Like the human body, masonry must be warmed and/or covered in the winter.
Cold weather means working with both cold mortar and cold masonry units–brick or block. Cold weather is a retarder of the chemical reaction time between cement and water. When air temperatures fall to near freezing, the setting time of mortar is perhaps eight to 10 hours. Compare this to 1.5 to three hours at 70 to 90 degrees Fahrenheit!
During the long setting times associated with cold conditions, some of the water in the mortar will be absorbed by the masonry units and some will evaporate. Both the absorbed water and the water remaining in the mortar are in a cold and sluggish state. Thus the absorbed water has a better opportunity to promote undesirable efflorescence because of longer contact time with soluble salts. Also, the water remaining in the mortar does not readily react with the cement. This results in a long setting time and a very slow rate of strength development.
Low temperatures and the water content of mortar during hardening are the governing factors for frost damage. Experience has shown that winter masonry work can proceed if, at the time of freezing, the mortar’s water content has been sufficiently lowered by absorption into the masonry units or the mortar has hardened prior to freezing.
Cold-weather masonry construction can proceed, even at temperatures below freezing, provided that the mortar ingredients are heated and, as the air temperature falls, the masonry units and the structure are kept above freezing during the initial hours after construction.
In-place mortar with moisture content in excess of about 6% will expand when frozen. Expansion will increase as the water content increases. Special efforts should be taken to reduce the initial water content from the usual 10-18% range to less than 6%. This can be achieved with proper weather protection and by selecting the correct masonry units for the job.
Cold-weather damage can occur when low-temperature, low-humidity air dries the mortar before a good bond can be achieved. On a damp winter day, masonry mortar may remain relatively soft for many hours; but if the air temperature rises while the mortar is still damp, the chance for a strong bond between the brick and the mortar is relatively good. However, in the presence of extremely cold, dry winds, the surface area between the mortar and the masonry units may lose water so rapidly that there is little opportunity for the development of a good bond. A warm rain may follow a cold period and the mortar will harden, but this does not assure that a good bond was obtained. Mortar that dries out before hardening cannot be expected to develop an adequate bond. “Dry-outs” are particularly objectionable because they are one of the possible contributing factors to leaky masonry walls.
Project planning for cold-weather masonry work should include the following measures:
- Think ahead. Have the proper equipment, manpower and protective measures in place well ahead of time.
- All masonry materials should be completely covered to prevent wetting by rain or snow.
- The tops of all walls not enclosed or sheltered should be covered with a strong weather-resistive material at the end of each workday. Partially completed walls should be covered at all times when construction is not in progress. The cover should be draped over the wall and extend a minimum of two feet out from both sides and securely held in place.
- Never use wet or frozen materials. Only use dry, frost-free masonry units and sand.
- The optimum mortar temperature has been found to be 70 degrees F +/- 10 degrees. The selected mixing temperature should be maintained within 10 degrees F.
- At air temperatures of 40 degrees F to 32 degrees F: Sand and mixing water should be heated to produce mortar temperatures between 40 degrees F and 120 degrees F. Completed masonry walls and raw materials should be protected from rain or snow for 24 hours after construction by covering with weather-resistive membrane materials.
- At air temperatures of 32 degrees F to 20 degrees F: Sand and mixing water should be heated to produce mortar temperatures between 40 degrees F and 120 degrees F. Maintain mortar temperatures on boards above freezing. Completed masonry walls should be kept above freezing for at least 24 hours after construction by providing weather-proof enclosures and auxiliary heat (when necessary).
- At air temperature of 25 degrees F to 20 degrees F: Sand and mixing water should be heated to produce mortar temperatures between 40 degrees F and 120 degrees F. Maintain mortar temperatures on boards above freezing. Salamanders or other sources of heat should be used on both sides of walls under construction. Windbreakers should be employed when wind speeds exceed 15 mph. Completed masonry walls should be kept above freezing for at least 24 hours after construction by providing weather-proof enclosures and auxiliary heat (when necessary).
- At air temperature of 20 degrees F and below: Sand and mixing water should be heated to provide mortar temperatures between 40 degrees F and 120 degrees F. Enclosure and auxiliary heat should be provided to maintain air temperatures above 32 degrees F around all walls and materials. Temperature of units when laid should be not less than 20 degrees F. Completed masonry walls should be kept above freezing for at least 24 hours after construction.
- When using artificial heat, make sure all exhaust gases are vented to the outside. Carbon dioxide from open salamanders, gasoline engines, generators or mixer engines may cause a chemical reaction known as carbonation which significantly reduces the strength and hardness of masonry surfaces.
- Admixtures such as antifreezes and accelerators are often encountered in cold-weather masonry construction. Most commercially-available masonry “antifreeze” admixtures are actually accelerators rather than freezing-point depressants. These may negatively impact masonry bond and compressive strengths if used in significant amounts. Calcium chloride is the most commonly used mortar accelerator. If used, it is recommended that it be limited to no more than 2% of the Portland cement in the mortar by weight.
For futher information regarding cold-weather masonry construction, contact us through our web site, www.CollinsArchitect.com .