Why should stones with smaller particle size be used when preparing high strength concrete?

Coarse aggregate plays a restrictive role in cement shrinkage in concrete. Because the elastic modulus of coarse aggregate is different from that of cement paste, tensile stress occurs in concrete. The tensile stress increases with the increase of coarse aggregate particle size, and will lead to the decrease of concrete strength.

The test shows that the width of the interface crack around the coarse aggregate with particle size of 15 ~ 25mm in concrete is about 0.1mm, the crack length is 2 / 3 of the particle size perimeter, and the interface crack is more connected with the crack in the surrounding cement slurry. In 5 ~ 10mm coarse aggregate concrete, the width of interface crack is relatively uniform, only 0.03mm, and the crack length is only 1 / 6 of the perimeter of particle size.