Masonry Magazine June 1968 Page. 25
Concrete masonry basement walls should be parged with portland cement plaster applied in two coats of ¼ in. thickness. The wall surface should be dampened before application of the first coat. The first coat should be roughened after it has partially set and then let harden for at least 24 hours before the second coat is applied. After dampening the first coat, the second coat is applied and kept moist for at least 48 hours following application. When curing is complete it is followed by two coats of bituminous waterproofing. A concrete drain tile embedded in gravel or stone fill at the outside base of the footing completes the construction (see NCMA TEK 1).
Selection of the type of coating to be used for a specific job should be based on an evaluation of information such as contained in Table 1. In some cases more than one type of coating will be found acceptable from a technical standpoint. Since the cost of the materials only approximates about 20% of the total cost, it is important to consider the labor involved in applying the coating to the surface. Here the method of application may be an influencing factor in selecting one coating in preference to another. A criterion for selection should be total cost per square foot per year of expected protection.
Coatings meeting the standards of those listed in the specifications suggested at the end of this publication will provide good service. For those not desiring to use technical specifications, the products of a reliable manufacturer, sold by a dealer in whom the buyer has confidence, and which are specifically recommended for the job, should give satisfactory results. Where possible, the performance of the selected material on other similar jobs should be observed as a guide.
The color range of waterproof coatings for concrete masonry walls varies from the clear transparent coatings to the completely opaque bituminous coatings. On jobs where a particular color effect is desired it is recommended that paint selection be determined by painting test patches on an inconspicuous section of the wall. This is important because the resultant color and texture will vary somewhat depending upon the kind of aggregate in the masonry, the rate and amount of absorption, the method of curing, and other factors. Clear coatings are desirable for waterproofing when the original texture and color are to be retained. In the case of opaque coatings, two-tone effects can be produced by applying first and second coats of different but harmonizing colors. A first coat, for example, could be light brown and the second eggshell. The second coat is painted lightly over the first, with care given to touch only the high spots and avoid brushing paint into the depressions. A mottled effect can be obtained by rubbing off part of the second coat. A variety of pleasing effects can be achieved through different color combinations depending upon the skill of the painter and the decorative effect desired.
Surface Conditioning
Regardless of the type of coating selected for use, success or failure can be dependent upon the adequacy of surface preparation. The masonry must be free of dirt, dust, grease, oil, and efflorescence. Dirt and dust may be removed by blowing with compressed air, scrubbing, hosing, or brushing, dependent upon the job conditions. Grease and oil may be washed off with solvent or strongly alkaline solutions such as lye. Thorough rinsing of the surface with water should follow lye treatment. Efflorescence is a white powdery material on the surface of masonry which is caused by deposit of salts from water moving from the interior of the masonry and evaporating on the surface. If water continues to enter the masonry after painting, efflorescence is likely to develop again and disrupt the paint film. The source of moisture should therefore be eliminated if possible. Most efflorescence can be removed by dry brushing or washing with a commercial masonry wall cleaner followed by thorough rinsing with water.
If oil base paint is to be employed, the alkalinity found in new concrete must be reduced because of its tendency to cause saponification (changing to soap) of the oils or alkyds in the paint. Seasoning or aging the masonry reduces the surface alkali through carbonation. Six months of aging is desirable. In lieu of aging, successful pretreatment has been achieved by painting the surface first with 3% phosphoric acid and then with 2% zinc chloride. The procedure, however, has become very rare due to the increased use of latex paints, and the good performance of portland cement base paints, both of which have caused a decline in the use of oil base paints.
If water-thinned paint such as latex is used, stains may develop on the painted surface when in contact with metals. Thus, iron or other metallic objects embedded in the masonry surface or adjacent to it should be covered with an anti-corrosive primer before being painted with water-thinned paint.
Preparation and Application
Thorough mixing or agitation of protective coatings in accordance with the manufacturer's instructions will help assure homogeneity of the material necessary for uniform application. Thinning should be done only with the recommended thinners as specified by the manufacturer. Discretion must also be exercised if tinting materials are added, to assure that the components of the added colors are compatible with the basic coating. Application of coatings should be accomplished when the temperature is within the range specified by the producer. The method of application selected should be consistent with the manufacturer's directions and should also be based upon the intended service of the coating. Brushing is often the most successful means of working the coating into irregularities which must be sealed for effective waterproofing. Sometimes more than one method of application will produce acceptable results if the work is well executed. The methods of usual application for various types of coatings as listed in Table I may be used as a guide. Coverage at the recommended rate is an important factor in obtaining a durable coating. No coating should be applied over another coating of a different type without the assurance that the two are compatible. Therefore, the manufacturer's recommendations relating to limitations on the use of the coating should strictly be followed.
FIGURE 1. FLOW OF MOISTURE IN MASONRY WALLS
INTERIOR
EXTERIOR
INTERIOR
EXTERIOR
MOISTURE FLOW
MOISTURE FLOW
IMPERMEABLE
FILM
PERMEABLE
FILM
PERMEABLE
FILM
IMPERMEABLE
FILM
MASONRY WALL ABOVE GRADE
MASONRY WALL BELOW GRADE