcu < σmu) σcu = σmu (1-Vf)

when Vf < Vmin, fiber content in the composite is so small that fibers break before ultimate strength of matrix is reached; fibers act as if there are voids in place of them and in fact decrease strength of matrix by a factor equal to (1-Vf).

when Vf > Vcr, fibers become effective in increasing the strength of composite above that of matrix. From practical point of view,there is no meaning to use a fiber ratio less than this criticalvalue.

307

For discontinuous fibers

my

fuc

rl

τσ

=my

fuc

dl

τσ2

=or andcll

=α

for which tensile strength in the fiber will be reached at the same forceas the yield strength of matrix on the interface

If α > 1, then matrix will yield first before fiber breaks in tension

308

Average value of tensile stress in the fiber is found as

)11(α

βσσ −−=

−

ff factor efficiencyfiber ==

−

ησσ

f

f

where β is related to stress distribution on fiber. β= 0.5 for fully plastic stress distribution β = 1.0 for fully elastic distribution

309

Strength of composite for discontinuous parallel fibers

)1(')11( fmffucu VVd

−+−

−= σα

βσσ for Vf > Vmin

If α > 5 and β = 0.5 then 9.0≥cu

cud

σσ

If fibers are not too short, their discontinuity may be neglected

310

ExampleProperties of matrix and fiber materials for a glass fiber reinforced composite with continuous fibers of 30% volume fraction are as follows

E (GPa) σfu (MPa) εu Polymer matrix 2 200 0.10 (yield) Glass fibers 140 2800 0.02 a) Determine the modulus of elasticity of the composite for tension

parallel to fibersb) Assuming linear elastic behavior of fibers until failure and elastic-plastic

behavior of matrix, determine ultimate tensile strength of compositec) What minimum fiber volume ratio should be used to obtain a reinforcing

effect

311

Durability of polymer composites

Polymer composites change with time and most significant factorsareElevated temperaturesFireMoisture Adverse chemical environmentsNatural weathering when exposed to sun’s ultra-violet radiation

312

TemperatureFluctuating temperatures have a greater deterioration effect on GRP. Difference in coefficients of thermal expansion of glass and resin may cause debonding.Exposed to high temperatures a discoloration of the resin may occur composite becoming yellow. Both polyster and epoxy show this effect. As a result of exposure to high temperatures, the composite becomes brittle.

FireA composite material must meet appropriate standards of fire performanceSome mineral filler, calcium carbonate can improve mechanical properties. Aluminum trihydrate and antimony trioxide are used as fillers to enable flame-retardant properties

313

MoistureCross-linked polymers absorb water which may cause a decrease in strength and modulus of elasticity. Absorption of water by polyesters and epoxies leads to swelling of the laminate.Water will also cause some surface flaws on fibers and reduce strength, long-term water absorption may cause weakening of the bond between fiber and polymer

314

Natural weathering causes some deterioration of GRP composites. Sunlight degrades both polyester and epoxy resins. As a result of discoloration, loss of light transmission occur. UV absorbers and stabilizers are added to resin formulations. A rise of temperature accelerates chemical reaction and hence degradation.Weathering can affect mechanical properties of GRP composites through surface debonding. Because weathering is a surface effect, thickness of laminate becomes important. 3mm thick laminate shows 12-20% reduction in flexural strength after 15 years exposure, while 10mm laminate shows only ~ 3% reduction after 50 years exposure

Weather

315

Use of polymers and polymer composites

Use of polymers and polymer composites in construction industry falls into three categories;

Non - load - bearingSemi - load – bearingLoad - bearing

Unreinforced polymers

Fiber – reinforced polymers

316

Selection of most appropriate resin should be done considering particular end use since every material does not possess all thefollowing characteristics

High light transmissionInfinite texture possibilitiesMinimum maintenance requirementsInfinite design possibilitiesResistance to water and corrosionHigh specific strengthHigh impact resistance

