Resources

Water Repellents for Concrete Masonry Walls

August 2018

INTRODUCTION

Water repellents are used on exterior walls to provide resistance to wind-driven rain. In addition, water repellents can also reduce the potential for efflorescence and staining from environmental pollutants, and enhance the color or texture of a wall.

When applied in accordance with manufacturer’s recommendations, water repellents effectively control water penetration. Water repellents are generally recommended for use on single wythe concrete masonry walls exposed to the weather. The choice of water repellent will depend on the surface to be protected, the exposure conditions, and on aesthetics. A wide variety of water repellents is available, offering many choices of color, surface texture, glossiness, and application procedures.

WATER RESISTANCE

Water penetration resistance of concrete masonry walls is dependent on wall design, design for differential movement, workmanship, wall maintenance, and the application of water repellents. This TEK focuses on water repellent products for above grade walls. The other factors are discussed in CMUTEC-009-23, TEKs 19-04A and 19-05A (refs 3, 5, and 4).

The effectiveness of water repellents can be evaluated in several ways. In the laboratory, Standard Test Method for Water Penetration and Leakage Through Masonry, ASTM E 514 (ref. 9), is currently the only standard test method for water penetration. The test simulates 51/2 in. (140 mm) of rain per hour with a 62.5 mph (101 km/h) wind for a duration of 4 hours. This test is often used to evaluate water penetration before and after application of a water repellent, or to judge the relative performance of several water repellent systems.

TYPES OF WATER REPELLENTS

There are two general types of water repellents: surface treatment repellents and integral water repellents. Surface treatment repellents are applied to the weather-exposed side of the wall after the wall is constructed. In addition to water repellency, surface treatment repellents also improve the stain resistance of the wall, by preventing dirt and soot from penetrating the surface, causing deep stains.

When used on new construction, choose water repellents that are able to resist the alkalinity of the fresh mortar. As an alternative, an alkali-resistant fill coat can be applied to the wall first, or the wall can be allowed to weather for about six months until the alkalinity is reduced.

In general, surface treatment repellents should allow for vapor transmission to ensure that interior humidity within the wall and structure can escape. Treatments which are impermeable to water vapor tend to fail by blistering and peeling when moisture builds up behind the exterior surface.

When choosing a surface treatment repellent, manufacturer’s guidelines should be consulted regarding appropriate substrates and applications for a particular product.

Regardless of the type of surface treatment chosen, it should be applied to a sample panel or on an inconspicuous part of the building to determine the appearance, application method, application rate, and compatibility with the masonry surface. Surface treatment repellents will require reapplication after a period of years to ensure continuous water repellency.

Integral water repellents are added to the masonry materials before the wall is constructed. The water repellent admixture is incorporated into the concrete mix at the block plant. This way, each block has water repellent throughout the concrete in the unit. For mortar, the water repellent is added to the mix on the jobsite. It is critical when using integral water repellents that the repellent is incorporated into both the block and the mortar to ensure proper performance of the wall.

The following sections describe in more detail the characteristics of various generic surface treatment repellents and integral water repellents.

SURFACE TREATMENT REPELLENTS

Cementitious coatings:

Coatings such as stucco or surface bonding mortar can be used to increase the water resistance of a wall, as well as to significantly change the texture of the finished wall surface. Consideration should be given to differential movement which may transmit stress into the coating. Further information on stucco is found in TEK 09-03A (ref. 8).

Paints:

Paints are colored opaque coatings, used when color uniformity of the wall is important for aesthetic reasons. Paints are a mixture of pigment, which hides the surface, and resin, which binds the pigment together. The proportion of pigment to resin, and the type of resin will affect the fluidity, gloss, and durability of the paint.

The pigment volume concentration (PVC) compares the amount of pigment in a paint to the amount of binder. As the PVC increases, the paint has more pigment and less binder. High PVC coatings are used where limited penetration is desired, such as for fill coats on porous materials. High PVC paints generally brush on easier, have greater hiding power, and usually cost less than low PVC paints. Low PVC paints are generally more flexible, durable, washable, and are glossier.

Fill Coats:

Fill coats, also called primer-sealers or fillers, are sometimes used to smooth out surface irregularities or fill small voids before application of a finish coat. Common fill coats include latex coatings and portland cement. In addition, acrylic latex or polyvinyl acetate is sometimes combined with portland cement for use as a fill coat. Fill coats should be scrubbed vigorously into the masonry surface using a relatively short stiff fiber brush.

Cement-Based Paints:

Cement-based paints contain portland cement as the binder, which creates a strong bond to the masonry and is not subject to deterioration from alkalis. Cement-based paints effectively fill small voids so that large amounts of water are repelled. Durability is excellent.

