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Hot Weather Setting Practices: Design Tips – Technical Bulletin #48

Special precautions must be taken when setting Cast Stone in hot weather. The stone setter must take measures to ensure that the quality of the installation does not suffer from high temperatures. Hot weather is defined to be temperatures above 90 degrees F (32 degrees C). 

The primary concern, to the masonry contractor during hot weather, is evaporation of water from the mortar. If sufficient water is not present, bond between the stone and mortar will be compromised. The effects of high temperature are not as damaging to the performance of the masonry as are low temperatures. The increased rate of hydration of the cement and favorable curing conditions in hot, humid weather will help develop masonry strength if sufficient water is present at the time of construction.

Temperature of the materials can be adjusted to aid the construction of quality masonry in hot weather conditions. ACI 530.1 specifies construction methods to produce quality masonry in hot weather conditions.

Cast Stone units are the material in masonry construction least affected by hot weather. However, the interaction between the Cast Stone and the mortar or grout is critical. Warmer stones will absorb more water from the mortar. Lower bond strength results if enough water is not present in the mortar when the units are laid. Thus, lower absorption units may be desirable since they allow more complete hydration of the mortar.


According to the Brick Institute of America, mortar in hot weather will tend to lose its plasticity rapidly due to evaporation of the water from the mix and the increased rate of hydration of the cement. The use of admixtures to increase plasticity is not recommended unless their full effect on the mortar is known. Mortar with high lime content and high water retention should be used. Retempering of the mortar should be permitted. Mortar mixed at high temperatures often has higher water content, lower air content, and a shorter board life than those mixed at normal temperatures.

Temperature of the mortar should be maintained between 70 degrees F and 120 degrees F (21 degrees C and 49 degrees C). Temperatures above 120 degrees F (49 degrees C) may cause flash set of the cement. Cold water may be used to help control the temperature of the mortar. Ice is highly effective in reducing the temperature of the mix water. When used, ice should be completely melted before combining the water with any other ingredients. In any case, mortar should be used within two hours of initial mixing.

During periods of hot weather the temperature of the materials should be controlled for best results. Storing brick and sand under cover of shade will help control heat gain of the materials. Sand should be stored on a raised platform and not in contact with a cover during the hot part of the day. This prevents ground moisture from rising, then condensing on the cover after temperatures cool down, thus contaminating the materials. 

When possible, shade should also be provided for laborers, whose productivity decreases with increasing temperature and humidity. Starting work earlier in the day and scheduling masonry construction, avoiding the hot, mid-day periods can reduce the effects of high temperatures on laborers and materials.

The following items are suggested for hot weather masonry construction. These items can be incorporated in the specifications of the project where applicable:

1. Maintain temperature of mortar and grout between 70 degrees F and 120 degrees F (21 degrees C and 49 degrees C). 

2. Ice may be used to lower the mix water temperature and must be completely melted before adding the water to the other ingredients. Cold water should always be used when mixing mortar and grout.

3. Cast Stone should always be thoroughly drenched with potable water prior to setting. This procedure is even more critical during hot weather.

4. Limit the spread of mortar beds to 4 ft (1.2 m) when temperatures are 100 degrees F (38 degrees C) or above, or 90 degrees F (32 degrees C) with an 8 mph (3.6 m/s) wind.

5. Place Cast Stone within one minute of laying mortar. Fog the joints with a water mist during the day.

6. Cover the units at the end of the day with plastic sheets to control moisture evaporation.

Note: Construction requirements, while work is in progress, are based on ambient temperatures. Protection requirements, after masonry is placed, are based on mean daily temperatures.

The information and suggestions contained in this Technical Bulletin are based on the available data and the experience of the staff of Continental Cast Stone. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of architectural Cast Stone.  Final decisions on the use of the information contained in this Technical Bulletin are not within the purview of the Continental Cast Stone and must rest with the project architect, engineer and owner.