AGGREGATES

Inert or chemically inactive materials

which when bonded together with cement

forms concrete

DEFINITION:

70% to 80% of concrete is composed of

aggregates

Properties of aggregates affect properties of

concrete.

Avoids cracking and gives strength to

concrete.

Eg: natural sand, gravel, crushed stone etc.

According to nature of formation Natural aggregates Artificial aggregates

According to size Coarse aggregates Fine aggregates All-in aggregates

According to shape Rounded aggregates Irregular or partially rounded aggregates Flaky aggregates Angular aggregates Elongated aggregates

CLASSIFICATION

According to nature of formation

Natural aggregates:obtained from natural sources

Artificial aggregates:manufactured from raw

materials by fusing in rotary kiln

According to size:Coarse aggregates:

retained on 4.75mm sieve.max. size for plain concrete –

150mmmax. size for reinforced concrete

– 60mm or less

Coarse aggregates:

Graded aggregates

Single sized aggregates

Fine aggregates:passing through 4.75mm seive

and retained on 0.15mm sieve.

Fine aggregates:

Natural sand

Crushed stone sand

Broken brick fine aggregate

All – in aggregates

Comes from pit or river bed

Used without seperating into different

sizes.

Contains fine and coarse aggregates

Used for unimportant works

Rounded aggregates:Surface area minimumConcrete requires lesser cement – low strength

According to shape

Irregular aggregates:natural irregularity due to attrition with rounded edges

Flaky aggregates:usually angular – thickness is small relative to other dimensions

Angular aggregates:- well defined edges- concrete – more cement – high strength and durability

Elongated aggregates:length is considerably greater than

other two dimensionsconcrete – poor quality and less durable.

Pit Sand

River Sand

Sea Sand

Sources of aggregates:

Pit Sand:pits dug at a depth of 1.5m to 2m in soilsharp, angular porous and free from harmful chemicalsfor making mortar

River Sand:obtained from river beds and banksbright and clear – sharp or roundedfor mortar and plastering

Sea Sand:seashoresbrown and roundedcontains salts – not used for construction

Manufactured Sand:manufactured in stone crushersless impuritiesbetter control over size and quality

Hard, strong, durable

Should not react with cement or steel

Angular and rough surface

Free from organic substances

Contains stone gravels and sand or in various

combinations

Requirements:

Representation of particle size in a sample of

aggregates

Good grading – all standard sizes in required

proportions

Sieve analysis – determination of proportions

of particles in an aggregate by seperation on

sieves

Grading of aggregates

80 mm40 mm20 mm10 mm4.75 mm2.36 mm1.18 mm600 microns300 microns150 micronspan

Sieve analysis

Sieve analysis

Weight of residue on each sieve% weight retainedCummulative % weight retained

Fineness modulus =

WR – sum of cummulative % retained

100WR

Sands are classified as:

type Fineness modulus

Fine sand 2.2 to 2.6

Medium sand 2.6 to 2.9

Coarse sand 2.9 to 3.2

Based on gradationwell graded – good representation of all sizes

Poorly graded:excess of of certain sizes

Gap graded:one or intermediate sizes are missing