Aggregate (elements of civil engineering)

Aggregate •Aggregates are the important constituents

in concrete. They give body to the concrete,

reduce shrinkage and effect economy.

•The aggregates occupy 70 – 80% of the

volume of concrete.

•Aggregates were considered chemically

inactive and acting as only filler material.

But, now it is recognized that many types of

aggregates form chemical bonds with

cement paste.

•Aggregates are made of Granite , Basalt(લાવા નો બનેલો ભરૂો-લીલો પથ્થર), sandstone, Quartzite etc

Classification of aggregates

(A) Classification of aggregates

based on weight :

Normal weight aggregate

1. Natural aggregates :-Sand, Gravel(ક ાંકરી), Granite , Basalt,

Quartzite, Sandstone(રેતિયો પથ્થર).2. Artificial Aggregates :- Broken bricks,

Air-cooled Slag

Light weight aggregate

1. Natural aggregates:- Pumice, Scoria,

Rice husk, Saw dust

2. Artificial aggregates :- Foamed slag,

Bloated clay, expanded shale, sintered

fly ash.

Heavy weight aggregate

Barite, Magnetite, limonite, Hematite

1. Normal weight aggregate :- density of concrete produced is 2300 to 2600 kg/m3

2. Light weight aggregate :- density of concrete produced is 1200 to 1850 kg/m3

3. Heavy weight aggregate :- density of concrete produced is 4000 to 5000 kg/m3

(b) Classification of aggregates based on

size :

1. Find aggregate size < 4.75 mm

Bulking is less.

It fills the voids between coarse

aggregates.

e.g. sand, rock dust

1. Coarse Aggregate size > 4.75 mm

Bulking is more.

It gives strength to the concrete

e.g. gravel

(c) Classification of aggregates based on shape :

1. Rounded

2. Irregular

3. Angular

4. Flasy

(d) Classification of aggregates based on texture :

1. Glassy

2. Smooth

3. Granular

4. Crystalline

5. Honey combed and Porous

Requirements of good aggregates.

Aggregate should be hard, strong and durable.

It should be free from organic impurities.

It should be free from grass and roots.

Clay content should not exceed 4%.

Aggregate should have good soundness. (Good resistance to volume change)

It should be well graded.

Uses of coarse aggregates.

Used as inert material in PCC and RCC.

Used as base course material for road works.

Used as railway ballast.

Tests on aggregate :

The various tests on aggregates are :

Aggregate crusing value test

Aggregate impact value test

Aggregate abrasion value test

Ten percent fines value test

Specific gravity test

Bulk density test

Absorption and moisture content

Fineness modulus test

Flankiness index

Elongation index

Aggregate crushing value test:

The ‘aggregate crushing value’ gives a

relative measure of the resistance of an aggregate to

crushing under a gradually applied compressive load.

The aggregate crushing value should not be more

than 45% for aggregates used for concrete other

than for wearing surfaces.

The aggregate crushing value should not be more

than 30% for aggregates used for wearing surfaces

such as roads, runways and airfield pavements.

Aggregate impact value test :

The ‘aggregate impact value’ gives a relative

measure of the resistance of an aggregate to

sudden. Shoke or impact loading.

The aggregate impact value should not be more than

45%for aggregates used for concrete other than for

wearing surfaces.

The aggregate impact value should not be more than

30% for aggregates used for wearing surface such as

roads, runways and airfield pavements.

Aggregate abrasion value test :

Resistance to wear offered by aggregate is termed

as ‘aggregate abrasion value’. This test is very

important for the aggregate used for concrete

used in construction of roads, floors of warehouses,

pavements, etc.

The aggregate abrasion value should not be more

than 50% for aggregates used for concrete other

than for wearing surfaces.

The aggregate abrasion value should not be more

than 50% for aggregates used for concrete used for

wearing surfaces such as roads, runways and airfield

pavements.

Specific gravity

The specific gravity is required for the calculations of

the yield of concrete or of the quantity of aggregate

required for a given volume of concrete. It gives

valuable information on its quality and properties . It

is seen that the higher the specific gravity of

aggregate, the harder and stronger it will be.