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.

The concept behind these procedures is to place and maintain the grout temperatures long enough for the material to properly hydrate, and for any free water not involved in the hydration process to evaporate. These cold weather procedures involve heating the water and masonry materials, using Type III portland cement (which gains strength faster than Type I), avoiding the use of excess water in the grout, and protecting the newly completed walls from cold temperatures and wind to reduce the rate of heat loss.

The warmer the grout materials and masonry are at the time of installation, the faster hydration occurs. Since the masonry is generally built long before being grouted, it is typically not practical to heat the masonry units. Electric blankets or other techniques can be used to heat the walls, but this is typically done only when the masonry is very cold because this approach is expensive.

Heating the grout actually involves using materials that are warm at the time they are mixed. If the grout begins at a warm enough temperature and is covered to prevent heat loss, the heat generated by hydration prevents the grout from freezing. The ideal temperature of grout is between 75º F and 120º F at the time of placement. This range can be accomplished by heating the sand and water.

The water can be heated within a 55-gal. drum by using specially made electric blankets that cover the outside of the drum, installing immersion heaters, or placing heaters that fit on the exterior surface.

Electric blankets also can be used. The most efficient way is to place the electric blanket on the ground and cover it with sheet metal for protection. The sand is then piled on top of the sheet metal and covered with another insulating blanket.

As a general rule, grout using Type III portland cement sets in approximately half the time of grout containing Type 1. Therefore, Type III should be considered when grouting during cold weather. (For further information regarding cold weather masonry construction procedures and protection, refer to ACI 530.1-ASCE 6-TMS 602.)

Once the grout has frozen and cracked the face shells of the masonry units, there is little that can be done to repair the damaged masonry. In most cases, especially where the face of the units is exposed, the masonry is typically removed and replaced. Sometimes, where the masonry will be concealed in the completed wall, it may be possible to design permanent pilasters or supplemental reinforcing to strengthen the damaged masonry.

In many cases, however, such strengthening can often cost as much as rebuilding the damaged masonry by the time engineering fees and the cost of the reinforcing schemes are considered.

Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.