Aggregates
are mainly used in concrete for the purpose of providing economy to the overall
cost of the concrete. Aggregates do not react with water or cement and act as
filler only.
However,
they comprise of features or properties which affects the composition of the
resultant concrete mix. Given below are few of the important properties –
- Composition
- Size and Shape
- Surface Texture
- Specific Gravity
- Bulk Density
- Voids
- Porosity and Absorption
- Bulking of Sand
- Fineness Modulus of Aggregate
- Surface Index of Aggregate
- Deleterious Material
- Crushing Value of Aggregate
- Impact Value of Aggregate
- Abrasion Value of Aggregate
Composition
Aggregates
which comprise of materials that are supposed to react with alkalies in the
cement and result in extraordinary cracking and expansion of the concrete mix
should not be used ever. Therefore, it is important to test aggregates to get
aware about the presence of such materials in the aggregate.
Size
and Shape
Aggregates
which comprise of materials of aggregate should be less than
- 1/4 of the minimum dimension of the concrete member
- 1/5 of the minimum dimension of the reinforced concrete member
- SE-Test
Aggregates comprise of materials of minimum
clear spacing between reinforced bars or 5 mm less than the minimum cover
between the reinforced bars and form.
Surface
Texture
The
surface texture is responsible for the creation of the hard bond strength
within the cement paste and the aggregate particles. So, in the event surface
area is porous and rough, bond strength is maximum. Also due to the
availability of the cement paste in the pores, the bond strength increase in
porous surface aggregates.
Specific
Gravity
Aggregates which comprise of materials ratio of weight of oven dried aggregates maintained for twenty four hours
at a temperature of 100-1100C, to the weight of equal volume of
water SE-Test displaced by saturated dry surface aggregate is known as specific
gravity of aggregates.
Aggregates which comprise of Specific gravities are primarily of two types.
- Apparent specific gravity
- Bulk specific gravity
Bulk
Density
The
weight of the aggregate that is needed to fill a container of unit volume is
defined as the bulk density. This bulk density is generally defined in
kg/litre.
Bulk density of aggregates depends upon 3 factors.
- Degree of compaction
- Grading of aggregates
- Shape of aggregate particles
Voids
Voids
are basically the empty spaces between the aggregate particles. The volume of each
void is equal to the difference of volume occupied by the particles and the
gross volume of the aggregate mass.
Porosity
and Absorption
Porosity
and Absorption can exist in
any of the 4 conditions.
- Very dry aggregatehaving no moisture
- SE-Test Dry aggregate contain some
moisture in its pores
- Saturated surface dry aggregate pores completely filled with moisture
but no moisture on surface of aggregate
- SE-Test wet aggregates (pores are
filled with moisture and also having moisture on surface
Fineness
Modulus
Fineness
Modulus is basically utilized for the purpose of getting an idea into how fine
the aggregate is. The more the fineness modulus, the coarser will be the
aggregate, the lesser the fineness modulus, the finer will be the aggregate.
Fineness Modulus is basically used to get an idea of how coarse or
fine the aggregate is. SE-Test Fineness Modulus value indicates that the aggregate is coarser and small value
of fineness modulus indicates that the aggregate is finer.
Specific
Surface Of Aggregate
Specificsurface denotes the surface area per unit weight of particular material. It is
basically an indirect method of denoting aggregate grading. Specific surface
gets increased with the decrease in the volume of the aggregate particle. Also,
fine aggregate’s specific surface area is more when compared to the coarse
aggregate.
Deleterious
Materials
SE-Test Aggregates should not contain
any harmful material in such a quantity so as to affect the strength and
durability of the concrete. SE-Test Aggregates materials
are called deleterious materials, Deleterious materials may cause one of the
following effects
- To interfere hydration of cement
- To prevent development of proper bond
- To reduce strength and durability
- To modify setting times
Deleterious materials generally found in aggregates,
may be grouped as under
- Organic impurities
- Clay, silt an dust
- Salt contamination
Bulking
Of Sand
SE-Test can be
defined as in increase in the bulk volume of the quantity of sand (i.e. fine
aggregate) in a moist condition over the volume of the same quantity of dry or
completely saturated sand. The ratio of the volume of moist sand due to the
volume of sand when dry, is called bulking factor.
Crushing
Value
The
crushing value of the aggregate provides a comparative measure of the resistance
of an aggregate in respect to the crushing under the compressive load which is
applied gradually. The crushing strength value serve as the useful factor for
knowing the behaviour of the aggregate in respect to compressive loads.
Impact
Value
The
aggregate impact value provides a comparative measurement of the aggregate’s
resistance to sudden impact or shock. This value is at times used as a
replacement to its crushing value.
Aggregate Abrasion Value
SE-Test aggregate abrasion value gives a relative measure of resistance
of an aggregate to wear when it is rotated in a cylinder along with some
abrasive charge.
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