QQP f2w
Solution: Step 1: For the degree of quality control specified, namely, good the value of standard
deviation read from Table 11.7 = 5.3 N/mm^2. Hence, the target mean strength for the desired
characteristic compressive strength
=25 + 1.65 × 5.3
= 33.745 N/mm^2
Step 2: Corresponding to this target mean strength the water cement ratio is read from the
appropriate curve corresponding to the 28 days strength of cement (Fig. 11.2). For a cement
strength of 51 N/mm^2 , curve D is selected and the water-cement ratio 0.46 obtained. This value
has now to be checked against the maximum limit of the water-cement ratio for the given
exposure condition. Table 11.1 for moderate exposure and reinforced concrete the maximum
water-cement ratio recommended is 0.50. Hence value of 0.46 obtained is acceptable.
Step 3: For maximum size of aggregate of 20 mm, the air content is taken as 2.0 per cent
(Table 11.11). Since the required grade of concrete is M 25 which is lower than M 35 grade, water
content per m^3 of concrete = 186 litres and sand as percentage of total aggregate by absolute
volume = 35 (Table 11.9).
Since Table 11.9 is based on certain specific conditions, adjustments as per Table 11.10 have
to be made for any deviations in these values as given below.
Adjustment in Adjustment in
water, sand content
content, per cent per cent of total
aggregates
by volume
(i) For sand conforming to nil – 1.5 per cent
grading zone III
(ii) Increase in value of
the compacting factor nil
by(0.85–0.80) = 0.05
= 1.5
(iii) Decrease in the value
of water-cement ratio by nil
(0.50–0.46) = 0.04
Overall adjustment + 1.5 per cent –2.8 per cent
Thus after incorporating the above adjustments the sand content = 35 – 2.8 = 32.2and the water content = 1.86 + 1.5 ×186
100 = 188.8 lit.
Step 4: Determination of cement content:
Water-cement ratio = 0.46
Quantity of water after adjustment = 188.8 litersTherefore, cement content =^1 88.^8
0.46= 410.43 kg0.05
0.1× 3
–0.04
0.05