Technical Notes 1 - All-Weather Construction
March 1992

Abstract: This Technical Notes describes how extremes of cold and hot weather can influence brick masonry construction. Information on weather prediction necessary for construction planning is provided. Cold and hot weather are defined, and the reaction of clay brick masonry materials to these extreme conditions is described. Recommendations are provided for continuing construction in these severe exposure conditions.

Key Words: absorption, brick, climatology, cold weather, evaporation, freezing, grout, hot weather, meteorology, mortar.

INTRODUCTION

Periods of cold and hot weather have a tremendous impact on the construction industry and the national economy. This is reflected in several ways. Cold weather can cause temporary delays and work stoppages on construction sites. Productivity and the quality of construction on job sites may be reduced if workers become too attentive to personal comfort during extremes in temperature. Proper material protection and handling can increase construction costs, although contractors and owners alike may benefit in the long run. Completed work not properly constructed during, or protected from, cold and sometimes hot weather may have to be removed and rebuilt. Investigations to evaluate the performance of suspect construction are an added expense which may be necessary. Owners and businessmen can suffer from lost rentals and business revenue when buildings are not completed on time. Furthermore, the seasonal influence on construction results in idle production facilities, large material inventories and high rates of unemployment during the winter months. Stopping work on a project due to extremes in weather conditions is not economically desirable.

The purpose of this Technical Notes isto describe how masonry materials react to cold and hot weather conditions. It also describes provisions which should be made to ensure that construction does not decrease in quality and can continue without interruption. Although "normal", "cold", and "hot" are relative terms, normal, used in this Technical Notes, will be considered to be any temperature between 40 ⁰F and 90 ⁰F (4 ⁰C and 32 ⁰C). Cold will be considered to be any temperature below 40 ⁰F (4 ⁰C), and hot any temperature above 90 ⁰F (32 ⁰C).

WEATHER PREDICTION

To successfully build during periods of abnormal weather conditions, designers and contractors must have advance knowledge of local meteorological conditions as well as knowledge of historic climatological information for a given area. Meteorology may be defined as current state atmospheric conditions, while climatology may be defined as the historic record of the averages and extremes of weather representative of an area. When in the planning stages for a project, designers are usually concerned with climatological data such as the average and extreme daytime and nighttime temperatures or average wind velocity for use in designing mechanical or structural systems. Contractors, however, are more concerned with meteorological conditions during construction, such as hourly temperatures and mean daily temperature, as well as the predicted temperatures and wind velocities for the next few days. Mean daily temperature is determined by adding together the maximum temperature for each day (24 hours, midnight to midnight) and the minimum temperature for the same day and dividing by two. Ambient temperature as used in this Technical Notes is the outdoor temperature at the time considered.

Meteorological information can be obtained from the National Weather Service, a branch of the National Oceanographic and Atmospheric Administration (NOAA). The National Weather Service has information centers located at major airports in cities throughout the country. These centers provide current weather information and regularly scheduled weather forecasts for the region under consideration. Climatological information can be obtained from the National Climatic Data Center, also a branch of NOAA. The National Climatic Data Center usually provides climatic information in the form of maps as shown in Figure 1. These maps contain daily, monthly and annual data for a region and may be obtained for a nominal fee by contacting the Center [5].

Examples of Climatic Data Available
FIG. 1a

Examples of Climatic Data Available
FIG. 1b

EFFECTS OF COLD WEATHER

Cold weather during masonry construction affects the materials and labor used. Successful construction will consider both in the planning, scheduling and set up of the masonry work. In addition to anticipating the specific weather conditions, the contractor must determine what the probable effects of the weather will be on the materials and the workers, how to protect materials and workers, how to store the materials, and what procedures should be used to meet the requirements specified in the construction documents.

In the United States, all model building codes have requirements relating to the construction of masonry during cold weather. While not identical, each of the building codes have similar general requirements regarding material protection, heating of materials, use of frozen materials and protection of completed work.

