Key Words: colorless coatings, film former, graffiti-resistant, penetrant, water penetration, water repellent.

Colorless coatings are available in many types and are designed for a variety of uses. The type of coating applied to brick masonry is dependent on the desired effect of the coating and its chemical and physical properties. Products promoted for use on brick masonry are not always appropriate. Some can be detrimental to brick masonry. Function of the brickwork plays an important role in coating selection. Coatings suitable for interior brick masonry may not be suitable for exterior exposures. Similarly, coatings applied to floors or pavements are subject to conditions different from coatings applied to brick masonry walls. Coating selection is also influenced by the material properties of the substrate. Recommendations for clear coatings on other masonry materials are not necessarily appropriate for brick masonry. Clay brick masonry is considerably different than stone, concrete and concrete masonry in both physical and chemical properties. Brick masonry has a different pore structure, is generally less absorptive, less permeable and is not as alkaline as concrete masonry. The recommendations included herein are applicable only to clay brick masonry.

This Technical Notes examines the various types of colorless coatings often applied to brick masonry. Specific recommendations are found under RECOMMENDATIONS FOR USE. Coatings for below grade masonry, such as damp proofing or waterproofing coatings, are not addressed. Other Technical Notes in this series cover the painting of brick masonry.

Clear coatings may be applied to brick masonry in an effort to facilitate cleaning, to resist graffiti, to provide gloss or to reduce water absorption or penetration. Many times a single product is used to achieve several of these objectives. Selection of a coating should be based on the desired appearance, resistance to water penetration, application, material substrate, economics, life span or other criteria set by the designer or user. In addition, the disadvantages of using colorless coatings should be considered during selection.

It is desirable to minimize the absorption and penetration of water in a brick masonry wall system for many reasons. Water absorption can lead to staining and efflorescence when soluble salts are liberated by moisture in a masonry wall. Water penetration is responsible for many of the problems encountered in walls. Water which freezes in nearly saturated brick can cause deterioration over time. Corrosion of metal ties, metal studs and other metal items due to prolonged contact with water can lead to structural or serviceability failures of a building wall. Wood members and sheathing are vulnerable to rotting and mold growth when wetted. Water penetrating an exterior wall can damage interior finishes.

To effectively minimize water penetration, care must be exercised in the material selection, design and detailing of brick masonry. Quality construction and proper maintenance are also important. Under normal exposures, it is nearly impossible for significant amounts of water to pass directly through the brick units or mortar. Most water penetrating a brick masonry wall occurs at separations and cracks between brick and mortar or at junctures with other materials.

Clear water repellent coatings are sometimes suggested to reduce water absorption and reduce the amount of water that penetrates the exterior brick masonry wythe. Research indicates the varied effectiveness of clear water repellents in reducing water leakage through a brick masonry wythe [4,8,11]. Change in the absorption properties of masonry may have no effect on water penetration of a masonry wall system. Clear water repellents can seldom stop water penetration through cracks over 0.02 in. (0.5 mm) in size. Clear water repellents cannot stop water penetration through incompletely filled mortar joints or from sources such as ineffective sills, caps or copings. Their effectiveness under conditions of wind-driven rain is questionable. As a result, the use of clear water repellent coatings to eliminate water penetration in a wall with existing defects is often futile. Clear water repellents are most effective at reducing the amount of water absorbed by the brick masonry. Thus, they can help reduce staining and efflorescence caused by moisture absorption, particularly on highly absorptive masonry.

The applications and limitations of clear water repellents are described in the sections that follow. Methods for ascertaining the effectiveness of clear water repellents are discussed under PERFORMANCE CRITERIA.

New Construction Applications. Clear water repellents are sometimes specified for newly constructed brick masonry as an "insurance measure" against water penetration and related problems. The thinking is that clear water repellents will prevent water penetration which would have otherwise occurred due to imperfections in construction. Such thinking should be discouraged, because clear water repellents cannot compensate for poor construction or design. As discussed previously, their effectiveness in stopping water penetration has limitations. Furthermore, most brick masonry wall systems do not require a clear water repellent to remain impervious to rain water. For these reasons, the use of clear water repellents on newly constructed drainage walls is not recommended.

