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November 15, 2004 7:53 AM CST

Turning Up the Heat on Masonry Construction

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The arrival of cold temperatures is no reason to stop masonry construction. By taking some extra precautions, mason contractors can continue to build masonry projects during cold weather and avoid seasonal delays, making the best use of all their resources, particularly labor. This article provides an overview of accepted industry practices for effectively working with masonry in cold weather.1

The 'Low-down' on Low Temperatures
Temperatures that are comfortable for people are similar to those that are good for masonry construction. When weather turns colder, you protect yourself with additional clothing; likewise, you have to protect masonry that is under construction or newly completed. When you're building with masonry and ambient temperature falls below 40?F (4.4?C), cold weather construction practices apply.

Photos courtesy of Jeep
Cold weather affects masonry materials in various ways. First, cold temperatures slow reaction rates between cement and water, leading to delays in tooling and setting times, and reduce strength gain rates. Also, cold masonry units can lower the temperature of mortar placed in contact with those units. If the units are cold enough, mortar temperature may drop below freezing. As water in the mortar freezes, disruptive expansion of the mortar occurs. Finally, wet or ice-covered unit surfaces prevent development of a good bond between the mortar and unit.

Cold weather also affects your laborers and may hinder the productivity and workmanship of masons. During cold weather, masons must first ensure their personal comfort and safety, then attend to normal construction tasks and any additional materials preparation, handling and protection of masonry. Unfortunately, these extra activities consume more time as temperatures continue to drop.

A good cold weather construction plan eliminates or minimizes the undesirable effects of cold temperatures on materials and people in a cost-effective manner.

Table 1. Strategies for Addressing Cold Temperatures during Masonry Construction
As temperature drops Strategies to Integrate into Your Plan:
. Optimize the selection of masonry materials for cold weather performance.
Protect materials.
Heat materials (see Tables 2 and 3).
Protect or enclose work areas.
Heat work areas and in-place work.

Change Materials with the Change in Weather?
You might modify your masonry materials for cold weather construction, but you probably won't switch materials. The masonry units themselves are typically selected on the basis of appearance or strength, not because they are suited to cold weather construction. Mortar type is also generally determined by structural or other performance criteria. The best strategy is to know how mortar and unit properties interact in cold weather, enabling the mason contractor to modify construction procedures to accommodate the specified materials.

Another variable that can be controlled is the water content of mortar. Typically, water content is established by masons or other crewmembers in the field, which permits adjustments to suit the ambient temperature. Keep in mind that mortar has water-retentive properties that affect its rate of moisture loss and stiffening. Also, mortars having high lime content or fine sands tend to have higher water demands and higher water retentivity than higher strength mortars or mortars made using well-graded sands. Although air entrainment increases water retentivity, it also reduces initial water demand required to achieve a workable consistency and has been shown to reduce susceptibility of mortar to damage by early freezing. The use of higher fineness cements (such as ASTM C 150 Type III) or accelerators increases reaction rates. These materials can be used in mortar to augment ? but not substitute for ? other cold weather construction practices.

Admixtures: Truth and Fiction
Admixtures should be used cautiously as part of a cold weather masonry program. While accelerators are sometimes useful in cold weather, they do not eliminate the need for other cold weather construction practices. Be very careful: Unless project specifications call for the use of an accelerator, the mason must request permission from the specifier in order to use one. ASTM C1384 provides criteria for evaluating admixtures, including accelerators, for use in masonry mortars.

Accelerators are sometimes mistakenly called "antifreeze" admixtures. Their function is not to reduce the freezing point of mortar, but to increase the rates of early-age strength development. Thus, they don't eliminate the need to protect mortar from freezing, but may limit the amount of time that protection is required.

Calcium chloride ? at a limit of 2% by mass of cement ? is commonly used in concrete as an accelerator, but its use in mortar is prohibited by the Specification for Masonry Structures because it contributes to corrosion of embedded metal such as wall ties, anchors and joint reinforcement.1 Only non-chloride based accelerators, as verified by the admixture manufacturer to meet ASTM C1384, should be allowed.

Protection, Storage and Heating
All masonry materials should be protected from rain, snow and ice. Masonry units and packaged mortar materials should be securely wrapped with canvas or polyethylene tarpaulins and stored above the reach of moisture migrating from the ground. Sand piles should also be covered and care taken to avoid contamination of the sand with mud and clay.

