Masonry Magazine October 1993 Page. 21
COLD WEATHER
MASONRY TIPS
Principal concern in cold weather construction is to keep newly constructed masonry from freezing.
PLANNING a masonry project this winter? Below normal temperatures (40°F and below) does not necessitate any significant changes of the mortar mixture or the masonry units. Rather, your principal concern will be to keep the newly constructed masonry from freezing. Here are some pointers:
Mortar-Ideal temperatures for the placement and curing of masonry mortar is the range of 70°F ± 10°F. In cold weather (40°F and below) mortar materials need to be heated, otherwise the mortar is likely to exhibit slower setting times and lower early strengths.
Water acts as a lubricant in the plastic mortar and is required for hydration of the portland cement contained in the mortar. While mortar stiffens as water is absorbed by units and evaporates, the hardening of mortar is a result of the reaction between the portland cement and water. This reaction rate is temperature dependent and is slowed or stopped when the cement paste is below 40°F.
During the critical early curing stages when the free water content of mortar is high (above 6%) the temperature of the in place masonry should be maintained above freezing to avoid disruptive expansion in the mortar due to freezing. While measures to reduce initial water content of the mortar and increase the dispersion rate of the free moisture in the masonry limits the time that the masonry is vulnerable to disruptive freezing expansion, such measures must be appropriately balanced with other considerations of good masonry practice. For example: The use of a clean well graded sand will reduce the water requirements of mortar without the detrimental loss of workability that may occur as a result of arbitrarily cutting back on the water content. Heating the mortar materials and providing heated enclosures for newly constructed masonry will increase both the dispersion rate of the free moisture and the reaction rate with the cement compounds.
Masonry Units-Only dry masonry units should be used during cold weather masonry construction. Wet units may become frozen before construction and impair the performance of the mortar and consequently the wall assembly. Further, dry units should not be excessively cold because they will cool the mortar rapidly and could cause freezing. Cold masonry units that are wet and frozen must be thawed, but carefully, to prevent overheating. Preheated masonry units exhibit all the usual performance characteristics of units used during normal construction, except the heated unit may absorb more water from the mortar.
Absorptive masonry units do have an advantage in cold weather over units with very low absorption. The absorptive unit can absorb excess
ADMIXTURES FOR COLD WEATHER MORTAR
"Antifreeze" is a misnomer applied to admixtures for cold weather mortar that are really accelerators, and not depressants of the mortar freezing point. These mixtures may decrease the compressive and bond strengths of the masonry if they are used in quantities that significantly lower the freezing point of the mortar. Antifreeze compounds have little benefit and are not recommended. Accelerators may reduce the time that newly constructed masonry needs to be protected from cold temperatures, but they do not eliminate the need for such protection.
Accelerators normally used include soluble carbonates, silicates and fluosilicates, aluminous cements, calcium aluminate, and some organic compounds such as triethanolamine. Calcium chloride is sometimes used as an accelerator, but may produce side-effects such as corrosion of steel (reinforcement, ties, anchors). For that reason, calcium chloride should not be permitted for use in mortar.
Corrosion inhibitors in accelerators are not recommended because they have not been evaluated in cold weather masonry construction.
Air-entraining admixtures can increase mortar workability and impede freeze/thaw deterioration. However, an excessive amount of air-entrainment should not be used in cold weather masonry construction because it will result in lower compressive strength and lower bond strength.
Coloring agents should be limited to finely ground metallic oxides, in quantities that do not exceed 10% of the cement content by weight. Carbon black, if used, should be limited to 2% of the cement content by weight.