Hot & Cold Weather Masonry Construction

Materials

General. Hot weather construction does not require drastic changes in masonry units, mortar, or grout mixes. Materials selected for normal temperatures will generally require little if any change during construction at temperatures above 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 hot weather construction. Absorption characteristics of units differ and may require adjustment of preparation and construction procedures. For example, clay brick units having a high initial rate of absorption (IRA) can contribute to rapid dryout of mortars in hot weather. Fired clay units that have an IRA over 30 g/min/30 in2 (30 g/min/194 cm2) should be wetted prior to use to reduce the absorption characteristics of the units when laid. Unit surfaces should not be dripping wet, since a saturated surface will inhibit development of bond between mortar and unit. A saturated, surface-dry condition is optimum.

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. A concrete masonry wall can be fog sprayed after the wall is constructed to assure the availability of adequate curing moisture.

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

Mortar Selection and Preparation. Water retention and board life are the most important properties of plastic mortar in hot weather. High strength mortars tend to exhibit reduced workability, board life, and water retentivity. The specifier should consider specifying Type N mortar in hot weather, provided it is structurally adequate for the given application. ASTM C 270 specifies that mortar should be mixed in a mechanical mixer for between three and five minutes. During hot weather, improved workability will be obtained with a 5 minute mix time. Since mortar will stiffen more rapidly in hot weather, it should be mixed with the maximum amount of water consistent with workability and retempered as needed to maintain that workability. ASTM C 270 indicates that mortar can be retempered and used during the first 2-1/2 hours after mixing. During hot weather it is advisable to limit the time period to 2 hours or less.

Portland Cement. Requirements for portland cement are contained in ASTM C150 for Type I or Type II cement.

Type III, high early strength cement, is not recommended for hot weather construction. Its early strength developing characteristics may further increase the rate of stiffening of the plastic mortar in hot weather.

Blended Cements. Requirements for blended cements are contained in ASTM C 595. All of the Types listed in ASTM C 270 are considered to be acceptable for use during hot weather construction.

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 putty prepared from slaked quicklime is also acceptable, but seldom used due to the convenience of packaged dry hydrated lime. Lime increases the water retention of mortar.

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, undersanded mortars tend to exhibit poor board life.

Water. Mix water used in mortar and grout should be potable. Only cool water should be used during periods of above-normal temperatures. When ice is used to cool the water, it must be completely melted before mixing with other materials.

Summary. Compliance with appropriate standards is necessary to ensure that desired properties of the materials are achieved. Avoid specifying high strength mortars when not structurally required. Wetting high-absorption fired clay units prior to use can reduce the water loss of mortars to these units. Concrete masonry units should not be wetted prior to installation, although concrete masonry walls may be fog sprayed after construction. Only cool water should be used in mortar and grout mixes.

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.

Retarders. In hot weather construction, the admixtures most often considered for use are retarders. Retarders delay the set time of the mortar, but they do not reduce evaporation rates. Proper curing is still needed to develop required hardened mortar properties. Therefore, the use of retarders is not a substitute for recommended hot weather construction practices.

Pigments. Pigments are considered to have little impact on hot weather performance of mortars provided they meet the requirements for normal construction.

Air Entraining Agents. The addition of air entraining admixtures is not recommended due to difficulty maintaining field control of the addition rate and subsequent mortar air content. This does not preclude the use of airentraining cementitious materials.

Summary. Admixtures should not be included in masonry mortars unless they are laboratory tested at the temperature extremes presumably requiring their use. Further, their use should produce the desired effect while used in the construction. Specifiers should be aware of the side effects, such as corrosion and retardation, of the admixture containing mortar before selecting any admixture.

Grout. Requirements for grout are contained in ASTM C 476 Standard Specification for Grout for Masonry. Slump should be maintained as high as possible for proper placement.

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