Clear water repellents can be useful for brick masonry walls which are particularly vulnerable to water penetration, especially in climates which receive large amounts of rain. These wall types include barrier walls, chimneys and parapets. When a clear water repellent is considered for use on these elements, the benefits must be weighed against the possible disadvantages. Past successful performance of the proposed coating, for a number of years in the same exposure conditions and on the same brick and mortar types, should be required. In climates which experience freezing and thawing cycles, the effect of a colorless coating on the durability of the masonry is a particular concern. For exterior brick pavements subject to repeated freeze/thaw cycling, the possibility of accelerated deterioration outweighs any benefits of clear water repellents.

Whenever applying clear water repellents to newly constructed masonry walls, a minimum of one month should pass after close-in of the building before application of the coating. This period will allow the walls to cure sufficiently and, more importantly, allow the evaporation of moisture from the building materials to occur naturally, unimpeded by a coating on the masonry. In fact, a relatively dry substrate is recommended by many colorless coating manufacturers. A delay of one year is preferred so that efflorescence due to water absorbed during construction, often known as "new building bloom," is not entrapped by the coating. For a more complete discussion of efflorescence, refer to Technical Notes 23 Series.

Remedial Applications. The most common reason for application of a clear water repellent coating is to reduce or eliminate water penetration in a building experiencing water penetration problems. The inability of a coating to eliminate water penetrating through larger than hairline cracks in the masonry has already been discussed. When a clear water repellent is being considered as a means to eliminate water penetration in a brick masonry wall, several items must be addressed prior to application of a coating. It is paramount that the source of water penetration be determined. The application of a clear water repellent in lieu of ascertainable repairs is not recommended. Common repairs which may be necessary and should be completed include:

1. Removal of defective sealant and cleaning, priming and replacement with a good grade of elastomeric sealant at all windows, copings, sills, expansion joints between brick masonry and other materials;

2. Repointing of incompletely filled, cracked or disintegrated mortar joints;

3. Removal and replacement of spalled or cracked brick;

4. Surface grouting of separations between the brick units and mortar.

These remedial measures are described in Technical Notes 7F [15].

Other repairs, which are generally more difficult and costly to complete, include:

1. Clearing mortar blockage from weep holes and the interior drainage system;

2. Removal and replacement of damaged, omitted or improperly installed flashing.

These latter repairs are argued by some people to be unnecessary or uneconomical if using a clear water repellent. However, all of these repair techniques can reduce water penetration and are long-term solutions. Application of a clear water repellent is not considered a long-term solution. Long-term solutions are preferred.

After remedial measures have been completed and inspected, it is usually advisable to wait a period of several months to determine if additional corrective steps are necessary. Many times the moisture penetration problems will be corrected by these initial repairs and further consideration of coatings can be dismissed.

If water penetration remains a problem, or long-term solutions are judged to be uneconomical despite their benefits, the application of a clear water repellent can be considered. If water absorption appears to be the problem, a clear water repellent can be particularly effective. However, clear water repellents are not a permanent solution and will require reapplication. See the discussion under the Durability section for further information on the life span of coatings.

The use of colorless coatings can reduce the amount of water absorbed by a brick masonry wall and may help reduce staining and efflorescence. As a result, colorless coatings are sometimes used on brick masonry which has a relatively high absorption, such as walls which have been sand-blasted. Brick manufacturers sometimes apply colorless coatings to units during manufacture to reduce staining or initial rate of absorption. ASTM C 216 Specification for Facing Brick requires that the brick manufacturer report the presence of such coatings. Selection of a coating for any of these uses should be based on demonstrated successful performance in similar usages and exposures. Staining and efflorescence may not be completely eliminated by application of a coating. If staining or efflorescence occurs on masonry which is treated with a colorless coating, the stains and salts may be difficult or impossible to remove.

Another common reason for using a colorless coating is to achieve a darker, wet or glossy appearance. Gloss is created by changing the reflectance of the brick masonry surface. In some cases, change of appearance may be the undesired side effect of an improperly applied coating or of poor coating selection. Some coatings may give an undesired sheen or gloss. See Figure 1. Other coatings may impart a yellow appearance to the masonry. Satisfactory appearance of a treated surface is best judged by examining a sample panel or test area of masonry before and after treatment.

