Protecting Your Concrete Skin: Evaporation Retardant
Imagine spending a day in the sun without using a sun block. Or a day in a steady wind without a windbreaker. Or in a house heated by a forced air pump without a humidifier. What would the condition of your skin be after prolonged exposure to these conditions? Would you expect to see blisters? Cracking? Scaling?
The surface of fresh concrete reacts very much the same way as your skin to the exposure of severe environmental conditions. Unprotected from sun, wind, and low humidity, the surface of fresh concrete will start to dry faster than the underlying concrete. Such a condition will cause blisters if the surface is sealed before the bleed water has evaporated. Cracks will occur as the surface shrinks prematurely. Scaling or dusting may occur if the finisher "blesses" the slab with water. In addition, it can be very difficult to achieve a flat surface as the slab begins to carpet. Carpeting is the condition where the surface of the concrete is rolling or pushing under the weight of the finisher and equipment leaving ridges over the finished slab.
Concrete and finishers are particularly prone to these types of problems in the spring when the evaporation rate exceeds 0.2lb/SF/hour. The subbase or deck is cool in the morning, which slows the setting times. The concrete surface is treated to sun and wind and low humidity, which dries it at a faster rate than the concrete mass. To the unsuspecting finisher the surface appears to be ready to finish due to the rapid moisture loss. But doing so may be premature and may create the problems discussed above.
Evaporation retardants are now available to aid finishers against the leading causes of rapid moisture loss. Evaporation retardants, such as L&M Construction Chemicals, Inc. "E-Con," are applied through a mist sprayer immediately following the strike-off and bull float operation. The spray mist creates a monomolecular film on the surface, which prevents rapid moisture loss and extends the workable time of the concrete. The film completely dissipates as finishing progresses and no residue remains after the concrete has hardened. Adequate and proper curing is still required.
It is important that finishers recognize when environmental conditions are right for rapid moisture loss. The attached chart , provided by L&M Construction Chemicals, Inc., illustrates when such conditions do exist. Anytime the evaporation rate exceeds 0.2lb/SF/hour, the finisher needs to consider protecting the concrete "skin," whether through the use of wind screens, sun shielding or evaporation retardants . Just as you would use sunscreens and windbreakers to protect your own skin against the elements, you must respect and protect the skin of your concrete.
Bloem, Delmar, "Plastic Cracking of Concrete," Engineering Information, National Ready Mix Concrete Association, July 1960, 2 pp. Additional information on rapid moisture loss and plastic shrinkage prediction can be found in ACI Committee Report 302, "Floor and Slab Construction", section 8.4, and Committee Report 305, "Hot Weather Concreting," American Concrete Institute Manual of Concrete Practice.
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