Aggregate and its types

What comes in your mind if you listen the word aggregate? Pebbles extracted from natural sources or the broken pieces of stone, right? Have you ever feel sand is also included in aggregate? Beside this the broken pieces of brick and other construction materials can be also used as aggregate.

 Aggregates

Aggregate is an inert material (but sometime chemically active) dispersed throughout the cement so as to produce large volume of concrete and also reduce shrinkage. It includes both the fine aggregate and coarse aggregate. It includes about 70-75% of total proportion of concrete mix.

Types of aggregate

On the basis of size:

1. Fine aggregate

The aggregates with particle size less than 4.75 mm are categorized under this. It is used to fill the voids in between the coarse aggregates in concrete so as to increase the resistance against crushing.

Sand is commonly used as fine aggregate in preparation of the cement as well as lime mortar and concretes. It is formed by decomposition of sandstone under the effect of weathering agencies. It may be either natural sand (eg. River sand, stream sand, pit and sea sand) or artificial sand (prepared by crushing stones and gravel to powder). Normally, sand extracted directed from stream and river is used for small scale construction whereas for medium and large construction project, the largely extracted sand is washed and separate according to the size of it in crusher and transported in the construction site.

Property of sand:

A good sand should have following properties:

• Well graded with fineness modulus between 2 to 3.
• Strong and durable
• Chemically inert
• Clean and free from clay and silt
• Free from organic matter and vegetation
• Free from salts and chemicals

Why do we use sand in mortar and concrete?

Function of sand

• It doesn’t increase the strength of mortar but it is used as an adulterant for economy
• It reduces the shrinkage of binding materials and hence cracking of mortar during setting is avoided.
• It increases the resistance of mortar against crushing
• It fills the void present in the coarse aggregate in concrete
• It helps in setting/hardening of cement by allowing water to penetrate through its voids.
• It sub-divides the paste of the binding materials into a thin film and then more surface is offered for spreading and adhering
• It helps in making mortar of any desired strength by varying the proportion with the binding materials.

Size of sand used for different purpose

Plastering – Fine sand which passes through screen with opening 1.58 mm

Masonry work – coarse sand which passes through screen with opening 3.18 mm

Concrete mix – gravelly sand which passes through screen with opening 7.62 mm

2. Coarse aggregate   

      The aggregates which retain in 4.75 mm IS sieve size are coarse aggregate. Coarse aggregates cover the greater percentage of concrete. Aggregate greatly influences the crushing strength and water tightness of the concrete and its resistance to wear and tear.

      In most of the construction, pebbles or broken stone are used as coarse aggregate. Coarse aggregates are prepared manually or mechanically. In the country where there is no stone, they use some artificially made aggregate like broken bricks, and some others.

Function of coarse aggregate:

1. It imparts greater volumetric stability and duration to concrete.
2. It makes a solid and hard mass of concrete (with cement and sand).
3. It increases the crushing strength of concrete.
4. It is cheaper that cement and directly helps in achieving economy in concrete.

On the basis of shape:

1. Rounded Aggregate

• Rounded shaped aggregate particle (river or sea shore gravel) with minimum voids ranging from 32 to 33%.

• It requires minimum cement paste to make good concrete since it gives minimum ratio of surface area to the volume

• The interlocking between its particles is less, and hence poor bond development, making it unsuitable for high strength concrete and pavement.

2. Irregular aggregates

• Partly rounded shape aggregate particles (pit sand and gravel) with higher percentage of voids ranging from 35 to 38 %.

• It requires more paste for a given work-ability.

• The interlocking between particles, though better than that obtained with the rounded aggregate, is inadequate for high strength concrete.

3. Angular aggregates

• Sharp angular and rough aggregate particles (crushed rock) with maximum percentage of voids ranging from 38 to 40%.

• The aggregates require more paste to make workable concrete of high strength.

• The interlocking between particles is good, providing a good bond thus these aggregate is suitable for high strength concrete and pavements subjected to tension.

4. Flaky and elongated aggregates

• An aggregate is termed flaky when its least dimension (thickness) is less than three-fifth (0.6 times) of its mean dimension.

• The particle is said to be elongated when its greatest dimension (length) is more than nine-fifth (1.8 times) of its mean dimension.

On the basis of density:

1. Light weight aggregate ( bulk density ~ 1200 kg/m³ )
2. Medium weight aggregate ( bulk density ~ 1520 kg/m³ )
3. Heavy weight aggregate ( bulk density ~ 2080 kg/m³ )

Types of aggregates		Uses
Light weight	Expanded clay, shale or slate, crushed brick	For making light weight concrete for structure, also for its insulating properties.
Medium weight	Crushed limestone, sand, river gravel, recycled concrete	Used for normal concrete projects.
Heavy weight	Steel or iron shot, steel or iron pellets	For making high density concrete for shielding against radiation.

Some terms related to aggregates:

Fineness modulus of aggregate:

            It is the index number used to indicate the average size of particles in the aggregates. It can be empirically calculated by adding the cumulative percentage of sample of an aggregate retained on each of specified series of sieve, and dividing the sum by 100.

Bulking of sand

            The increase in the volume of given weight of sand caused by the thin films of water around the sand particles pushing these particles apart is called bulking of sand. The extent of bulking depends upon the percentage of moisture presence in sand and its fineness. Fine aggregates shows more effect of bulking than coarse aggregate.

Segregation:

            Segregation is the process of separation of ingredient materials of concrete to make the heterogeneous mixture of freshly prepared concrete mix within a batch. Coarse aggregate, fine aggregate or cement paste separate from the concrete mix. Segregation can occurs at any stage of working as it may occurs during proportioning, mixing, transporting, placing and finishing. Segregation may cause the concrete weaker, less durable and will leave a poor surface finish. For more detail, CLICK HERE

Bleeding:

            Bleeding is the particular form of segregation in which some form of water appear in the surface of fresh concrete after it has consolidated and before it is set. Bleeding is predominantly observed if the concrete mix is with high water content, badly proportioned and insufficiently mixed concrete. For more details, CLICK HERE.

Soundness of aggregate

Soundness of aggregate is defined as the ability of aggregate to resist excessive change in volume. Aggregates which are porous, weak and containing any undesirable extraneous matters undergo excessive volume change in different physical condition. These physical conditions are moisture change, variation of temperature, freezing and thawing, alternate wetting and drying and so on.

            The fine and coarse aggregates used in concrete which are liable to be exposed to the action of frost should be subjected to soundness test.

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