Building Materials, Third Edition

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Mix design must be planned well in advance otherwise considerable time will be lost if the
28-days strength of cement is assessed in the laboratory first, and then the 28-days compressive
strength of concrete and trial mixes are obtained. In practice the 28-days strength of cement
may be assumed on the basis of the grade of cement (33, 43, or 53 grade) and its specified
strength. To further reduce this time lag, the compressive strength of the concrete at 28-days
may be estimated from the 7-days strength on the basis of a general relationship between the
two strengths which varies and depends on the chemical composition of the cement type and
quality of the aggregates, and the aggregate-cement ratio.

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Concrete is essentially a mixture of cement, water, coarse and fine aggregates which consolidates
into a hard mass due to chemical reaction between cement and water. Each of the four
constituents has a specific function. Besides most optimum use of ingredients, the selection of
right kind of ingredients having properties which help to achieve desirable properties of
concrete is of utmost importance. A brief discussion about the ingredients follows.

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The cement used should have a minimum compressive strength at different ages according to
the relevant IS specification–IS:269–1987 (33 grade OPC), IS: 8112–1982 (43 grade OPC),