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Decreasing the concrete temperature will also decrease the evaporation rate. Q: What other factors contribute to crazing?Ī: Sometimes crazing occurs even when finishing procedures, ambient conditions at placement, and curing methods are optimized. This increases the shrinkage of the surface paste and corresponding risk of crazing. 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.Īlso, along with potentially increasing the evaporation rate, unvented fossil fuel heaters such as torpedo heaters used during placement can cause early-age carbonation. Initiating curing procedures as soon as possible maintains early-age slab moisture. Q: What about sprinkling water on the surface after the slab hardens?Ī: The curing environment plays a critical role in the occurrence of crazing. It will increase the water-cement ratio and decrease the strength of the surface paste, which increases the risk of crazing and dusting. Excess water (whether bleedwater, applied water, or applied evaporation retardant) should never be finished into the surface. The intent is to allow the water to evaporate, leaving behind the protective film. However, as these products are mostly water, do not work them into the surface. Another way to slow evaporation is by applying an evaporation retardant to the slab surface to form a monomolecular film. Fogging increases the relative humidity of the air do not apply water to the slab.
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Do not add water to the surface while the concrete is still plastic. Q: If evaporation is too fast during finishing, will sprinkling water on the surface help?Ī: No. Erecting wind breaks also decreases evaporation from the concrete surface. For example, fogging is often performed to increase the ambient relative humidity which will reduce concrete moisture evaporation. Q: How can the contractor adjust ambient conditions?Ī: Any of the variables in the evaporation rate nomograph can be adjusted to decrease the evaporation rate. You can adjust ambient conditions, finishing procedures, and curing application. But there are procedures to minimize the risk of crazing. Sometimes, they are difficult to see and often only appear after wetted and drying. They are not uncommon on finished surfaces, especially when high early-strength concrete is used. Likewise, when the evaporation rate is high, there is a risk of crazing shortly after concrete hardens.Ī: Crazing cracks are difficult to prevent. 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. The nomogragh is used with ambient conditions and concrete temperature to estimate the evaporation rate from concrete. Is there a way to predict when crazing will occur?Ī: You can assess the risk of crazing by using the evaporation rate nomograph included in ACI 305 Hot Weather Concreting. There are several more slabs scheduled to be poured. Q: We understand crazing is not structurally significant but the owner does not like them. 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.
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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.Ĭrazing occurs at a very early age due to a rapid loss of moisture similar to plastic shrinkage cracks. By definition, plastic shrinkage cracking occurs early while the concrete is still plastic and typically while the slab is still being finished. Q: Is crazing considered to be plastic shrinkage cracking or drying shrinkage cracking?Ī: Crazing cracks fall in between these two broad categories. Crazing cracks are typically less than 1/8- to ¼-inch deep and are generally not structurally significant. They are associated with early surface drying or cooling, causing the immediate surface to shrink differently than the underlying concrete. Are these cracks a structural problem?Ī: Craze cracks are small pattern cracks occurring in a slab surface. 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.