AGGREGATE

What is an

AGGREGATE?

Aggregate: the inert filler

materials, such as sand or

stone, used in making

concrete

AGGREGATE

• It's a rock like material of various sizes and shapes,

used in the manufacturing process of concrete

• Generally occupy 70-80% of the volume of

concrete.

• Have important influence on concrete properties

• Aggregate should have the following character:

a) Hard and strong

b) Free of undesirable impurities (clean)

c) Chemically stable

d) Durable

Bond of Aggregate

• Partly due to interlocking

• Partly due to physical & chemical

characteristics of the aggregate

• Other factors include the cement paste

FUNCTION OF AGGREGATE

• They reduce the cost of concrete

• Well graded aggregates produce workable yet

cohesive concrete

• They reduce the heat of hydration of concrete

since they are normally chemically inert and act

as a heat sink for hydrating cement

• They reduce the shrinkage of concrete since

most aggregates are not affected by water and

they restrain shrinkage of the hydrating cement.

OTHER FUNCTION

• Control of surface hardness

- most aggregates have better abrasion

resistance

than hydrated cement

• Colour or light reflecting properties

• Control of density

• Control of fire

MAIN TYPES OF AGGREGATE

a) Crushed stone

- obtained by mechanically crushing rocks

- It is angular & have rough surface texture

b) Gravel

- produced by erosion of mountainous bedrock,

and surficial materials & following

transportation & abrasion, finally deposited at

the bank of the rivers.

- It is round and smooth edges

c) Sand

- it’s sharp & angular particles

CRUSHED STONES

GRAVEL

SAND

PROPERTIES REQUIRED FOR

MIX DESIGN

• Shape

• Size

• Specific gravity / density

• Sieve analysis

• Soundness

• Source

• strength

• Texture

• Thermal properties

• Bulk density

• Bulking factor

• Cleanliness

• Chemical properties

• Moisture content

• Grading

• Durability

Sizes

• Coarse aggregate e.g.

Gravel – 4.76mm or more

(5mm)

• Fine aggregate e.g. sand –

less than 4.76mm

• Silt – varies from 0.02mm

– 0.06mm

• Clay – much finer than

0.02mm

MAXIMUM AGGREGATE SIZE

• The higher the maximum aggregate size, the

lower the paste requirement for the mix.

• Increase in the max. aggregate size, will improve

concrete durability because there will be less

paste subject to chemical and physical attack.

• Strength of concrete increases with increasing

aggregate size because w/c ratio can be lowered.

• More water is required for smaller aggregates

than for larger maximum sizes

• Larger sizes require less water & therefore

less cement, so it’s more economical.

• Harsh sand often produce unworkable mixes

• Very fine sands often produce uneconomical

concretes because more water is required

and more cement is needed.

A) Petrological(source)

• 1.) Igneous - Granites, basalts dolerites,

gabbros & porphyries – Hard, tough, dense –

Excellent aggregates

• 2.) Sedimentary - Sandstones - When

hard & dense suitable - Siliceous better than

calcareous which are liable to acid attack

Limestones - Sedimentary rocks chiefly composed

of calcium carbonate Harder, denser types are

suitable

Cont’d

Shales - Poor aggregates – weak, soft, laminated

& absorptive

• 3.) Metamorphic - Variable character –

marbles & quartzites usually massive, dense &

adequately tough & strong

Schists & slates are often thinly laminated &

therefore unsuitable

Specific gravity or Density

• 1.) Heavy – SG>4.0i) Magnetic, natural iron ore (Fe3O4)

ii) Barytes (BaSO4)

Cont’d

• 2.) Normal SG 2.5-3.0i) Sands & gravels – division is arbitrarily on size – sands pass

4.76 mm BS sieve

ii) Granites & basalts – hard & tough

iii) Sandstone – hard & dense types

iv) Limestone – hard & dense types

v) Broken bricks – must be free of plaster – low sulphur

content

vi) Air cooled blast furnace slag

Cont’d

• 3.) Lightweight - SG<2.5i) Sintered PFA

ii) Foamed slag

iii) Vermiculite

iv) Expanded polystyrene

v) Pumice

vi) Wood waste

C) Shape & Texture

• Particle shape classified to BS 812 : 1975

• Terms used:

• Well rounded, rounded, sub-rounded, sub-angular, angular

Cont’d

• Surface texture classified to BS 812 : 1975• Relates to the degree of polish or dullness, smoothness or

roughness of particles surfaces

• Depends on hardness, grain size and pore character

• Note: BS 812 & BS 882 have been replaced

by BS EN 12620

SHAPE OF AGGREGATE

SHAPE OF AGGREGATE

• It could effect workability and mechanical

properties of concrete.

