Ce6110 lecture 1-aggregate

CE6110: Advanced Concrete Technology 27/7/2016

by Dr. M. Jahidul Islam 1

Major Md. Jahidul Islam, PhD, Engrs e-mail: [email protected]

PART B

Lecture # 01

Aggregate

2 CE6110: Dr M. Jahidul Islam

mailto:[email protected]



Aggregates:

In ordinary structural concretes,

the aggregates occupy about 65 to

75 % of the total hardened

volume.


Aggregate is relatively inexpensive and does not enter into complex chemical reactions with water;

It has been customary, therefore, to treat it as an inert filler in concrete

However, due to increasing awareness of the role played by aggregates in determining many important properties of concrete, the traditional view of the aggregate as an inert filler is being seriously questioned

Aggregate characteristics that are significant for making concrete include porosity

grading or size distribution

moisture absorption

shape and surface texture

crushing strength

elastic modulus

the type of deleterious (harmful) substances present

CE6110: Dr M. Jahidul Islam 4



Aggregate characteristics are derived from mineralogical

composition of the parent rock (which is affected by

geological rock-formation processes), exposure conditions

to which the rock has been subjected to before mining,

and the type of equipment used for producing the

aggregate.

Therefore, fundamentals of rock formation, classification

and description of rocks and minerals, and industrial

processing factors that influence aggregate characteristics

are of importance.


Classification of aggregates according to particle size,

bulk density, or source have given rise to a special

nomenclature, which should be clearly understood.

For instance, the term coarse aggregate is used to

describe particles larger than 4.75 mm (retained on No. 4

sieve), and the term fine aggregate is used for particles

smaller than 4.75 mm

Typically fine aggregates contain particles in the size

range 75 μm (No. 200 sieve) to 4.75 mm, and coarse

aggregates from 4.75 to about 50 mm, except for mass

concrete that may contain particles up to 150 mm.




Most natural mineral aggregates, such as sand and gravel,

have a bulk density of 1520 to 1680 kg/m3 (95 to 100

lb/ft3) and produce normal-weight concrete with

approximately 2400 kg/m3 (150 lb/ft3) unit weight

For special needs, aggregates with lighter or heavier

density can be used to make correspondingly lightweight

and heavyweight concretes

Generally, the aggregates with bulk densities less than

1120 kg/m3 (70 lb/ft3) are called lightweight and those

weighing more than 2080 kg/m3 (130 lb/ft3) are called

heavyweight


For the most part, concrete aggregates are comprised of

sand, gravel, and crushed rock derived from natural

sources. These are referred to as natural mineral

aggregates.

On the other hand, thermally processed materials such as

expanded clay and shale, which are used for making

lightweight concrete, are called synthetic aggregates.

Aggregates made from industrial by-products (e.g.,

blastfurnace slag and fly ash) also belong to this category.

Municipal wastes (such as plastic, rubber, etc.) and

recycled concrete from demolished buildings and

pavements are also being investigated for use as aggregate

for fresh concrete.




Natural mineral aggregates form the most important class of

aggregates for making Portland cement concrete

Most common coarse aggregate consumed by the concrete

industry is either gravel or crushed rock.

Carbonate rocks comprise about two-thirds of the crushed

aggregate; sandstone, granite, diorite, gabbro, and basalt make

up the rest

Natural silica sand is predominantly used as fine aggregate, even

with most lightweight concrete

Natural mineral aggregates are derived from rocks of several

types and most rocks are themselves composed of several

minerals

A mineral is defined as a naturally occurring inorganic substance

of more or less definite chemical composition and usually of a

specific crystalline structure CE6110: Dr M. Jahidul Islam 9

According to their origin, rocks are classified into three

major groups: igneous, sedimentary, and metamorphic;

these groups are further subdivided according to

mineralogical and chemical composition, texture or grain

size, and crystal structure.





Igneous rocks are formed by cooling of the magma

(molten rock matter) either above, or below, or near the

earth’s surface. The degree of crystallinity and the grain

size of igneous rocks, therefore, vary with the rate at

which magma was cooled at the time of rock formation. It

may be noted that grain size has a significant effect on

the rock characteristics; rocks having the same chemical

composition but different grain size may behave

differently under the same condition of exposure.


