Hot weather defined: “Normal” masonry construction is generally thought of as having ambient temperatures between 40 and 90 degrees F. The American Concrete Institute, ACI 530.1, considers hot weather conditions when the ambient temperature exceeds 100 degrees F or 90 degrees F with a wind velocity greater than 8 mph. Of course, there may be building codes and specifications that vary.
The Brick Industry Association, in their Tech Note 1 states, “The primary concern during hot weather is rapid evaporation and absorption of water from the mortar. ……without sufficient water, cement hydration slows or stops, which reduces the bond strength and extent of bond between brick and mortar”. Naturally compressive strength suffers as well. And, remember, the purpose of mortar is to bond the masonry units together.
In hot temperatures the mortar actually requires more water than at cooler temperatures to achieve a given workability or plasticity. Despite this increased initial water demand, the mortar may be more difficult to use and the board life and stiffening time is shortened. This is due to the increased water loss brought on by the higher temperatures of the masonry units, their associated higher absorption rates, and naturally, the higher evaporation rate into the air.
Significant problems that are brought on by hot temperatures include:
Both scenarios can create non-durable and low quality mortar and can lead to reduced buckling strength of a wall that is concentrically loaded. It also reduces the wall strength under horizontal and wind loading.