Masonry Magazine January 1970 Page. 38
Long as the sand is not scorched or contaminated. Mortar sand is often heated by placing it around a section of large diameter smokestack pipe or culvert in which a slow-burning fire is built. If steam is available, steam coil pipes or steam jets can be placed under the sand to heat it. Steam boilers are an economical source of heat for winter jobs. For heating materials, the following data may be useful:
(1) One boiler h.p. (33,500 BTU's per hr) will raise the temperature of 36 gallons of water about 100°F. in one hour.
(2) One boiler h.p. will raise the temperature of 1 ton of moist unfrozen sand about 65°F. in one hour.
(3) One boiler h.p. will raise the temperature of 1 ton of frozen sand about 35°F. in one hour.
Concrete masonry units may be heated by stacking around salamanders. Another method employed on large projects consists of using oil burning or electrical hot-air heaters combined with a blower (Fig. 1).
Protection
The problem of protection from the elements in cold-weather masonry construction is related to two phases. One is during the construction itself when both the masonry and the masons must be considered and the other is after the masonry is completed but not yet cured to adequate strength. Construction projects greatly differ in size, location, height, and many other factors. As a result, the most economical means of protecting and heating a particular project must be determined from a detailed study of the job itself. The contractor should therefore be given a wide choice in deciding the specific protective methods to be used. General requirements for protection of masonry during construction are listed in Table 4.
Temporary enclosures for concrete masonry construction range from simple tarpaulins to elaborate shelters (Fig. 2) containing entire buildings. Where weather is severely cold the majority of concrete masonry construction will require protection in the form of complete enclosure. A great advantage in enclosing all of the work is that it permits operations to be carried on without interruption. Factors bearing on the enclosure of choice involve the type of building and its design, scaffolding, masonry work under construction, and severity of the weather. Materials used for temporary enclosures include canvas, and reinforced polyethylene plastic sheeting available as clear or opaque material. Corrugated reusable fiberglass panels are also available. The use of clear plastics enables light to enter the construction area and permits solar radiation to contribute to warming the enclosed space. The temperature inside enclosures covered with polyethylene may be as much as 45 degrees above outside air temperatures during sunny weather, which may be all the heat required during the daytime. Materials for enclosures ideally possess the properties of high strength, durability, transparency, flexibility, fire resistance, and ease of installation. In some cases an area enclosure, constructed of 2 x 4-in. wood framing and plastic sheets, can be moved from place to place during a series of repeated building operations. Scaffolds can be enclosed with material (Fig. 3) or panels prefabricated with plastic coverings may be attached to steel scaffolding.
It is important that wind and snow load be considered in designing enclosures and that loads on enclosures not transmit excessive stresses to the building during construction. Precautions such as anchoring of scaffolds and enclosures must not be overlooked.
General requirements for protection of masonry after work is complete are listed in Table 5. It should be noted that the temperature which determines the protective requirements is the mean daily air temperature. The type of heating system selected will depend on the availability of equipment, size of the project, weather conditions, etc. Unvented heaters such as salamanders and direct fire heaters which give off products of combustion to the heated area may be satisfactory when a complete enclosure is not being used. Vented heaters such as oil- or gas-fired space heaters with electric blowers should be used for complete enclosures to assure that smoke and fumes are removed in order to protect both masons and masonry. Infrared lamps have been successfully employed in heating walls and are advantageous in not requiring attention during the period of use. Electric blankets for commercial use also have the same advantage. Whatever type of heating system is employed, care should be taken to distribute heat evenly to both sides of masonry walls since overheating a particular area may dry the mortar before hydration can occur. Hydration may also be adversely affected by reduction of relative humidity in the heated space as the temperature rises.
Heat requirements for enclosures may be estimated from the formula:
Heat Required Number of Cubic
(BTU's) Feet (in thous.) x
Degree Temp. Rise
x 140
Example: How much heat would be
necessary to maintain an enclosure
100' x 100' x 10' at 32°F. with an out-
door temperature of 2°F?
Heat Required Cu. Ft. in thous. x
Degree Temp. Rise
x 140
Heat
Required=100 x 30 x 140
Heat Required=420,000 BTU
Special Techniques
Mortar for cold-weather construction should be mixed in smaller quantities than usual to avoid excessive cooling before being placed. Metal mortar boards with built in electrical heaters may be employed but care must be used to avoid overheating or drying the mortar. Excessive cooling of heated masonry units may be avoided by not stockpiling on the scaffolding too far in advance of use. Only dry concrete units should be used.
Mortar should never be placed on snow- or ice-covered base, since bond will not be developed and because of danger of movement upon thawing. It is a good practice to heat the surface of existing masonry to the same temperature of the masonry units to be added. Heat should be sustained long enough to thoroughly dry the existing masonry units. When work is stopped for any length of time the top of the wall should be protected to prevent rain and snow from entering the cores of the concrete masonry blocks.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. BOX 9185, ROSSLYN STATION, ARLINGTON, VIRGINIA 22209