Granite Basalt



Sedimentary rocks are stratified rocks that are usually

laid down under water but are, at times, accumulated by

wind and glacial action. The siliceous sedimentary rocks

are derived from existing igneous rocks. Depending on

their method of deposition and consolidation, it is

convenient to subdivide them into three groups: (1)

mechanically deposited either in an unconsolidated or

physically consolidated state, (2) mechanically deposited

and consolidated usually with chemical cements, and (3)

chemically deposited and consolidated


Limestone

Metamorphic rocks are igneous or sedimentary rocks that

have changed their original texture, crystal structure, or

mineralogical composition in response to physical and

chemical conditions below the earth’s surface. Common

rock types belonging to this group are marble, schist,

phyllites, and gneiss. The rocks are dense but frequently

foliated. Some phyllites are reactive with the alkalies

present in Portland cement paste.


Marble: Limestone Quartz sandstone



Aggregates that weigh less than 1120 kg/m3 (70 lb/ft3) are generally considered lightweight, and find application in the production of various types of lightweight concretes

The light weight of the aggregate is due to the cellular or highly porous microstructure

It may be noted that cellular organic materials such as wood chips should not be used as aggregate because they would not be durable in the moist alkaline environment within Portland-cement concrete

Natural lightweight aggregates are made by crushing igneous volcanic rocks such as pumice, scoria, or tuff

Synthetic lightweight aggregates are manufactured by thermal treatment of a variety of materials, for instance, clays, shale, slate, diatomite, pearlite, vermiculite, blast-furnace slag, plastic and fly ash





PUMICE

The most widely used natural L.W.A. Usually whitish gray to yellow in color but may also be brown red or black. It is porous in structure.

EXPANDING: Materials are passed through a rotary kiln at about 1090C. Gasses within the material expand, forming thousands of tiny air cells within the mass.


SCORIA

It is also of volcanic origin, resembles industrial cinders and is usually red to black in color. (Cinders are residues from high-temperature combustion of coal in industrial furnaces). The pores in scoria are larger than those of pumice and more or less spherical shape.




EXPANDED PERLITE

It is an aggregate derived from crushing perlite and then expanding the resulting particles in a kiln by driving the water out.

It is used to replace natural sand in lightweight concrete manufacture and has very good insulating properties.

Concrete made with this aggregate has limited strength as well as high shrinkage.

Perlite is also used in the manufacture of cement mortar.


VERMICULITE

It is a type of mica, and also used in the manufacture of lightweight concrete.

It is produced by heating the raw material until it expands up to 20 times its original volume.

It is too soft and weak a material to be used in concrete that requires strength, but is used in plaster as a replacement for sand.

The bulk density of vermiculite is 64 to 192 kg/m3 which is nearly same as that of perlite.

Concrete made with vermiculate or perlite has low compressive strength and high shrinkage, but excellent insulating properties.




EXPANDED SHALE, CLAY,

and SLATE

They are in

manufactured

lightweight aggregate

category, and are

produced by crushing

the raw materials and

heating them to 1350C,

when they become soft

and expand (up to 600

to 700% of original

volume) because of

entrapped gasses.


EXPANDED SHALE

EXPANDED CLAY

22

Some of the L.W.A., especially the fine portions of crushed aggregates, have highly angular, unfavorable particle shape.

This has harmful effects on; Workability

Finishing

Bleeding on concrete

These can be reduced by AIR-ENTRAINMENT (up to 10%), increased cement content, use of mineral admixtures, or partial substitution of fine, light particles by normal-weight concrete sand or that recommended for masonry mortar.

CE6110: Dr M. Jahidul Islam



SEGREGATION

The lighter bulk specific gravity

of the aggregate can also cause

problems because it can produce

segregation of the coarse

particles from the concrete mass

during mixing, shipping, placing,

and compaction.

During the vibration of freshly

mixed concrete, the coarse

particles have a tendency to

move upward.

The danger of segregation can be

reduced by;

careful proportioning,

proper handling of the fresh

concrete.

ABSORPTION

High absorption value and the

high rate of absorption of most

L.W.A. can also be a problem if

not checked frequently and

counter balanced in the

proportioning.

The high water absorption can

be a problem in connection with

the frost resistance of L.W.A.

concretes.


