Q: I'm relatively new to the concrete polishing business. My company has been involved in treating stone and tile floors for years, and recently began to polish more concrete surfaces. Recently, we were asked to bid on a project that featured a concrete floor with a number of small cracks spread like spider webs throughout the surface.
How should we approach this project? Do we need to repair the cracks before we begin polishing?
A: From what you describe, the concrete surface has a deformation known as craze cracks. The American Concrete Institute defines this condition as fine random cracks or fissures in the surface of plaster, cement paste, mortar, or concrete. When you look at the surface, you'll see that craze cracks are small pattern cracks occurring just on a slab's surface. The presence of this type of crack is associated with early surface drying or cooling. The immediate surface shrinks differently than the underlying concrete. Craze cracks typically form grids of about 2-inch diameters.
The cracks themselves have no real width or depth, and outside of appearance, they do not affect the performance of a slab. Many craze cracks are not even visible until the surface of a floor gets wet and starts to dry.
Ron Sturm, senior petrographer for CTLGroup, Skokie, Ill., says craze cracks are found on the surface of a slab. They are typically 1/10th of a millimeter (or 0.004 inches) or less in thickness and 1 millimeter (about 1/32-inch) in depth. Be sure to look closely at the surface. Normally there are no surface delaminations between the cracks.
You typically do not repair crazing. It does not deteriorate over time, so a repair is not necessary. Appearance is generally the main criticism; they are generally not structurally significant. In some cases, you can apply sealers and surface hardeners, but these products often accentuate the appearance of the crazing.
For concrete polishers, craze cracking can be a blessing or a curse. It is important to communicate the presence of the surface defect with the owner or specifier. It would be also prudent to conduct a test polish area.
—This is an excerpt from “Evaluating Concrete Surfaces,” a presentation editor-in-chief Rick Yelton gave at the International Concrete Polishing and Staining Conference in October. The entire presentation is posted atwww.concreteconstruction.net in the CONCRETE SURFACES section.