Q: The owner of a recently constructed manufacturing facility contacted us to help solve a problem with the concrete floor's surface. The floor is a slab on grade that appears to be structurally sound. It seems crews followed proper joint patterns and curing procedures. The building contains a steel cold-forming machine and a heat treat furnace. It is unheated and there are no ventilation problems. There is limited forklift activity and all have rubber tires. The owner wants us to resolve the floor surface's dusty condition, which existed before the owner moved in. The surface has no chipping or flaking, but there are some random cracks. In some areas when the surface is slightly wet, there are spiderweb-like patterns of small microcracks. The owner encourages us to apply a surface coating on the floor to mitigate the dust. But I'm concerned this is an expensive solution. I believe it's best to solve the root cause of the dusting before applying a coating of any sort—and I want to eliminate any chance that the coating will peel off. Can you offer a solution?
A: How to repair the floor depends on the quality of the concrete. In most cases, the dusting your potential client is experiencing results from the abrasion of a thin surface layer of material.
There are two potential causes: This thin layer probably occurred 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.
According to ACI, when this reaction occurs the concrete undergoes irreversible carbonation shrinkage, which can be the cause of the surface crazing you mention. Also when freshly placed concrete surfaces are exposed to carbon dioxide from improperly vented combustion heaters—used to keep concrete warm during the winter—in the first 24 hours, they are susceptible to dusting.
The floor's condition appears to be sound, with random cracks probably from plastic shrinkage cracking. Even so, you should inspect the slab to determine if the control joints are located properly throughout the area.
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 is extending the life of the concrete slab by using surface treatments containing certain chemicals, including sodium silicate and magnesium flu silicate. Remember that sealing products will not convert a poor slab into a quality floor, but can upgrade a dusty floor and improve its wear resistance.
When using these products, remember the depth of penetration into the slab will vary with the porosity of the surface and concrete moisture content at the time of application. Products in this group are not specifically formulated for curing applications and do not meet the requirements of either ASTM C 309 or ASTM C 1315 for liquid membrane forming compounds. Although they offer some desirable benefits when you apply after curing, do not apply them on fresh concrete.
If you plan to apply either of these surface treatments on new concrete floors, moist-cure the floor for at least seven days and allow it to air dry according to the product manufacturer's recommendations. Remove liquid membrane-forming curing compounds before applying the surface treatment, as the membrane could prevent penetration of the liquid. The lone exception would be using compatible curing and sealing products from a single manufacturer.
Consider several items before selecting a liquid surface treatment as your solution. ACI recommends determining the jobsite conditions. Also, consult the building's owner to agree on the final appearance of the surface.
When a diluted solution of sodium metasilicate (water glass) or silicofluorides soaks into a floor's surface, the silicate reacts with calcium compounds to form a hard, glassy substance within the pores of the concrete. The degree of improvement depends on how deeply the silicate solution penetrates.
For this reason, dilute the solution with water to make it penetrate deeply. Apply three or four coats, allowing each coat to dry before applying the next. For the first two coats, use four parts water to one part silicate. The third coat should be a 3:1 solution. The treatment is completed as soon as the concrete surface gains a glossy, reflective finish.
Apply zinc, sodium, and magnesium silicofluoride sealers the same way as water glass. Use these compounds individually or in combination, but a mixture of 20% zinc and 80% magnesium yields excellent results.
For the first application, dissolve 1 pound of silicofluoride in 1 gallon of water. For subsequent coatings, use 2 pounds/gallon of water. Mop the floor with clean water shortly after the preceding coat has dried to remove encrusted salts. Because they are toxic, follow all safety precautions when applying silicofluorides.