• Vary between rounded (implying water-worn

material) and angular (material with clearly

defined edges, produced by crushing).

• Angular material tends to produce concrete of

lower workability but higher strength for a

given w/c ratio.

• Rough textured or flat aggregates require more

water to produce a workable concrete than

rounded or cubical, well-shaped aggregate.

SURFACE TEXTURE

• A rough texture aggregate will provide an

extremely good key to cement.

• A glassy surfaces do not form a strong bond

with cement.

• Although rougher surface will tend to reduce

workability, they also result in increased

strength.

• Rough textured aggregates require more water

to produce a workable concrete

SPECIFIC GRAVITY / DENSITY

• Specific gravity of porous material such as

wood and aggregate can be calculated under

different moisture condition.

• Density of the aggregates is required in mix

proportioning to establish weight-volume

relationship.

SOUNDNESS

• Soundness of aggregate

is its resistance to chemical

attack, or to repeated

physical changes, such as

freeze-thaw cycles.

The ability of aggregates to resist

excessive change in volume

(soundness) due to physical changes in

the environment is also of importance.

Knowledge of these potentially harmful

factors will ensure that precautions can

be taken at the mix design stage of a

project.

The soundness of aggregates to

physical changes caused by the

environment is important to the

long-term durability characteristics

of concrete. Excessive changes in

volume can be caused by freezing

and thawing, thermal changes at

temperatures greater than freezing,

and cycles of wetting and drying.

SIEVE ANALYSIS

Sieve analysis helps to determine the particle size

distribution of the coarse and fine aggregates. This is

done by sieving the aggregates as per IS: 2386 (Part I) –

1963. In this we use different sieves as standardized by

the IS code and then pass aggregates through them and

thus collect different sized particles left over different

sieves.

PRINCIPLE

By passing the sample downward through a series of

standard sieves, each of decreasing size openings, the

aggregates are separated into several groups, each of

which contains aggregates in a particular size range.

APPARATUS

A set of IS Sieves of sizes - 80mm,

63mm, 50mm, 40mm, 31.5mm, 25mm,

20mm, 16mm, 12.5mm, 10mm,

6.3mm, 4.75mm, 3.35mm, 2.36mm,

1.18mm, 600μm, 300μm, 150μm and

75μm .

ii) Balance or scale with an accuracy to

measure 0.1 percent of the weight of

the test sample

Importance of sieve analysis of

aggregates are,

• Particle size distribution in a sample of

aggregate.

•To neglect particle interference.

•Good gradation of aggregate.

•To know about fineness modulus of sand.

BULKING

• Due to moisture in each particle

of sand, sand gets a coating of

water due to surface tension

which keeps the particles apart.

This causes an increment in

volume of sand known as

bulking.

DryMoisture content

3 – 5 %

Saturated

The presence of moisture in sand increases the volume of

sand. This is due to fact that moisture causes film of water

around the sand particles which result in the increase of

volume of sand. For a moisture content of 5 to 8 percent, the

increase in volume may be about 5 to 8 percent, depending

upon the grading of sand. The finer the material, the more

will be the increase in volume for a given moisture content.

This phenomenon is known as bulking of sand. When

moisture content is increased by adding more water, sand

particles pack near each other and the amount of bulking of

sand is decreased. Thus the dry sand and the sand

completely flooded with water have practically the same

volume.

•A container is taken and it is filled two third with the sample of

sand to be tested.

•The height is measured, say 20cm.

•Sand is taken out of container

•The container is filled with water

•Sand is then slowly dropped in the container and it is thoroughly

stirred by means of a rod.

•The height of sand is measured say 16cm,

• the Bulking of sand= [(20-16)/16] X100

below.

.

Thermal properties

• There are 3 thermal properties that may be

• significant in the performance of concrete

• – coefficient of thermal expansion,

• – specific heat

• – Conductivity

• • The last two are of interest in mass

• concrete, but usually not in ordinary

• structural works

Thermal properties

• The coefficient of thermal expansion of

• aggregate determines the corresponding

• value of concrete especially when we

• consider the fire safety

• • Its influence depends on the aggregate

• content of the mix and on the mix

• proportion of concrete in general

GRADING

• This term used to describe the relative proportion

of various particle sizes between the nominal

maximum aggregate size and the smallest

material, which passes a 150 m sieve.

