Concrete Vapor Barriers
There are few elements of concrete court construction that create as much conversation and controversy as the use and placement of vapor barriers (vapor retarders as they are now known) under concrete surfaces.
The USTC&TBA and most surfacing suppliers have long recognized the need for vapor barriers under concrete courts. There is seldom complete agreement for the proper techniques to minimize the transmission of moisture and vapor through concrete slab. There is universal agreement that preventing vapor transmission is a desirable goal. The tennis court surfacing industry is not unique in its disagreements as to the correct techniques. However, there is a growing consensus that was recently outlined by the American Concrete Institute in their Committee 302 update dated April 1, 2001. It details that the vapor retarder should be placed directly below the concrete slab to be effective when any coating system is to be applied.
A question that comes to mind for many designers and general contractors is “Why do I need any vapor barrier”? The answer is that regardless of the coating system, acrylic, urethane or epoxy, all are affected by vapor drive through a concrete slab. Unless the designer and general contractor properly addresses the issue of vapor drive it is not an issue of if a problem will occur, but when it will occur.
Once we agree that a vapor retarder is essential, it is equally important to install a vapor barrier that is effective. Many in the concrete industry assume a 6 mil. poly film is adequate as a vapor barrier. The current standard set forth by ASTM E1745 indicates it would not meet the standard for puncture resistance or vapor transmission. The current thinking is that a minimum thickness of 10 mils is required. Although it will effectively retard vapor transmission, it has little resistance to the stress placed on it by cable or rebar placement or normal construction activity. Why go to the effort of placing the vapor barrier if you compromise it with the reinforcement system? It would be our strong recommendation that vapor barrier material meet the standard of Class B in ASTM E1745 for its added puncture resistance and strength.
Another consideration when placing the vapor barrier material is its ultraviolet light stability. Most polyethylene films have limited UV stability. Even two weeks of UV exposure can compromise many polyethylene sheets. Once again material selection is a real issue. These issues should be discussed with both concrete and site work contractors.
After the effort has been made to minimize vapor transmission, it is still very important to address the issues of water/cement ratios, finish texture, perimeter drainage, cure time, and proper surface preparation.
Concrete is a durable and versatile surface, however one must be aware of its unique properties for the coating system to perform to its full capabilities.
There are many experts in this field. Mr. Howard Kanare of Construction Technology Laboratory, Skokie, IL, provided information for this article.