Building Materials, Third Edition

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p & 2 pX Concrete takes up the shape of the form, A variety of looks can be had to
the architects imagination. By careful preparation of formwork, proper mix design and good
workmanship smooth surfaces can be achieved. Prefabrication units can be produced to a fine
finish.

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X The type of surface treatment depends upon the purpose for which the
concrete surface is to be used. For example a pavement surface should be plane but with
sufficient roughness to exhibit skid resistance. The concrete after levelling is rocked, broomed
or scratched to make the surface rough.

e2pX The exterior surfaces of concrete elements can be modified to give a pleasant
look. The concrete surface is roughened, cleaned and wetted. Over this a cement mortar of ratio
1:3 is applied. This mortar rendering can be given a number of surface finishes such as sand
facing, rough cast finish, peeble dash etc.

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The water-cement ratio (w/c) is one of the major factors but not the only one influencing the
strength of concrete. It is responsible mainly for the porosity of the hardened cement paste.
Water-cement ratio is the water used to
the quantum of cement in the mixture
by weight. For proper workability the
w/c ratio varies from 0.4–0.6. However,
maximum strength is derived at w/c =
0.4 at which minimum capillary cavities
are expected to form. It may be noted
that for complete hydration of cement
under controlled conditions the water
requirement is about 38 per cent. When
it is decreased to less than 0.4 there is
improper consistency and workability
of cement and honeycombed structure.
However, concrete compacted by
vibrator displays higher strength even
up to w/c = 0.3 as shown by dotted line
in Fig. 10.13. At w/c ratio more than 0.6,
the increase in volume of hydrated
products will not be able to occupy the
space already filled with water. Hence, porosity increases and strength decreases. In arriving
at the w/c ratio values it is assumed that aggregates are saturated with the surfaces in dry
condition. Suitable adjustments should be made for dry aggregates. Sometimes cement-water
ratio versus strength is plotted (Fig. 10.14). Being a straight line it is supposed to be a better
mean to interpret the results.

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