What is the effect of coarse aggregate size in compressive strength of concrete?

What is the effect of coarse aggregate size in compressive strength of concrete?

When we design mix the concrete, its target strength is much less than the compressive strength of the aggregates used in the formation. It can also be clearly seen in the mix design procedure that greater the maximum  size of coarse aggregate used, less is the cement required to achieve the target strength.

So, undoubtedly, greater the maximum size of coarse aggregate, greater is the compressive strength of concrete made with 2 precautionary conditions:-

1. The coarse aggregates should be properly graded, and the gradation becomes more and more important as we increase the maximum size of coarse aggregate because of greater voids, which needs to be filled to attain the desired compressive strength. As void fraction increases on increasing the size of aggregates.

2. Maximum size of aggregate used in the mix design should not be greater than 1/5th of the least dimension. So when we design a slab, its thickness comes in the range of 4-6 inches(say 100-150 mm), so the max size of aggregate should not be greater than 20 or 30 mm. Since 20 mm is commercially available, and we cannot use 40 mm for slabs below 200 mm thickness, we usually use 20 mm coarse aggregate in all the designs.

If the max size of aggregate is greater than 1/5th of the least dimension, the chances of failure in shear are extremely high.

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