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Concrete Technology, 11563
Lecture 1: Introductionand Repetition
2012
Björn Johannesson, BYG
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Concrete technologyREPETITION basic course
Björn Johannesson, DTU
Guggenheim Museum (NY)
Øresund Bridge
Airport
Concrete bridge in southernFrance (The Millau Viaduct)
Casting
Mold
Concrete construction
Casting
11563
3BJÖRN JOHANNESSON, DTU
Course 11563 Concrete Technology2012
Introduction
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Staff: Construction Materials, DTU
Ole Mejlhede Jensen
Mette Geiker
Björn Johannesson
Staffan Svensson
Kurt Kielsgaard Hansen
Lisbeth Ottosen
Judith Selk Albertsen
Ebba Schnell
Concrete
Wood
Transport and sorption
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Assistant teachers
Mia Schou Møller [email protected]
Frederik Marthedal [email protected]
Jens Hamann [email protected]
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Teachers
Juan Manuel Paz García, Ph.D. studentLectures 4, 5 and 10 (Concrete chemistry)
Wu Min, Ph.D. studentLecture 8 (Moisture fixation)
Mads Mønster Jensen, Ph.DLectures 7, 9 and 12 (Porosity development, moisture transport and durability)
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My background – Concrete research
1988
2003
2005
2006
Master of Science: Lund 1993Ph.D.: Lund 2000
Björn Johannesson, BYG
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Outline
• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
Björn Johannesson, BYG
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Learning objectives
Knowing about the main components of concrete (and their properties)
Knowing the approximate mass concentrations of different components of concrete
Knowing the main chemical components of cement (and some of their properties)
Björn Johannesson, BYG
Knowing the basics of hydration of different cement clinker minerals
Knowing about the importance of treatment of concrete after casting
Knowing about the importance of workability of fresh concrete
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
Björn Johannesson, BYG
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The cement paste, i.e., cement mixed with water, forms a solid product.
The hardened cement paste can be regarded as the ’glue’ joiningthe sand and gravel (stones) together in the concrete.
Concrete
The higher concentration of the paste, i.e., the lower water content in mix, the greater the strength.
Ca. 10 mm
Björn Johannesson, BYG
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Concrete as a Material
Hydro-electric power damthat spans the Yangtze in Sandouping, China. It is the largest hydro-electric powerstation in the world.
Ship locks for river traffic to bypass the Three Gorges Dam, May 2004
Three Gorges DamOil platform
Three Gorges Dam
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Advantages
Ability to be cast (any shape)EconomicDurableSound-insulatingFire-resistantEnergy-efficientAesthetic properties
Disadvantages
Low tensile strengthLow ductilityVolume instabilityLow strength-to-weight ratio
Advantages – disadvantages with concrete as a material
Björn Johannesson, BYG
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Compressive strength 35 MPaFlexural strength 6 MPaTensile strength 3 MPa
CompressionTension
Compressive strength10 times higher thantensile strength
10 × 10 × 10 cm concrete cube
Basic failure criterions of normal hardened concrete
Björn Johannesson, BYG
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The annual consumption of concretein the US corresponds to covering central Copenhagenwith an 80 m thick layer.
Example: Production quantitiesof concrete in the US
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Property ValueElastic modulus 28 GPaTensile strain at failure 0.001Coefficient of thermal expansion 10 × 10−6 /°CDensity 2300 kg/m3
Typical engineering properties of concrete
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Material Density
(kg/m3)
Tensilestrength(MPa)
Elasticmodulus(GPa)
Thermalconductivity(W/(mK))
Steel 7800 500 210 50
Wood 510 75 5 0.5
Plastic 1000 50 3 0.1
Rock 2600 20 50 3
Concrete 2300 3 25 3
Comparison of properties of differentbuilding materials
Concrete: “man-made rock”
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Limitations
Concrete is brittle with low tensile strength
Volume stability can be a problem (caused by moisture changes andtemperature changes)
Creep can be a problem (deformation changes at constant mechanical loads)
Björn Johannesson, BYG
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Concrete properties – shapeable, ‘high’ strength and durable
Taipei 101
106 stories Taiwan, 2002508 m
One of the tallestconcrete buildings
Björn Johannesson, BYG
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Concrete properties – shapeable, ‘high’ strength and durable
Burj Khalifa (2010)
828 m high164 stories (156 in concrete)Vertically pumped concrete, 700 mElevator 10 m/s
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Concrete properties –shapeable,
strength and durability
Complementing building to Ordrupgård museum, Copenhagen
Årsta Bridge, Stockholm
Björn Johannesson, BYG
22Mechanical test of reinforcedconcrete column
Concrete properties –shapeable,
strength and durability
Björn Johannesson, BYG
CN Tower i Toronto, 1975The world’s highest slip mold constructionHeight = 553.34 m !