Cement-based paints are sold either premixed, or in dry form and mixed with water just before use. They should be applied to a damp surface using a stiff brush, and kept damp for 48 to 72 hours, until the cement cures. If the cement-based paint is modified with latex, however, wet curing is not necessary. White and light colors tend to be the most satisfactory.

Latex Paints:

Latex paints are water-based, with any one of several binder types. They are inherently resistant to alkalis, have good hiding characteristics, and are durable and breathable, making them a good choice for concrete masonry walls. Butadiene-styrene paints and polyvinyl acetate emulsion paint are both categorized as latex paints. Latex paints can be applied to either damp or dry surfaces, and dry quickly, usually within 1 to 1 ½ hours. They are generally inexpensive and easy to apply by brush, roller, or spray.

Alkyd Paints:

Alkyd paints are durable, flexible, have good gloss retention, are low in cost, but have low alkali resistance. They should be sprayed on, since they tend to be difficult to brush apply. They dry quickly once applied.

Clear Surface Treatment Repellents:

Clear treatments are used to add water resistance to a wall without altering the appearance. These treatments are classified by the resin type, such as silicone or acrylic.

Clear treatments can be classified as either films or penetrant repellents. Penetrant repellents are absorbed into the face of the masonry, lining the pores. They adhere by forming a chemical bond with the masonry. Penetrant repellents do not bridge cracks or voids, so these should be repaired prior to applying the treatment. Silanes and siloxanes are penetrant repellents. Films, such as acrylics, form a continuous surface over the masonry, bridging very small cracks and voids. Because of this, films can also reduce the vapor transmission of a concrete masonry wall. Films tend to add a glossier finish to the wall surface, and may intensify the substrate color.

Silicones: Silicones can be further subdivided into silicone resins, silanes, and siloxanes. These treatments change the contact angle between the water and the pores in the face of the masonry, so that the masonry repels water rather than absorbing it. Silicones have been found to reduce the occurrence of efflorescence on concrete masonry walls.

Silicone resins: These are the most widely used silicone-based water repellents for masonry. They can penetrate the surface of masonry very easily, providing excellent water repellency. Silicone resins should be applied to air dry surfaces, and are usually fully dry after 4 to 5 hours.

Silanes: Like silicone resins, silanes have good penetration characteristics. Although volatility of silane has been a concern, the absorption of silane by masonry generally occurs at a much faster rate than evaporation of the silane. Silanes, unlike silicone resins, can be applied to slightly damp surfaces.

Siloxanes: Siloxanes have the benefits of silanes, i.e., good penetration and ability for application on damp surfaces. Siloxanes are effective on a wider variety of surfaces than silanes, and dry relatively quickly. Costs are comparable to silanes, and are slightly higher than silicone resins.

Acrylics: Acrylics form an elastic film over the surface of masonry to provide an effective barrier to water. Acrylics dry quickly and have excellent chalk resistance. Acrylics should be applied to air-dry masonry surfaces. Costs tend to comparable to silicone resins.

OTHER TREATMENTS

Epoxy, Rubber, and Oil-Based Paints:

These paints form impervious moisture barriers on concrete masonry surfaces. This makes for an excellent water barrier, but does not allow the wall to breathe. As such, these paints are generally not considered water repellents. These treatments are better limited to interior walls, since they can blister and peel when used on exterior walls.

Oil-based paints adhere well to masonry, but are not particularly resistant to alkalis, abrasion, or chemicals. Rubber and epoxy paints offer high resistance to chemicals and corrosive gases, and are generally used in industrial applications.

APPLICATION OF SURFACE TREATMENT REPELLENTS

This section contains some general guidelines for application of surface treatments. In all cases, refer to manufacturers’ literature for final recommendations and procedures. Surface treatments should typically be applied to clean, dry walls. Wall surfaces should be cleaned in accordance with manufacturer’s instructions to ensure good adhesion and penetration. The wall should be allowed to dry for 3 to 5 days between cleaning or rain and application of the repellent. All cracks and large voids should be repaired prior to applying the repellent. If caulk is used in the repair, the caulk should be compatible with the surface treatment repellent and fully cured before treatment application.

Weather can have a significant effect on the application and curing of water repellents. It is usually recommended that the repellent be applied when temperatures are expected to remain above 40°F (4 °C) during the two to four days after application. There should be little or no wind during sprayon applications, to avoid an uneven coating and drift of the treatment onto other materials. Adjacent landscaping should be protected during application, and, depending on the surface treatment, it may also be necessary to protect other building materials, such as aluminum or glass.