Compared to normal-weight aggregate

concrete with a typical unit weight of 2400

kg/m3 (150 lb/ft3), heavy-weight concretes

weigh from 2900 to 6100 kg/m3 (180 to 380

lb/ft3), and are primarily used for making

nuclear radiation shields

Heavy-weight aggregates (i.e., those that

have a substantially higher density than

normal-weight aggregate) are used for the

production of heavy-weight concrete

Natural rocks suitable for heavy-weight

aggregate consist predominately of two

barium minerals, several iron ores, and a

titanium ore


Natural Barium Sulphate

Iron ores

Perovskite rock (titanium ore)




Slow cooling of blast-furnace slag in ladles, pits, or iron molds yields a

product that can be crushed and graded to obtain dense and strong

particles suitable for use as concrete aggregate.

The properties of the aggregate vary with the composition and rate of

cooling of slag.

Acid (siliceous) slags generally produce a denser aggregate, and basic

(limey) slags tend to produce a vesicular or honeycombed structure

with a lower apparent specific gravity (2 to 2.8).

On the whole, the bulk density of slowly cooled slags, which typically

ranges from 1120 to 1360 kg/m3, is somewhere between normal-weight

natural aggregate and structural lightweight aggregate.

These aggregates are widely used for making precast concrete

products such as masonry blocks, channels, and fence posts.




Fly ash consists essentially of small spherical particles of

aluminosilicate glass, which is produced by combustion of

pulverized coal in thermal power plants

As large quantities of the ash remain unutilized in most

countries of the world, attempts have been made to use

the ash for making lightweight aggregate

In a typical manufacturing process, fly ash is pelletized and

then sintered in a rotary kiln, shaft kiln, or a traveling

grate at temperatures in the range 1000 to 1200°C

Variations in the fineness and carbon content of fly ash are

a major problem in controlling the quality of sintered fly-

ash aggregate


Rubble from demolished concrete buildings yields

fragments in which the aggregate is contaminated with

hydrated cement paste, gypsum, and minor quantities of

other substances

The size fraction that corresponds to fine aggregate

contains large amounts of hydrated cement and gypsum,

and it is unsuitable for making fresh concrete mixtures

However, the size fraction that corresponds to coarse

aggregate, although coated with cement paste, has been

used successfully in several laboratory and field studies




In our day to day life, plastic becomes one of the most

widely used items. More than 299 million tons of plastics

were produced in 2013 alone.

Although it has made our life comfortable but it also

create a large waste disposal problem. Because of its non-

biodegradable nature it creates congestion in ground and

water system. In Bangladesh 750 thousand tons of recycled

plastic waste was created during 2010 – 2011.

There is a potential to adopt waste plastic as a partial

replacement for coarse or fine aggregate and achieve

higher strength than the regular concrete.


A knowledge of certain aggregate characteristics (i.e.,

density, grading, and moisture state) is required for

proportioning concrete mixtures

Porosity or density, grading, shape, and surface texture

determine the properties of plastic concrete mixtures

In addition to porosity, the mineralogical composition of

aggregate affects its crushing strength, hardness, elastic

modulus, and soundness, which in turn, influence various

properties of hardened concrete containing the aggregate





Generally, aggregate properties affect not only the concrete mixture proportions but also the behavior of fresh and hardened concrete

Due to considerable overlap between the two, it is more appropriate to divide the study of aggregate properties into three categories that are based on microstructural and processing factors:

Characteristics dependent on porosity: density, moisture absorption, strength, hardness, elastic modulus, and soundness

Characteristics dependent on prior exposure and processing factors: particle size, shape, and texture

Characteristics dependent on chemical and mineralogical composition: strength, hardness, elastic modulus, and deleterious substances present




For the purpose of proportioning concrete mixtures it is not necessary to determine the true specific gravity of an aggregate

Natural aggregates are porous; porosity values up to 2 percent are common for intrusive igneous rocks, up to 5 percent for dense sedimentary rocks, and 10 to 40 percent for very porous sandstones and limestone

For the purpose of mix proportioning, it is desired to know the space occupied by the aggregate particles, inclusive of the pores existing within the particles

Therefore, determination of the apparent specific gravity, which is defined as the density of the material including the internal pores, is sufficient.

