ACI 302.1R-15 Guide to Concrete Floor and Slab Construction.
6.1—Soil-support system preparation The soil-support system should be well drained and provide adequate and uniform load-bearing support. The ability of a slab to resist loads depends on the integrity of both the slab and full soil-support system. As a result, it is essential that the full soil-support system be tested or thor- oughly evaluated before the slab is placed (Ringo 1958). The in-place compaction and moisture content of the subgrade, the subbase (if used), and the base should meet the minimum requirements imposed by the specifcations, project-specifc geotechnical report, or local building code. The base should be free of frost before concrete placing begins and able to support construction traffc such as concrete trucks (Fig. 6.1). The base should normally be dry at the time of concreting. If protection from the sun and wind cannot be provided, or if the concrete is placed in hot, dry conditions, the base should be lightly dampened with water in advance of concreting. There should be no free-standing water on the base or muddy or soft spots when the concrete is placed. The day before slab placement, testing should be conducted to establish the moisture content of the stone base and the subgrade material supporting the stone base. Exces- sive moisture below the concrete can aggravate differential top-to-bottom drying of the slab, which enhances curling potential. Keeping a record of the moisture content could help explain excessive curling should it develop later.
6.1.4 Base material—The use of the proper materials is essential to achieve the tolerances recommended in 6.1.3 (Suprenant and Malisch 1999a). The base material should be a compactible, easy-to-trim, granular fll that will remain stable and support construction traffc. The tire of a loaded concrete truck mixer should not penetrate the surface more than 1/2 in. (13 mm) when driven across the base. The use of so-called cushion sand or clean sand with uniform particle size, such as concrete sand meeting requirements of ASTM C33/C33M, will not be adequate. This type of sand will be diffcult, if not impossible, to compact and maintain until concrete placement is completed. A clean, densely graded granular material with a balanced fne content that produces a low-friction surface while mini- mizing any wicking of moisture generally provides the best support system. These densely-graded crushed products are commonly called crusher-run materials. The following material properties have proven to be adequate: a) Any local state department of transportation approved road base material with 100 percent passing the 1-1/2 in. (38 mm) sieve, 15 percent to 55 percent passing the No. 4 (4.75 mm) sieve and less than 12 percent passing the No. 200 (75 μm) sieve b) Material that satisfes the requirements of ASTM D1241 with the modifed allowance of less than 12 percent passing the No. 200 (75 μm) sieve c) Material passing the No. 200 (75 μm) sieve should be clean granular fll with less than 3 percent clay or friable particles.ACI 302.1R pdf download.