Q: We just completed a concrete floor that is acceptable in every respect except one: There are numerous craze cracks over much of the surface. The floor flatness meets specification requirements, and there are few random shrinkage cracks. However, the owner is concerned that the craze cracks will cause progressive floor deterioration. Is there any information in the literature to show that floors with craze cracking can still be durable?
A.: Crazing is generally believed to be a cosmetic problem. The cracks are unsightly, but seldom affect surface durability or wear resistance.There are some exceptions, however. We know of one laboratory study in which crazing was purposely induced by applying a dry cement shake to a nearly two-hour-old slab with bleedwater on the surface (Ref. 1). After the shake had absorbed bleedwater for five minutes, an experienced finisher steel troweled the slab surface. This surface and others in the experiment were exposed to rolling friction, and depth of wear was measured. For 28-day-old specimens, depth of wear for the crazed surface was comparable to that of another steel-troweled surface without craze cracks. The researcher noted that when a 90-day-old craze-cracked specimen made with the same concrete was tested, it lost the shake-coat layer after only one test cycle of the abrasive wheel. This failure was attributed to a loss of bond between the high-shrinkage shake and the lower-shrinkage base concrete. Does this mean that slab surfaces with craze cracks are more susceptible to delamination or other failure modes? Based on observations of existing slabs, that seems unlikely.
Photo A shows a craze-cracked convention-center floor that has received heavy hard-wheel traffic for many years. The floor is still in service and the craze cracks haven't affected its performance. Photos B and C show the same exterior concrete slab in a northern climate. Photo B was taken in 1984 and Photo C in 1996. The slab has been exposed to freezing and thawing and deicing agents during the interim. Note that no further deterioration has occurred.This anecdotal evidence supports the statement in Reference 2 that craze cracks don't affect the structural integrity of concrete and in themselves should not lead to subsequent deterioration of the concrete. There are many other examples of craze-cracked slabs that still perform their function, despite a possibly displeasing appearance. Reference 2 further states: "Crazing is often self-healing and should not lead to problems of durability. If, however, the crazing is so severe that it leads for example to frost damage on a concrete paving, then it is likely that (a) it is so deep that it cannot strictly be called crazing and (b) the water content and the permeability of the concrete are so high that frost damage is inevitable even without the presence of cracks that look like crazing." To that latter statement, we say, "Amen."
- Blake Fentress, "Slab Construction Practices Compared by Wear Tests," ACI Journal, July 1973, pp. 486-491
- Non-Structural Cracks in Concrete, Concrete Society Technical Report No. 22, The Concrete Society, 1982, pp. 26-27
Over the years, we have researched problems our customers have had with scaling of exterior concrete. Our winters are sometimes harsh, which causes an abundance of freeze-thaw cycles.Recently, we have noticed some scaling in areas where the concrete surface is crazed. The mixes used for these areas contained at least 5 1/2 sacks of portland cement per cubic yard and 5% to 7% entrained air. They were placed at slumps of 4 to 5 inches. In most of the scaled areas, the matrix covering the aggregate is 1/16 to 1/8 inch thick.Everything we've researched on the subject of craze cracking suggests the cracks will not affect the structural integrity of concrete, But if you have cracks in exterior concrete, can't moisture penetrate into them, freeze and possibly scale the surface? Won't the hydraulic pressure created by the frozen moisture promote problems such as scaling? If the answer to these questions is yes, we suggest that crazing can lead to surface defects, which can indeed affect the structural integrity of the concrete.
-Clay Allen Central Pre-Mix Concrete Co. Spokane, Wash.