The apparent specific gravity for many commonly used rocks ranges between 2.6 and 2.7; typical values for granite, sandstone, and dense limestone are 2.69, 2.65, and 2.60, respectively


For the purpose of concrete mixture proportioning, in

addition to apparent specific gravity, data are usually

needed on bulk density which is defined as the mass of the

aggregate fragments that would fill a unit volume

The phenomenon of bulk density arises because it is not

possible to pack aggregate fragments together such that

there is no void space

The term bulk is used since the volume is occupied by both

aggregates and voids

The approximate bulk density of aggregates commonly

used in normal-weight concrete ranges from 1300 to 1750

kg/m3 (80 to 110 lb/ft3)




Various states of moisture absorption in which an aggregate

particle can exist are shown in the following slide

When all the permeable pores are full and there is no water film on

the surface, the aggregate is said to be in the saturated-surface

dry condition (SSD); when the aggregate is saturated and there is

also free moisture on the surface, the aggregate is in the wet or

damp condition

In the oven-dry condition, all the evaporable water has been driven

off by heating to 100°C

The amount of water in excess of the water required for the SSD

condition is referred to as the surface moisture


Absorption capacity is defined as the total amount of moisture

required to bring an aggregate from the oven-dry to the SSD

condition

Effective absorption is defined as the amount of moisture required

to bring an aggregate from the air-dry to the SSD condition

The absorption capacity, effective absorption, and surface moisture

data are invariably needed for correcting the batch water and

aggregate proportions in concrete mixtures made from stock

materials

As a first approximation, the absorption capacity of an aggregate,

which is easily determined, can be used as a rough measure of

porosity and strength





Damp sands may suffer from a phenomenon known as

bulking

Depending on the amount of moisture and aggregate

grading, a considerable increase in the bulk volume of the

sand can occur

Because the surface tension of water keeps the particles

apart, fine sands show more bulking

Since most sands are delivered at the job site in a damp

condition, wide variations can occur in the batch

quantities if batching is done by volume

For this reason, proportioning of concrete mixture by mass

has become the standard practice in most countries





The crushing strength, abrasion resistance, and elastic modulus

of aggregate are interrelated properties that are greatly

influenced by porosity

Aggregates from natural sources that are commonly used for

making normal-weight concrete, are generally dense and strong;

therefore they are seldom a limiting factor to strength and

elastic properties of concrete

Typical values of the crushing strength and dynamic elastic

modulus for most granite, basalt, trap rock, flint, quartzitic

sandstone, and dense limestone aggregates are in the range 210

to 310 MPa (30 to 45 × 103 psi) and 70 to 90 GPa (10 to 13 × 106

psi), respectively

With regard to sedimentary rocks, the porosity varies over a

wide range, as will the crushing strength and related

characteristics 40 CE6110: Dr M. Jahidul Islam



An aggregate is considered unsound when the volume changes in

aggregate induced by weather (e.g., alternate cycles of wetting

and drying, or freezing and thawing), result in the deterioration

of concrete

Unsoundness is shown generally by rocks having a certain

characteristic pore structure

Concretes containing some cherts, shales, limestones, and

sandstones have been found susceptible to damage by frost

action or by salt crystallization within the aggregate particle

Although high moisture absorption is often used as an index for

unsoundness, many aggregates such as pumice and expanded

clays can absorb large amounts of water but remain sound

Unsoundness is therefore related to pore size distribution rather

than to the total porosity of aggregate


A pore size distribution that allows the aggregate particles

to get saturated on wetting (or thawing in the case of frost

attack), but prevent easy drainage on drying (or freezing)

is capable of causing high hydraulic pressure within the

aggregate particles

In the case of frost attack, in addition to pore size

distribution and degree of saturation there is a critical

aggregate size below which high internal stresses capable

of cracking the particle will not occur

For most aggregate, this critical aggregate size is greater

than the normal size of coarse aggregates used in the

construction practice; however, for some poorly

consolidated rocks (sandstones, limestones, cherts,

shales), this size may be in the 12 to 25 mm range




Grading is the distribution of particles of a granular material

among various size ranges, usually expressed in terms of

cumulative percentage larger or smaller than each of a series of

sizes of sieve openings, or the percentage between certain

range of sieve openings

There are several reasons for specifying grading limits and

maximum aggregate size; the most important is their influence

on workability and cost

For example, very coarse sands produce harsh and unworkable

concrete mixtures, and very fine sands increase the water

requirement (therefore, the cement requirement for a given

water-cement ratio) and are uneconomical

Aggregates that do not have a large deficiency or excess of any

particular size produce the most workable and economical

concrete mixtures


0.1 1 10Sieve Size (mm)

