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

Learn about dusting, also known as chalking, on concrete surfaces, its formation, characteristics, and why it occurs. Understand the causes of weak concrete floors and their effects.