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.
Q: How can the contractor adjust ambient conditions?
A: Any of the variables in the evaporation rate nomograph can be adjusted to decrease the evaporation rate. For example, fogging is often performed to increase the ambient relative humidity which will reduce concrete moisture evaporation. Erecting wind breaks also decreases evaporation from the concrete surface.
Q: If evaporation is too fast during finishing, will sprinkling water on the surface help?
A: No. Do not add water to the surface while the concrete is still plastic. Fogging increases the relative humidity of the air; do not apply water to the slab. Another way to slow evaporation is by applying an evaporation retardant to the slab surface to form a monomolecular film.
However, as these products are mostly water, do not work them into the surface. The intent is to allow the water to evaporate, leaving behind the protective film. Excess water (whether bleedwater, applied water, or applied evaporation retardant) should never be finished into the surface. It will increase the water-cement ratio and decrease the strength of the surface paste, which increases the risk of crazing and dusting.
Q: What about sprinkling water on the surface after the slab hardens?
A: The curing environment plays a critical role in the occurrence of crazing. Initiating curing procedures as soon as possible maintains early-age slab moisture. But if water or liquid curing compounds are applied to the slab for curing, they should not be more than 10° F cooler than the slab surface to avoid rapid temperature contraction.
Also, along with potentially increasing the evaporation rate, unvented fossil fuel heaters such as torpedo heaters used during placement can cause early-age carbonation. This increases the shrinkage of the surface paste and corresponding risk of crazing.
Q: What other factors contribute to crazing?
A: Sometimes crazing occurs even when finishing procedures, ambient conditions at placement, and curing methods are optimized. The concrete mixture is also a factor. Decreasing the concrete temperature will also decrease the evaporation rate. This will have a beneficial effect against plastic shrinkage cracking, but it slows the strength gain and prolongs the critical period when crazing can occur.
In many cases, the design or construction schedule requires a rapid strength gain so you can perform post-tensioning or remove forms. It's not uncommon to use a high cement content for these mixtures which reduces the amount of bleedwater.
Also, overusing water-reducing admixtures results in low water-cement ratios and higher strength. But these admixtures, especially high-range water reducers, typically reduce the water content so that there is not enough bleedwater to overcome plastic shrinkage cracking.
Accelerators also reduce the bleed-water but are not often used if adverse weather conditions risk plastic shrinkage cracking and crazing.
Sometimes surface retarders are used to provide more working time during adverse weather. But these extend the amount of time for plastic concrete to lose moisture and risk cracking.
Q: Is there anything that should be done to repair crazing once it occurs?
A: You typically do not repair crazing. As most crazing does not deteriorate over time, a repair is not necessary. In some cases, you can apply sealers and surface hardeners, but these products often accentuate the appearance of the crazing. As appearance is generally the main criticism, these products may not address the owner's concern.