Crazing occurs after the surface has hardened and penetrates to a very shallow depth.
Fogging increases the surrounding relative humidity, so there is no need to apply water to the concrete.
This month, we turned to Scott Tarr, a partner with Concrete Engineering Specialists, for this month's Problem Clinic topic. You can E-mail him at STarr@concreteES.com, or telephone him at 603-953-5815.
Q: We are dealing in a dispute between a contractor and an owner over the placement of a post-tensioned slab that has exhibited some craze cracking. The owner does not like the cracks. Are these cracks a structural problem?
A: Craze cracks are small pattern cracks occurring in a slab surface. They are associated with early surface drying or cooling, causing the immediate surface to shrink differently than the underlying concrete. Crazing cracks are typically less than 1/8- to ¼-inch deep and are generally not structurally significant.
Q: Is crazing considered to be plastic shrinkage cracking or drying shrinkage cracking?
A: Crazing cracks fall in between these two broad categories. By definition, plastic shrinkage cracking occurs early while the concrete is still plastic and typically while the slab is still being finished. Drying shrinkage cracking (and temperature contraction cracking) occurs after the slab has hardened and is shrinking due to the loss of moisture or a drop in temperature.
Crazing occurs at a very early age due to a rapid loss of moisture similar to plastic shrinkage cracks. But crazing generally occurs after the surface has hardened and penetrates to a very shallow depth, while both plastic shrinkage cracks and drying shrinkage cracks are significantly deeper than surface crazing.
Q: We understand crazing is not structurally significant but the owner does not like them. There are several more slabs scheduled to be poured. Is there a way to predict when crazing will occur?
A: You can assess the risk of crazing by using the evaporation rate nomograph included in ACI 305 Hot Weather Concreting. The nomogragh is used with ambient conditions and concrete temperature to estimate the evaporation rate from concrete. As stated in ACI 305, an evaporation rate exceeding 0.2 lb/ft2/24 hr risks plastic shrinkage cracking as surface moisture evaporates faster than bleeding replenishes it. Likewise, when the evaporation rate is high, there is a risk of crazing shortly after concrete hardens.
Q: Can we prevent crazing cracks?
A: Crazing cracks are difficult to prevent. They are not uncommon on finished surfaces, especially when high early-strength concrete is used. Sometimes, they are difficult to see and often only appear after wetted and drying. But there are procedures to minimize the risk of crazing. You can adjust ambient conditions, finishing procedures, and curing application.