Hot & Cold Weather Masonry Construction

Materials

General. Cold weather construction does not require drastic changes in mortar mixes or masonry units. Materials selected for normal temperatures will generally require little if any change during construction at temperatures below normal.

Masonry Units. Masonry units should conform to the appropriate ASTM Standard for the type specified.

All masonry units used during normal temperature masonry construction may be used during cold weather construction. Low absorption clay brick units (IRA of less than 5 or 6 g/min/30 in2) (5-6 g/min/194 cm2) remove less water from the mortar and may necessitate extending protection requirements for a longer period of time until mortar water content is sufficiently low to prevent expansion on freezing. Units with a higher IRA are advantageous in some instances because they remove more water from the mortar and reduce the possibility of damage by early freezing.

Concrete masonry units should not be wetted before use. Concrete masonry units expand when wet and shrink as they dry. Thus, placing wet concrete masonry units in a wall contributes to increased shrinkage.

Mortar. Requirements for mortar are contained ASTM C 270, Standard Specification for Mortar for Unit Masonry.

Mortar Selection and Preparation. The increased strength gain of high strength mortars and reduced water retentive properties may be desirable, particularly in combination with low absorption masonry units. The specifier may wish to consider use of a Type S mortar under such circumstances. ASTM C 270 indicates that mortar should be mixed in a mechanical mixer for between 3 and 5 minutes. During cold weather, avoid extended mixing times which can increase air contents. A 3-minute mix is adequate to achieve uniform dispersion of mortar ingredients within a batch.

Portland Cement. Requirements for portland cement are contained in ASTM C 150 for Type I , Type II, or Type III cement.

Type III, high early strength cement, may provide some benefit in cold weather construction due to its more rapid strength development. Its use can be considered in mortars made by mixing portland cement and lime, however it is not a substitute for other cold weather recommendations. Hydration of cement contained in mortar is greatly reduced when mortar temperatures are below 40°F(4.4°C) regardless of whether the cement used is Type I, II, or III.

Blended Cements. Requirements for blended cements are contained in ASTM C 595.

Although all of the types listed in ASTM C 270, except slag cement (Type S or SA), are considered to be acceptable during cold weather construction, the specifier should be aware that blended cements often have slower strength development characteristics than portland cement. Slag modified portland cement and pozzolan-modified portland cement will generally more closely approximate the strength development characteristics of portland cement.

Masonry Cement and Mortar Cement. Requirements for masonry cements and mortar cements are contained in ASTM C 91 and ASTM C 1329 respectively.

The Type designation should correspond to the mortar type specified.

Lime. Requirements for Type S hydrated lime are contained in ASTM C 207.

Lime increases the water retention of mortar. Lime putty prepared from slaked quicklime possesses a slightly greater water content than the dry hydrate and is not recommended for cold weather construction.

Aggregates. Requirements for sand for mortar are contained in ASTM C 144. Evenly graded sand provides optimum workability, water retentivity, and strength development. Over-sanding or under-sanding the mortar mix should be avoided. While increasing sand content may increase board life, over-sanded mixes will have poor water retention and strength development characteristics. Conversely, under-sanded mortars tend to exhibit poor board life. Avoid the use of sands containing a large fraction of fine particles (minus #50 mesh) that increase mortar water demand.

Water. Mix water used in mortar and grout should be potable. Heated water should be used to produce mortar and grout temperatures above 40°C (4.4°C).

Summary. Compliance with appropriate standards is necessary to ensure that desired properties of the materials are achieved. Prior to commencing construction, clay brick masonry units with suction rates greater than 5 to 6 grams of suction per minute per 30 square inches (194 square centimeters) should be selected, especially if prolonged periods of below normal temperature construction are predicted. Concrete masonry units should not be wetted prior to installation. Mortars should meet the requirements of ASTM C 270 for the Type designated. During cold weather, the use of Type III cement in portland cement-lime mortars mixed at the jobsite may be beneficial. The specifier should consider the use of Type S mortars rather than Type N mortars when low absorption units are used. Sand should not contain a high fraction of fine particles.

Admixtures. Admixtures are materials added during the initial mixing of mortar to modify one or more of the properties of the mortar in the plastic and/or hardened state. They should be used only when specified and only when their compatibility with other mortar ingredients and required mortar properties have been confirmed by laboratory tests. In cold weather masonry construction the admixtures most often encountered are anti-freezes and accelerators.

Anti-freeze. Most of the commercially available "antifreeze" mixtures for mortar are misidentified. They are accelerators rather than mortar freezing point depressants. Some actual antifreeze admixtures are available, which include several types of alcohol. If used in quantities that will significantly lower the freezing point of the mortar, the compressive and bond strengths of the masonry may decrease rapidly. Since antifreeze compounds have little benefit, they are not recommended.

Accelerators. The primary purpose of an accelerator is to hasten the hydration of the portland cement in the mortar. As such, they do not eliminate the need to protect mortar from freezing, but they may limit the length of time protection is required. The compounds commonly used as accelerators are: calcium chloride, soluble carbonates, silicates and fluosilicates, aluminous cements, calcium aluminate, and other organic compounds such as triethanolamine and calcium formate.

Calcium Chloride (CaCI,) is the most commonly used accelerator. It is also the main ingredient in most proprietary cold weather admixtures. Calcium chloride should not be classified as an antifreeze, because the amount required to significantly lower the freezing point of mortar would be so large as to have deleterious effects on mortar properties.

Calcium chloride is an effective accelerator, but may produce undesirable side effects, such as corrosion failures of joint reinforcement, door bucks, metal ties, and anchors in masonry. Also, excessive salts can contribute to efflorescence and may cause spalling of the masonry.

Calcium chloride should not be permitted in masonry containing metal (ties, anchors, and reinforcement). If calcium chloride is used, it is recommended that it be limited to amounts not to exceed 2% of the portland cement, and 1 % of the masonry cement, by weight.

Corrosion Inhibitors. Because accelerators are associated with corrosion failures of metal embedded in masonry, many commercial accelerators now contain corrosion inhibitors. These are usually found in proprietary compounds. Their effect on masonry and cold weather masonry construction has not been evaluated. For these reasons, the use of accelerators containing corrosion inhibitors cannot be recommended.

When accelerators are desired for use in masonry, use only non-chloride based accelerators, as certified by the admixture manufacturer.

Pigments. The possible effect of coloring agents on the performance of both fresh mortar and masonry should be considered. Mortar colors should meet the requirements for normal construction.

Some coloring agents contain dispersing agents to speed the distribution of the color throughout the mortar mixture. These dispersing agents may act as cement retarders and, consequently, affect the early strength gain of the masonry during cold weather masonry construction. During cold weather, this retarding effect may result in slow-setting, slow-strength-developing mortar, and masonry with a tendency toward efflorescence.

Air-Entraining Agents. Air-entraining admixtures are sometimes used in mortar to increase workability. Data suggest that laboratory air-entrained mortar specimens are less susceptible to deterioration due to freezing and thawing in the presence of moisture. However, excessive airentrainment in mortar will result in lower compressive and lower bond strength in masonry. Therefore, air-entraining admixtures should not be used in cold weather masonry construction. This recommendation does not preclude the use of air-entraining cementitious materials.

Summary. Accelerators, if specified and used, should be nonchloride based. Their use does not eliminate the need to implement recommended cold weather practices.

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