317

Disadvantages of polymers in construction areHigh cost of materials. However, low density and reduced foundation size make them competitiveLow stiffness and strength (use in composites)Poor scratch resistanceDegradation under UV light (stabilizers used)Low resistance to fire and high temperatures (additives used)

Non-load bearing thermoplastic polymers such as polyethylene have been used to manufacture pipes for transportation of water, oil and gas

318

Geotextiles• Main fibers used in geotextiles are polyethylene, polypropylene,

polyester and polyamide. Fibers are the load-bearing elements in geotextiles and framing technique determines the structure and physical and mechanical characteristics of the system

Geomembranes• Manufactured in impermeable sheet form from thermoplastic

polymers or bituminous materials either reinforced or unreinforced. Matrix can be reinforced by textiles.

Geo-linear elements• Long, slender strips or bars consisting of a unidirectional filament

fiber core made from a polyester, aramid or glass fiber in a polymer sheath of a low-density polyethylene or a resin. Fiber provides strength and deformation characteristics and matrix protects fiber and provides bonding with the soil

Thermoplastic polymers known as geosynthetics are used in soil environment in five categories

319

Geogrids• Grid-like structures of thermoplastic polymer material.

They form a quasi-composite system in conjunction with soil grid structure in fiber and soil is assumed to be the matrıx

• Two forms– Cross-laid strips– Punched thermoplastic polymer sheets

Geocomposites• Hybrid systems. Two or more different types of

thermoplastic polymer systems. They form a drainage passage along the water course with polymer core as drainage channel and geotextile skin as the filter

320

Marine applicationsDominant materials for pleasure craft. Effective in replacing wood

Truck and automobile systemsSports – car bodies and truck cabsStrength, stiffness, toughness, corrosion resistance and high-quality finish are physical and mechanical properties that must be satisfied. However, economy is the crucial consideration

Aircraft and space applicationsUsed for making components in aircraft industryThe fin of European airbus is a component made from a sandwich construction with carbon fiber/epoxy resin face material. The Westland helicopter rotor blades are made from carbon fiber composite material

321

Pipes and tanks for chemicalsCritical consideration is the corrosive resistance and ease of fabrication of complex shapes

Civil engineering structuresApplications have started with GRP structures

A dome structure erected in 1968 in Benghazi, LibyaRoof structure in Dubai Airport built in 19721970-1980 prestigious buildings in UK(Morpeth School, Mondial House, Covent Garden Flower Market etc.)

These buildings are built as a composite system, with either steel or reinforced concrete structural system and GRP composite as load-bearing infill panelsIn 1970, a classroom system, using only GRP, by folding flat plates into a folded plate system so that stiffness is provided by the structural shape.In 1980’s more ambitious structural elements were produced

322

FRP radar dome (25m diameter, factory made FRP sandwich panels are bolted together on site)

Mondial house (clad with FRP panels)

FRP modular classroom

323

MATERİAL Relative density

Diameter thickness ratio (microns)

Length (mm)

E (GPa)

Tens. Str. (MPa)

Failure strain (%)

Volume in composite (%)

Mortar matrix 1.8-2.0 300-5000 - 10-30 1-10 0.01-0.05 85-97

Concrete matrix 1.8-2.4 10000-20000 - 20-40 1-4 0.01-0.02 97-99.5

Asbestos 2.55 0.02-30 5-40 164 200-1800 2-3 5-15

Carbon 1.16-1.95 7-18 3-cont. 30-390 600-2700 0.5-2.4 3-5

Glass 2.7 12.5 10-50 70 600-2500 3.6 3-7

Polyethylene HDPE filament 0.96 900 3-5 5 200 - 2-4 High modulus 0.96 20-50 Cont. 10-30 > 400 > 4 5-10 Polypropylene (Monofilament) 0.91 20-100 5-20 4 - - 0.1-0.2

Polyvinyl alcohol (PVA) 1-3 3-8 2-6 12-40 700-1500 - 2-3

Steel 7.86 100-600 10-60 200 700-2000 3-5 0.3-2.0

Performance is controlled byvol. fraction of fibersproperties of fibers and matrixbond between the two

324

Table shows that elongations at break of all fibers are two or three orders of magnitude greater than strain at failure of matrix. Hence matrix cracks before fiber strength is approached

Most organic fibers have modulus of elasticity less than five times that of the matrix. Low modulus fibers are used in situations where matrix is expected to be uncracked. Large Poisson’s ratio of these fibers may cause debonding and pull-out, woven meshes or networks of fibers are necessary for efficient composites.