• Grading done to assist in the production of

concrete with satisfactory plastic properties

(workability, cohesion and resistance to

bleeding), as well as satisfactory hardened

properties (strength, durability & surface finish),

using as little cement as possible.

• Gradation of aggregate is determined from sieve analysis.

• A representative sample of the aggregate is passed through a series of sieves and the weight retained in each sieve - expressed as a percentage of the sample - is compared with the grading limits specified.

• A sieve is an apparatus (round / square) with square openings.

• It is identified either by the size (clear) of the opening or by a number.

• The higher the number, the smaller the opening

Sieve an apparatus used for sieve analysis

• The percentage of weights retained in each

sieve used to draw a graph called “Particle

Size Distribution Curve”

• It’s drawn by plotting

a) X-axis : Log of the opening size

b) Y-axis : Percentage of particles by weight

, coarser than or finer than the

particular sieve

• Grading is needed so that

- aggregates fill as much as possible of the total

space. Voids between larger particles can be

filled efficiently with slightly smaller ones.

- aggregates & cement being much denser than

water, tend to settle while the mixing water

tends to rise. So a well graded aggregate will

ensure that there are no large volumes of

cement paste and that settlement of solids is

minimised by particle interference.

• The grading and maximum size of aggregates

affect relatively aggregate proportion, cement

and water requirements, workability, economy,

porosity and shrinkage of concrete.

MOISTURE CONTENT

• Almost all aggregates contain moisture

• Some of the aggregate's moisture may contribute for

workability and some aggregate will absorb

water added for concrete mixing

• 2 types of moisture measurement recognized in

aggregate particles are:

a) Absorbed moisture

- Moisture retained within the pores of aggregate

b) Surface moisture

-Moisture that is held on the surface of aggregate

Moisture Condition

• Oven-dried, Absorption, Moisture Content

• Net effect

• Bulking of Sand ( 5% MC is worst)

Moisture Condition

• Water absorption – the weight of water absorbed

by aggregate particles in reaching a moisture level

or condition called Saturated Surface Dry (SSD)

• SSD – represents state of the moisture when all the

pores within a particle filled with water and the

particle's surface stays dry.It won't absorb

moisture / contribute moisture to surroundings.

• Based on the moisture level, aggregate can be

divided to 4 states that is

• a) Oven-dry c) Saturated, surface dry

b) Air-dry d) Wet

• Oven-dry :

All moisture is removed from the aggregate by

heating in an oven at 105°C. All pores are empty.

• Air-dry :

There's no free moisture & surface layers are dry.

Occurs in upper parts of aggregate stockpiles in

dry weather

• Saturated Surface Dry :

This is the ideal state for an aggregate for

concrete , since it requires no alteration to mixing

water.

• Wet : Surplus moisture is present.

• Oven-dry and air-dry will need extra water to

be added into the mixer.

• Aggregate is termed wet when :

there is free moisture on the aggregate surface

• Wet condition usually exists after rains

• Crushed stone & gravel (coarse aggregate)

absorb very little & also hold little water on

the surface.

• Fine aggregates have high absorption capacity

& even higher surface moisture.

CLEANLINESS

• IMPURITIES IN AGGREAGATES

• SiltThis material are often of a clayey nature, and they decrease the bond between aggregate and cement, reducing the strength of concrete.

• Organic ImpuritiesSuch material being acidic, reduces the alkalinity of cement paste that is essential for its hydration, thereby effecting setting time and strength.Impurities occur in form of top soil or leaves

• Other Impurities

Soluble Salts present in marine aggregate &

will leave a thin deposit on each particle

unless washed in fresh water.

Salt accelerate hydration of cement also can

cause corrosion of embedded metal.

• Reactive or Unsound Inclusions

Some aggregates contain mineral particles

that react to high alkali levels. Such as ASR

• Certain substances in limestone & dolomite

aggregate called “reactive silica” will react

with the cement especially in warm & moist

environment.

• Chemical reaction between silica in aggregate

will react with alkalis in cement will create

map like cracks in the concrete that will allow

for moisture penetration.

Damage Caused By Alkali Silica Reaction

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