Many structures used by the public are built with brick masonry. Schools, government buildings, libraries and noise barrier walls are applications where brick masonry is chosen for its appearance and low maintenance. Resistance to graffiti and ease of cleaning can be important. Colorless coatings are sometimes applied to brick masonry to help achieve these goals. Such coatings work by keeping graffiti or dirt on the masonry surface for easier removal. Glazed brick are often used in similar applications to provide the same benefits.

Colorless coatings for brick masonry can be classified into two general categories: film formers and penetrants. The two types have significantly different physical properties and performance. As the name implies, film formers produce a continuous film on the surface of the masonry. Penetrants enter up to 3/8 inches. (10 mm) into the brick masonry and do not form a surface film.

Most colorless coatings are available in two formulations: water-borne and solvent-borne. Normally, better penetration and performance is attained using solvent-borne solutions. However, manufacturers are increasingly using water-borne solutions which have lower volatile organic compound (VOC) content. Increasing the solids content of the coating can also reduce VOC content.. VOC content is regulated by the Environmental protection Agency nationwide because of its connection with poor air quality. Effective September 1999, the VOC content of architectural coatings, including water repellents, cannot exceed 600 grams per liter. Only California has more stringent requirements (400 g/l). Because the 600 g/l limit is lower than the VOC content of many products currently on the market, many changes to available products are expected. Product data and test results should be examined carefully to compare performance. Carrier type influences permissible application conditions. Temperature range, substrate moisture content, environmental regulations and adjacent materials and vegetation must be considered.

Colorless coatings are discussed in the following sections according to generic chemical type. Most colorless coating manufacturers will provide information on the generic chemical chemical composition of their products. In addition, handbooks are available which classify many proprietary coatings according to their generic chemical composition [6].

Acrylics, stearates, mineral gum waxes, urethanes and silicone resins are among the products which form a film when applied to brick masonry. The large molecular size of these products prevents them from penetrating into the masonry. Typically, film-forming products work by adhering a film to the surface of the brick masonry substrate. Surface preparation can be important in achieving satisfactory adhesion of a film-forming coating. Film materials, continuity and product concentration determine the performance characteristics.

Film-forming products are effective at preventing water from penetrating into brick masonry. Film formers can bridge the small hairline cracks which are commonly the source of water penetration. However, their ability to exclude water from the exterior also inhibits evaporation of water within the masonry through the exterior face. This reduction in the water vapor transmission rate, or lack of breathability, through the brick masonry is of special concern in exterior brick masonry subject to freezing and thawing cycles. Thus, film-forming products are generally not recommended for brick masonry in such environments.

A film on a masonry wall may facilitate cleaning by keeping surface contaminants from penetrating into the masonry. This characteristic leads to their use as graffiti-resistant coatings. When an appearance change is desired, film formers are typically used. Film-forming products, by their nature, tend to produce a sheen or gloss when applied. In some cases, they may darken the appearance of a wall (the wet look) when used in high concentrations.

Acrylics. Acrylics can be effective as water repellents. However, they are vulnerable to cracking due to thermal fluctuations. They are often used when a high gloss is desired. Acrylics are available in two forms, water-borne and solvent-borne. Acrylic emulsions are water-borne. Acrylic solutions are solvent-borne. Because of increasing regulation of solvent-borne products, acrylic emulsions are more widely used. Coating manufacturers typically recommend that acrylics be applied to substrates that are thoroughly dry. If applied to a damp substrate, the acrylic film can separate from the masonry, giving it a cloudy, or whitened, appearance. Acrylics in particular tend to create a slippery surface. This is a concern in pavement applications. When stabilized against degradation in ultraviolet (UV) light, acrylics can last five to seven years.

Stearates. Stearates promoted for use on masonry are generally aluminum or calcium stearates. They are sometimes known as metallic soaps. Stearates form a water repellent surface by reacting with free salts in mineral building materials and plugging the pores. Some formulations are used as integral water repellents in concrete masonry and mortar. Their effectiveness as water repellents varies, and typically film-forming stearates must be reapplied every year. Stearates also have the potential to turn cloudy if moisture gets behind the coating.

