Building Materials, Third Edition

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to the volume, and the cement paste required is minimum. Poor interlocking bond makes it
unsuitable for high strength concrete and pavements.

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X 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.

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X2They 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.

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X2These 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.
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Aggregates are classified as normal-weight, heavy-weight and light-weight aggregate depending
on weight and specific gravity as given in Table 6.1

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The properties to be considered while selecting aggregate for concrete are strength, particle
shape, specific gravity, bulk density, voids, porosity, moisture content and bulking.

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 X The strength should be at least equal to that of the concrete. Rocks commonly used
as aggregates have a compressive strength much higher than the usual range of concrete
strength. A typical stress-strain curve for aggregate is shown in Fig. 6.1. The test conducted for