Q: My company is about to begin a project that will be built during very hot, dry, windy weather. We are concerned that there may be problems with the masonry that are associated with installation during these extreme weather conditions.
What potential problems should we look for in masonry installed under these conditions, and how can we avoid them?
A:Specifications for Masonry Structures (TMS 602/ACI 530.1/ASCE 6) includes several requirements for hot weather masonry construction and protection. The hot weather procedures do not apply until the air temperature exceeds 100º F, or 90º F with an 8-mph or stronger wind.
Mortar sets very quickly during hot weather. Hot units absorb water more quickly than cooler ones. Water evaporates from the wall quickly, especially when hot weather is accompanied by dry winds, which can result in several problems.
Loss of moisture from the mortar can make it stiff during construction. If the mortar is spread out on the unit too far, the top surface may stiffen before the next unit is placed on top of it. This situation disrupts mechanical keying of the mortar into the masonry units and can result in pockets of entrapped air that reduce the contact area between the mortar and units. Dry and stiff mortar often results in visible bond separations on the top of the bed joints.
Another problem that occurs during hot weather is rapid drying of the mortar after it is placed, which can cause shrinkage cracking. This cracking can appear as transverse lines in the mortar, especially along the length of the bed joints. If water evaporates too quickly from the mortar, hydration can be significantly disrupted. Incomplete hydration can make the surface of the mortar joint friable or easy to crumble when rubbed, and can be easily eroded by wind and rain.
There are many approaches that help avoid hot weather masonry construction problems. If the contractor knows early on that the masonry work will occur during hot weather, steps can be taken during the selection of materials to minimize potential problems. For example, mortars with high water retention should be selected, such as using Type N instead of Type S or Type M. In climates not subject to significant freeze/thaw problems, Type O mortar can be used. Select brick units with a low initial rate of absorption, which reduces the amount of water drawn out of the mortar during installation.