What is Crazing in Concrete?
Crazing is a phenomenon that occurs in concrete, resulting in the formation of a complex network of fine cracks or fissures on the surface. This is primarily due to the shrinkage of the surface layer, which causes the concrete to develop these cracks. These cracks are usually not very deep and rarely exceed a depth of 3mm.
The cracks are more noticeable on over floated or steel-troweled surfaces. In such cases, the cracks develop in irregular hexagonal shapes, with areas enclosed by the cracks typically no more than 40mm wide. However, in unusual instances, these areas may be as small as 10mm. Figure 1(a) and (b) provide visual representations of this phenomenon.
Craze cracks typically appear soon after concrete is placed and are usually noticeable within the first week. Sometimes they are not immediately apparent, but become visible once the surface is wet and begins to dry. Although they do not impact the structural strength of the concrete and generally have little effect on durability, the appearance of a crazed surface can be unattractive.
Causes of Crazing in Concrete
Crazing in concrete is a common problem that occurs due to incorrect construction practices. One of the main reasons for crazing is poor or inadequate curing. Curing of concrete is crucial to maintain the moisture content when the concrete starts to set and gain strength. However, if the evaporation rate from the concrete surface is higher than the moisture gain from curing, it can result in crazing cracks in the concrete. Factors such as direct sunlight, low humidity, or drying winds can contribute to this issue.
Another construction practice that can lead to crazing in concrete is intermittent wet curing and drying. When concrete is intermittently cured, the surface is allowed to dry for some time, which can cause crazing to occur.
Excessive floating during the concrete finishing process can also lead to crazing. This occurs when the cement paste accumulates on the top of the concrete, while the coarse aggregate settles down. The moisture accumulation at the top of the concrete can cause crazing when it dries up.
Other practices that can contribute to crazing in concrete include excessive laitance on the surface, finishing with a float when there is bleed water on the surface, and sprinkling cement on the surface to dry up the bleed water. These practices can create a weak surface on the concrete due to the concentration of fines on the surface.
Lastly, over vibration during the construction process can cause extra bleed and laitance to accumulate on the surface of the concrete, which can also lead to crazing. Overall, it is essential to avoid these construction practices and follow proper curing and finishing techniques to prevent crazing in concrete.
Preventive Measures for Crazing in Concrete
Proper and early curing of concrete is essential to prevent moisture loss and promote proper hydration. It also helps prevent the appearance of crazing on the concrete surface. Crazing occurs when the surface of the concrete dries too quickly, causing small cracks to form. To prevent crazing, it is recommended to use a curing compound on the surface of the concrete, which prevents rapid evaporation of moisture. Sprinkling dry cement or a mixture of cement and fine sand on the surface of the plastic concrete should be avoided as it can cause crazing.
To further prevent crazing, a low water-cement ratio should be used, consistent with adequate compaction. Workability enhancing air-entrained concrete with a moderate slump can also be used. Air-entrainment reduces the rate of bleeding in fresh concrete, which in turn reduces the likelihood of crazing. Higher slump concrete should be avoided as it can cause the mixture to segregate, resulting in a weak surface layer.
It is also important to avoid steep moisture differences between the concrete surface and the interior of the concrete. Troweling the surface as little as possible and avoiding the use of a steel float is recommended. Rich finishing mixes should be avoided, and over-vibration should be prevented to avoid bringing too much slurry to the top or side. Grouting processes or rubbing the surface with neat cement paste should also be avoided.
These recommendations are based on industry standards, including the ACI 302.1R Guide for concrete floor and slab construction, IS177T Concrete slab surface defects – Causes, prevention and repair by the Portland Cement Association, and Ward Malisch’s article “Avoiding Common Outdoor flatwork problems” in Concrete Construction from July 1990. By following these guidelines, concrete can be properly cured and maintained, preventing the appearance of crazing on the surface.