Masonry Units

Masonry units are the material in masonry construction least affected by below-normal temperatures. The physical properties of masonry units are essentially the same in cold weather except that a cold unit will have a slightly smaller volume than one at normal temperatures. However, the absorption characteristics of the masonry unit and its temperature contribute to the rate of freezing of masonry during cold weather. Under normal conditions of construction, using masonry units with initial rates of absorption (IRA) less than or equal to 30 g/min/30 in.² (30 g/min/194 cm²) at the time of laying improves the bond between the brick and mortar which leads to increased moisture resistance of the wall assembly. In cold weather, brick having an initial rate of absorption of 25 g to 30 g/min/30 in.²(25 g to 30 g/min/194 cm²) may be desirable.

Using brick with a higher IRA reduces the risk of freezing by more rapidly absorbing water from the mortar or grout. If a brick with a low IRA is used, then the water content of the mortar should be the minimum necessary for workability. If suction or other measures reduce the water content to less than 6 percent of the total mortar volume prior to freezing, the mortar will not experience disruptive expansive forces upon freezing. Further, significant reductions in transverse or compressive strength of the masonry assemblage will not occur.

The temperature of the masonry unit also contributes to the rate of freezing of masonry. A cold unit will more rapidly withdraw the heat of hydration from the mortar and thus increase the rate of freezing. Masonry units preheated prior to laying minimize cold weather effects on the hydration process of the mortar by maintaining the heat within the mortar. Masonry units should be heated to a temperature of approximately 40 ⁰F (4 ⁰C) prior to laying when ambient temperatures are below 20 ⁰F ( -7 ⁰C). Heating units to temperatures above 40 ⁰F (4 ⁰C) is seldom necessary. It may be advantageous to heat units even when ambient temperatures are above 20 ⁰F ( -7 ⁰C). Preheated units will exhibit the same absorption characteristics as units laid during normal weather conditions.

Units which are frozen should be thawed and dried completely before use. Frozen masonry should not be built upon. Completed masonry which is frozen may be moistened after thawing to reactivate the hydration process and continue to develop strength [7,10].

Mortar

Mortar mixed with cold materials have properties quite different from those at normal temperatures. Cold weather retards the hydration of the cement in the mortar mix. Mortar mixed during cold weather often has lower water content, increased air content, and reduced early strength compared with those mixed during normal temperatures. For these reasons, mortar is often mixed with heated materials to produce performance characteristics associated with mortar mixed at normal temperatures, or with admixtures which may improve the early strength and plasticity of the mix. Water, sand, or both may be heated for use in mortar. Heating prepackaged materials such as portland cement and hydrated lime can be difficult. Specific recommendations are a function of temperature and are found in later sections of this Technical Notes.

Mortar materials and the proportion of ingredients, within the permissible ranges, can also be modified for cold weather conditions. A higher sand content provides a stiffer mortar which will better support the weight of subsequently laid masonry. A lower lime content will allow the water content of the mortar to decrease more rapidly, just as a brick with a higher IRA. High-early-strength (Type III) portland cement may be used to increase the rate of early strength gain. Admixtures, although not recommended, may be used to accelerate the rate of set.

Freezing of the mortar should be avoided in all cases. Mortar which freezes is not as weather-resistant or as watertight as mortar that has not been frozen [6]. Furthermore, significant reductions in compressive and bond strength may occur. Mortar having a water content over 6 to 8 percent of the total volume will experience disruptive expansive forces if frozen due to the increase in volume of water when it is converted to ice. Thus, the bond between the unit and the mortar may be damaged or destroyed. Mortar in newly completed masonry should be protected from freezing. Specific requirements are found in Table 1.

Grout, although made from similar materials, should not be confused with concrete. Typically smaller aggregate is used in grout for easier placement and consolidation. Concrete uses a minimum amount of water, whereas the water-cement ratio for grout is high, because grout is placed in absorptive molds of brick. Furthermore, high water content is necessary in grout for ease of flow, but it greatly increases the amount of volumetric expansion which can occur upon freezing. Thus grout, like mortar, should be mixed with heated materials to prevent the damaging effects of freezing. High-early-strength (Type III) portland cement may be used to increase the rate of early strength gain of the grout. Admixtures may also be used, but protection of the grouted masonry is still required.