Most manufacturers recommend applying clear surface treatments using a saturating flood coat, with a 6 to 8 in. (152 to 203 mm) rundown below the contact point of the spray. It is sometimes recommended that a second coat be applied when the first is still wet. Coverage rates vary from 75 to 200 ft²/gallon (1841 to 4908 m²/m³) depending on the surface treatment repellent used and the type and condition of the masonry.

When applying a water repellent over a previously treated wall, ensure that the new treatment is compatible with the old. With some surface treatments, masonry should be uncoated for proper adhesion. In these cases, the old treatment can be allowed to weather off, or, if time does not permit this, a pressurized wash followed by high pressure water rinse can remove previous surface treatments from masonry.

The durability of a coating is a function of the type of coating, the application procedure, the rate of application, the surface preparation, and the exposure conditions. For this reason, it is difficult to predict how the various surface treatment repellents will perform under field conditions.

INTEGRAL WATER REPELLENTS

Integral water repellents are usually polymeric products incorporated into the masonry products prior to construction. Because integral water repellents are evenly distributed throughout the wall, they do not change the finished appearance. In addition, integral water repellents are effective at reducing efflorescence, since water migration throughout the block is reduced.

As stated earlier, it is essential that an integral water repellent admixture be incorporated into the mortar at the jobsite, as well as into the block and any other masonry wall components, such as precast lintels. The same brand of water repellent admixture should be used in the mortar as was used in the block, to ensure compatibility and bond.

Questions often arise regarding the effect of integral water repellents on mortar bond strength, due to the decreased water absorption. Research has shown that bond strength is primarily influenced by the mechanical interlock of mortar to the small voids in the block.

When walls containing integral water repellents are grouted, the grout produces a hydrostatic pressure which forces water into the surrounding masonry unit, allowing proper curing of the grout.

Generally, the use of other admixtures in conjunction with integral water repellents is not recommended. Some other admixtures, especially accelerators, have been shown to reduce the effectiveness of integral water repellents.

Some integral water repellents are soluble when immersed in water for long periods of time. Conditions which allow standing water on any part of the wall should be avoided. For this reason, mortar joints should be tooled, rather than raked. In addition, walls incorporating integral water repellents should not be cleaned with a high-pressure water wash.

REFERENCES

Clark, E. J., Campbell, P. G., and Frohnsdorff, G., Waterproofing Materials for Masonry. National Bureau of Standards Technical Note 883. U. S. Department of Commerce, 1975.
Clear Water Repellents for Above Grade Masonry, Sealant, Waterproofing, and Restoration Institute, 1990.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Flashing Strategies for Concrete Masonry Walls, TEK 1904A, Concrete Masonry & Hardscapes Association, 2008.
Flashing Details for Concrete Masonry Walls, TEK 19-05A, Concrete Masonry & Hardscapes Association, 2008.
Fornoville, L., Water Repellent Treatment of Masonry, Proceedings of the Fourth Canadian Masonry Symposium, University of New Brunswick, Canada, 1986.
McGettigan, E., Application Mechanisms of Silane Waterproofers, Concrete International, October 1990.
Plaster and Stucco For Concrete Masonry, TEK 09-03A. Concrete Masonry & Hardscapes Association, 2002.
Standard Test Method for Water Penetration and Leakage Through Masonry, ASTM E 514-05a. ASTM International, 2005.

Plaster and Stucco for Concrete Masonry

August 2018

INTRODUCTION

Portland cement-based plaster has many useful applications: as a moisture resistant coating for concrete masonry walls; as an interior wall finish in residential and commercial structures; and as an exterior architectural treatment for buildings of all types.

The terms cement plaster and cement stucco are used interchangeably. They both describe a combination of cement and aggregate mixed with a suitable amount of water to form a plastic mixture that will adhere to a surface and preserve the texture imposed on it.

When freshly mixed, plaster is a pliable, easily workable material. It can be applied either by hand or machine in two or three coats, although two-coat applications are more typical when plaster is applied to newly constructed concrete masonry.

While plaster may be used as an interior or exterior finish for most building materials, some type of metal reinforcement or mechanical keying system is usually required to effectively attach the plaster to the substrate. Concrete masonry, however, provides an excellent base for plaster without the need for reinforcement. Since block is manufactured of the same cementitious material as that in the plaster, the two have a natural affinity.

