Monday, March 7, 2011

Cold Weather Masonry

Planning and preparation are the keys to successful construction of masonry in cold weather. While some changes in procedures, equipment, and supplies are required, extension of the construction season into cold weather avoids seasonal construction delays and permits better utilization of a mason contractor's resources, particularly manpower. With careful planning and implementation of an effective cold weather construction program, successful masonry construction can proceed despite cold weather. The Specification for Masonry Structures (ACI-530. 1-95/ ASCE 6-95/TMS 602-95)considers cold weather construction to exist when ambient temperature falls below 40' F (4.5' C). As the temperature of mortar materials falls below normal:

• Water requirement to reach a given consistency is reduced.
• A given amount of air-entraining agent yields more entrained air.
• Initial and final set of the mortar is significantly delayed.
• Heat-liberating reaction rates between portland cement and water are substantially reduced and become minimal as mortar temperatures drop below 40' F (4.5' C).
• Strength gain rates are reduced.

Cold masonry units lower the temperature of mortar placed in contact with those units. As noted above, this will slow reaction rates between cement and water, reduce strength gain rates, and delay tooling and setting times. If the units are cold enough, the temperature of the mortar may rapidly drop below freezing and result in disruptive expansion of the mortar as water in the mortar freezes. Wet or ice covered unit surfaces prevent development of good bond between mortar and unit.In addition to affecting the performance of masonry materials, cold weather may also affect the productivity and workmanship of masons.During cold weather, in addition to attending to normal construction tasks, masons are concerned with personal comfort and safety, additional materials preparation, handling, and protection of masonry. These extra activities consume more time as temperatures continue to drop.
The goal of a cold weather construction plan is to eliminate or minimize the undesirable effects of cold weather on materials and people in a cost-effective manner. Strategies for accomplishing that goal can include optimizing the selection of masonry materials for cold weather performance, protecting materials, heating materials, protecting or enclosing work areas, or heating work areas and in place work. Combinations of these strategies may be required depending on the severity of weather. Selection of techniques to implement these strategies is usually the responsibility of the mason contractor, who must evaluate the effectiveness and practicality of techniques in the context of the specific project and weather conditions encountered .