Steel fibers of varying cross-sections or bond ends provide anchorage; glass fiber bundles may be penetrated with cement hydration products to give effective bondingDmax of mortars or concrete affect efficient fiber distributionConcrete with a Dmax = 10mm is preferred. Dmax > 20mm is never allowed. To avoid shrinkage, at least 50% by volume of inert filler

(aggregate, fly ash, or limestone dust) should be used

325

Structure of fiber-matrix interface

Properties of fiber reinforced cementitious materials depends on microstructure of interface

Interface is initially water-filled zone which develops to a microstructure

different than bulk matrix

There are 3 layers in interface;a) Very thin (less than one micron) Ca(OH)2 rich rather

discontinuous, directly in contact with fiberb) Massive Ca(OH)2 layerc) Porous zone (up to 40 microns) consisting of CSH and some

ettringite

326

Structure and post-cracking composite theory

In hardened cement-based composites, fibers bridging the cracks contribute to the increase in strength, failure strain and toughness. Limited volume fraction of low modulus fibers do not contribute to the modulus of elasticity of the composite

327

εεfuεmu

matrix

σmu

σcu

σfu

0

σcu

σfu

σmu

0 Vmin Vcr Vf 1.0 Vf

fiber Fibercontrolled

σ′fVf

σfuVf

compositeσ′f

σmu (1 – Vf) + σ′f Vf

σcu

σ cu =

σ fu V f

σcu = σmu (1-Vf) + σ′ fVf

(uncracked)σcu = σ′f Vfcracked)

σ′f = σmu

σ′f < σmu

328

Modulus of elasticity of the composite1) Where the matrix and fibers behave elastically

Ec = Ef Vf + Em (1-Vf)

2) After the point where the matrix starts cracking

ff

fc V

dd

E .⎟⎟⎠

⎞⎜⎜⎝

⎛=

εσ

329

Ultimate tensile strength1) Matrix fails and later composite carries stress at a decreased

level. This mode of failure is called a “matrix controlled” failure. This occurs under the condition that Vf < Vcr

σcu = σmu (1-Vf) + σ’f Vf (uncracked)σcu = σ’f Vf (cracked)σ’f = stress in the fiber when matrix cracks (ε = εmu)

2) Matrix fails and later composite carries stress at an increased level. This mode of failure is called “fiber controlled” failure. This occurs under the condition that Vf > Vcr, σcu = σfu VfThen the critical volume fraction for fibers is found as

fmufu

mucrV

'σσσσ

−+=

330

On the other hand, there is a fiber volume ratio that determines the strengthening effect of fibers in the composite

fu

muVσσ

=min

It is observed that Vcr is a function of σ’fIf σ’f = σmu, then Vcr = Vmin

If σ’f < σmu, then Vcr < Vmin

331

When discontinuous fibers are used there is a critical length for fibers so that critical bond shear stress along the fiber-matrix interface does not lead to bond failure, that is, fibers will break rather than pull-out

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=φφνσ

φυ tantan

lntan ncrL fu

cr

where, r = radius of fiberυ = Poisson’s ratio of fiberc = cohesionφ = Angle of fraction between matrix and fibern = normal pressure exerted on the surface of fiber due to

shrinkage of matrix

332

Determination of critical length of discontinuous fibers is donethrough experimentation

When discontinuous fibers of short length are used (L < Lcr)Then we may have a third mode of failure which is due to reaching the bond strength at the fiber-matrix interface. In this case the strength of the composite is determined not by fiber strength but the adhesion between the fiber and matrix

