Early Random Cracking in Floors

Hotline Problem

A supplier in the sun belt reported that a floor built by a contractor customer had developed many random cracks within 2 or 3 weeks after construction. The floor was built in June, and during the subsequent weeks the ambient temperatures were between 105 and 110 degrees F.

The supplier thinks the slab might have been about 130 degrees F during this time. The 4-inch slab was made with a 5- to 6-inch slump concrete containing 15-20 percent fly ash. A hand tool was used to make the score marks ¼- to ½-inch deep, dividing the floor into 12-foot squares. A reddish colored curing compound was used, but it was not applied uniformly. The supplier wanted to know what had caused the cracking.

Troubleshooting Response

The main trouble appears to be inadequate joints. The depth of the joints should be ¼ thickness of the concrete so that they will perform their function of controlling cracking. In this case they should be a full 1-inch deep.

An auxiliary contributor to the random cracking described is not enough care taken in application of the curing compound. The slab undoubtedly dried nonuniformly because of the uneven application, and rapidly because of the high temperature, causing random cracks to form.

A joint spacing of 12 feet would be satisfactory for many slabs, but it is too large for this one. A good rule of thumb is that the joint spacing in feet should be 2 to 3 times the slab thickness in inches. According to this rule, the joints for a 4-inch slab should be spaced 8 to 12 feet apart. However, it is also suggested that the intervals between joints should be shorter whenever there is reason to expect shrinkage to be high or the range of temperature to be great. It would have been wise to space the joints 8 feet apart of less.

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