It's called craze cracking or “alligator cracking,” and the more serious form "map cracking," which effects serviceability. Craze cracking is primarily a cosmetic issue but store and warehouse owners sometimes raise objections when they see craze cracks on their floors fearing that it will lead to serviceability issues over time. But this isn't the case because the cracks have no significant width or depth.
Ron Sturm, a senior petrographer for CTLGroup, Skokie, Ill., says craze cracks are very small and are found in the top surface regions of a slab. They are typically 1/10th of a millimeter (or 0.004 inches) or less in thickness and 1 millimeter (approximately 1/32-inch) in depth. Also there aren't surface delaminations between the cracks. Many craze cracks aren't even visible until the surface of a floor gets wet and starts to dry.
It seems there is a greater incidence of craze cracking on floors now but we may be more aware of them because more owners are using concrete floors as the finished surface. Craze cracking happens to everyone who installs floors. The goal is to minimize them with these suggestions.
Why it happens
As a contractor you can work to minimize craze cracks but you can't eliminate or prevent them. The natural process of carbon dioxide (CO2) from the air combining with calcium hydroxide and other cement compounds to produce calcium carbonate causes a small amount of shrinkage on the slab surface. You can reduce the width and depth of craze cracks to avoid map cracking related problems.
Work on your mix designs. There is a trend toward using well-graded mix designs with large top-sized aggregates for retail and warehouse floor construction to produce durable floors with less shrinkage and curling. Concrete for floor slabs must be finish-able and sometimes additional small aggregate and cementitious material is excessive, resulting in more and deeper craze cracks. In addition, many floor mixes now include fly ash or slag as replacements for some of the portland cement in the mix. The Blaine fineness of these particles (the particle size or fineness of a cement determined from air permeability tests) is higher than portland cement, causing increased water demand and more shrinkage.
Terry Fricks, a consultant to The Fricks Co., Fort Worth, Texas, says his company always is trying to find ways to minimize craze cracking in its floors. They minimize both the amount of water and cement in the concrete. He adds that they used to see craze cracking a lot in their topping mixes because the mixes commonly had 850 pounds of cement per cubic yard. Their goal is also to lower the water-cement (w/c) ratios.
Finishing considerations. The potential for craze cracking increases with the thickness of the cement paste and fine aggregate at the surface of a slab. The potential increases by using vibratory strikeoff tools or by early floating procedures. Large aggregate is driven down while more fine aggregate and paste rises to the surface resulting in more craze cracking. Fricks says that if you see “corn rows” developing between the blades or excessive cement spatter on a riding trowel, the finishing procedures started too early. Excessive bull floating immediately after strickoff can increase the thickness of the paste region at the surface too.
Curing. Without proper curing there is more shrinkage, especially on the top of a slab where drying occurs first. Good curing reduces the potential for significant craze cracking. “Burned” trowel finishes compact concrete even more, further reducing the amount of water. So it's important to keep a slab moist starting immediately after the last finishing operation followed by wet curing. Wetting and drying a slab surface afterward isn't curing and doesn't help.
A final thought
In preconstruction meetings, educate owners about the possibility of craze cracking and what it is. They need to know more so that confrontations are avoided when the work is finished.