Building Materials, Third Edition

g™ PTS

p2IHIR2‚—2˜02ƒ'2—2‡—Eg2‚—22g™

In 1897 Feret proposed a rule defining strength of concrete paste in terms of volume fractions
of the constituents as

S =




c^2

K

c wa

Where c = volume of cement
w = volume of water
a = volume of air
K = a constant
In this expression the volume of air is also included, which means the voids in concrete are
taken into account in estimating the strength.

h

2e˜—2v—

Abrahm presented his classic law in 1918 as “For plastic mixtures using neat and clean aggregates
the strength of concrete under specified conditions is governed by the net quantity of water
mixed per bag of cement”. He gave the following equation to estimate the strength of concrete.

S = Ax
B
where S = strength at 28 days, and constants, A = 14000 lbs/sq.in and B = 7.
According to Abrahm’s law it is evident that strength of concrete depends only upon w/c
ratio provided the mix is workable. Recently some modifications have been suggested by
various researchers. However, the truth of statement could not be challenged. The effect of
w/c ratio on the composition of cement paste is shown in Fig. 10.15.