2. 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 ભૂરો-લીલો પથ્થર), sandstone, Quartzite etc
3. 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
5. 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
6. (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
7. (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
8. 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.
9. 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
10. 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.
11. 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.
12. 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.
13. 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.
