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Materials Engineer, Roorkee
It is supposed that concrete should give satisfactory service to its entire life. However problems arises, if care is not taken during construction blemish appears on the surface of a concrete slab, it will likely to be one of these: bilisters, cracking, crazing, curling, delamination, discoloration. DUSTING, efflorescence, low spots, popouts, scaling or spelling. This paper will give the details about dusting.
Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking and this is composed of water, cement and fine particles. The concrete surface powder under any kind of traffic, and also surface can be easily scratched with nail or even by sweeping.
A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by: any finishing operation performed while bleed water is on the surface or before the concrete has finished bleeding. Working this bleed water back into the top 6 mm of the slab produces a very high water-cement ratio and, therefore, a low strength surface layer.
REASONS FOR CONCRETE DUSTING
Floating and troweling concrete with bleed water on it mixes the excess water back into the surface, further weakening the concrete strength and wear resistance and giving rise to dusting. Dusting may also be caused by:
1. Water applied during finishing
2. Exposure to rainfall during finishing
3. Spreading dry cement over the surface to accelerate finishing
4. A low cement content
5. Too weak mix
6. Lack of proper curing.
Especially in summer allowing rapid drying of the surface
7. Carbonation of surface
8. Freezing of the surface
9. Dirty aggregates.
when the concrete contractor who placed the slab finished it too early. The finishing operation brought fines to the top, which hardened to a rather weak concrete with low abrasion resistance. Another potential cause for dusting is a chemical reaction. When there’s carbon dioxide in the ambient air, it combines with the calcium silicate hydrate and calcium hydroxide in hydrated cement paste to form calcium carbonate.
HOW TO PREVENT DUSTING
1) Proper selection of concrete materials, proper mix design and construction practice to give required slump, without excessive bleeding and/or segregation and given required strength and durability.
2) Never sprinkle or trawel dry cement into the surface of plastic concrete to absorb bleed water. Remove bleed water by dragging a garden hose across the surface. Excessive bleeding of concrete can be reduce by modifying mix proportions.
3) Provide proper curing. Insufficient or no curing of the concrete surface often results in a soft concrete surface, which will easily dust under traffic.
Good curing is extremely important in hot climate as the conditions often favors rapid loss of moisture from the surface. One method that is often used for slab is to cover the surface with wet hessian to reduce temperatures build-up. Alternatively, curing compound is used to protect the concrete from drying till it has reached the final set. Wet curing either by making bunds or with hessian cloth or by sprinkler arrangement should start immediately after the concrete has attained its final set. Do wet curing for at least 10 days under hot arid conditions.
HOW TO REPAIR DUSTING CONCRETE
Several solutions exist to solve this problem. One is to grind off or shotblast off this thin layer to expose the solid concrete underneath and potentially a new surface with good wear resistance. Another method is to apply a surface hardener. This treatment will not convert a basically bad concrete slab into a good one; it will, however, improve wearability and reduce dusting of the surface.
The major ingredient in many floor-surface hardeners is sodium silicate (water glass) or a metallic silicofluoride (magnesium and zinc fluosilicates are widely used). The treatment is usually applied in two or three coats, letting the surface dry between each application. Another method is to paint concrete surface with acrylic, urethane, or exposy.
The Health Risks of Concrete Dust
Concrete and concrete blocks are made with sand and gravel which contain “free” silica, or crystalline silica. Fine particles of crystalline silica are abraded from the surface and released into the air we breathe. The most common silica particles are quartz, while the other two forms are cristobalite and tridymite. Mortar and stucco also release silica dust.
Many cements contain other hazardous substances like lime, nickel, cobalt, and chromium compounds. Alkaline substances are corrosive to human tissues and small amounts of chromium can cause allergic reaction.
Concrete dust may irritate the nose, throat, and respiratory tract by mechanical abrasion, causing coughing, sneezing, and shortness of breath. Concrete dust may cause chronic bronchitis or aggravate pre-existing lung disease such as emphysema, bronchitis, and asthma.
After prolonged exposure, respirable concrete dust can cause severe damage to human lungs. Crystalline silica has been classified as a human lung carcinogen. In addition, it can cause silicosis, which is incurable. More commonly, silica dust causes the formation of scar tissue in the lungs, reducing their ability to take in oxygen.
RULES TO PREVENT DUSTING
1. Use moderate slump concrete not exceeding 125 mm
2. Do not start finishing operation while the concrete is bleeding.
3. Do not broadcast cement or sprinkle water on concrete prior to or during finishing operations.
4. Ensure that there is adequate venting of exhaust gases from gas-fired heaters in enclosed spaces.
5. Use adequate curing measures to retain moisture in concrete for the first 3 to 7 days and protect it from the environment, especially freezing.
6. Use design mixes, followed by good construction practice.
1. Taylor, Peter C., Detwiler, Rachel J., and Tang, Fulvio, J., Investigation of Discoloration of concrete slabs (Phase 2), S. No. 2228b. Portland Cement Association, 2000.
2. Guide for Concrete Floor and Slab Construction, ACI 302. IR, American Concrete Institute, Farmington Hills, MI.
3. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177. Portland Cement Association, Skokie, IL.
4. Trouble Shooting Guide for Concrete Dusting, Concrete Construction, April 1996.
We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting this paper to us. We are hopeful that this will be of great use to all civil engineers who are looking for information on “Dusting of Concrete Slab Surface”
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