Mineral Gum Waxes. Paraffin wax and polyethylene wax are commonly referred to as mineral gum waxes. These products are typically solvent-borne and can be good water repellents, able to bridge hairline cracks. However, they have been known to darken the substrate and, in cases where moisture gets behind the coating, turn the surface a milky white. Figure 2 illustrates a case where a mineral gum wax promoted for use on brick masonry was applied to a building which had water penetrating through cracks in the masonry and at flashing failures. The sources of moisture were not addressed, and the coated masonry became moist. This led to clouding and eventual spalling of the masonry, as shown in Fig. 3.

Silicone Resins. Known chemically as polydimethylsiloxanes (PDMS), silicone resins may form a film in the presence of moisture when their solids content is between five and ten percent. The PDMS comes in many weights and forms. The properties of the resin depend on the molecular weight and structure of the PDMS. Early silicone resins were known for reducing the durability and changing the appearance of masonry. They often yellowed and were hard to clean. There are PDMS manufactured today which do not yellow or contribute to soiling. Silicones do not chemically bond with the substrate, and as a result, have a short life. Many silicones require reapplication on a yearly basis, although some last longer.

Penetrating type coatings are characterized by their penetration into the substrate, typically to depths up to 3/8 in. (10 mm). They repel water by changing the capillary force, or contact angle, of the pores in the face of the masonry from positive (suction) to negative (repellency). Penetrating coatings are typically more resistant to UV degradation because of their chemical composition and because they penetrate below the masonry surface. Because they coat the pores rather than bridge them, penetrants tend to have better water vapor transmission characteristics. The solids content of these materials commonly ranges from five to forty percent by weight. Penetrants can be categorized into five groups: silanes, siloxanes, silicates, methyl siliconates and blends of these.

Silanes. Silanes used as clear water repellents are more accurately known as alkylalkoxysilanes or alkyltrialkoxysilanes. Their small molecular structure allows good penetration on even dense substrates. They are used in relatively high concentrations (typically 20 percent or greater solids content). Silanes chemically bond with silica- or alumina-containing materials to make the material water repellent. Silanes are best applied to slightly damp substrates. An alkaline substrate, such as concrete or concrete masonry, acts as a catalyst to speed the reaction to form a water repellent surface. Chemical catalysts are also used with silanes to improve the chemical reaction on less alkaline substrates such as brick.

Siloxanes. The clear water repellent coatings known as siloxanes are actually oligomerous alkylalkoxysiloxanes or silsesquioxanes. Although they have a larger molecular structure than silanes, good penetration and water repellency can also be achieved by siloxanes. Like silanes, siloxanes chemically bond with silica- or alumina-containing materials such as brick. This results in a long life, up to ten years or more, and makes them more difficult to remove. Siloxanes can also be applied to a damp surface. Siloxanes are less volatile than silanes and react with chemically neutral substrates without a chemical catalyst. Siloxanes are typically used in solutions having five to twelve percent solids by weight. Siloxanes have been known to work well on certain brick masonry installations. However, siloxanes are highly reactive with silica and will bond with glass which is not properly protected.

Silicates. Ethyl silicates are commonly used as consolidants on natural stone and occasionally brick masonry. They are generally used for restoration of deteriorated masonry. Consolidants are designed to react with and stabilize the substrate to which they are applied. Their use on brick is uncommon. Sodium silicates and potassium silicates are sometimes used in the concrete industry as curing compounds and accelerators. None are effective water repellents and are not recommended for this use on brick masonry.

Methyl Siliconates. Methyl siliconates are alkaline solutions which react slowly with silica-containing materials in the presence of carbon dioxide to form a water repellent surface. Siliconates are sometimes injected into brick masonry to form a horizontal barrier to rising damp. However, because of their slow reaction time, they make poor water repellents and are not recommended for surface application to exterior brick masonry walls.

Blends. Colorless coatings are also made from blends of the materials listed above. Blends are created to produce products with the benefits of the constituent materials. As such, they reflect the properties of the constituent materials, but the properties will be modified somewhat. Thus, it is important that the user review product data and test results for blended products.

