As stated above, coarse aggregates are produced by the disintegration and crushing of rock. Coarse aggregates are usually those particles which are retained on an IS 4.75 mm sieve.
Stones that are hard and durable such as granite, basalt, quartzite provide good coarse aggregate. Naturally available river gravels and shingle obtained from sea beaches after removing shale may be used.
Blast furnace slag is used for making light-weight concrete. Brick-bats are used for lime concrete or for temporary or cheap concrete work. For reinforced concrete only crushed rock aggregates of size 20 mm are generally considered as satisfactory.
The aggregates should be absolutely clean, free from organic matter and other impurities. The aggregate must be capable of resisting weather.
The grading of coarse aggregate is very important for getting good quality concrete. Good grading of coarse aggregate implies that the quantity of aggregate used should contain all standard fractions of aggregate in required proportions such that the sample contains minimum voids. Thus, a sample of well-graded aggregate (both coarse and fine) containing minimum voids requires a minimum paste phase. Minimum paste require less quantities of cement and water. This is further to mean increased economy, higher strength, lower shrinkage and greater durability.
Fineness modulus is obtained by adding the cumulative percentages of aggregates retained on each sieve (ranging from 80 mm to 150 micron) and dividing their sum by 100. Fineness modulus is an empirical factor, larger the figure, coarser is the material. For a good concrete, the value of fineness modulus of coarse aggregate should be between 6 and 8.5.
One of the methods of arriving at the practical grading is by the trial and error method. Different size fractions of aggregates can be mixed, and the one that gives the maximum weight or minimum void may be used.
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