MATERIALS IN COLD WEATHER CONSTRUCTION

Protection

Although the temperature of the materials used in masonry construction is one of the factors which should be adjusted for cold weather construction, adjustments in construction practices may also be necessary. ACI 530.1/ ASCE 6/TMS 602 , Specifications for Masonry Structures, addresses material heating as well as requirements for protection of masonry constructed in cold weather [3]. Protection is one of the most necessary adjustments to make in construction practices. Construction materials should be carefully covered to remain dry. ACI 530.1/ASCE 6/TMS 602 requires protection such as the use of insulating blankets and forced air heaters. However, protection may also include special light-weight, warm work clothes worn by laborers or standard construction equipment adapted to unique cold weather protection uses. This approach is common in northern Europe where cold weather may last up to six months.

All masonry materials should be kept dry and free from ice and snow by covering with tarpaulins or clear polyethylene sheets. Sand and masonry units should be covered and stored on raised platforms to avoid contact with the ground. Careless material storage increases the cost of laying masonry because removal of ice and snow and thawing of masonry units are necessary before construction may begin. Partially completed or exposed walls should be covered at the end of each day's work with a weighted tarpaulin which extends a minimum of 2 ft (1 m) down each side of the wall to prevent contamination by water, ice, or snow (Fig. 2).

Brick Noise Barrier Wall with Cold Weather Protection
FIG. 2

Workers should also be protected from the cold weather to maintain their productivity. Recommended protection will vary with weather conditions from warmer clothes to complete enclosure of the work site. Masons may work in the open with forced air heaters as a heat source at mean daily temperatures no less than 20 ⁰F ( -7 ⁰C). Heated enclosures should be provided at temperatures below 20 ⁰F ( -7 ⁰C). By providing wind breaks or temporary shelters, workers can remain productive at outside temperatures well below freezing. If a shelter or enclosure is used both the workers and the materials benefit from a warmer environment. The masons' comfort and productivity are improved, and the materials need less preparation prior to laying (i.e. heating).

There are many types of equipment which are available as sources of heat for cold weather construction. The type selected will depend upon availability of equipment, fuel source and economics, size of project and severity of exposure. Salamanders are widely used as a source of heat on scaffolds. Commercial electric blankets may be used to cover walls during the curing period. When complete enclosure of the work area is provided, space heaters are recommended. The enclosure should allow circulation of warm air on both sides of the masonry wall.

Contractors have used several different methods for complete and partial enclosures of buildings. Large tents, temporary wood structures covered with clear plastic, and shelters built of prefabricated panels covered with clear plastic sheets are examples of complete enclosures (Figs. 3 and 4). Partial enclosures often consist of enclosed swinging scaffolds which may be moved from floor to floor when necessary (Figs. 5 and 6).

Enclosure in Place
FIG. 3

Interior of Enclosure
FIG. 4

Scaffold Enclosure
FIG. 5

Tubular Scaffold Enclosure
FIG. 6

Mortar and Grout Admixtures

Accelerators. Accelerators are admixtures used to speed the setting time of mortar and grout. By increasing the rate of hydration of the cement, accelerators increase the rate of early strength gain. The most common accelerators are inorganic salts such as calcium chloride, calcium nitrate, soluble carbonates and some organic compounds. Any accelerator should be evaluated for deleterious effects on masonry strength and materials. Admixtures must not contribute to staining or efflorescence or cause corrosion of metal accessories used in construction of the masonry. Indiscriminate use of accelerators can adversely affect the in-place performance of the completed masonry. Accelerators alone are not suggested treatment for cold weather construction problems. Mortar and grout containing accelerators must still be protected from freezing.

Calcium chloride, while highly effective as an accelerator and widely used in the past, causes corrosion of metals used in masonry due to the chloride content. For this reason, chlorides should not be used in mortar or grout in contact with metals (i.e. ties, anchors and reinforcement). Also, the incidence of efflorescence may be increased when excessive salts are present. If a chloride accelerator is used, it is recommended that it be limited to amounts not to exceed two percent of the weight of Portland used in the mortar mix or one percent of the weight of masonry cement.