Any coating applied to brick masonry will change the physical properties of the masonry. Appropriate performance criteria for colorless coatings are not well defined. Currently, coating manufacturers rely on test methods developed for common substrates, such as concrete and brick, to measure the performance of their products. For example, water repellency is often measured by comparing the cold water absorption of untreated and treated brick using the method described in ASTM C 67 Test Methods of Sampling and Testing Brick and Structural Clay Tile. While easy to perform, the interpretation and use of such laboratory results, for a single material, has not been determined. Until these tests are validated with research and correlated to masonry wall performance, one must rely on good judgment and experience in establishing performance criteria limits. The following criteria can be useful in comparing several colorless coating alternatives. The most critical properties of colorless coatings to be evaluated are water vapor transmission, water penetration and repellency, durability, reapplication, gloss, slip resistance, graffiti resistance and environmental considerations. These properties are discussed in the following sections. A summary of several properties of colorless coatings is found in Table 1.

The most important property to consider when selecting a coating for application on exterior brick masonry is the water vapor transmission rate. The water vapor transmission, or breathability, of exterior brick masonry is important in determining the rate and amount of water which can evaporate through the face of the masonry. Coatings which do not breathe well, those not having a high water vapor transmission rate, can entrap water within the brickwork leading to clouding of the coating. See Figs. 2 and 4. a minimal reduction in the water vapor transmission rate through coated brick masonry reduce the possibility of masonry deterioration due to freeze-thaw cycles or salt crystallization by ensuring that natural evaporation can occur. Otherwise, water-soluble salts which may be normally deposited on the surface of the masonry as efflorescence may be trapped beneath the coating. As salts crystallize, they grow significantly in size. Accumulation of entrapped salts can lead to spalling. The potential for these actions may preclude the use of coatings on brick masonry. Additionally, older brick masonry structures may be subject to damaging moisture problems, such as rising damp and condensation, which are only aggravated by the application of a colorless coating.

A method to evaluate the potential of a colorless coating to entrap damaging salts and cause spalling is proposed by Binda [3]. Individual brick units are treated with the colorless coating on their exposed face. The sides of the unit are sealed with rubber to prevent evaporation except through the treated face. The units are subjected to cycles of immersion in a salt solution for 4 hours and air drying for 44 hours. The cross-sectional size is measured after each cycle. Deterioration is typically by delaminations of the treated brick face. Hence, a reduction in brick cross section. A correlation of the number of cycles to deterioration in this test to the durability of a masonry assemblage has not yet been established. However, this method is one means of assessing salt crystallization damage potential when evaluating colorless coatings.

Others have also tried to establish the effect of colorless coatings on the water vapor transmission rate and durability of brick masonry. At present, there is no definitive test. The water vapor transmission rate can be measured using ASTM E 96 Test Methods for Water Vapor Transmission of Materials, desiccant (dry) cup method. A comparative measurement can be made between an untreated and a treated brick unit sample, or the water vapor transmission rate of the coating itself can be determined. Comparative testing of an untreated and a treated brick sample is advised. For comparative testing, a maximum ten percent reduction in the rate of vapor transmission is the recommended limit.

Water penetration resistance is an important criterion when selecting a clear water repellent. However, most clear water repellent manufacturers measure performance of a clear water repellent based on water absorption. The ASTM C 67 test for cold water absorption is often used to compare the absorption of treated and untreated brick units. This approach has two significant limitations. Testing an individual brick is not a measure of the performance of a clear water repellent on the brick masonry assemblage. Secondly, changing the absorption of a brick unit may have no beneficial effect on the water penetration resistance of the brickwork. Performance of a coating is more accurately evaluated by measuring water penetration resistance of the constructed brick masonry wythe.

The water penetration resistance of clear water repellent coatings should be measured by comparative laboratory testing in accordance with ASTM E 514 Test Method for Water Penetration and Leakage Through Masonry. Testing should be performed on a minimum of three identical wall specimens of the intended materials and construction. The amount of water penetration should be measured on each specimen in accordance with ASTM E 514 before and after coating with the clear water repellent. The percentage reduction in water penetration is a measure of the water repellent effectiveness. A 90 percent reduction in maximum leakage rate, percent area of dampness on the back face of the wall and total water collected after 24 hours of testing [4] as compared to the untreated wall panel is recommended. However, a specified reduction under laboratory conditions may not necessarily reflect performance of the clear water repellent on the actual masonry construction. Variables in construction can have a significant impact on water penetration resistance of a masonry wall. Thus, a 90 percent reduction rate for a laboratory test does not automatically translate into a 90 percent reduction in water leakage through the exterior brick wythe of a constructed building.

