Hot weather wall work

Q: I have a question regarding the masonry walls on a warehouse building our company recently completed. The portion of the masonry work performed during hot weather seems to have hairline cracks at the bond line between the mortar and brick, especially at the top of the joint. It seems like the mortar dried too quickly and had excessive shrinkage.

How can this type of problem be avoided?

A: Without examining the wall and removing masonry prisms for laboratory analysis, it is not possible to know what actually happened. There are several possibilities.

Laying masonry during hot weather can be very difficult, especially when it is dry and windy. The mortar sets and stiffens much faster in these conditions than in cooler weather. The mortar does not bond well to the unit placed on top of it because it is stiff. Instead of having good contact over the surface, air can be trapped at this bond interface.

Depending on how much water has evaporated, the rate of hydration of the mortar may be greatly reduced. The rapid drying following construction also can lead to cracks and separations because of shrinkage and poor hydration.

To avoid these problems, smaller batches of mortar should be used so that it can be placed faster following its initial mixing. It is also important to string out less mortar. If you string out mortar for too long of a distance, it dries out and stiffens on the surface.

Checks can be made during construction to test the bond being achieved. Lift up a unit right after it is placed to see if the mortar is adhering to the unit. If the surface of the mortar is dry, the brick comes off leaving most of the material on the unit below.

If feasible, within a few hours following construction, lightly mist the wall with water to help reduce the rate of drying. The misting must be controlled to avoid applying excessive moisture that can result in staining.

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