CONCRETE knowledge from a CONCRETE source!

The National Ready Mixed Concrete Association (NRMCA) defines scaling as local flaking or peeling of a finished surface of hardened concrete as a result of exposure to freezing and thawing. Scaling will normally begin as a small area but can expand to cover large areas.

Although it is likely that scaling will never be eliminated due to the many variables that contribute to it, the Portland Cement Association (PCA), NRMCA and ACI International have developed the following guidelines to help minimize scaling.

1. Use an air-entrained, low water/cement ratio (0.45 or less w/c, as delivered), moderate slump concrete (not exceeding 5 inches) with a 28-day compressive strength potential of at least 4000 psi. Except when absolutely necessary, do not retemper concrete before placement.

2. Avoid finishing practices that reduce or eliminate the air-entrained voids in the wearing surface layer, or that result relative to the bulk concrete at lower depths. Do not perform any finishing operations with water present on the slab surface. Delay finishing until all bleed water has risen to, and disappeared from, the surface.

3. Provide adequate curing for the concrete.

4. Avoid late season concrete placement, where concrete can experience freezing conditions and/or exposure to deicing salts before (a) the concrete has reached 4000 psi, and/or (b) the slab has had at least 30 days of air drying.

5. Protect first year concrete from the harsh winter environment. Prevent newly placed concrete from becoming saturated with water prior to freeze and thaw cycles during the winter months by applying a silane or siloxane-based breathable concrete sealer (do follow the manufacturer's application recommendations). Do not use deicing salts in the first year after placing the concrete. Use clean sand for traction. When conditions permit, hose off salt accumulations deposited by vehicles on newly placed drivways, approaches, and garage slabs. Never use ammonium sulfate or ammonium nitrate as a deicer; these are chemically aggressive and destroy concrete surfaces.

6. Provide proper drainage. Poor drainage permits water or deicer and water to stay on the concrete surface for extended periods, increasing the severity of exposure for the concrete and promoting scaling, especially in driveway, sidewalk, and curb and gutter applications.

Adequate curing is a must for the development of strength, water-tightness and durability in hardened concrete. It is very important to provide and maintain satisfactory moisture and temperature conditions immediately after concrete placement and finishing, and for as long as practical. As a general rule, the length of the curing period for concrete flatwork in temperatures above 40 degrees F should be a minimum of 7 days, or the time necessary for the concrete to reach 70% of its specified compressive strength. The later in the season it gets, the longer it will take to reach these points.