MATERIALS

Of primary importance to the performance of the finished surface is the selection and use of proper materials. Each must be evaluated on its ability to provide serviceability, durability, and satisfactory appearance. Standard Specification for Application of Portland Cement-Based Plaster, ASTM C 926 (ref. 3) includes specifications for materials for use in plaster

Cement

Cement should comply to one of the following product specifications:

Blended hydraulic cement —ASTM C 595 (ref. 4)
Types IP, IP(M), IS, IS(M), and their air-entrained
counterparts IP-A, IP(M)-A, IS-A, IS(M)-A
Masonry cement—ASTM C 91 (ref. 5) Types M, S, N
Portland cement—ASTM C 150 (ref. 6)
Types I, II, III, and their air-entrained counterparts IA, IIA,
IIIA
Plastic cement—UBC 25-1 (ref. 1)
White portland cement—ASTM C 150 (ref. 6) Types I, IA,
III, IIIA

Aggregates

Aggregates used in plaster should conform to the chemical and physical requirements of ASTM C 897, Standard Specification for Aggregate For Job-Mixed Portland Cement Plasters (ref. 2), except as noted below. Recommendations for gradation of the sand to be used in the base coat are listed in Table 1.

Aggregates used for finish coats need not comply with the gradation requirements of ASTM C 897. Various sizes and shapes can be evaluated with test panels to obtain special textures or finishes. As a starting point, all aggregates for finish-coat plaster should be below a No. 16 sieve and uniformly graded. Uniform gradation produces plaster that is easier to apply. If necessary, larger aggregate may be added to obtain the desired appearance.

MIXTURES

Properly proportioned mixtures can be recognized by their workability, ease of application, adhesiveness to the base, and resistance to sagging.

The combinations of cementitious materials and aggregates shown in Table 2 have proven to provide satisfactory performance. These proportions are recommended for first and second coat applications.

Considerations in selecting the plaster mix include suction of the masonry, its surface irregularities, climate extremes, extent of surface exposure, and method of application. For economy and simplicity, it is better to select the same plaster type for both scratch (first) and brown coat (second coat in a three coat application) applications, adjusting the proportions for the brown coat to allow for a larger aggregate to cement ratio.

The finish coat can be varied in appearance by changing the size and shape of the aggregate, by adding color, by changing the consistency of the finish mix, and by the application method. For the finish coat, a factory prepared mixture may be used or the finish coat may be proportioned and mixed at the jobsite. Job-mixed finish coat plaster will provide a truer color and more pleasing appearance if white portland cement is used in conjunction with a fine-graded, light colored sand. Recommendations for job mixed finish coat proportions are listed in Table 3.

The success of plastering depends on proper batching and mixing of the individual and combined materials. Water is placed in the mixer first, after which half of the sand is added. Next the cement and any admixtures are added. Finally, the balance of the sand is added and mixing is continued until the batch is uniform and of the proper consistency, which usually takes 3 or 4 minutes.

Although batching by shovelfuls remains the most commonly used method in the field, shovelful batching should be checked daily by volume measures to establish both the required number of shovelfuls of each ingredient and the volume of mortar in the mixer when a batch is properly proportioned. Water additions should also be batched using containers of known volume. Proper mixing should result in a uniform blend of all materials.

PLASTER APPLICATION

Open textured concrete masonry units, laid with flush (nontooled) joints, should be specified on walls intended to be plastered. The open texture promotes a good mechanical bond between the plaster and the masonry. New concrete masonry walls should be properly aligned and free from any surface contamination, such as mortar droppings or sand. It is important that the wall be properly cured and carrying almost all of its design dead load before the plaster is applied. Existing masonry walls should be inspected for alignment, and any coatings or surface treatments other than portland cement paint be should removed by sandblasting prior to plastering.

Plaster may be applied by hand or machine in two or three coats in accordance with the thicknesses given in Table 4. Two-coat application is most often used when plaster is applied directly to concrete masonry, and for horizontal (overhead) plaster application.

The scratch coat can be applied either from the bottom to the top of the work area, or from top to bottom. The plaster must be applied with sufficient force to fully adhere it to the masonry. Excessive troweling or movement of the scratch coat must be avoided, because too much action will break the bond between the plaster and masonry. The applied plaster must be brought to the required thickness and the surface made plumb. The thickness is established by the use of screeds and grounds. A rod or straightedge is used to even the surface when the area between the screeds and grounds is filled with plaster. The rod can bear on the screeds or contact the grounds and be moved over the surface, cutting off high spots and showing up the hollow spaces, which must be filled and rodded again.

Scratch-coat plasters are scored or scratched to promote mechanical bond when the brown coat is applied. The scratch coat should be scored in a horizontal direction; shallow scratching is adequate.

Brown-coat plasters are applied, rodded, and floated to even the surface, provide a uniform suction throughout the basecoat plaster, and provide a desirable surface for the finish coat.

