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Hot Weather Construction: Hot Weather Construction

During the summer as temperatures begin to rise, it is important to start implementing hot weather masonry construction requirements on your projects. High temperatures will affect the properties of masonry units, mortar, and grout. Brick and concrete block that have been sitting in the sun will absorb more moisture from mortar. Mortar workability and set time decreases, and a fixed amount of air entraining agent will provide less entrained air. All of these conditions reduce the amount of water available for cement hydration, which can affect mortar and grout strength and in turn, the strength of the masonry wall. In the worst case mortar and grout may “flash-set” or harden prematurely due to high temperature, leading to lower strength than if those materials were cured in normal conditions.

Hot weather construction procedures are outlined in TMS 602 Specification for Masonry Structures and should be put into effect when the ambient air temperature is greater than 100°F or exceeds 90°F with a wind velocity greater than 8 mph. The requirements are separated out into three parts.
  1. Preparation before masonry work begins: Sand piles have to be kept damp and loose, and mixed mortar must be less than 120°F. Sand does not have to be saturated, just spray the pile often so that evaporating water cools the pile.

    When the ambient air temperature exceeds 115°F or 105°F with a wind velocity greater than 8 mph, all materials and mixing equipment are also required to be shaded from direct sunlight. That means putting up tarps or building a partial enclosure to keep the sun off.

  2. During masonry construction: Mortar and grout temperatures must remain below 120°F . All equipment that comes in contact with mortar materials and mixed mortar, such as the mixer, the mortar board, and the container used to transfer mortar between the two, must be sprayed down with cool water first. Mortar can be retempered with cool water to maintain a workable consistency, but all unused mortar must be discarded two hours after initial mixing.

    When working with AAC masonry units, do not spread thin-bed mortar more than four feet ahead, and AAC units must be set within one minute after the thin-bed mortar has been spread.

    When the ambient air temperature exceeds 115°F or 105°F with a wind velocity greater than 8 mph, cool water is also required to be used when mixing mortar and grout. Ice may be added to the water as long as it is completely melted before being used to mix mortar.

  3. Protection of newly constructed masonry: When the mean (average) daily temperature exceeds 100°F or 90°F with a wind velocity greater than 8 mph, newly built masonry must be fog sprayed until damp at least three times a day until the masonry is three days old. This provision ensures there is enough moisture in the masonry wall for the mortar and grout to gain strength.
The best strategy for meeting these requirements is to provide shade for materials and mixing equipment, flushing equipment with cold water before mixing, and using cool water to mix mortar. It is ok to use ice to cool down mix water for mortar and grout, but do not add ice directly to the mixer. Store materials to allow for air circulation. Keep water in a light colored, open container that is out of the sun to permit evaporative cooling. Avoid using water from hoses that have been sitting in the sun.

Use mortar that retains water well and has a longer board life. It may be possible to use a Type N instead of a Type S mortar. Retemper the mortar often. Admixtures such as retarding agents are not recommended since they delay cement hydration and do not reduce water evaporation.

Tips for mixing and placing grout in hot weather include using a higher slump (10 to 11 inches) and fog spraying walls first to reduce the amount of water that is absorbed from the grout into the walls.

Store masonry units in conditions to keep them cool and prevent excessive suction of water from mortar during construction. Clay brick with a high initial rate of absorption (greater than 30 g/30 in2/min) should be pre-wetted by spraying down the brick approximately eight hours before use and covering the pile with a tarp or plastic so that the brick are at a saturated surface dry condition when placed in the wall. Do not pre-wet concrete block since this will lead to shrinkage when the block dries.

Providing protection from the sun and wind can also reduce rapid drying, to keep moisture in the wall to properly hydrate cement in mortar and grout. Shades and windbreaks can block the direct sun and wind drying effects. Moisture can also be kept in the wall by covering or wrapping them in wet burlap.

Construction during hot weather conditions can also be minimized by working during hours that avoid the hottest midday times. Utilizing early morning hours has the added benefit of taking advantage of materials that have cooled overnight. All of these measures can help with meeting hot weather masonry construction requirements.
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