Masonry Magazine June 1989 Page. 20
through masonry walls. Figure 1 illustrates the migration of moisture through masonry walls above and below grade. It also indicates the positions where barriers should generally be located.
Above grade, moisture from inside of masonry structures will tend to migrate to the drier area outside. An exterior barrier designed to prevent penetration of rain should not interfere with the normal movement of moisture to the exterior of the wall or trouble may result. An example of this is the blistering of oil paint applied to an exterior masonry surface, with crazing of the masonry from freezing and thawing of the entrapped moisture.
Below grade, moisture migrates in the reverse direction. It moves from the damp soil to the drier area inside the building. Again, the surface through which the moisture escapes to the inside should be somewhat pervious to prevent the buildup of water pressure. A solid impervious barrier is desirable on the exterior wall surface to prevent the entry of moisture.
Waterproofing
Above Grade
The materials listed below are suitable for application to exteriors of above grade concrete masonry walls. They are effective in preventing the infiltration of rain, and afford a permeable surface which permits transpiration of entrapped moisture to the outside air.
Clear Sealers
Primer-Sealers
Latex Paints
Portland Cement Paints
Stucco
Clear coatings may be applied directly to concrete masonry walls without a filler or a primer, if the natural color and texture of the block are to be retained, and if water repellency is the only requirement. Since silicone based coatings function by reducing the absorbency and capillary action of the masonry surface, they do not assure complete waterproofing under conditions of high wind or continuous rainfall. Application of clear coatings is accomplished by flooding the surface by means of a low pressure spray head.
Primer-sealers, also called fill coats or fillers, contain portland cement, finely graded siliceous sand, and sometimes latex. Primer-sealer coats are brushed into the voids of concrete masonry to reduce the permeability. Primer-sealers not containing latex must be applied to dampened surfaces, and require moist curing for hydration of the portland cement content. Those primer-sealers containing latex retain sufficient moisture for hydration so that moist curing is not a requirement.
Latex paints are composed of resinous materials in water emulsions. They include styrene-butadiene, polyvinyl acetate, and acrylic types. Latex paints are ideally suited for application to concrete masonry because they may be used on damp or dry surfaces and do not require moist curing. They also have the advantage of drying as soon as the water of emulsion has evaporated, usually within 1½ hours. Although somewhat higher in cost than other types of latex, acrylic resins have superior resistance to rain penetration and show greater overall durability.
Portland cement paints generally contain from 65% to 80% portland cement by weight. Available in a variety of colors, they are mixed with water just before use. Portland cement paints can also be obtained with siliceous sand fillers. The paint sets by hydration of the cement content, which bonds to the masonry surface. Portland cement paints are applied to moist surfaces by stiff brush, and are moist cured following application. They have a long history of successful waterproofing when properly used.
Stucco, applied either by hand or by machine spraying, can be used to reduce the permeability of old and new concrete masonry walls. Good bond between the stucco and the masonry depends on mechanical key and suction. The texture of concrete masonry affords both good mechanical key and suction, thereby providing an excellent bonding surface. Typical stucco mixes are one part by volume of portland cement to from three to five parts of damp, loose sand. The variations in sand requirements are due to different gradations in sand mixes throughout the country. Trial mixes are required to assure the proper combinations. Hydrated lime may be added to the mix as a plasticizer, but the amount used should not exceed ten percent by weight or twenty five percent by volume of the portland cement content. Two or three coat applications are commonly used. The total thickness of the stucco application is approximately 5/8 inch thick.
Surface
Preparation
Concrete masonry surfaces scheduled to receive waterproof coatings should be clean and free of harmful substances in order to assure successful results. The masonry surface must not contain accumulations of dirt, oil, or efflorescence. Dirt and dust may be removed by scrubbing, hosing, brushing, or blowing with compressed air. Substances deeply ingrained in the masonry surface which may prevent adhesion of waterproof coatings can be removed by sandblasting or with a stiff bristle brush. Washing with solvent or highly alkaline solutions such as lye serves to remove grease and oil. Efflorescence may be removed from the surface by dry brushing or by washing with commercial wall cleaners.
Cracks in concrete masonry should be repaired before application of waterproof coatings, however, fine cracks may be left untouched for sealing with fillers. To prevent the recurrence of cracking, an analysis should be performed to determine if the use of control or isolation joints is warranted. Those cracks which are confined to the mortar joints can be repaired by tuckpointing. Cracks which cross the masonry units themselves, should be undercut to about 1/2 inch width at the surface and 1/2 to 3/4 inch width at a depth of 1/2 inch. After brushing away all loose mortar and dirt, the notch should be filled with a dry pack mortar. A stiff mix of one part portland cement to two parts of masonry sand may be used. The area around the crack should be dampened before packing to minimize absorption of water from the mortar.
Coatings for waterproofing should be mixed thoroughly as recommended by the manufacturer. They should be applied only when the temperature is within the limits specified by the manufacturer. Brushing is the usual means of applying fill coats and portland cement paints. Latex paints may be applied by brush, roller, or spray. Clear coatings are normally applied by flooding spray. Coverage must be