The life span of colorless coatings is an important criterion in material selection. The effective life is influenced by the thickness of the film or depth of penetration into the brickwork, exposure to UV light, the severity of weather exposure and the form of masonry construction. There are several test methods which simulate outdoor exposure. ASTM G 53 Practice for Operating Light- and Water-Exposure Apparatus (Fluorescent UV-Condensation Type) for Exposure of Nonmetallic Materials is one often specified. The difficulty with using such tests to measure life span of a coating is trying to correlate laboratory test results to field performance. Coating characteristics, such as gloss or water repellency, can be measured before and after exposure and the results compared, but such tests have not been correlated to the actual life expectancy of the coating. Greater depth of penetration or film thickness and greater resistance to degradation in UV light and harmful environments imply longer life for an exterior applied colorless coating. Many clear water repellents are warranted by the coating manufacturer to last ten years or more.

In addition to laboratory testing, monitoring field performance by comparative testing can determine the effectiveness of a clear water repellent after several years of service. The water penetration of a newly coated wall can be measured and recorded in a specified area using a field test, such as a Masonry Absorption Test (MAT) tube [11], RILEM tube or other technique [87]. Periodic comparisons can be made using the same technique in the same location. Such evaluation will indicate if the coating has met its warranted life and also when recoating may be necessary.

The durability of a coating applied to an interior brick pavement is characterized by resistance to abrasion and yellowing. It is common for film-forming products to require reapplication to brick floors every few years, depending on the amount of traffic. Evaluation of a coating's resistance to abrasion is difficult, because there are no direct test methods for measurement on brick.

Reapplication of clear coatings is another consideration. Most coatings must be reapplied every 7 to 15 years, and some last considerably shorter periods of time. Compatibility of coatings can be a problem. For example, a penetrating coating cannot be applied over an existing film-forming coating. In some cases, reapplication of the same coating may cause clouding. In other cases, the hydrophobic nature of a coating makes reapplication difficult or impossible. It may be necessary to strip the treated surface of any prior coating according to the coating manufacturer's recommendations before a new or different coating can be applied. This procedure may involve hazardous chemicals often regulated or restricted from use by local, state or federal environmental regulations. Thus, an existing coating may have to remain in place until it wears off, even if deterioration of the masonry calls for its removal.

Gloss can be measured by determining the reflectance of a coating using Federal Test 141a, Method 6121. However, desirable gloss is a subjective matter. Gloss is best evaluated by examining a representative test area of brick masonry including the entire range of colors and textures, one-half of the area treated with the specified coating, the other half untreated. Acceptable appearance should be determined by the designer or owner. An accepted test area should be retained as a means for judging acceptability of other treated areas.

A colorless coating can adversely affect the slip resistance of an interior brick floor or exterior brick pavement. Coated floors or pavements should be evaluated for slipperiness for safety reasons, especially in public access areas and in areas where water may contact the floor or pavement. The slip resistance of a coating is often measured using ASTM D 2047 Test Method for Static Coefficient of Friction of Polish-Coated Floor Surfaces as Measured by the James Machine. Nonhazardous surfaces have a minimum static coefficient of friction of 0.5 as measured by this test method [1].

Graffiti-resistant coatings may be permanent or sacrificial in nature. In addition to the properties already mentioned, they should be evaluated on the basis of their resistance to chemicals and degradation in UV light. A high resistance to common chemical cleaners is a necessary property of a permanent coating, so that the graffiti can be removed without removing the coating. The effect of cleaning chemicals on the coating can be evaluated by applying the cleaning chemical to a sample area of coated brick masonry. Always consult the coating manufacturer prior to such testing for safety precautions, as reactions between the cleaner and the coating may be hazardous. Many graffiti-resistant coatings are sold with a special cleaner designed for use with that coating. If the coating is designed as a sacrificial layer, ease of removal is important and should be documented by the product manufacturer and evaluated by application to a test area. Satisfactory performance is indicated by successful removal of purposely applied graffiti. Resistance to weathering and UV stability can be evaluated using ASTM Test Method G 53 to simulate exterior exposure, followed by evaluation of the desired performance characteristics, such as ease of cleaning.

