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Dusting concrete surfaces
Surface chalking or powdering under traffic due to a weak wearing layer.
Chalking or powdering at the surface of a concrete slab is called dusting. Characteristics of such surfaces are:
- They powder under any kind of traffic.
- They can be easily scratched with a nail or even by sweeping.
A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by:
- Finishing operations performed while bleed water is on the surface: working bleed water back into the top ¼ inch produces a high water-cement ratio and therefore a weak surface layer.
- Placement over a non-absorptive subgrade or polythene, which reduces normal absorption and increases bleeding.
- Insufficient or no curing: often results in a soft surface skin that will easily dust under foot traffic.
- Floating and/or troweling of condensation moisture from warm humid air on cold concrete, particularly in basement floors where cold concrete sets slowly.
- Inadequate ventilation: carbon dioxide from open salamanders, gasoline engines or power buggies may cause carbonation, greatly reducing the strength and hardness of the concrete surface.
- Inadequate protection of freshly placed concrete from rain, snow or drying winds.
Use concrete with moderate slump (not over 5 inches). Never sprinkle or trowel dry cement into the surface; remove bleed water by dragging a garden hose across the surface. Do not perform any finishing operation with water present on the surface.
Avoid placing concrete directly on polythene or non-absorptive subgrades; place 1–2 inches of damp sand first. On absorptive subgrades, dampen the surface just prior to concrete placement.
Provide proper curing using a liquid membrane curing compound. When placing concrete in cold weather, use warm concrete with an accelerator.
To minimise or eliminate dusting, apply a chemical floor hardener such as zinc or magnesium fluorosilicate in compliance with manufacturer's directions on thoroughly dried concrete. If dusting persists, use hardeners with cementitious properties such as latex formulations, boiled linseed oil or paint.
In severe cases, wet grind the top surface followed by properly bonded placement of a topping course. If this is not practical, installation of a floor covering such as carpeting or vinyl tile is the least expensive solution.