Masonry Materials
Selection: Masonry units are typically selected on the basis of aesthetic or structural properties rather than consideration of performance in cold weather construction. Mortar type is also often determined by structural or other performance criteria. However, knowledge of how mortar and unit properties interact in cold weather masonry construction enables the mason contractor to modify construction procedures to accommodate the specified materials. The initial water content of mortar required for workability is in the range of 11 to 16 percent. Mortar used to lay units stiffens as mixing water contained in the mortar is absorbed by units, evaporates, or reacts with the portland cement in the mortar. Water content of mortar needs to be below 6 percent to avoid disruptive expansion upon freezing. Units having high initial rates of absorption (suction) will accelerate stiffening by absorbing water from the mortar. Low absorption or wet units remove very little water from the mortar. The water retentive properties of a mortar also affect the rate of moisture loss and stiffening. 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. Air entrainment increases water retentivity, but 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 rate at which portland cement reacts with water is primarily influenced by the temperature of the mortar. The use of higher fineness cements, such as Type lll cement, or accelerators increases reaction rates. These materials can be used in mortar to augment, but not substitute for, other cold weather construction practices. 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 they may limit the amount of time that protection is required. Calcium chloride (at a limit of 2% by weight of cement) is commonly used in concrete as an accelerator, but its use in mortar is prohibited by the Specification for Masonry Structures (ACI 530.1-95/ASCE 6-95/TMS 602-95) because it con tributes to corrosion of embedded metal such as wall ties, anchors, and joint reinforcement. ASTM C270 indicates that admixtures are not to be used unless specified. Therefore, unless the project specifications call for the use of an accelerator, the mason must request permission from the specifier in order to use an accelerator. Only non-chloride based accelerators, as verified by the admixture manufacturer, 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. When placed, the mortar temperature should be in the range of 40' F (4.5' C) to 120' F (49' C). At temperatures of less than 40? F (4.5' C), cement hydration necessary for strength development is minimal. At temperatures of 120' F (49' C) or higher, flash set is imminent. If ambient temperatures are falling below freezing, a minimum mortar temperature of 70' F (21' C) is recommended. Water is often the first material to be heated for two reasons: it is the easiest material to heat and it can store much more heat, pound for pound, than the other materials used in mortar. Although recommendations vary as to the highest temperature to which water should be heated, Some specifiers put a maximum of 180' F (82' C) because higher temperatures pose a personnel safety hazard and could result in flash set. To avoid flash set, heated water should be combined with cold sand in the mixer before adding the cement. Sand is typically delivered to the project and used in a damp loose condition. Therefore, even though sand piles are covered, it may be necessary to heat sand to thaw frozen lumps when temperatures fall below freezing. Generally, sand is heated to about 50' F (10' C), although higher temperatures are permissible as long as the sand is not scorched and as long as resultant mortar temperatures do not exceed 120' F (49' C). Masonry units should not have any visible ice on bedding surfaces when used, nor should the temperature of masonry units be less than 20' F (- 6.5' C) to avoid rapid lowering of mortar temperatures. Better productivity is often attained using units having a minimum temperature of 40' F (4.5'C) . Masonry units should be kept dry, except that very high-absorption fired-clay brick may need to be wet ted, but not saturated, prior to use. Various techniques can be used to heat mortar materials. Water is often heated in barrels or tubs. Sand piles can be heated using electric heating pads, by placing sand over a heated pipe, or by using steam heating systems. Masonry units are usually heated on pallets in an enclosure or stored in a heated area.
Protecting Work Areas and Construction
Wind breaks, heated wall coverings, enclosures, or heated enclosures are used to maintain adequate mortar temperatures and to improve the comfort and efficiency of masons and laborers. The level of protection required will depend on the severity of weather encountered. The Specification for Masonry Structures (ACI 530. 1 -95/ASCE 6-95/-95TMS 602-95) defines certain cold weather construction requirements as summarized in Table 1. It includes provisions needed during the work day while masonry is being laid, as well as protection requirements for newly constructed masonry. Several means of implementing these provisions are available to the mason contractor, recognizing that regional climatic differences and project-specific factors must be taken into account when selecting the most effective methods of protection for a given project. Basic principles required for satisfactory cold weather masonry construction are well established as indicated in this publication and the referenced documents. The use of innovative construction and protection techniques based on these established principles can often improve the effectiveness and efficiency of a cold weather construction program.

Table 1 - Cold Weather Construction Requirements
Provisions for work in progress
Ambient temperature above 40° F (4.5° C)
Ambient temperature below 40° F (4.5° C) or temperature of units below 40° F (4.5° C).

Ambient temperature is between 25° F (-4° C) and 20° F (-7°C)

Ambient temperature is below 20° F (-7° C). Requirement
Normal construction practice. Cover stored materials.

Heat mortar materials to produce mortar temperatures between 40°F (4.5° C) and 120° F (49° C) at time of mixing. Maintain mortar above freezing until used in masonry. If units have a temperature below 20° F (-7° C), heat to above 20° F (-7° C). Remove visible ice from units.

Heat masonry under construction from both sides. Install wind breaks when wind velocities reach 15 mph (24 km/h).

Provide heated enclosure for masonry under construction and maintain temperature above 32° F (0° C) within that enclosure.
Protection of newly completed work

Mean daily temperature above 40° F (4.5° C)

Mean daily temperature between 40° F (4.5° C) and 25° F (-4° C)

Mean daily temperature between 25° F (4° C) and 20° F (-7° C)

Mean daily temperature below 20° F (-7° C) Requirement

Normal construction practice. Cover top of unfinished masonry work to protect it from weather.

Cover completed masonry with weather resistive membrane to protect from rain or snow for 24 hrs. after construction.

Cover masonry with insulating blankets or equivalent protection for 24 hrs. after construction.

Maintain temperature of masonry above 32° F (O° C) for 24 hrs. after construction.

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