Masonry Magazine January 1975 Page.18
Mortar
Performance
General
Hydration and strength development in mortar 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 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 their normal-temperature counterparts. Mortars mixed at low temperature have lower water contents, longer setting and hardening times, higher air contents and lower early strength than those mixed at normal temperatures. Heated mortar materials produce mortars with performance characteristics identical to those in the normal temperature range.
Effects of Freezing
Frozen mortar takes on all 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 mortar during freezing is considered a significant factor affecting its freezing characteristics. Mortars possessing water contents in excess of 6 to 8% expand on freezing. Expansion increases as the water content increases, so every effort should be made, through the selection of masonry unit or protection, to reduce the mortar water content from the initial 11% to 16% range to some value below 6% to avoid the disruptive expansive force.
Loss of Water
The early freezing of mortar does not significantly reduce either transverse or compressive strength. The effect of cure condition on the strength of masonry that is subjected to long periods of freezing, where freeze-drying or evaporation reduces the water content, is unknown. Masonry once frozen and dried may be expected to suffer a strength reduction. 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.
Masonry Unit
Performance
General
A cold masonry unit will exhibit all performance characteristics of a heated unit, except that the volume of the unit is, or may be, the smallest it will be within the masonry. If the cold masonry unit is wet and frozen, the absorption characteristics may be decreased due to ice blockages in the pores. Preheated masonry units would exhibit all performance characteristics of units as used during normal construction, except the heated unit may withdraw more water from the mortar because of absorptive characteristics of a cooling body.
Selection
The architects' selection of the masonry unit should consider the performance characteristics of the unit during cold weather masonry construction. An absorptive unit will withdraw water from the mortar and lessen the possible disruptive expansions that may occur in the mortar or initial freezing. Conversely, a very low absorption unit will not withdraw sufficient water to prevent mortar and masonry expansion. The temperature of the masonry unit and the ratio of its volume and density to those of the mortar joint are further contributors to the freezing rate of masonry. Masonry units that are dry, but excessively cold, will rapidly withdraw heat from the mortar and increase the rate of freezing.
Masonry
Performance
During cold weather masonry construction, as discussed previously, the rate with which masonry freezes is influenced by the mortar's temperature and properties, the masonry units' temperature and properties, and the severity of the temperature and the wind. Although few researchers have related performance characteristics of masonry to freezing, their findings are too general to warrant specific conclusions. Nevertheless, masonry should be constructed in such a manner that it will develop sufficient strength and that the mortar will lose sufficient water to prevent expansion of the masonry upon freezing. All masonry frozen during the early periods of construction should also be moistened to reactivate the hydration process.
Materials
Cold weather masonry construction does not dictate any drastic changes of the mortar mixtures or the masonry units. Materials selected for normal temperature construction will generally require little change during construction at low temperatures. The masonry units and mortar should conform to requirements of applicable ASTM Specifications.
Masonry Units
As mentioned previously, low absorption units may require additional or earlier protection from freezing. The effect of the unit is considered insignificant, however, if freezing occurs after the moisture within the mortar has been decreased sufficiently low to prevent expansion on freezing.
Mortars
During cold weather masonry construction, it may be advisable to substitute Type III, High-Early Strength Portland Cement as a direct replacement for Type 1, Normal Portland Cement, in the mortar. Type III cement, because of its more rapid early age strength development characteristics, will provide greater internal protection for the masonry mortar.