23Concrete road produced around 1930 located outside Lund, Sweden
Concrete properties –shapeable,
strength and durability
Björn Johannesson, BYG
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
Björn Johannesson, BYG
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Concrete composition
cement + water + sand + stone + (admixtures)
cement paste
cement mortar
concrete
aggregates
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Portland cement and its chemical composition
SiO2 Al O2 3
CaO
0
0
0
0.5
0.5 0.5
1.0
1.0
1.0
0.68
0.10
0.22 Portland cement
Cement + water + sand + stone + (admixtures)
PC most common cement in use
Sand rich in SiO2
Limestone rich inCaO
Clay rich inAl2O3
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Drinkable water can be used as water in concrete
Öland BridgeSeawater
Cement + water + sand + stone + (admixtures)
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The aggregates may not contain humus.(especially the small fractions, i.e., the sand).
There should not be clay particles onthe surface of the aggregates since it lessens the adherence of cement.
Typical alkali-silica damages
Clay particles on aggregates
Clay
Humus
Porous flint with a high content of opal
One should avoid aggregates containingreactive silica (e.g. opal-containing flint).This type of silica may react with thealkalis (Na+ and K+) in the hardened cement and cause serious cracks.
Cement + water + sand + stone + (admixtures)
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Term Type Sizes‘Singels’ (shingles) stones 32-64 mm
‘Nøddesten’ (nut stones) stones 16-32 mm
‘Ærtesten’ (pea stones) stones 8-16 mm
‘Perlesten’ (pearl stones) stones 4-8 mm
‘Betonsand’ (concrete sand) sand 0-4 mm
Filler* (filler) sand <0.25 mm
*Also lime, stone powder, fly ash
Cement + water + sand + stone + (admixtures)
One distinguishes between ‘sømaterialer’ and ‘bakkematerialer’ (lake and hill materials, respectively)‘Bakkematerialer’
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Danish terminology
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Surface-active admixtures1. Air-entraining agents – To avoid frost damages2. Water-reducing agents – Improves the fluidity of fresh concrete
Aggregate
Air bubble
1. Accelerators – For example winter casting2. Retarding agent – Casting of thick constructions
Setting- and hydration-regulated admixtures
The ‘new’ water-reducing agents made it possible to develop theso-called ‘high-strength’ concrete and ‘self-compacting’ concrete.
Winter castingTunnel element
Massive construction
Cement + water + sand + stone + admixtures ACHEMICAL ADMIXTURES
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Fly ash and silica fume
SiO2
Al O2 3
CaO
0
0
0
0.5
0.5 0.5
1.0
1.0
1.0
Portland cement
Fly ash = by-productfrom burning coal
Silica fume = by-productfrom silicon alloys
0.12
0.30
0.58
Fly ash and silica fume give improvements of some properties of concrete
Cement + water + sand + stone + admixtures BMINERAL ADMIXTURES
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Effect of plasticizers (water-reducing agents) and fly ash
Hardened concretewith fly ash and silicafume can obtain acompression strengthas high as 200 MPa!
Very fluid-like fresh concrete
Fly ash × 2000
The spherical shapeof the fly ash particlespromotes the fluidityof fresh concrete
Cement + water + sand + stone + admixtures
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Prestressed cables
Cement + water + sand + stone + admixtures
+
Reinforcement = Reinforced concrete
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Remember: no structural concrete without reinforcement!