Calcium nitrite and calcium nitrate are inorganic nonchloride compounds also used as accelerators. These compounds require higher dosage by weight and are more costly than calcium chloride, but will not corrode metals or contribute to efflorescence.

Antifreeze. An antifreeze lowers the freezing point of the substance to which it is added. Most commercial mortar "antifreeze" admixtures do not do this, but are instead accelerators. However, some true antifreeze admixtures are available. These admixtures are alcohols or combinations of salts. If used in the quantities required to be effective, significant reductions in mortar compressive and bond strengths usually result. For this reason, use of antifreeze compounds is not recommended.

Heating

In freezing weather, ice may be present in mixing water and moisture in the sand may turn to ice. Ice in the mixing water must be melted before it can be added to the mixer. Sand which contains frozen particles or frost cannot be used. It must first be thawed by heating in an appropriate manner. Further heating may also be beneficial.

As stated earlier, both water and sand used in the mortar and grout may be heated to provide proper temperatures for construction. Water is the easiest method to heat. It is also the best material to heat because of its high specific heat. Sand may also be heated. This may be done by placing an electric heating pad on top of the sandpile and covering with a weather-resistant tarpaulin (Fig. 7). The electric pad can safely heat the sand overnight without exceeding a temperature of 100 ⁰F (38 ⁰C). A more labor intensive method of heating the sand is to place the sand over a heated pipe or to pile the sand around a horizontal metal culvert or smoke stack section, in which a slow fire is built (Fig. 8). Other methods for heating sand involve the use of a steam lance or other steam heaters. Careful attention to the fire or other heat source and the sand is required. Sand should be heated slowly to avoid scorching.

Heating of a Sandpile with an Electric Blanket
FIG. 7

Heating of a Sandpile with a Metal Pipe
FIG. 8

In an alternate approach, an electric rod can be used to heat mixing water and sand simultaneously. The electric heating rod is placed in a drum of water in the center of a sandpile. The rod heats the water over several hours. The sand surrounding the drum slowly absorbs heat from the drum and insulates the drum from further heat losses.

Materials heated for use in mortar should have a minimum temperature of 70 ⁰F (21 ⁰C) and a maximum temperature of 160 ⁰F (71 ⁰C) to avoid flash set. Scorched sand (with a reddish cast) must not be used in mortar.

In cold weather, mortar should be mixed in smaller amounts so it can be used before it cools. In any case, mortar must be used within 2 1/2 hours from the time of initial mixing. After combining all ingredients, the temperature of the mortar should be between 40 ⁰Fand 120 ⁰F(4 ⁰C to 49 ⁰C). Mortar temperatures over 120 ⁰F (49 ⁰C) may lead to flash set, resulting in lower compressive strength and reduced bond strength. Once a mortar temperature is selected, steps should be taken to mix successive batches to the same temperature. Mortar may be placed on electrically heated mortar boards to help maintain proper temperature. However, use caution to avoid excessive drying of the mortar with the heater.

Grout should be placed at a minimum temperature of 40 ⁰F(4 ⁰C) and a maximum temperature of 120 ⁰F(49 ⁰C) within 1 1/2 hours of mixing. As with mortar, water or aggregate may be heated to produce a heated mixture. Water temperature should not exceed 160 ⁰F (71 ⁰C). The sand may be heated following recommendations for heating sand used in mortar. Masonry receiving grout should have a minimum temperature of 40 ⁰F(4 ⁰C).

COLD WEATHER CONSTRUCTION RECOMMENDATIONS

Special Precautions

There are two reasons why masonry should never be placed on a snow or ice-covered base or bed. There is danger of movement when the base thaws, and bond cannot be developed between the mortar bed and frozen supporting surfaces.

If the walls are properly covered when work is halted, ice or snow removal from walls should not be necessary. However, in the event that the covering is displaced, the top course may be thawed with steam or a portable blowtorch, carefully applied. The heat should be sustained long enough to thoroughly dry out the masonry. If portions of the masonry are frozen or damaged, defective parts should be replaced before progressing with new work.