Masonry materials may need to be heated prior to use to assure cement hydration in mortar. At temperatures of less than 40?F (4.4?C), cement hydration necessary for strength development is minimal. At temperatures of 120?F (48.9?C) or higher, flash set is imminent. This means that freshly mixed mortar should be in the range of 40?F to 120?F (4.4?C to 48.9?C) and kept above freezing until used in masonry. (Note: For grout, if ambient temperatures are falling below freezing, a minimum temperature of 70?F (21.1?C) is recommended at the time of placement.) See Tables 2 and 3 for specific recommendations.

Table 2. Cold Weather Practices for Masonry Under Construction
When Ambient Temperature is:
. 32?F to 40?F (0?C to 4.4?C) 25?F to 32?F (-3.9?C to 0?C) 20?F to 25?F (-6.7?C to -3.9?C) 20?F (-6.7?C) and below
Mortar Heat sand or water
. Heat sand AND water AND keep mortar above freezing until it is used
. . ... In addition, heat masonry surfaces under construction to 40?F (4.4?C) or higher and use wind breaks or enclosures when wind exceeds 15 mph (24 km/h)
. . . ... In addition, provide an enclosure and auxiliary heat to maintain air above 32?F (0?C) within enclosure
Grout Heat materials ONLY if below 32?F (0?C)
. ... In addition, heat grout aggregates or water to produce grout between 70?F and 120?F (21.1?C and 48.9?C)
. . ... In addition, heat masonry to a minimum of 40?F (4.4?C) prior to grouting
Source: Specification for Masonry Structures (ACI 530.1-02/ASCE 6-02/-95TMS 602-02)

Table 3. Heating Masonry Materials
Material Notes Temperature
Water
  • Water can be heated in barrels or tubs.
  • It is the easiest material to heat and can store much more heat (per unit mass) than the other materials used in mortar.
  • To avoid flash set, heated water should be combined with cold sand in the mixer before adding cement.
  • Heat water to a maximum of 140?F (60?C)
    Sand
  • Sand is typically used in a damp loose condition.
  • It may be necessary to heat sand to thaw frozen lumps when temperatures fall below freezing.
  • Heat with electric heating pads, over a heated pipe, or by using steam heating systems.
  • Heat sand to 50?F (10?C) and higher as needed
    Masonry units
  • Masonry units should not have any visible ice on bedding surfaces when used, and the units can't be below 20?F (-6.7?C).
  • Heat on pallets in an enclosure or stored in a heated area.
  • The units should be kept dry, although very high absorption fired-clay brick may need to be wetted ? but not saturated ? prior to use.
  • Heat units to at least 40?F (4.4?C)

    Protecting Work Areas and Construction
    Wind breaks, heated wall coverings, and plain or heated enclosures are used to maintain adequate mortar temperatures and to improve the comfort and efficiency of masons and laborers. If the need arises, entire buildings can be enclosed with scaffolding and tarps. The level of protection required will depend on the severity of weather encountered as defined by anticipated mean daily temperatures.

    Reference 1 includes provisions needed during the workday while masonry is being laid, as well as protection requirements for newly constructed masonry. Mason contractors can choose from a variety of means to accomplish this. The main point is that masonry construction need not be disrupted by the arrival of colder weather.

    In Summary
    Cold weather adds a few challenges for mason contractors. While materials won't be changed, modifying mortars may help prevent negative impacts of cold temperatures ? keeping in mind that admixtures have to be used very cautiously, if at all. The greater the temperature drop, the more strategies you have to employ to protect newly constructed masonry or masonry under construction.

    References

    1. Specification for Masonry Structures (ACI 530.1-02/ASCE 6-02/-95TMS 602-02)
    2. Cold Weather Masonry Construction, PCA IS248
    3. Hot and Cold Weather Masonry Construction, PCA LT232 (published by the Masonry Industry Council)
    4. Standard Specification for Admixtures for Masonry Mortars, ASTM C 1384


    About the Author

    Jamie Farny is the Program Manager of Masonry and Special Products for the Portland Cement Association, coordinating research and promotion activities regarding cements for masonry and white cement. He participates on committees on concrete, plastering, mortars, and masonry units of the American Society for Testing and Materials and the American Concrete Institute. He holds a B.S. in Civil Engineering from the Illinois Institute of Technology.

     

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