Hot & Cold Weather Masonry Construction

Planning and Construction

General. During cold weather masonry construction, the rate with which masonry freezes is influenced by the mortar's temperature and properties, the masonry unit's temperature and properties, and the severity of the temperature and wind. The difference between masonry and air temperatures affects the rate at which the masonry freezes. The mortar and masonry properties affect the possibility of damage when the masonry freezes.

Immediately after masonry is laid during cold weather, at least two counteracting forces begin. Absorptive masonry units draw water from the mortar and the mortar, through its water-retentive characteristics, holds the water within the mortar. At the same time, the surrounding air may be chilling the masonry and withdrawing water through evaporation. The balancing of these forces governs the masonry strength development, and the degree of disruptive expansion accompanying freezing.

When masonry freezes, two conditions are identifiable: (1) masonry frozen while the mortar is in the wet condition (moisture content greater than 6%); and (2) masonry frozen while the mortar is in the dry condition (moisture content less than 6%).

Masonry frozen while the mortar is in the wet condition (greater than 6% moisture) contains enough water to cause disruptive expansion because of the 9% increase in volume that occurs when the water is converted to ice. The expansion upon freezing is related to the mortar water content. In the frozen condition the masonry develops high bond and compressive strengths - both the hydrated cement and ice contribute to these properties. Upon thawing, the masonry returns to its pre-frozen state except for damage caused by expansion, which may tend to reduce the bond and compressive strength.

Masonry frozen while the mortar is in the dry condition (less than 6% moisture) undergoes freezing without disruptive forces. As the mortar freezes, the unfrozen water moves to allow for the freezing water expansion. Cement hydration slows and stops. The frozen water contributes to the load carrying and tensile bond strengths of the masonry. Upon thawing, the masonry returns to its pre-frozen state.

Although a few researchers have related the performance characteristics to freezing, their findings are too general to warrant specific conclusions.

Summary. When construction takes place at below-normal temperatures, masonry needs to be constructed in such a manner that it will develop sufficient strength so the mortar will lose enough water to prevent expansion of the masonry upon freezing. Further, all masonry frozen during the early periods after construction should subsequently be moistened either naturally or artificially after thawing to reactivate the cement hydration process, which in turn will promote further strength development of the masonry.

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