Question: I am about to install a 10,000-square-foot floor and the owner believes that wet curing the floor will result in no curling. Is this true? The subgrade under the floor is wet so a vapor barrier was specified to prevent the possibility of wet floors after the building has been opened for use.

Answer: Wet curing concrete slabs will improve the characteristics of the concrete, but preventing curling isn't one of them. In fact, wet curing will probably increase the amount of curling.

Curling results when one side of a floor slab dries more quickly than the other side. In this situation, the drier surface shrinks more than the wet side does. The drier side is almost always the top of a floor so curling occurs at construction and contraction joints. While the floor is being wet cured, there will probably be very little activity with regard to curling. It will begin when the floor starts to dry out.

The installation of a vapor barrier is a good idea but it will trap moisture on the bottom of the slab, keeping it moist longer and therefore causing increased curling. However, the most serious problem with regard to wet curing is that some water from the curing process will flow down through the contraction and construction joints and pool on top of the vapor barrier. This reservoir of water will keep the bottom of the slab wet for a longer period of time, resulting in even more curling.

To achieve the least amount of curling, it would be best to place a well-graded aggregate concrete mix with a reduction in the amount of portland cement and ultimately reduce the total water in the concrete. Cure the floor by covering it with poly or use a good curing material.

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