Masonry Magazine June 1994 Page. 22
Mortar
to provide balanced performance compatible with units used in construction and prevailing weather encountered. Whenever mortar is accepted under the property specifications, laboratory tests must be performed to confirm that the selected mortar materials and proportions meet ASTM C270 requirements.
The use of retarding admixtures in conventional mortar systems is not recommended. Retarders delay set time of the mortar, but they do not reduce evaporation rates. Therefore, don't expect retarders to solve all hot-weather problems. Proper curing is still needed to develop required hardened mortar properties.
Storage and Preparation of Materials
Keep materials as cool as possible. Cover or shade units and mortar materials from direct sunlight, whenever practical. Maintain sand in a damp-loose condition, both to provide evaporative cooling and to assure consistent batching of mortar materials when proportioning by volume. Whenever sand moisture drops below the 4 to 8% that is typical for sand delivered in a damp, loose condition, the stockpile should be sprinkled to restore moisture and increase evaporative cooling.
Use cool water to mix the mortar. Water stored in a light colored, open barrel is cooled to some extent by surface evaporation. Coupled with shading of the barrel from direct sunlight, evaporative cooling may be sufficient. Water can also be cooled by maintaining a steady flow or adding ice. Avoid using water from an unshaded water hose of any significant length. When exposed to sunlight, long water hoses become effective water heaters, compounding difficulties associated with hot-weather construction.
Mortar can also absorb heat from contact with metal equipment. Cooling mixers, wheelbarrows, mortar pans, and other metal equipment by flushing with cool water helps reduce such heat gain. Wooden mortar boards tend to be quite absorptive and should also be flushed with water prior to contact with fresh mortar.
Mix mortar from three to five minutes in a mechanical mixer using the maximum amount of water consistent with good workability. It should also be retempered as needed to maintain a workable consistency. Discard mortar that has not been used within 2½ hours of mixing and mix fresh mortar.
Clay Brick
Clay Brick units having a high initial rate of absorption (IRA) can contribute to the rapid dry-out of mortars in hot weather. Wetting of high absorption units (IRA of over 30 g/min-30 sq in.) is recommended to reduce the suction of the brick. As indicated in Fig. 3, brick can be fully saturated, surface wet, surface dry, or completely dry. The optimum condition for hot-weather construction is the "surface dry" condition where the unit has sufficient suction to establish good bond with the mortar without contributing to rapid drying of the mortar. The surface wet condition may also yield desired performance provided adequate depth of moisture exists and the surface is not dripping wet. A saturated condition will inhibit good bond between mortar and unit and, as noted previously, dry units with high IRA's units contribute to the rapid dry-out of mortars in hot weather.
Concrete Masonry
Concrete masonry units should not be wetted before use. Concrete masonry units expand when wet and shrink as they dry. Thus, placing wet concrete masonry units in a wall contributes to increased shrinkage. When necessary to assure adequate curing moisture, apply a fog spray after the concrete masonry wall is in place.
Other Construction Practices
During hot-weather construction, mortar beds must not be spread more than 4 ft (1.2 m) ahead of the masonry being placed and masonry units must be placed within one minute of spreading the mortar (see ACI 530.1). Since wind and low relative humidities increase evaporation, the use of wind screens and fog sprays can effectively reduce the