QPR f2w
A graph should then be prepared showing the relationship between the cement-water ratio
(reciprocal of water-cement ratio) along the X-axis, and compressive strength at 7 days and 28
days along the Y-axis. The exact water-cement ratio is then calculated from the graph
corresponding to the required design strength of the concrete. That trial mix which meets all
the specified parameters for the various properties of concrete, both fresh and hardened, is
finally selected as the mix design to be used.
Whenever there is a change in the source of fine or coarse aggregate it is necessary to conduct
tests for sieve analysis, specific gravity, water absorption, etc. and readjust the mix proportions
in the light of the results obtained. It is also advisable to use cement from the same plant to
ensure that the variability in strength and other properties is kept to the minimum.
The mix design so obtained gives the weights, in kg, of the different ingredients required to
make 1 m^3 of compacted concrete. After the proportions of the ingredients are finalised, about
0.03 m^3 of concrete is prepared for casting of 6 cubes of 150-mm size and for conducting the
slump and V-B time test. The tests for the physical properties of the ingredients must be
conducted first and then the cubes cast. There cubes are required for the compressive strength
test at 7 days and three for the test at 28 days. The individual weights, in kg, of the ingredients
can be obtained by multiplying the respective batch weights of m^3 of concrete by 0.03 or by 0.05
if 3 days strength is also required.
If the aggregates are batched in a surface wet condition the quantity of water added to the
mix or the free water should be calculated. For example, if the surface water in the fine
aggregate is 3 per cent and the quantity of fine aggregate is 500 kg/m^3 , the quantity of water
should be reduced by 500 ×^3
100
= 15 kg or 15 litres per m^3 of concrete while the weight of fineaggregate should be increased by the corresponding quantity, in this case, 15 kg. Thus the
batching quantity of the fine aggregate will be 515 kg/m^3 (500 + 15) of concrete.
If the aggregates are batched in a dry condition the batch weight of aggregates should be
reduced and the weight of the mixing water increased by the corresponding amount. For
example if the water absorption of coarse aggregates is 2 per cent and batch weight 1,300 kg/
m^3 and the water absorption of fine aggregate 0.4 per cent and batch weight 500 kg/m^3 the
quantity of mixing water per m^3 of concrete should be increased by
20.4
1300 500
100 100 = 26 + 2.0 = 28.0 kg.Thus the quantity of coarse aggregate should be decreased by 26 kg and of the fine aggregate
by 2.0 kg. If the above quantity of water is not added the dry aggregate will absorb water from
the mix, thus reducing the effective water-cement ratio, which will in turn, adversely affect the
workability of the mix.
There are four stages of moisture in the aggregate from bone dry condition to moist condition.
The intermediate stage is a saturated surface dry (SSD) condition which is the ideal condition
to use in mix design. In SSD condition the aggregate will neither absorb mixing water making
the mix harsh, nor will they contribute extra surface water to the mixing water resulting in an
increase in the water-cement ratio and consequently a loss in the strength. For this reason, in
mix design aggregate are assumed to be in a SSD condition and water recommended for
mixing is calculated from the free water-cement ratio and recommended cement content in the
mix.