Concrete and reinforcement
The concrete and the reinforcement have the same temperatureexpansion coefficient. This is a necessary condition for avoiding
cracking of reinforced concrete structures
About 1–5 % of volume is reinforcement BJO, BYG
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Cement
Water
Stone
Sand
Air
Volume portions of constituents in concrete
BJO, KKH, BYG
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Fresh concrete is a mixture of:
• 20–30 vol% cement and water
• 70–80 vol% aggregates
• 1–2 vol% air
• possibly: admixturesBJO, BYG
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The most important mixtureparameter is the weight ratio between
water and cement in mix
Low water-cementratio
High strengthLow permeability
High water-cementratio
Low strengthHigh permeability
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Composition of stone sizesfor concrete aggregates
The total composition of theaggregates should be such that
it is optimally packed
Goodmixtureof aggregatesof differentsize
Badmixtureof aggregatestoo many bigstones
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
BJO, BYG
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Workability of fresh concreteSlump (Danish: ‘Sætmålsprøvning’)
’Normal’ concrete
Very fluid concrete
Danish terminology: Slump:(‘jordfugtig konsistens’) 0–30 mm (‘stiv konsistens’) 30–60 mm(‘plastisk konsistens’) 60–100 mm(‘tyktflydende konsistens’) 100–150 mm(‘flydende konsistens’) >150 mm
Slump testCasting of concretewith fluid properties(‘flydende konsistens’)
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In some casesa low slump valueis desirable
Casting of concretewith low slump (0–30 mm slump)
Slump measurement
0–30 mm Slump(‘jordfugtig beton’)
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Casting - Workability
Casting – the vibrationis performed whenthe concrete has beenplaced
Casting and vibration of concrete
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Result of improper compacting offresh concrete
Concrete surface
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
BJO, BYG
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Belite hydration
Alite hydration gives early strength2C3S + 7H → C3S2H4 + 3Ca(OH)2
OHH;OFeF;OAlA;SiOSCaO;C 232322 =====
2C2S + 5H → C3S2H4 + Ca(OH)2
(Water) (C-S-H)(Tricalciumsilicate)
(Calciumhydroxide)
(Water) (C-S-H)(Dicalciumsilicate)
(Calciumhydroxide)
~ −100 KJ/mol
~ −50 KJ/mol
Aluminate hydration2C3A + 21H → C4AH13 + C2AH8
(Water) (Calciumaluminatehydrate 1)
(Tricalciumaluminate)
(Calciumaluminatehydrate 2)
~ −400 KJ/mol Normallyunwanted!
Cement and water reactions
The tricalcium aluminatecontent should be low in asulfate-resistant cement
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Aluminate hydration2C3A + 21H → C4AH13 + C2AH8
(Water) (Calciumaluminatehydrate 1)
(Tricalciumaluminate)
(Calciumaluminatehydrate 2)
~ −400 KJ/mol Toofast reaction,too muchheat
Gypsum is almost always added tocement to curb the reaction speed
Aluminate and gypsum hydrationC3A + 3CS*H2 + 26H → C6AS3H32
(Water) (Ettringite)(Tricalciumaluminate)
3*
232322 SOS O;HH;OFeF;OAlA;SiOSCaO;C ======
*
(Gypsum) O2HCaSO 24 ⋅Gypsum
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Cement + water = Cement paste
Hardened cement under a microscope
Calcium Hydroxide (Plates)
Ettringite (Needles)
C-S-H (Amorphous ~ no shape)
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Alite hydration2C3S + 7H → C3S2H4 + 3Ca(OH)2
(Water) (C-S-H)(Tricalciumsilicate)
(CalciumHydroxide)
Cement with admixturesfly ash and silica fume
Ca(OH)2 + SiO2 → C-S-H
(Calcium-hydroxidfrom thenormalcement reaction)
Silicon oxidefrom fly ashand/orsilica fume
New formed(C-S-H)
due to thepuzzolan
puzzolanThe silica fumehas a so-called micro-filler effect.The silica fumeparticles are about100 nm indiameter (same sizeas cigarette smokeparticles)
Gives high strength and low permeability
The concrete in the Øresund Linkcontains both fly ash and
silica fume
Fly ashSilica fumeSTEP
1ST
EP 2
OBSERVE: SiO2 present as sand (as quartz) is not the same as SiO2 present as silica. Sand is inert with respect to cement while silica is not (it reacts with CH)
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
BJO, BYG
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EfterbehandlingHigh risk of cracking!Avoid early drying using membranes
Treatment after casting
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Setting and Bleeding: fresh concreteSetting: defined as the onset of rigidity in fresh concrete
Hardening: describes the development of useful and measurable strengthBJO, BYG
Bleeding (a special form of segregation) may be defined as the upward movement of water after concrete has been consolidated but before it has set (undesirable phenomena). Can be avoided using cement with high fineness.
