Cementitious coating bonding is critical to successful in-place performance. Therefore, extreme care should be taken in preparing substrates for coating application. Sample applications for bond strength should be completed if there is any question regarding the acceptability of a substrate, especially with remedial waterproofing applications.
Poured-in-place or precast concrete surfaces should be free of all honeycombs, voids, and fins. All tie holes should be filled before coating application with nonshrink grout material as recommended by the coating manufacturer. Although concrete does not need to be cured before cementitious coating application, it should be set beyond the green stage of curing. This timing occurs within 24 hours after initial concrete placement.
With smooth concrete finishes, such as precast, surfaces may need to be primed with a bonding agent. In some instances a mild acid etching can be desirable, using a muriatic acid solution and properly rinsing substrates before the coating application. Some manufacturers require a further roughing of smooth finishes, such as sandblasting, for adequate bonding.
On masonry surfaces, voids in mortar joints should be filled before coating installation.
With both masonry and concrete substrates, existing cracks should be filled with a dry mix of cementitious material sponged into cracks. Larger cracks should be sawn out, usually to a 3 4 in minimum, and packed with nonshrink material as recommended by the coating manufacturer.
Moving joints must be detailed using sealants designed to perform under the expected movement. These joints include thermal movement and differential movement joints. The cementitious material should not be applied over these joints as it will crack and “alligator” when movement occurs.
If cracks are experiencing active water infiltration, this pressure must be relieved before coating is applied. Relief holes should be drilled in a substrate, preferably at the base of the wall, to allow wicking of water, thus relieving pressure in the remainder of work areas during coating application. After application and proper curing time (approximately 48–72 hours), drainage holes may then be packed with a nonshrink hydraulic cement material and finished with the cementitious coating.
After substrate preparations are completed and just before application, substrates must be wetted or dampened with clean water for adequate bonding of the coating. Substrates must be kept continually damp in preparation for application. The amounts of water used are dependent on weather and substrate conditions. For example in hot, dry weather, substrates require frequent wettings. Coatings should not be applied in temperatures below 40°F or in conditions when the temperature is expected to fall below freezing within 24 hours after application.
Cementitious coatings should be carefully mixed following the manufacturer’s recommended guidelines concerning water ratios. Bonding agents should be added as required with no other additives or extenders, such as sand, used unless specifically approved by the manufacturer. With smooth surfaces such as precast concrete, an additional bonding agent is required.
Cementitious coatings may be applied by brush, trowel, or spray. Stiff, coarse, or fiber brushes are used for application. Brush applications require that the material be scrubbed into a substrate, filling all pores and voids. Finish is completed by brushing in one direction for uniformity.
Spray applications are possible by using equipment designed to move the material once mixed. Competent mechanics trained in the use of spray equipment and technique help ensure acceptable finishes and watertightness (Fig. 3.11).
|FIGURE 3.11 Spray application of cementitious membrane on negative side.|
With textured masonry units such as split face or fluted block, additional material is required for effective waterproofing. On this type of finish, spraying or brush applications are the only feasible and effective means.
The amount of material required depends upon the expected water conditions. Under normal waterproofing requirements, the first coat is applied at a rate of 2 pounds of material per square yard of work area. The finish coat is then applied at a coverage rate of 1 lb/yd2. In severe water conditions, such as below-grade usage with water-head pressures, materials are applied at 2 lb/yd^2. This is followed by a trowel application at 2 lb/yd2. Clean silica sand is added to the second application at 25 lb of silica to one bag, 50 lb, of premixed cementitious coating.
With all applications, the second material coat should be applied within 24 hours after applying the first coat. Using these application rates, under normal conditions, a 50-pound bag of coating will cover approximately 150 ft 2 (1 lb/yd2 , first coat; 2 lb/yd2 , second coat). The finish thickness of this application is approximately 1 /8 in.
Trying to achieve this thickness in one application, or adding excessive material thickness in one application, should not be attempted. Improper bonding will result, and material can become loose and spall. To eliminate mortar joint shadowing on a masonry wall being visible through the coating, a light trowel coat application should be applied first, followed by a regular trowel application.
The cementitious coating beginning to roll or pull off a substrate is usually indicative of the substrate being too dry; redampening with clean water before proceeding is necessary. Mix proportions must be kept constant and uniform, or uneven coloring or shadowing of the substrate will occur.
After cementitious coatings are applied they should be cured according to the manufacturer’s recommendations. Typically, this requires keeping areas damp for 1–3 days. In extremely hot weather, more frequent and longer cure times are necessary to prevent cracking of the coating. The water cure should not be done too soon after application, as it may ruin or harm the coating finish. Chemical curing agents should not be used or added to the mix unless specifically approved by the coating manufacturer.
Typically, primers are not required for cementitious coating applications, but bonding agents are usually added during mixing. In some cases, if substrates are especially smooth or previous coatings have been removed, a direct application of the bonding agent to substrate surfaces is used as a primer. If there is any question regarding bonding strength, samples should first be applied both with and without a bonding agent and tested before proceeding with the complete application.
Cementitious coatings should not be applied in areas where thermal, structural, or differential movement will occur. Coatings will crack and fail if applied over sealant in control or expansion joints. Cementitious-based products should not be applied over substrates other than masonry substrates such as wood, metal, or plastics,