Possible environmental hazards are also of concern when considering a colorless coating. Often the chemicals used in colorless coatings are highly reactive and can etch glass, damage paint, kill vegetation and emit harmful vapors. This requires attention to worker safety and proper protection of adjacent surfaces.

Selection of a specific product should be based on recommended performance criteria described herein and any other criteria set by the designer to address the particular conditions involved. The possible advantages of colorless coatings should be weighed against the disadvantages associated with their use. In addition, the brick manufacturer should be consulted for recommendations on the use of colorless coatings prior to selection of any coating. Colorless coatings change the physical properties of the brick masonry on which they are applied. This may be beneficial or, in many cases, detrimental to the performance of the masonry.

In exterior brick masonry walls, one of the foremost concerns is water penetration. The many factors important in determining the water penetration resistance of brick masonry walls are discussed in Technical Notes 7, 7A and 7B [15]. Clear water repellents should never be used as a substitute for good design, construction and maintenance.

When a colorless coating is considered for a brick masonry wall exposed to freezing, the water vapor transmission rate is critical to the durability of the masonry. In addition, a condensation analysis, as described in Technical Notes 7C and 7D [15], should be performed to determine the effect of the coating on the location of condensation within the wall system before applying a colorless coating.

Because they permit more rapid water vapor transmission, penetrating coatings are preferred over film-forming coatings for exterior brick masonry walls. If a clear water repellent is to be used, siloxanes are recommended. Siloxanes provide the advantage of good water repellency and long term performance and have been shown to be effective on many brick masonry walls. Silanes containing chemical catalysts have also been used successfully.

Because of the effect of a film on the breathability of masonry, film-forming coatings should be used cautiously, particularly in freezing environments. Only products with known performance in a similar climate, wall type and exposure on brick masonry with similar physical properties should be used. In areas where freeze-thaw cycles is a concern, the possible disadvantages of a colorless coating may preclude their use. For this reason, film-forming coatings should be avoided on exterior brick masonry walls in freezing climates.

Prior to applying a colorless coating on any exterior wall, the wall should first be thoroughly inspected and necessary repairs made. The inspection should determine the condition and suitability of: caps and copings, flashing, weep holes, sealant joints, mortar joints and general execution of details. Technical Notes 7F provides an inspection checklist for areas of concern. The source of the problems should be identified and all deficiencies should be carefully repaired or corrected. For further discussion see the earlier section entitled Remedial Applications. Only when remedial repairs have been made, and problems still exist, should a clear water repellent coating be considered for brick masonry.

Drainage Walls. Drainage-type masonry walls, such as brick veneer and cavity walls, are designed to accommodate water penetration of the exterior brick wythe without damage to the interior components of the wall system. Thus, when designed and constructed properly, a clear water repellent is not necessary on newly constructed brick masonry drainage walls.

When the elements essential to successful performance of a drainage wall are not achieved and water penetration is a problem, a clear water repellent coating may be appropriate. When a drainage wall is treated with a colorless coating, the use of vents at the top and bottom of all wall cavities is recommended to promote evaporation of moisture from the brick masonry.

Barrier Walls. Barrier walls are exterior single wythe walls and multiwythe walls with filled collar joints, which do not contain any provision for internal drainage of water which penetrates the brick wythe(s). These walls rely solely on the mass of the masonry to prevent moisture penetration. Proper design, good workmanship and maintenance are exceedingly important.

Sometimes such walls contain flashing and drainage spaces formed by interior finishes. For walls without a provision for drainage of water which does penetrate, prevention of water entry becomes even more important. Walls without drainage spaces, and walls in areas subject to large amounts of rain, are prime candidates for the application of a clear water repellent. A clear water repellent can limit the amount of moisture which is absorbed by the wall and help reduce the potential for water penetration.

Chimneys and Parapets. Chimneys and parapets are other elements where clear water repellents can sometimes be recommended. These walls can be subject to early deterioration because of severe exposure. They are often exposed to wind-driven rain, water rundown on the exterior walls from the crown or coping, water penetration from failures in the chimney crown or wall coping and incorrect flashing installations. Good detailing of precast chimney crowns and parapet copings, properly constructed with overhangs, wall flashing and a drip, prevents many water penetration problems. However, because of the large amount of moisture which can contact the surface of a chimney or parapet wall, a clear water repellent coating can sometimes be effective in reducing water-related problems. Conditions in which a clear water repellent may be recommended on chimneys and parapet walls include climates with a driving rain index above three (see Fig. 5) and on sloped or horizontal projections of such elements.

