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
1.2 Sampling and testing
Test for course aggregates
Test |
Code |
Frequency |
Gradation |
IS: 2386 (Part 1) |
One test per 40 m3 or part thereof for each source. |
Flakiness index |
IS: 2386 (Part 1) |
- do - |
Deleterious constituents |
IS: 2386 (Part 2) |
If in doubt, one test per source. |
Water absorption |
IS: 2386 (Part 3) |
Once for each source. |
Aggregate impact value |
IS: 2386 (Part 4) |
One test per source. |
Soundness |
IS: 2386 (Part 5) |
- do - |
Property |
Code |
Frequency |
Gradation |
IS: 2386 (Part 1) |
One test per 40 m3 or part thereof for each source. |
Deleterious constituents |
IS: 2386 (Part 2) |
If in doubt, one test per source. |
Silt content |
IS: 2386 (Part 1) |
One test per 40 m3 or part thereof for each source. |
Bulking |
IS: 2386 (Part 3) |
- do - |
RELATED CODES
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
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