Aggregate is an important constituent in concrete. It occupies about three-quarters of the volume of concrete, it contributes significantly to the structural performance of concrete, especially strength, durability, and volume stability.
What makes aggregates so special that it is used in a huge amount in concrete?
It is normal to raise a question in the mind of fresh engineers that Why aggregates are used in a huge amount in concrete or construction works? For that, we have to understand or know the properties of aggregate. Let’s discuss the properties of aggregates briefly.
Properties of aggregate
i) Size of aggregates
Based on the size of particles, aggregates are classified as follows:
- Coarse aggregate: It passes through an 80mm sieve and retained on a 4.75mm sieve.
- Fine aggregate: It passes through a 4.75mm sieve and retained on a 75-micron sieve
. Particlesbetween 0.075mm and 0.002mm are classified as silt and particles of size smaller than 0.002mm as clay.
- Silt and clay are undesirable particles and should be removed from aggregates used for making concrete.
ii) Shape of aggregate
- Aggregates are classified as rounded, irregular or partly rounded, angular and flaky.
- The shape of the particles affects the voids in compacted aggregate.
- Aggregate of rounded shape is very appropriate from workability consideration, but it results in poor bonding between the passes and the cement paste.
- Flaky and elongated particles are not desirable for making concrete.
A flaky particle has thickness less than 0.6 times of mean sieve size. Mean sieve size is defined as the arithmetic average of a sieve size through which it passes and over which it is retained. Elongated particles have their longest dimension 1.8 times the mean sieve size. These particles have the highest surface area and hence reduce the workability of concrete. They tend to orient in a plane during compaction resembling with stratified rock and so affect adversely the durability of concrete.
iii) Surface Texture of aggregate
The surface texture describes the nature of the surface of the aggregate. The aggregates are classified as glossy, smooth, granular, rough, crystalline, Honey-combed and porous. The shape and surface texture of aggregates influence the strength of concrete considerably.Aggregates of a rough surface increases the strength because of the higher surface area resulting in greater adhesive force between the particles and cement paste.
However, it reduces workability on account of more water required to wet the surface of the aggregates.
iv) Compressive strength of aggregate
Generally, the strength of aggregate is greater than the strength of normal concrete. The strength of aggregate is required to be made in the following situations:
- For the production of high strength and ultra high strength concrete.
- When contemplating to use aggregates manufactured from weathered rocks.
- Aggregate manufactured by industrial purpose.
iv) Modulus of Elasticity of aggregate
Modulus of Elasticity of aggregate depends on its composition, texture, and structure. The modulus of elasticity of aggregate will influence the properties of concrete w.r.t. shrinkage and elastic behavior and to a very small extent creep of concrete. Many studies have been conducted to investigate the influence of modulus of elasticity of aggregate on the properties of concrete.
One of the studies indicated that the ‘E’ of aggregate has a decided effect on the elastic property of concrete and that the relation of ‘E’ of aggregate to that of the concrete is not a linear function, but maybe expressed as an equation of exponential type.
v) Specific gravity of aggregate
In concrete technology, the specific gravity of aggregates is made use of in design calculation of concrete mixes. With the specific gravity of each constituent known, its weight can be converted into solid volume and hence a theoretical yield of concrete per unit volume can be calculated.
vi) Bulk density of the aggregate
The bulk density or unit weight of
Bulk density shows how densely the aggregate is packed when filled in a standard manner. The bulk density depends upon the particle size distribution and shape of the particles.
vii) Water absorption and surface moisture
Aggregates contain permeable holes through which moisture can permeate. There are four conditions of moisture content:
- Bone dry: When an aggregate does not contain any moisture. The condition is attained by drying aggregate at 100 degrees.
- Air-dry: When a saturated and surface dry aggregate is allowed to dry in air, a part of moisture evaporates and part of it is retained in the pores. This is known as air-dry aggregates. C for 24 hours.
- Saturated and surface dry: When all the pores in the aggregate are completely filled with moisture and there is no free moisture on its surface, it is said to be saturated and surface dry condition. This condition of aggregate is considered for the water cement ratio in the concrete mix design.
- Moist: When an aggregate has free moisture on its surface in addition to all pores filled, they are said to be in moist condition.
Soundness is the resistance of aggregates to physical disintegration forces such as heating and cooling, wetting and drying, freezing and thawing due to climate changes. The aggregate is said to be unsound when its volume changes due to physical disintegration forces resulting in deterioration of the concrete.
ix) Durability of aggregates
Durability is the resistance to aggregates to chemically disintegrating forces. The most common chemical reaction, causing disintegration, is between the active silica constituents of the aggregate and alkalies in the cement.
These aggregates contain veins or inclusions of reactive silica, which when used with cement having a high alkali content forms alkali
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