AGGREGATES

AGGREGATES

Aggregates are crushed oruncrushed, derived from natural sources such as river terraces, river beds, rocks,boulders, gravels etc., for use in production of mortar or concrete for normal structuralpurposes including mass concrete works.

Classification of Aggregates:

On the basis of Geological Origin the aggregates may be classified into natural aggregates and artificial aggregates.

Natural Aggregates :These are obtained by crushing from quarries of igneous, sedimentary ormetamorphic rocks. Gravels and sand reduced to their present size by the natural agencies alsofall in this category. The most widely used aggregate are from igneous origin. Aggregatesobtained from pits or dredged from river, creek or sea are most often not clean enough or wellgraded to suit the quality requirement. They therefore require sieving and washing before theycan be used in concrete.

Artificial Aggregates: Broken bricks, blast furnace slag and synthetic aggregates are artificialaggregates. Broken bricks known as brick bats are suitable for mass concreting, for example, infoundation bases. They are not used for reinforced concrete works. Blast furnance slag aggregateis obtained from slow cooling of the slag followed by crushing. The dense and strong particlesas obtained are used for making precast concrete products. The sp. gr. of these range between2–2.8 and bulk density 1120–1300 kg/m3.Synthetic aggregates are produced by thermally processed materials such as expandedclay and shale used for making light weight concrete.

On the Basis of size :

According to size aggregates are classified as coarse aggregate, fine aggregate and all-in- aggregate

Coarse Aggregate : Aggregate retained on 4.75 mm sieve are identified as coarse. They areobtained by natural disintegration or by artificial crushing of rocks. The maximum size ofaggregate can be 80 mm. The size is governed by the thickness of section, spacing ofreinforcement, clear cover, mixing, handling and placing methods.

All-in-aggregate:Naturally available aggregates of different fractions of fine and coarse sizesare known as all-in-aggregate.

Graded Aggregate: Aggregate most of which passes through a particular size of sieve areknown as graded aggregate. For example, a graded aggregate of nominal size 20 mm means anaggregate most of which passes IS sieve 20 mm.

Fine Aggregate: Aggregate passing through 4.75 mm sieve are defined as fine. They may benatural sand—deposited by rivers, crushed stone sand—obtained by crushing stones andcrushed gravel sand. The smallest size of fine aggregate (sand) is 0.06 mm. Depending uponthe particle size, fine aggregates are described as fine, medium and coarse sands. On the basisof particle size distribution, the fine aggregates are classed into four zones; the grading zonesbeing progressively finer from grading zone I to grading zone IV (IS: 383).

On the Basis of Shape

Aggregates are classified as rounded, irregular, angular, and flaky.

Rounded Aggregates :These are generally obtained from river or sea shore and produceminimum voids (about 32 per cent) in the concrete.

Irregular Aggregates :They have voids about 36 per cent and require more cement paste ascompared to rounded aggregate. Because of irregularity in shape they develop good bond andare suitable for making ordinary concrete.

Angular Aggregate:They have sharp, angular and rough particles having maximum voids(about 40 per cent). Angular aggregate provide very good bond than the earlier two, are mostsuitable for high strength concrete and pavements

Flaky Aggregate :These are sometimes wrongly called as elongated aggregate. However, bothof these influence the concrete properties adversely. The least lateral dimension of flaky aggregate(thickness) should be less than 0.6 times the mean dimension.

1.1 Requirements

1. Fine aggregate consist of natural clean sand, gravel, manufactured sand produced by crushing rock/gravel or a combination of the above, conforming to the requirements of physical and chemical properties stipulated in BIS codes and subject to Engineer's acceptance.
2. Coarse and fine aggregates shall consist of tough, hard, durable and uncoated particles containing no harmful material in quantities sufficient to adversely affect the concrete or reinforcing steel, and shall contain no materials likely to cause staining or otherwise disfigure the concrete surface.
3. Coarse and fine aggregates which shall be obtained from a source approved by the Engineer, shall comply with the requirements of BIS codes and subject to Engineer’s acceptance. If required by the Engineer, the aggregates shall be washed with water.
4. The nominal maximum size of coarse aggregate shall be as large as possible within the limits specified but in no case greater than ¼ of the minimum thickness of the member or ¾ the minimum clear spacing between individual reinforcing bars or wires.
5. The gradation of fine aggregate for concrete and mortar shall be in accordance with the gradation designations in IS: 383 and subject to Engineer’s acceptance.
6. Each batch of aggregate delivered to the Site shall be kept separate from previous batches, and shall be stored to allow for inspection and tests to be carried out.
7. The storage area for the clean washed sand shall be shaded from the direct rays of the sun and shall be screened for protection from dust. The area in the neighbourhood of stockpile/mixing plant shall be watered as necessary, to reduce the rising of dust.
8. The aggregates shall be properly stored and labelled without intermingling as classified at any storage area.
9. No aggregate deliveries shall be made to the Site until the Engineer has approved the samples as complying with the specification.
10. The Contractor shall provide means of storing aggregate at each point where concrete is made such that:

1. Each nominal size of coarse aggregate and the fine aggregate shall be kept separated at all times.
2. Building stockpiles are to prevent harmful segregation and breakage.
3. Stockpiles shall be on hard and clean surfaces with not more than 5% slope.
4. Contamination of the aggregates by the ground or other foreign matter shall be effectively prevented at all times.
5. Each heap of aggregate shall be capable of draining freely.
6. Stockpiles shall be protected from direct sunlight.
7. Intermingling of aggregates shall not be approved.
8. Unloading of aggregate shall prevent harmful segregation and breakage.

11. The preparation, location and size of any stockpiles and the methods ofsegregation shall be approved by the Engineer.
12. Wet aggregates in stockpiles shall be taken frequently to determine the moisturecontent and the amount of water to be added to the mix.

1.2 Sampling and testing

1. The samples of both fine and coarse aggregate shall be in accordance with the requirements of IS: 2430 for testing at least two weeks before delivering to the Site.
2. All samples shall be taken in the presence of the Engineer.
3. Samples of aggregates shall be tested in accordance with the requirements of IS: 2386.

Test for course aggregates

1.IS:383:Specification for coarse and fine aggregates from natural sourcesfor concrete

2.IS: 2386: Methods of tests for aggregate for concrete:

(Part 1);Particle size and shape

(Part 2);Estimation of deleterious materials and organic impurities

(Part 3);Specific gravity, density, void, absorption and bulking

(Part 4);Mechanical properties

(Part 5);Soundness

(Part 6);Measuring mortar making properties of fine aggregate

(Part 7);Alkali aggregate reactants

(Part 8);Petrographic examinations

3.IS: 2430;Methods for sampling of aggregates

*****