QPT f2w
However, there are other factors which also affect the properties of concrete, for example, the
quality and quantity of cement, water and aggregates; method of transporting, placing,
compacting and curing of the concrete. Hence, the proportions arrived at for the mix design
should be considered only as a basis for a trial, subject to modification in the light of experience
and the actual materials that will be used at site.
In the IS method the general principles and requirements of basic data remain unchanged.
That is, specifying characteristic strength, workability recommendations in terms of compacting
factor Vee-Bee time or slump, type and grade of cement, maximum size of aggregate used,
grading of aggregates according IS: 383–1970, and durability requirements in terms of minimum
cement content and maximum water-cement ratio for various exposure conditions (Table 11.1).
The procedure of target strength of laboratory trial mixes is also followed.
2
The IS method uses the value of standard deviation based on the degree of quality control to
be selected depending upon the infrastructure and practices adopted at site, such as ‘very
good’, ‘good’ and ‘fair’ (Table 11.7).
‘Very good’ degree of control would require the following conditions at site: Fresh cement
from a single source and regular tests, weigh-batching of all materials, aggregate supplied in
single sized, control of aggregate grading and moisture content, control of water added,
frequent supervision, regular workability and strength tests, and field laboratory facilities.
‘Good’ degree of control would require the following conditions at site: Carefully stored
cement and periodic tests, weigh-batching of all materials, controlled water, graded aggregates,
occasional grading and moisture tests, periodic checks of workability and strength, intermittent
supervision, and experienced workers.
‘Fair’ degree of control would require the following conditions at site: Proper storage of
cement, volume batching of all aggregates, allowance for bulking of sand, weight-batching of
cement, water content controlled by inspection of mix, and occasional supervision and tests.
The values of standard deviation are then used for working out the target strength of the mix
design whenever results of a sufficiently large number of tests, say at least 30, are not available.
As mentioned earlier, it can be seen that standard deviation increases as the grade of concrete
increases for a given degree of quality control.
2 2Eg2
As mentioned earlier, different cements and different types, size gradings and shapes of
aggregate can produce concrete of different compressive strengths for the same water-cement
ratio. Hence, the IS method recommends that a relationship be established between compressive
strength and the free water-cement ratio for the materials actually to be used. If such data is not
readily available or not easily obtainable, the standard has drawn up curves giving the
relationship between water-cement ratio and compressive strength for six different strengths
of cement, (Fig. 11.2). These range from 31.9 to 36.8 N/mm^2 (325 to 375 kg/cm^2 ) to 56.4 to 61.3
N/mm^2 (575 to 625 kg/cm^2 ) and in essence cover the three different grades of cement,
manufactured in the country, namely 33 grade, 43 grade and 53 grade OPC.
After fixing the target strength of the mix the corresponding water-cement ratio can be found
from the curve the corresponding 28-days strength of cement to be used in the mix (Fig 11.2).