Introduction
Aggregates are the materials basically used as filler with binding material in the production of mortar and concrete. They are derived from igneous, sedimentary and metamorphic rocks or manufactured from blast furnace slag, etc. Aggregates form the body of the concrete, reduce the shrinkage and effect economy. They occupy 70-80 per cent of the volume and have considerable influence on the properties of the concrete. It is therefore significantly important to obtain right type and quality of aggregates at site. They should be clean, hard, strong, durable and graded in size to achieve utmost economy from the paste.
Earlier aggregates were considered to be chemically inert but the latest research has revealed that some of them are chemically active and also that certain types exhibit chemical bond at the interface of aggregates and cement paste. To increase the bulk density of concrete aggregates are used in two markedly different sizes—the bigger ones known to be coarse aggregate (grit) and the smaller ones fine aggregate (sand). The coarse aggregate form the main matrix of concrete and the fine aggregate from the filler matrix between the coarse aggregate.
Classification of aggregates.
1. On the basis of geological origin.
The aggregates may be classified into natural aggregates and artificial aggregates.
1a. Natural aggregates.
These are obtained by crushing from quarries of igneous, sedimentary or metamorphic rocks. Gravels and sand reduced to their present size by the natural agencies also fall in this category. The most widely used aggregate are from igneous origin.
Aggregates obtained from pits or dredged from river, creek or sea are most often not clean enough or well graded to suit the quality requirement. They therefore require sieving and washing before they can be used in concrete. Broken bricks, blast furnace slag and synthetic aggregates are artificial aggregates.
1b. Artificial aggregates.
Broken bricks known as brick bats are suitable for mass concreting, for example, in foundation bases. They are not used for reinforced concrete works. Blast furnance slag aggregate is obtained from slow cooling of the slag followed by crushing. The dense and strong particles as obtained are used for making precast concrete products.
The sp. gr. of these range between 2–2.8 and bulk density 1120–1300 kg/m3. The blast furnace slag aggregate has good fire resisting properties but are responsible for corrosion of reinforcement due to sulphure content of slag.
Synthetic aggregates are produced by thermally processed materials such as expanded clay and shale used for making light weight concrete.
2. On the basis of size.
According to size aggregates are classified as
Coarse aggregate
Fine aggregate and
All-in- aggregate.
2a. Coarse aggregates.
Aggregate retained on 4.75 mm sieve are identified as coarse. They are obtained by natural disintegration or by artificial crushing of rocks.
The maximum size of aggregate can be 80 mm. The size is governed by the thickness of section, spacing of reinforcement, clear cover, mixing, handling and placing methods. For economy the maximum size should be as large as possible but not more than one-fourth of the minimum thickness of the member. For reinforced sections the maximum size should be at least 5 mm less than the clear spacing between the reinforcement and also at least 5 mm less than the clear cover.
Aggregate more than 20 mm size are seldom used for reinforced cement concrete structural members.
2b. All-in- aggregates.
Naturally available aggregates of different fractions of fine and coarse sizes are known as all-in-aggregate. The deficiency of any particular fraction can be corrected for use in the mix but they are not recommended for quality concrete.
2c. Graded aggregates.
Aggregate most of which passes through a particular size of sieve are known as graded aggregate. For example, a graded aggregate of nominal size 20 mm means an aggregate most of which passes IS sieve 20 mm.
2d. Fine aggregates.
Aggregate passing through 4.75 mm sieve are defined as fine. They may be natural sand—deposited by rivers, crushed stone sand—obtained by crushing stones and crushed gravel sand. The smallest size of fine aggregate (sand) is 0.06 mm. Depending upon the particle size, fine aggregates are described as fine, medium and coarse sands.
On the basis of particle size distribution, the fine aggregates are classed into four zones; the grading zones being progressively finer from grading zone I to grading zone IV (IS: 383).
3. On the basis of shapes.
Aggregates are classified as
1. Rounded
2. Irregular
3. Angular and
4. Flaky.
1. Rounded aggregates.
These are generally obtained from river or sea shore and produce minimum voids (about 32 per cent) in the concrete. They have minimum ratio of surface area
to the volume, and the cement paste required is minimum. Poor interlocking bond makes it unsuitable for high strength concrete and pavements.
2. Irregular aggregates.
They have voids about 36 per cent and require more cement paste as compared to rounded aggregate. Because of irregularity in shape they develop good bond and are suitable for making ordinary concrete.
3. Angular aggregates.
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 most suitable for high strength concrete and pavements; the requirement of cement paste is relatively more.
4. Flaky aggregates.
These are sometimes wrongly called as elongated aggregate. However, both of these influence the concrete properties adversely.
The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension. For example, the mean sieve size for an aggregate piece passing through 50 mm and retained on 40 mm sieve is (50 + 40)/2 = 45.0 mm. If the least lateral dimension is less than 0.6 × 45 = 27.0 mm, the aggregate is classified as flaky.
Elongated aggregate are those aggregate whose length is 1.8 times its mean dimension. Flaky aggregate generally orient in one plane with water and air voids underneath. They adversely affect durability and are restricted to maximum of 15 per cent.
5. On the basis of unit weight.
Aggregates are classified as normal weight , heavy weight, light weight depending upon the unit weight and specific gravity
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