Q.: Do some sands tend to cause concrete to crack more than others? We normally use a manufactured sand and a natural sand. The manufactured sand appears to cause a lot more cracking, especially plastic shrinkage cracking.

A.: All aggregates have their own characteristic effects on the amount of drying shrinkage. This is a matter of three characteristics of the aggregate: the modulus of elasticity, the particle shape and the aggregate grading. Possibly the two sands you are using differ in all these respects. Plastic shrinkage cracking, however, would be unrelated to modulus of elasticity but might be related to the particle shape and the sand grading. Particles that tend to be elongated or sliver-like have greater surface area and require more water per cubic yard of concrete. Concretes containing such aggregates are likely to shrink more and have more cracks.