General Requirements-Cold Weather

The following items are suggested in addition to the construction and protection requirements for cold weather masonry construction found in Table 1. These items can be incorporated in the specifications of the project where applicable.

1. Protect masonry units, cementitious materials and sand so that they are not contaminated by rain, snow or ground water.

2. Cover tops of masonry at all times when work is not in progress. Cover shall extend a minimum of 2 ft (1 m) down the masonry, and shall be securely held in place.

3. Units with higher initial rates of absorption (up to 40 g/min/30 in.² (40 g/min/194 cm²)) may be used to resist mortar freezing. However, units with suctions in excess of 30 g/min/30 in.² (30 g/min/194 cm²) shall be sprinkled, but not saturated, with heated water just prior to laying. Water temperature shall be above 70 ⁰F(21 ⁰C) when units are above 32 0F (0 ⁰C). If units are 32 0F (0 ⁰C) or below, water temperature shall be above 120 ⁰F (49 ⁰C).

4. Use a mortar with a higher sand content and a lower water retention, especially with brick units having a low IRA. If Type III portland cement is used, the protection period listed in Table 1 may be reduced from 48 to 24 hours.

5. Heat sand and water used in mortar and grout mixtures to a minimum temperature of 70 ⁰F(21 ⁰C) and a maximum temperature of 160 0F (71 ⁰C). Keep mortar temperature less than 120 ⁰F(49 ⁰C) to avoid flash set.

6. Maintain temperature of masonry units above 20 ⁰F (-7 ⁰C)when laid.

7. Place grout at a minimum temperature of 40 ⁰F(4 ⁰C) and a maximum temperature of 120 ⁰F(49 ⁰C). Maintain masonry receiving grout above 40 ⁰F (4 ⁰C). Maintain grouted masonry above 32 ⁰F (0 ⁰C) for 48 hours following placement of grout.

EFFECTS OF HOT WEATHER

Periods of hot weather may also adversely affect the construction of masonry. The contractor must take measures to ensure that the quality of masonry construction does not suffer from high temperatures. While hot weather has been defined to be temperatures above 90 ⁰F (32 ⁰C), temperature, wind speed, relative humidity and solar radiation all influence the absorption of masonry units, the rate of set, and the drying rate of mortar. The primary concern in controlling these properties in hot weather is evaporation of water from the mortar. If sufficient water is not present, bond between the brick and mortar will be sacrificed.

The effects of high temperature and high humidity are not as damaging to the performance of the masonry as are low temperatures and low humidity. The increased rate of hydration of the cement and favorable curing conditions in hot, humid weather will help develop masonry strength if sufficient water is present at the time of construction.

Temperature of the materials may be the easiest factor to adjust to produce performance characteristics associated with construction at normal temperatures. Adjustments in construction practices further aid the construction of quality masonry in hot weather conditions. ACI 530.1/ASCE 6/TMS 602 specifies construction methods to produce quality masonry in hot weather conditions.

Masonry Units

Masonry units are the material in masonry construction least affected by hot weather. However, the interaction between the masonry units and the mortar or grout is critical. Warmer units will absorb more water from the mortar. In hot weather conditions this is usually not a problem unless high suction brick are used (IRA over 30 g/min/30 in.² (30 g/min/194 cm²)). If high suction brick are used, they should be properly wetted prior to laying. Wetting may take place immediately before laying the units, but the preferred method is to wet the whole pallet 3 to 24 hours before use. The brick must be surface dry at the time of laying and should have an IRA less than 30 g/min/30 in.²(30 g/min/194 cm²). Lower bond strength results if not enough water is present in the mortar when the units are laid. Thus, lower absorption units may be desirable because they allow more complete hydration of the mortar.