0

20

40

60

80

100

Cu

mu

lati

ve

Per

cen

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assi

ng

(%

)

Sand

ASTM-Upper Limit

ASTM-Lower Limit

1 10 100Sieve Size (mm)

0

20

40

60

80

100

Cu

mu

lati

ve

Per

cen

t P

assi

ng

(%

)

PP CA

Brick CA

ASTM-Upper Limit

ASTM-Lower Limit




The maximum size of aggregate is conventionally designated by the

sieve size on which 15 percent or more particles are retained

In general, the larger the maximum aggregate size, the smaller will be

the surface area per unit volume which has to be covered by the

cement paste of a given water-cement ratio

Since the price of cement may be 10 to 15 times as much as the price

of aggregate, any action that saves cement without reducing the

strength and workability of concrete can result in significant economic

benefit

In addition to cost economy, there are other factors that govern the

choice of maximum aggregate size for a concrete mixture

According to one rule of thumb used in the construction industry, the

maximum aggregate size should not be larger than one-fifth of the

narrowest dimension of the form in which the concrete is to be placed;

also, it should not be larger than three-fourths of the maximum clear

distance between the reinforcing bars


As large particles tend to produce more microcracks in the

interfacial transition zone between the coarse aggregate

and cement paste, with high-strength concrete mixtures

the maximum aggregate size is generally limited to 19 mm

Similarly, aggregate grading has also considerable effect on

the cement paste requirement of a concrete mixture

In practice, low void contents are achieved by using

smoothly graded coarse aggregates with suitable

proportions of graded sand

In practice, an empirical factor called the fineness

modulus is often used as an index of the fineness of

aggregate




The shape and surface texture of aggregate particles influence

the properties of fresh concrete more than hardened concrete

Compared to smooth and rounded particles, rough-textured,

angular, and elongated particles require more cement paste to

produce workable concrete mixtures, thus increasing the cost

Shape refers to geometrical characteristics such as round,

angular, elongated, or flaky

Particles shaped by wearing down tend to become rounded by

losing edges and corners

Wind-blown sands, as well as sand and gravel from seashore or

riverbeds generally have a well-rounded shape

Crushed intrusive rocks possess well defined edges and corners

and are called angular


Laminated limestones, sandstones, and shale tend to

produce elongated and flaky fragments, especially when

jaw crushers are used for crushing

Those particles in which thickness is small relative to two

other dimensions are referred to as flat or flaky, while

those which are considerably bigger in length than the

other two dimensions are called elongated

Another term sometimes used to describe the shape of

coarse aggregate is sphericity which is defined as a ratio of

surface area to volume

Spherical or well-rounded particles have low sphericity,

but elongated and flaky particles possess high sphericity




Elongated, blade-shaped aggregate particles should be avoided or

limited to a maximum of 15 percent by mass of the total aggregate

This requirement is important not only for coarse aggregate but also

for manufactured sands (made by crushing stone), containing

elongated grains, which produce very harsh concrete

The classification of surface texture, defined as the degree to which

the aggregate surface is smooth or rough, is based on visual judgment

Surface texture of aggregate depends on the hardness, grain size, and

porosity of the parent rock and its subsequent exposure to forces of

attrition

There is some evidence that, during early age, the strength of

concrete, particularly the flexural strength, can be affected by the

aggregate texture; a rougher texture seems to help the formation of a

stronger physical bond between the cement paste and aggregate. At a

later age, with a stronger chemical bond between the paste and the

aggregate, this effect may not be so important


Deleterious substances are those that are present as minor

constituents of either fine or coarse aggregate but are capable of

adversely affecting the workability, setting and hardening, and

durability characteristics of concrete

For both fine and coarse aggregates, ASTM C 33 requires that

“aggregate for use in concrete that will be subject to wetting,

extended exposure to humid atmosphere, or contact with moist

ground shall not contain any materials that are deleteriously reactive

with the alkalies in the cement in an amount sufficient to cause

excessive expansion except that if such materials are present in

injurious amounts, the aggregate may be used with a cement

containing less than 0.6 percent alkalies or with the addition of a

material that has been shown to prevent harmful expansion due to

the alkali-aggregate reaction”


Education

Ce6110 lecture 1-aggregate