So, the preparation of concrete will require the presence of sand, the particles of which have sizes from 1.2 millimeters up to 3.5 millimeters. The use of sand with a smaller fraction is not allowed. Ideally, sand should differ in cleanliness, that is, it is not allowed to have particles of silt or clay. The content of clay or silty particles should not exceed 5 percent, because greater value has a very negative effect on the strength of concrete strength.