333

ExampleIt is planned to reinforce a concrete matrix of tensile strength σmu = 8 MPa with polymer fibers to increase its tensile strength. Fiber diameter is 0.25mm. Fiber tensile strength is 80 MPa and fiber-matrix bond strength is 0.2 MPa. Assuming constant bond shear stress distribution along the interface, calculate critical fiber length If discontinuous parallel fibers of 40mm length are used and a composite tensile strength of 11MPa is required, what fiber volume ratio will be necessary? In this case what will be the failure mode?If the moduli of elasticity of concrete and fibers are 20GPa and 50GPa, respectively, plot stress-strain diagram of composite in part b)For a continuous use of above fibers, and for the purpose of obtaining a reinforcing effect of the fibers for any volume ratio, what should be the lower limit of fiber modulus of elasticity.Calculate critical fiber ratio for value of Ef in c

334

Overall Outline


335

Metals, their differences and uses

Metals in Civil Engineering

Ferrous(iron + carbon) Non-ferrous

Cast iron(C > 1.7 %)

Steel(C < 1.5 %)

Structural steel(C ≈ 0.25 %) Aluminium Copper

336

1) Cast irons

Major consumption in pipes and fittingsIn civil engineering for tunnel segments and mine shaft tubingCarbon content is greater than 2 %. Hard and brittle.

2) SteelSteel is obtained through carbon reducing operations. Carbon steel is an alloy of iron and carbon. Amount of carbon within the lattice determines the properties of the steel. Alloys containing less than 0.008 % carbon are classed as irons. Steel has a carbon content less than 2.0 %

C ≤ 0.25 % ⇒ mild steel, low carbon steel (structural steel is in this category)0.3 % ≤ C ≤ 0.6 % ⇒ medium carbon steel, carbon steelC > 0.6 % ⇒ high carbon steel

Normally, Mn and S are added to steel during productionIf elements other than Mn and Si are added ⇒ alloy steelsIf elements like Cr and Ni are added ⇒ stainless steels

337

2.1 Structural steelFour grades of structural steelsGrade Min.ten.st.(MPa)

40 40043 43050 50055 550

All structural steels are readily weldable. For higher grades care is needed in welding

2.2 Heat treated steelsProperties of steel (C > 0.3 %) can be varied by heat treatment

Heating to high temperatureFast cooling by quenching in oil or waterFollowed by reheating to about 650ºC (tempering)

Fast cooling produces ⇒ hard, brittle microstructure (known as martensite)Used only for cutting tools, no use in structural engineering

338

2.3 Stainless steelsFerrow alloys: containing at least 12% Cr and some Ni and Mo. Chromiumproduces a stable passive oxide film.

Three basic types (grouped according to metallurgical structure):1. Martensitic: 13% Cr, low carbon, hard, used for cutlery, unweldable2. Ferritic: 13% Cr, low carbon, unweldable3. Austenitic: 18% Cr and 8% Ni, low carbon, most resistant to pitting type of corrosion, weldable

339

3. Aluminium and alloys

Used both structurally and decoratively in cladding, roofing, window frames, window and door furniture. Lightness of aluminium is an advantage in structures with high self-weight / live load. Such as roofs, footbridges and long span structuresHigh durability of aluminium makes it usable in polluted and coastal areasHigh cost limits its useHigh initial cost maybe offset by reduced maintenanceStructural sections are produced by extrusion

Eal = 70GPa & Est = 210 GPa ⇒ Aluminium deflects more under same loadHowever, specific moduli (E/ρ) is comparable(E/ρ)al = 20, (E/ρ)st = 29

340

4. Copper and alloysUsed in applications where high thermal and electrical conductivity needed such as domestic water services, heating, sanitation4.1 Brasses; copper – zinc alloys ⇒ enhanced strength and corrosion resistance4.2 Bronzes; copper – tin alloys ⇒ high corrosion resistance

