Building Materials, Third Edition

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(vi) Compressive strength of cement at 28 days, 51 N/ mm^2

Sieve analysis

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ƒX Step 1: For the degree of quality control specified, namely, good the value of
standard deviation
read from Table 11.7 = 6 N/mm^2. Hence, the target mean strength for the
desired characteristic compressive strength

= 30 + 1.65 × 6 = 39.9 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 day strength of cement (Fig. 11.2). For a cement
strength of 51 N/mm^2 curve D is selected and the water-cement ratio 0.4 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 the value of 0.40 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 30 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 sand

water content, content per cent of total

per cent aggregates by volume

(i) For sand conforming to nil –1.5

grading zone I per cent

(ii) Increase in the value of

the compacting factor by nil

(0.85–0.80) = 0.05

0.05
231.5

0.1