Cold-Weather Masonry and Mortar Tips

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Masonry work requires special attention when working temperatures are below 40 F. Very cold weather changes the behavior of mortar and can lead to cracking and other problems. Masons must act promptly and follow special steps to keep masonry warm and workable. This may include protecting raw materials from cold and ice, heating mortar during application, and insulating structures during the curing process.

How Cold Weather Affects Mortar

Cold weather slows the hydration of mortar. If the water in the mortar freezes, it creates a destructive change in volume, causing mortar expansion. If the mortar contains more than 6 percent water, the expansion due to freezing will be great enough to crack the mortar. In addition, moisture or ice on the surface of masonry units can prevent a good bond between the mortar and the units.

Tips for Working With Masonry and Mortar in Cold Weather

The general strategy to prevent cold-weather problems is to ensure the materials stay dry during storage and to keep the mortar above 40 F as long as possible.

Mixing Mortar:

  • Mix relatively small amounts of mortar mix so that water will not be absorbed by the materials and will not frost.
  • Keep a close eye on mortar temperature to prevent excessive drying of the mortar due to applied heat.
  • Heat the sand or water, as appropriate, to ensure the mortar is above 40 F. When using heated water, combine it with cold sand in the mixer to prevent flash set, before adding cement.
  • Employ speed hydration by using high-early cement or by using an accelerator or admixture.
  • Be aware that most commercial masonry ‘antifreeze’ admixtures are accelerators rather than freezing-point depressants. ASTM C1384 provides criteria for evaluating admixtures and accelerators in masonry mortars. Type III cement can change the color of mortar.

Storing and Handling Materials:

  • Place materials on planks and cover them with tarps. All materials should be completely covered to prevent wetting by rain or snow.
  • Use bulk-delivered dry mortar ingredients whenever possible.
  • Thaw frozen lumps of sand by heating and breaking up the clumps, if necessary.
  • Place masonry units on unfrozen surfaces because ice reduces bond; when ice melts, the masonry can move. Do not lay masonry units having either a temperature below 20 F or containing fro­zen moisture, visible ice, or snow on their surfaces.
  • Heat masonry materials before use, as needed, so that cement hydration can occur properly. Masonry units with high rates of absorption will accelerate stiffening.
  • Place mortar on heated surfaces, such as metal mortarboards, to prevent freezing during application.
  • Calcium chloride (at a limit of 2 percent by weight of cement) is commonly used in concrete as an accelerator, but the Specification prohibits its use in mortar for Masonry Structures.
  • Wet—do not saturate—masonry units with very high absorption, such as fired-clay brick. Other masonry should be kept dry prior to installation.
  • Do not install glass masonry units during cold periods.

Protecting New Masonry Structures:

  • Cover walls with plastic to prevent water from entering the masonry when temperatures are above 32 F.
  • Cover walls with 1/2-inch insulation blankets when the temperature is between 32 F and 20 F to prevent or reduce rapid heat loss and to prevent water from entering the masonry.
  • Cover walls with 1-inch insulation blanket or maintain a heated area to 40 F for two days following installation when temperatures are between 20 F and 0 F. Heat sources may be used on both sides of the masonry under construction.
  • Install windbreakers if wind speed is higher 15 miles per hour.