Masonry Magazine June 1989 Page. 21
accomplished at the recommended rate to assure a long lasting, effective coating. Before a coating is applied over another of a different type, it should be ascertained that the two are chemically compatible.
Waterproofing
Below Grade
Where subsoil conditions do not cause water to build up hydrostatic pressure, simply parging the exterior of concrete masonry walls below grade may be all that is required for adequate waterproofing. A portland cement and sand mix (1:3½ by volume), or Type M mortar may be used. The completed parging should be at least ½ inch thick. It should be trowelled to a smooth, dense surface extending from the footing to six inches above the finished grade. It should be beveled at the top to form a wash, and thickened at the bottom to form a cove between the base of the wall and the footing. If the parging is applied in two coats, the first coat should be roughened when partially set, hardened for 24 hours, and then moistened before the second coat is applied. The parging should be moist cured for at least 48 hours before backfilling. A bituminous coating may also be applied to the parging to increase the resistance to water penetration. Coal tar or asphalt based bitumens are available in solvent for hot application, or in emulsions for application at ambient temperatures. (Figure 2) Bituminous coatings may be applied in conjunction with felts or fabric to form a built-up membrane suitable for resisting water under hydrostatic pressure. Some authorities consider asphaltic and coal tar products to be incompatible with each other. As a precaution, the built-up membrane should therefore be constructed of either all coal tar products or all asphaltic products. Cotton, felt, asbestos felt, woven glass fabric, glass fibre mat, and woven burlap are available as saturated bituminous ply material. In construction of built-up membranes, the masonry surface is first coated with a bituminous primer. The surface is next mopped with hot bitumen or cold bituminous emulsion. A layer of fabric is then rolled or broomed into the bitumen. The fabric is normally overlapped a minimum of 12 inches at the sides and ends. The process is repeated until from two to six plies of felt or fabric are applied. The number of plies required depends upon the hydrostatic head to be resisted. When the membrane has been built up to the required number of plies, it is given a final coat of bitumen. Insulation board, or premolded bituminous membrane is then placed over the completed work to prevent damage during the backfilling process.
In place of using bituminous materials to build up a waterproof membrane, an elastomeric sheet system may be utilized. Elastomeric materials include butyl, neoprene, and ethelyne propylene diene monomers (E.P.D.M.). They are available in sheets sized up to 20 ft. x 100 ft. The largest sized sheet that can conveniently be handled should be selected, to minimize the field joining of sections. Cement recommended by the manufacturer is used to secure the sheets to the surface on which placed. Sheets are overlapped and spliced at joints with splicing tape.
Waterproofing of below grade walls should be applied to the exterior of the wall surface. The advantage of providing a seal against future cracking, as afforded by built-up membranes, cannot be assured by waterproofing the interior wall surface. In some cases, such as in existing structures, access to the exterior surface