Masonry Magazine June 1994 Page. 21
rtland Cement Association
pca
TROWEL TIPS
an aid to the masonry industry
Hot Weather Masonry Construction
Hot weather poses some special problems for masonry construction. These arise from higher temperatures of materials and equipment, more rapid evaporation of the water required for cement hydration, and accelerated cement hydration. ACI 530.1/ASCE 6/TMS 602 (ACI 530.1) defines hot-weather construction as occurring when ambient temperature exceeds 100°F (38°C), or 90°F (32°C) when the wind velocity is greater than 8 mph. Factors compounding hot-weather problems include low relative humidity and direct sunshine.
As the temperature of mortar increases:
• Workability is reduced; or, for a given workability, more water is required.
• A given amount of air-entraining agent yields less entrained air.
• Initial and final set occur earlier, and evaporation rates are generally faster.
• Units absorb more moisture from the mortar.
As a result, the mason will find it more difficult to place mortar and units. However, in addition to affecting workability, rapid drying can result in a lack of sufficient water for hydration of cement in the mortar. Since hydration of cement is necessary for normal strength development of mortar, a marked reduction in strength may occur under rapid drying conditions. Exposed mortar surfaces are particularly vulnerable. Evaporation removes moisture more rapidly from the outer surface of a mortar joint, while the interior retains moisture longer and develops greater strength. This difference in strength across the thickness of the wall can reduce the wall's buckling strength and resistance to wind or other horizontal loads.
Recognizing these potential problems is the first step in avoiding them. With careful planning and preparation, steps can be taken to minimize or compensate for the effects of hot weather on masonry construction.
Scheduling
Many of the problems associated with hot-weather masonry can be minimized by scheduling construction to avoid hot, midday periods. Use the early morning hours during the extended daylight season associated with hot weather. This will provide the benefit of working with materials that have cooled overnight. Also, the more comfortable working temperatures usually result in improved workmanship and productivity. In some instances, night construction may be an effective alternative - provided adequate lighting and support services are available.
Selecting a Mortar Mix
In hot-weather construction, properties such as water retentivity and workability should be given careful consideration when selecting mortar type and materials. For example, provided Type N and Type S mortars are both structurally adequate for a given masonry application, the increased water retentivity and workability generally associated with a Type N mortar make it a better choice for hot-weather construction.
The proportion and property specifications of ASTM C270 permit selection of mortar mix designs within prescribed ranges of sand contents. The sand content of a specific combination of sand and masonry cement or sand and portland cement-lime can be adjusted within these limits to optimize the board life, water retentivity, workability of the mortar. However, there is often a trade-off between different mortar properties. For example, increasing sand content tends to increase board life but can reduce water retentive properties and strength development of mortar. Reducing the sand content tends to reduce board life but increases the level of hydration achieved before water is lost by evaporation, thereby accelerating early strength gain. Mortar materials and proportions should be selected