g PUWPlowman has given the following values of constants A and B
28 day strength at 18°C Coefficients
(M=19800°Chr/kg/cm^2 )AB
<175 10 68
175-350 21 61
350-525 32 54
525-700 42 46.5Example 10.3 The strength of a fully matured concrete sample is found to be 500 kg/cm^2. Find
the strength of identical concrete at age of 7 days when cured at an average temperature of 20°C
in day and 10°C in night.
Maturity of concrete at the age of 7 days = (time × temperature)
= 7 × 12 × [20– (–11)] + [7 × 12 × (10–(– 11)]
= 4368°C hr
Now A = 32 B= 54, thereby
percentage of strength of concrete at maturity of 4368°Chr
= A + B log 10 4368
1000= 66.5%
Strength at 7 days = 500 ×66.5
100= 332.5 kg/cm^2IHV yuefsvs
In fresh concrete—concrete in the plastic state, which can be moulded into desired shape—the
theoretical w/c ratio to meet the requirements of water for chemical combination with cement,
and to occupy the gel-space is about 0.4 for maximum strength. The w/c ratio used at site may
vary because of:
(a) the presence of free surface moisture in the aggregates, and (b) the absorption of moisture
by the dry or porous aggregates.
Because of the above limitations another characteristic workability, which is again a reflection
of w/c ratio, becomes important.
The theoretical w/c ratio used will not give the maximum strength because of the reasons
stated above. 100 per cent compaction of concrete will give maximum strength and this can be
obtained by increasing the w/c ratio. The water lubricates the concrete which can be compacted
at site with the specified efforts. The lubrication required for handling concrete without
segregation, for placing without loss of homogeneity, for compacting with specified effort and
for easy finish are indications of workable concrete.
As per Road Research Laboratory U.K. workability is defined as the property of concrete
which determines the amount of useful internal work necessary to produce full compaction. It
can also be defined as the ease with which concrete can be compacted 100 per cent with regard
to mode of compaction and place of deposition.
Workability is different than consistency. The latter indicates degree of fluidity or mobility.
A concrete with high consistency need not be workable for a particular job. For example a