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Characteristics of Concrete Masonry Units with Integral Water Repellent

CMHA TEK 19-07

A concrete masonry unit’s characteristics are a function of the properties and proportions of the materials used, as well as the manufacturing processes. The unit characteristics do not singularly define the characteristics and performance attributes of a concrete masonry wall, but they certainly play a significant role in influencing those attributes. When used as part of a breathable exterior wall for an inhabited structure, or as a barrier for any conditioned or protected space, concrete masonry is expected to contribute to the water penetration resistance and moisture control of the wall assembly. Current model building codes include provisions intended to ensure that exterior walls provide adequate weather protection for the building (ref. 1).

Design of concrete masonry walls to mitigate or control moisture migration includes many considerations beyond the characteristics of the concrete masonry unit, such as flashing, weeps, workmanship, mortar or grout characteristics, vents, coatings, vapor barriers, air barriers, temperature differences, and accommodation of differential movement, plumbing and roof leaks, as well as other considerations. The potential for condensation, whether at the wall’s interior surface, weather-exposed surface, and/or interior of the wall, should also be considered. Proper design and construction of concrete masonry, considering all of these elements, is critical to the water resistant performance of the wall system. These topics are addressed in References 2 through 7 and in other literature sources.

Mortar joints are especially critical to a wall’s water penetration resistance. Achieving good bond between the mortar and the unit surfaces is essential and is largely influenced by the mortar material itself, tooling procedures, and joint profile as well as by the configuration of the concrete masonry unit. Ribbed units, for example, make it difficult to adequately tool the mortar joints. Reducing mortar’s absorption characteristic is also important for achieving success in moisture control in a concrete masonry wall. This can be achieved using integral water repellent admixtures in the preparation of the mortar.

While all of the aforementioned aspects significantly affect wall performance, this TEK focuses specifically on evaluating the water penetration resistance characteristics of concrete masonry units and their role in contributing to control of moisture in the wall.

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