CIP 1: Dusting Concrete Surfaces

Dusting Concrete Surfaces

• Dusting is the formation of loose powder resulting from the disintegration of the surface of hardened concrete.
• Characteristics of dusty surfaces: • Powder under any kind of traffic • Easily scratched with a nail or even by sweeping

Why Do Concrete Floors Dust?

• Concrete floors dust under traffic because the wearing surface is weak.
• Causes of weakness: • Finishing operations performed while bleed water is on the surface or before the concrete has finished bleeding • Poor finishing practices, such as broadcasting dry cement or sprinkling water on the surface during finishing • Floating and/or troweling operations following the condensation of moisture from warm humid air on cold concrete • Inadequate ventilation in enclosed spaces • Insufficient curing • Inadequate protection of freshly placed concrete from rain, snow, or drying winds • Allowing the concrete surface to freeze

How to Prevent Dusting

• Use concrete with the lowest water content and an adequate slump for placing and finishing
• Avoid using concrete with a high slump
• Use water-reducing admixtures to increase slump while maintaining a low water content
• Remove bleed water by dragging a garden hose across the surface
• Do not perform any finishing operations with water present on the surface or while the concrete continues to bleed
• Use air-entrained concrete to reduce excessive bleeding
• Modify mix proportions to reduce bleeding
• Accelerate the setting time to reduce bleeding

Special Cases

• In cold weather, use concrete with a moderate slump and an accelerating admixture
• Place concrete at a temperature exceeding 50°F [10°C]
• Use a trimable, compactible fill, such as a crusher-run material, over vapor retarders or non-absorptive subgrades prior to concrete placement
• Lightly dampen absorptive subgrades just prior to concrete placement to prevent high evaporation rates

Repairing Dusting

• Sandblast, shot blast, or use a high-pressure washer to remove the weak surface layer
• Apply a commercially available chemical floor hardener, such as sodium silicate (water glass) or metallic zinc or magnesium fluosilicate, in compliance with manufacturer's directions