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Plastic shrinkage BJO, BYG
When the evaporation rate exceeds the rate of bleeding and the free settlement period is ended, a hydrostatic tension begins to develop throughout the mass owing to the formation of menisci at the water surfaces in the capillaries. This results in vertical as well as lateral compressive forces and may be manifested in a slab (thick plate) by pattern cracking. It is called plastic shrinkage cracking.
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Approximatemethod ofcalculatingthe rate ofevaporationfrom a freshconcretesurface
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
BJO, BYG
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Concrete durability
Sulfate attacks the concrete
Water (with very few ions)- leaching
Chloridesfrom seawatercausereinforcementcorrosion
Frost damages - deicing agents
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Alite hydration2C3S + 7H → C3S2H4 + 3Ca(OH)2
(Water) (C-S-H)(Tricalciumsilicate)
(Calciumhydroxide)
Process of carbonation
Ca(OH)2 + CO2 → CaCO3 + H2O(Calcium-hydroxid
in the hardenedcement)
CarbonedioxideIn the air
Calciumcarbonate
Carbonation of Ca(OH)2 in concrete
Lowers the pH-value in concrete pore solutionINCREASING RISK OF REINFORCEMENT CORROSION
Fly ashSilica fumeSTEP
1ST
EP 2
Notwanted
Water
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Salt-frost damages on surfacestogether with reinforcement
corrosion
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Outline• Basic properties of concrete• Composition of concrete mixes• Workability of fresh concrete• Hydration mechanisms (hardening)• Treatment after casting• Concrete durability
• Description of the layout of the course
BJO, BYG
59BJO, BYG
Lectures 1 - 7
Table 1: Schedule part I, Concrete Technology 11563, fall 2012, Aud. 306/34. Lecture Date Time Title
1 05-09-2012 8:00-10:00 Introduction, Repetition 2 12-09-2012 8:00-10:00 History of concrete, General practice 3 19-09-2012 8:00-10:00 Raw materials of concrete 4 26-09-2012 8:00-10:00 Concrete mix design, Hydration 5 03-10-2012 8:00-10:00 Chemical equilibrium 1 – basics 6 10-10-2012 8:00-10:00 Chemical equilibrium 2 – application Vacation 7 24-10-2012 8:00-10:00 Pore structure development
60BJO, BYG
Lectures 8 - 13
Table 2: Schedule part II, Concrete Technology 11563, fall 2012, aud. 306/34. Lecture Date Time Title
8 31-10-2012 8:00-10:00 Moisture fixation 9 07-11-2012 8:00-10:00 Moisture transport
10 14-11-2012 8:00-10:00 Corrosion 11 21-11-2012 8:00-10:00 Mechanical properties 12 28-11-2012 8:00-10:00 Durability 13 05-12-2012 8:00-10:00 Repetition
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After each lecture it is time to exercise!
Time set aside: from after lecture until 12:00
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Exercises
ROOMS: 116/025 116/44 116/45
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Literature:
Notes provided (250 DKK)
Powerpoint slides provided
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