Q: Our company is building a warehouse in central Wisconsin. Several weeks after construction and grouting of the wall, we noticed several small cracks in the concrete masonry units. In some areas, the cracks were relatively large, were generally vertical, and in line with the center of the grouted cells.

The masonry was installed during cold weather; however, the walls were covered with tarps immediately after construction. It seems likely that the cracking developed as a result of freezing several days after the masonry was grouted.

What should we have done to avoid this cracking, and what can we do to repair the condition?

A: There is always a risk of frozen masonry when construction occurs during cold weather. Frozen grout can be much more of a problem than frozen mortar in cold weather construction. Grout generally contains considerably more water than mortar. As water freezes within the grout, the resulting expansion can crack the surrounding masonry.

Problems with frozen mortar occur when there is a considerable amount of free water within the material. Grout gains strength by hydration. During the hydration process, water chemically combines with the cement to form calcium hydroxide. Water that is used in the hydration process to form calcium hydroxide is no longer available to freeze.

Free water is any amount in excess of that needed for hydration, or water that has yet to be used in the hydration process. Therefore, grout used in cold weather construction should contain as little water as necessary; in other words, the amount needed for hydration plus enough extra for proper placement.

Obviously, the temperature of the grout must fall below 32º F to freeze. The hydration process itself generates heat. However, cold temperatures greatly reduce the rate of hydration. The process is very slow at temperatures below about 40º F, and virtually stops when the temperature of the grout approaches 32º F.

Typically grout freezes when the materials are cold at the time of initial mixing, or when the rate of heat loss exceeds the rate of heat generated by hydration of the grout, as is the case with very cold temperatures or during high winds and inadequate protection. The reduced rate of hydration not only can result in cracking when the grout freezes, but the walls are more susceptible to damage from wind loads during construction due to the lower strength gain.

There are generally two approaches to take when constructing grouted masonry walls during cold weather. One plan that has recently gained a lot of popularity is to build a heated enclosure. This approach provides comfort to the workers and increases productivity. Within the enclosure, the masonry sets quickly and excess water evaporates or is used by the hydration process. On most smaller projects, however, it is not economical to use heated enclosures.