Question:

What is concrete dusting? Why do concrete floors dust? This is a major problem to our company, we hope you have some answers.

Answer:

Per the National Ready Mixed Concrete Association. What is dusting? Chalking or powder at the surface of a concrete slab is called dusting. The characteristics of such a surface are: Powdery under any kind of traffic. Easily scratched with nail or stiff bristle broom.

Why do concrete floors dust? A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by the finishing operation performed over bleed water on the surface. Finishing or working this bleed water back into the top of the slab produces a low strength layer right at the surface. Placement of concrete over poly or some non absorbent surface, increases bleeding and as a result the risk of surface dusting.

Insufficient or no curing of the surface. This omission of curing often results in a soft concrete surface, which will easily dust under traffic. Finishing, floating or troweling of condensation moisture from warm humid air on cold concrete. In cold weather the concrete sets slowly, particularly cold concrete in below grade placements. If relative humidity is high, water will condense on the freshly placed concrete. This water condensation, if troweled into the surface, will cause dusting.

Inadequate ventilation in close quarters. Carbon dioxide from fossil fuel heaters and engines will cause a chemical reaction upon the surface of the fresh concrete known as carbonation. Carbonation greatly reduces the strength and hardness of the affected concrete surface.

Last, inadequate protection of freshly placed concrete from the elements, rain, snow, wind.

To prevent dusting, use concrete with a moderate slump. However, concrete with a higher slump may be used providing the mixture is designed to produce the required strength without excessive bleeding or segregation.

Never sprinkle or trowel dry cement powder into the surface of fresh concrete to absorb bleed water. Remove bleed water dragging a hose across the surface. Excessive bleeding of concrete can be reduced by using air entrainment or by modifying the mix proportions or adjusting the set time.

Do not perform any finishing operations with water present on the surface. Avoid direct placement of concrete on polyethylene. Also, remember to dampen the surface of a highly absorptive subgrade. Provide proper curing by using monomolecular films and liquid membrane curing compounds or covering the surface with a water retention sheet goods. When placing concrete in cold weather, use warm concrete, when possible.

How to repair dusting concrete.
To minimize or eliminate dusting, apply a chemical floor hardener like our L&M Seal Hard. If dusting is reduced but still is noticeable, retreat with L&M Seal Hard. In severe cases, a serviceable floor can be obtained by using the FGS system (FGS Conditioner and FGS Hardener Plus). This is a grinding and polishing system for the improvement or repair of distressed concrete floor surfaces. If this is not possible, grind the floor surface in preparation for a properly bonded, latex modified floor topping like our L&M Duratop.

The best idea is to avoid the dusting in the first place, call L&M if you can't.