Building Materials, Third Edition

PUT f   





2w—

—

where S = strength of concrete
w = volume of water
c = volume of cement
a = volume of air
For concrete which is to be compacted by vibrator a lower water to cement ratio may be used.

„ 2 — 2 ƒ3 2  2 e—
X The extent to which an aggregate will pack down and produce
a minimum void content is dependent on its particle shape. The crushed stone and gravels give
higher strength. A reduction in the void content of the coarse aggregate by better packing,
means that the amount of mortar can be reduced and hence sand and cement. Thus the coarse
aggregate to sand ratio is increased and although the overall mix may be leaner the mortar may
be richer, and by virtue of reduction in water/cement ratio which may thereby be permitted,
the strength of concrete may be increased.
A rounded spherical or cubical shaped aggregate when compacted contains less voids than
an irregular and flaky aggregate of the same nominal size. Therefore, the former gives higher
strength.
A common belief has been that the larger the maximum size of aggregate, the denser and
stronger will be the concrete. The large aggregates have lower total surface area and require
lower water to cement ratio resulting in higher strength. In practice it is not so. The larger
aggregates give lower surface area for development of gel bonds leading to lower strength.
Also large size aggregates give heterogeneous concrete causing nonuniform distribution of
load when stressed.
High strength concrete gives lower strength as compared to lean concrete if large size
aggregate is used. The influence of maximum size of aggregate on compressive strength of
concrete with different cement contents is shown in Fig. 10.26, and that with different water-
cement ratios is shown in Fig. 10.27.

p2IHPT s"™22w—"2ƒ8

2e—22PV2h—,2ƒ'2

g™22h2‚™'

p2IHPU s"™22w—"2ƒ8

2e—22'2ƒ'2

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