Mortar

Mortar in hot weather will tend to lose its plasticity rapidly due to evaporation of the water from the mix and the increased rate of hydration of the cement. The use of admixtures to increase plasticity is not recommended unless their full effect on the mortar is known. Mortar with a high lime content and high water retention should be used. Retempering of the mortar should be permitted. Mortar mixed at high temperatures often has higher water content, lower air content, and a shorter board life than those mixed at normal temperatures. Temperature of the mortar should be maintained between 70 ⁰F and 120 ⁰F (21 ⁰C and 49 ⁰C). Temperatures above 120 ⁰F (49 ⁰C) may cause flash set of the cement. Cold water may be used to help control the temperature of the mortar. Ice is highly effective in reducing the temperature of the mix water. When used, ice should be completely melted before combining the water with any other ingredients. In any case, mortar should be used within two hours of initial mixing.

Grout

Grout reacts to hot weather in a manner similar to mortar. Water more easily evaporates and thereby reduces the water-cement ratio. Grout requires a high slump, at least 8 in. (203 mm), for placement into the absorptive brick molds. Therefore, a high water-cement ratio should be maintained by reducing evaporation and initially mixing grout with adequate water. Furthermore, ACI 530.1/ASCE 6/TMS 602 specifies grout shall be used within 1 1/2 hours of mixing. As with mortar, ice may be used to lower the mix water temperature.

HOT WEATHER CONSTRUCTION RECOMMENDATIONS

Special Precautions

During periods of hot weather the temperature of the materials should be controlled for best results. Storing brick and sand under cover of shade will help control heat gain of the materials. Sand should be stored on a raised platform and not in contact with a cover during the hot part of the day. This prevents ground moisture from rising, then condensing on the cover after temperatures cool down, thus contaminating the materials. When possible, shade should also be provided for laborers, whose productivity decreases with increasing temperature and humidity. Starting work earlier in the day and scheduling masonry construction to avoid the hot, mid-day periods can reduce the effects of high temperatures on laborers and materials.

Adjusting masonry construction practices may effectively control hot weather problems. ACI 530.1/ASCE 6/TMS 602 limits the length that mortar may be spread to 4 ft (1.2 m) and requires masonry units to be placed within one minute of spreading the mortar. Wind breaks may prevent rapid drying of mortar during and after placement, and covering walls with a weather resistant membrane at the end of the work day will prevent rapid loss of moisture from the masonry assemblage. Wet curing or fog spraying may further improve masonry strength development during periods of high temperatures and low relative humidity.

General Requirements-Hot Weather

The following items are suggested in addition to the construction and protection requirements for hot weather masonry construction found in Table 2. These items can be incorporated in the specifications of the project where applicable.

1. Maintain temperature of mortar and grout between 70 ⁰F and 120 ⁰F (21 ⁰C and 49 ⁰C).

2. Cold water may be used when mixing mortar and grout. Ice used to lower the mix water temperature must be completely melted before adding the water to the other ingredients.

3. Masonry units with high suctions (IRA over 30 g/min/30 in.² (30 g/min/194 cm²)) should be properly wetted prior to use. Units with lower rates of absorption may be desirable.

4. Mortar with a high water retention is desirable.

5. Limit the spread of mortar beds to 4 ft (1.2 m) when temperatures are 100⁰F (38 ⁰C) or above, or 90 ⁰F (32 ⁰C) with a 8 mph (3.6 m/s) wind.

6. Place masonry units within one minute of spreading mortar.

7. Partially completed walls may be fog sprayed at the end of the work day to control moisture evaporation.

This Technical Notes describes how masonry materials react to extremes in weather conditions. Construction requirements and protection requirements are recommended for construction in both cold and hot weather to ensure that construction can continue without a decrease in quality. Performance characteristics associated with materials mixed and constructed during normal temperatures can be achieved by following the appropriate construction and protection recommendations addressed in this Technical Notes. Tables 1 and 2 summarize these recommendations for cold and hot weather construction.

The information and suggestions contained in this Technical Notes are based on the available data and the experience of the engineering staff of the Brick Institute of America. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of brick masonry. Final decisions on the use of the information contained in this Technical Notes are not within the purview of the Brick Institute of America and must rest with the project architect, engineer and owner.

REFERENCES