341

Design for minimum weight

To find what is the least amount of material needed to carry design loads

P Consider a cantilever beam of length L

End deflection isEI

PL3

3

=δ

4

3

3ExPL

=δ , x is a unit of length

P = Applied loadL = length of beam E = modulus of elasticityI = second moment of area

342

2/1

2/15

3 EPLW ρ

δ ⎟⎟⎠

⎞⎜⎜⎝

⎛=

for a given set of design condition (P, L, δ), W is minimized by minimizing 2/1Eρ

W = ρAL = ρLx2weight is

A = area of the cross-sectionρ = density of the material of which the beam is made

22

24

LWxρ

=

343

Materials in decreasing order of efficiency in bending

25.60Copper21.82Concrete17.17Steel12.86Titanium11.25GFRP10.0Aluminium4.55Timber Timber is oustanding

Then comes Al and GFRPThese compete in applications where weight iscostly (aircraft, sailing dinghies, racing cars, small-scale building, squash rackets, golf clubs, tables and chairs)

344

Overall Outline


345

TIMBERTimber has been one of the basic materials of construction since the earliest days of human kind. Today it has been largely superseded by concrete and steel. However, the use of timber remains extensive.

Wood – A Nature Made Composite

Wood contains; 60 % cellulose (C6H10O5)n28 % lignine (C41H36O6)12 % pectine (C6H8O6)n & some others

346

Internal structure of wood is made up of groups of long and slender cells, called “grain”, that look like a group of thin pipes. The walls of the cells are made up of “ligno-cellulose” and they are bonded to each other with “pecto-cellulose”. The length of the cells is between 1-6 mm and their diameter is about 1/100 of their length

Depending on internal structure, the trees may be separated into three groups;

1) Softwoods without any vessels or pores for the sap like pine, fir, spruce, cedar and cypress

2) Hardwoods with diffused vessels or pores like maple, beech, poplar 3) Hardwoods with ring-porous structure like oak, ash etc.

Even though chemical composition is the same, differences in internal structure result in differences of properties. Unit weights vary between 0,398 – 0,720 kg/dm3 and flexural strengths between 65 – 110 MPa.

347

Type of tree Unit weight (kg/dm3)

Compressivestr. (MPa)

Flexural str. (MPa)

Poplar (kavak) 0,398 41 69 Fir (köknar) 0,431 36 71 Spruce (ladin) 0,436 31 69 Yellow pine (sarıçam) 0,515 38 65 Cedar (sedir) 0,523 45 77 Black pine (karaçam) 0,568 48 110 Red pine (kızılçam) 0,577 38 73 Oak (meşe) 0,690 55 94 Beech (kayın) 0,720 53 105

Table 1. Mechanical properties of trees in Turkey in air-dried conditionand parallel to grains

Even though chemical composition is the same, differences in internal structure result in differences of properties.

348

Largest properties are obtained in the direction parallel to grain and in dry condition which is the condition in which wood is usually utilizedBeing an organic material, wood is affected by the moisture in the atmosphere, by the ultraviolet rays of sun and deteriorated by parasites such as fungus or worms. Wood has to be protected by coatings and poisonous preservativesIts anisotropical nature and the limitations of size make its use difficult. To correct those deficiencies, artificial wood products like plywood, fiberboard, etc. are manufactured and used to a great extend

Properties of timber change with- direction- moisture content

349

102030405060708090

10 20 30 40 50

100

60 70 80 w (%)

Comp.Str.(MPa)

Composite strength – moisture relationfor soft - woods with unit wt = 0.420 kg/dm3

350

3rd class 2nd class 1st class Strength (MPa) Pine Oak Pine Oak Pine Oak Tensile // - - 8.5 10.0 10.5 11.0 Compressive // 6.0 7.0 8.5 10.0 11.0 12.0 Compressive ⊥ 2.0 3.0 2.0 3.0 2.0 3.0

Timber specification (TS 647)Allowable stresses

Modulus of elasticityPine: // 10 GPa

⊥ 0.3 GPaOak: // 12.5 GPa

⊥ 0.6 GPa

Documents

Construction Materials in civil engineering