What are craze cracks?
Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer. Craze cracks are small pattern cracks on a slab’s surface. They are associated with early surface drying, causing the immediate surface to shrink differently than the underlying concrete. Crack patterns typically form grids of about 2-inch diameters. They are typically 1/10 millimeter (0.004 inches) or less in thickness, and 1 millimeter (1/32-inch ) or less in depth. Many craze cracks are not even visible until the surface of a floor gets wet and starts to dry.
Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete and rarely do they affect durability or wear resistance. However, crazed surfaces can be unsightly. They are particularly conspicuous and unsightly when concrete contains calcium chloride, a commonly used accelerating admixture.
What causes craze cracks and how to prevent it?
Crazing is caused by drying out of the concrete surface, so it is particularly common when the surface has been exposed during placement to low humidity, high air or concrete temperature, hot sun, or any combination of these. Concrete contractors can minimize or prevent craze cracks by starting to cure as soon as possible after final finishing, especially on a hard-troweled floor. Moist curing is best, although a spray-on monomolecular curing compound also can be effective. Using a drier, stiffer mix can reduce crazing as well. For exterior slabs, minimize the amount of working or troweling of the surface and use a broomed finish, which tends to mask minor cracks and surface blemishes.
Craze cracking is only one of the types of cracks that occur due to shrinkage of the concrete. Others are plastic shrinkage cracks and drying shrinkage cracks. By definition, plastic shrinkage cracking occurs while the concrete is still plastic and typically while the slab is still being finished. These cracks are deeper and farther apart and also are caused by drying, as surface moisture evaporates. Synthetic fibers can help prevent plastic shrinkage cracking, as can fogging to reduce evaporation.
Drying shrinkage cracks occur after the slab has hardened and are due to the loss of moisture in the concrete matrix. Reduce drying shrinkage cracks by using lower water-cement ratio mixes. Control drying shrinkage cracks by properly placing joints in the slab—2 1/2 to 3 times in feet the thickness of the slab in inches (so 15 to 18 feet apart for a 6-inch slab).
Is there anything you can do with crazing after the fact?
Typically, craze cracking is not repaired because it does not deteriorate over time. Sealers and surface hardeners actually can make crazing more obvious. If you really want to fix crazing, a thin overlay is about the only choice.
Follow These Rules to Prevent Crazing
- Use moderate slump (3-5 inches) concrete with reduced bleeding characteristics.
- Follow recommended practices and timing, based on concrete setting characteristics, for placing and finishing operations:
• Avoid excessive manipulation of the surface, which can depress the coarse aggregate, increase the cement paste at the surface, or increase the water-cement ratio at the surface.
• DO NOT finish concrete before the concrete has completed bleeding (look for the presence of a water sheen on the surface)
• DO NOT dust any cement onto the surface to absorb bleed water.
• DO NOT sprinkle water on the surface while finishing concrete.
- Cure properly as soon as finishing has been completed