Q: I'm the construction coordinator for a city in a freeze-thaw region. Deicer scaling and sheet scaling of city streets are both a concern to us because either form of this distress can affect pavement aesthetics and cause surface roughness. What are some of the causes and ways to prevent this kind of deterioration?
Although both problems are called scaling, the causes are different. Deicer scaling commonly occurs on exterior flatwork that's exposed to freeze-thaw cycles and applications of deicing agents. The finished surface flakes or peels off, usually in small patches that may later merge to expose larger areas.During freezing and thawing, hydraulic pressures that develop within the concrete may be sufficient to cause scaling. Deicing agents accelerate scaling, probably because applying the agents increases the number of freeze-thaw cycles and helps keep the concrete moisture content above a critical level at which scaling occurs. Deicers also dissolve in the concrete pore water, creating osmotic pressures that add to hydraulic pressures.To avoid deicer scaling:
Use air-entrained concrete (5% to 8% air content).
Limit concrete slump to 5 inches.
Don't perform any finishing operations when water is on the concrete surface.
Cure the concrete with a curing compound or watertight covering, then allow it to dry uncovered for at least 30 days before deicing salts are used.
Even if exterior flatwork isn't exposed to freeze-thaw cycles or deicers, it can exhibit sheet scaling. The surface peels off in areas ranging in size from several square inches to many square feet. Sheet scaling occurs after a layer of water and air collects beneath a dense-troweled surface. Troweling seals the surface while the underlying concrete is still bleeding or able to release air. Because air entrainment slows bleeding, finishers are more likely to trowel too soon, especially on hot, dry, windy days. Placing concrete on a cold surface also contributes to the problem because the concrete sets more slowly and thus bleeds longer. To avoid sheet scaling:
Prohibit steel troweling of air-entrained concrete. Instead, call for a float, broom, or burlap-drag finish.
Use heated or accelerated concrete when placing concrete on a cold surface.
Excellent summaries of scaling and sheet scaling are given in the Concrete in Practice (CIP) series, published by the National Ready Mixed Concrete Association, 900 Spring Street, Silver Spring, MD 20910 (301-587-1400). Ask for CIP 2, Scaling Concrete Surfaces, and CIP 20, Delaminations of Troweled Concrete Surfaces.