What causes crazing on architectural precast products? Does it affect structural integrity, or is it just an aesthetic problem? How do you minimize it without sacrificing the 5000-psi compressive strength usually specified for architectural precast products?
Craze cracks occur when the surface shrinks more than the interior of the precast unit. The condition may be aggravated by wetting-and-drying cycles. The cracks have no effect on strength or durability because they penetrate only as deep as the thin layer of cement paste on the surface. They may be an aesthetic problem, however, especially on horizontal surfaces where dirt can most easily enter the cracks. The cracks are most noticeable on surfaces of structural elements made with white cement. To minimize craze cracking, avoid smooth off-the-mold or as-cast finishes. Crazing generally won't appear when the outer cement skin has been removed by a surface finishing method. If your customer requests an off-the-mold finish, you'll need to use a minimum water and cement content (low water-cement ratio) in the concrete. You can still achieve the 5000-psi compressive strength by using a water-reducing admixture. Vibrate the concrete enough to remove bugholes, but not so much that you bring too much paste to the form surface. Uniform slow curing, with minimum moisture loss from the smooth surfaces, also helps to minimize crazing.
Architectural Precast Concrete, published by the Precast/Prestressed Concrete Institute, contains excellent advice for avoiding crazing and other surface defects in precast elements. You can order the book from The Aberdeen Group by phone (800-323-3550), fax (630.543.3112) or from our Web site (
www.tagbookstore.com). The price is $75 plus sales tax and shipping/handling costs.