When preparing concrete unit paver specifications, I still come across paver details indicating stone dust as the setting bed material. Some landscape architects continue to detail stone dust screenings as a setting bed for concrete pavers. However, the Interlocking Concrete Pavement Institute( ICP), as well as concrete paver manufacturers discourage the use of stone dust in lieu of sand due to the fact that stone dust reduces drainage capacity of the bed as well as consistent compaction throughout.
Bedding sand under concrete unit pavers should conform to ASTM C 33 or CSA A23.1. This material is often called concrete sand. Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust. The bedding sand should have symmetrical particles, generally sharp, washed, with no foreign material. Waste screenings or stone dust should not be used, as they often do not compact uniformly and can inhibit lateral drainage of moisture in the bedding sand.
ICPI Tech Spec 17-Bedding Sand Selection for Interlocking Concrete Pavements in Vehicular Applications provides additional guidance on selecting bedding sand. Bedding sand should be spread and screeded to a nominal 1 in. (25 mm) thickness. Frozen or saturated sand should not be installed. If there is an uneven base (due to inconsistent compaction or improper grading), the bedding sand should not be used to compensate for it. Over time, unevenness in the bedding sand will reflect through to the surface. Uneven areas on the base surface must be made level prior to placing the bedding sand. Once the base is complete, screed pipes or rails are placed on it and the bedding sand spread over them. The sand is screeded or smoothed across the pipes with a straight and true strike board. Screed pipes are removed and the resulting void filled with bedding sand. After the sand is screeded it should not be disturbed. Sufficient sand is placed and screeded to stay ahead of the placed pavers. Powered screeding machines that roll on rails and asphalt spreading machines adapted for screeding sand have been successfully used on larger installations to increase productivity.