The brown coat is applied in sufficient thickness to bring the surface to the proper plane. A few minutes after the plaster has been applied, the surface is rodded to the desired plane. The plaster thickness is properly gaged with plaster screeds or wood slats of proper thickness as the guides. After rodding, the surface is floated to give it the correct surface texture.

Floating of the brown coat is the most important part of plastering. Floating must be done only after the plaster has lost sufficient moisture so that the surface sheen has disappeared but before the plaster has become so rigid that it cannot be moved under the float. This interval is critical, since the degree of consolidation that occurs during floating influences the shrinkage-cracking characteristics of the plaster.

The full thickness of the base coats should be applied as rapidly as the two coats can be put in place. The second coat should be applied as soon as the first coat is sufficiently rigid to resist the pressures of second-coat application without cracking. Under certain conditions this may mean applying both first and second coats in a single day. The short delay, or even no delay, between the first and second coats promotes more intimate contact between them and more complete curing of the base coat. No stoppage of plaster should occur within a panel. The finish coat is applied to a predamped, but still absorptive, base coat to a thickness of about 1/8 in. (3.2 mm). The finish coat is applied from the top down and the whole wall surface must be covered without joinings (laps or interruptions). Table 4 summarizes the recommended nominal plaster coat thicknesses for both two and three coat work.

Differential suction between the masonry units and mortar joints may cause joint patterns to be visible in two coat applications if the first coat is too thin. This may also occur if the walls are plastered while the units contain excessive moisture.

CONTROL JOINTS

Cracks can develop in plaster from a number of causes: drying shrinkage stresses; building movement; foundation settlement; intersecting walls, ceilings, and pilasters; weakened sections in a wall from a reduction in service area or cross section because of fenestration; severe thermal changes; and construction joints.

To prevent such cracking, install control joints in the plaster coat directly over and aligned with any control joints in the base. Normally, cracking will not occur in plaster applied to uncracked masonry bases if the plaster bonds tightly to the base structure. If excessive cracking does occur, the application (particularly floating) procedure may not have provided adequate bond of plaster to concrete masonry. Altering application procedures or mechanically anchoring the plaster to the concrete masonry surface with mesh may be required.

CURING

To obtain the best results from the cementitious materials in cement plaster, moisture must be kept in the plaster for the first few days after application. The base coat should be moist cured until the finish coat is applied. Generally, fogging the surface with water at the start and again at the end of the work day will suffice. If it is hot, dry, and windy, the plaster surface should be moistened and covered with a single sheet of polyethylene plastic, weighted or taped down to prevent water loss through evaporation.

Immediately before finish-coat application, the base coat should be moistened. This moisture absorbed by the base coat and the ambient relative humidity provides total curing of the finish coat plaster (particularly colored finish coats) so that it is not necessary to further moist-cure the finish coat.

MAINTENANCE OF PLASTER

Minimal care will keep plaster attractive for many years.
Washing will keep the surface clean and the color bright.
Washing plaster wall surfaces consists of three steps:

Prewet the wall, saturating it. Start at the bottom and work to the top.
Use a garden hose to direct a high-pressure stream of water against the wall to loosen the dirt. Start at the top and wash the dirt down the wall to the bottom.
Flush remaining dirt off the wall with a follow-up stream.

Prewetting overcomes absorption and prevents dirty wash water from being absorbed and dulling the finish. A jet nozzle on a garden hose will usually clean effectively. Do not hold the nozzle too close to the surface because the high pressure stream of water may erode the surface.

Chipped corners and small spalls can be patched with premixed mortar. The patch area should be wetted before applying plaster. Prepare premixed mortar by adding water and mixing to a doughy consistency, then trowel into the patch area, and finish to match the texture of the surrounding surface.

A fresh, new look can be given to any exterior plaster wall by applying a surface treatment of paint, portland cement paint, or other coating. Portland cement paints are mixed with clean water to a brushable consistency and laid on heavily enough to fill and seal small cracks and holes. The surface should be dampened immediately before application.

REFERENCES

Plastic Cement, Uniform Building Code Standard 25-1, International Conference of Building Officials (ICBO), 1994.
Standard Specification for Aggregate for Job-Mixed Portland Cement-Based Plasters, ASTM C 897-00. American Society for Testing and Materials, 2000.
Standard Specification for Application of Portland Cement Based Plaster, ASTM C 926-98a. American Society for Testing and Materials, 1998.
Standard Specification for Blended Hydraulic Cements, ASTM C 595-02. American Society for Testing and Materials, 2002.
Standard Specification for Masonry Cement, ASTM C 91-
American Society for Testing and Materials, 2001.
Standard Specification for Portland Cement, ASTM C 150-
American Society for Testing and Materials, 2000.