Hot & Cold Weather Masonry Construction

Mortar and Grout Performance

General. As the ambient temperature falls below freezing, the individual mortar and grout ingredients become colder and their proper preparation becomes more involved. The heat-liberating reaction between cement and water is slowed or stopped when the cement paste is subjected to temperatures below 40°F (4.4°C). Hydration and strength development proceed only at temperatures above freezing and only when sufficient water is available. However, cold weather masonry construction may proceed at temperatures below freezing provided the mortar and grout ingredients are heated and, as the ambient temperature decreases, the masonry unit and the structure are maintained above freezing during the early hours after construction.

Mortars mixed using cold but unfrozen materials possess plastic properties quite different from those mixed at normal temperatures. Mortars mixed at low temperatures have a lower water demand, longer setting and hardening times, higher air contents and lower early age strength than those mixed at normal temperatures. Heated mortar materials produce mortars with performance characteristics identical to those in the normal-temperature range and, consequently, this construction practice is deemed desirable for masonry constructed during cold weather.

Mortars and grouts mixed to a particular temperature lose heat until they attain the ambient temperature. During this temperature loss, they undergo various stages of cooling, freezing, and additional cooling after freezing. They first cool to 32°F (0°C). The mortars or grouts then enter a supercooling period, where temperatures below 32°F (0°C) may be measured.

Freezing of mortars and grouts may not occur at 32°F (0°C) because dissolved cement compounds depress the freezing point of the solution in the systems. Freezing then begins, when the temperature falls below the depressed freezing point, and continues until all of the water in the mortar or grout is frozen. The temperature remains essentially constant during this stage. Once the water has frozen, the mortar or grout temperature decreases until it reaches the ambient temperature.

Increased wind velocities, combined with falling temperatures, increase the rate of cooling. Chill factor, the term relating these combined wind and temperature effects, is considered an important variable of cold weather masonry construction.

Effects of Freezing. When fresh mortar freezes, its performance characteristics are affected by many factors: water content, age at freezing, strength development prior to freezing, etc. The frozen mortar takes on an outward appearance of being hardened mortar, as evidenced by its ability to support loads in excess of its unfrozen counterpart, and its ability to bond to surfaces.

The water content of the mortar during freezing is considered a significant factor affecting its freezing characteristics. Mortars containing water in excess of 6 to 8% expand on freezing. Expansion increases as the water content increases, so cold weather practices are intended to reduce the water content from the initial 11% to 16% range to some value below 6% to avoid the disruptive expansive forces.

When grout freezes, the frozen grout may lose its ability to bond to the masonry and the reinforcement. Expansive forces can be of sufficient magnitude to fracture the masonry.

Loss of Water. Provided disruptive expansion has not occurred, the early freezing of mortars does not significantly reduce potential transverse or compressive strength. However, masonry that is subjected to freezing, where freeze-drying or evaporation reduces the water content, may be expected to suffer a strength reduction unless water is supplied to the mortar after the masonry thaws. Consequently, mortars, once frozen, should be supplied additional water or allowed to absorb natural water to reactivate the portland cement hydration process for further strength development.

Summary. The performance characteristics of masonry mortars and grouts are influenced by temperatures below normal. The changes in the performance characteristics are predictable and quality masonry construction may proceed with only minor changes in material preparation. Heating mortar and grout materials reduces setting time, increases rate of strength development, and extends the period before freezing to permit the mortar and grout to develop hardened properties comparable to those used in normal-temperature weather.

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