Colorless coatings are generally applied to interior walls to facilitate cleaning or provide a gloss. Water repellency and breathability of an interior wall is generally not a concern. While some penetrating coatings may provide the desired effect, a film-forming product will typically give best results when gloss and ease of cleaning are desired. Water-borne acrylics (acrylic emulsions) and urethanes have been found to produce the best results for interior brick masonry walls. Acrylics, in particular, are known to provide a high gloss. Both are durable in applications with no UV exposure. Finished appearance is best judged by applying the coating to a test area on the surface to be treated.

In the case of exterior brick masonry walls which have their interior face exposed, water vapor transmission may be a concern. Film-forming products should be used cautiously, only after the effect of the film on the water vapor transmission of the wall system has been evaluated.

Brick masonry pavements are significantly different from vertical brick masonry. Their exposure is different, as is their construction. An exterior brick masonry pavement is subjected to a more severe weathering exposure than an exterior vertical wall. Pavements have greater contact with moisture due to their horizontal orientation and are often not protected by overhangs. In addition, the lack of a drainage cavity or air space to aid in drying further increases the severity of exposure.

There are several disadvantages associated with the use of a colorless coating on pavement surfaces. Colorless coatings can decrease the slip resistance of the pavement or floor, especially when wet. Also, pavements and interior floors are subject to abrasion due to foot traffic which shortens the life expectancy of most coatings compared to vertical applications.

Exterior Pavements. By nature of their construction, pavements only allow evaporation of moisture from the masonry through one face, the wearing surface. As a result, the potential for problems associated with reduced water vapor transmission are significant. These disadvantages usually outweigh any potential benefit. For this reason, colorless coatings are not recommended for use on exterior brick pavements subject to freezing and thawing. In exterior environments not subject to freezing, water vapor transmission rate of the coating must be high. Clouding of the coating is a particularly common problem. See Fig. 4.

Interior Floors. Colorless coatings are often applied to interior brick floors to provide a glossy finish and to facilitate cleaning. Mortarless brick pavements are sometimes coated to help retain the jointing sand in the joints. Urethanes, acrylics, waxes and some penetrating coatings which meet the performance criteria discussed herein and those set by the designer can be used on interior brick masonry floors not subject to freezing. The primary disadvantage of most colorless coatings used on floors is their tendency to increase the slipperiness of the floor. Acrylics can be particularly hazardous; however, performance varies with formulation, and some acrylic solutions have been used successfully. New epoxy-based coatings show promise in this area. Past successful performance is the best measure of a satisfactory coating. Another caution is that film-forming coatings may separate from the brick paving and turn cloudy if moisture from the brickwork or supporting members migrates through the brick floor. Consequently, a film-forming coating should only be applied when the brick floor and supporting members are substantially dry.

Selection of a colorless coating for use on brick masonry should be based on the desired performance, the information discussed in this Technical Notes and literature from the coating manufacturer. Additional items to be considered prior to application of a colorless coating follow. Whenever possible, consult with the brick manufacturer for specific recommendations regarding coating of a particular brick. Properties of each brick are unique and can affect coating performance.

1. It is suggested that the designer or user require test reports for relevant performance criteria and a written warranty from the coating manufacturer for the performance of the coating over a designated period of time. The application contractor should know precisely the work to be performed and should protect adjacent and surrounding surfaces from over-spray as necessary. Qualifications of the contractor should be verified.

2. The coating should be that of a well-known manufacturer who has been in business for a period of at least five years. It is suggested that a brand name be used that has a good track record over a period of at least five years. References of projects with similar applications, materials and exposure should be investigated.

3. The coating should be applied at the application rate and under the climatic conditions recommended for clay brick masonry substrates by the coating manufacturer. Typically, temperatures above 40^oF (4^oC) and below 100^oF (38 ^oC) are required. Application on windy days should be avoided when possible.

4. Repair and replacement of brick and mortar joints and other necessary repairs should be completed prior to applying a colorless coating.

5. There should have been no efflorescence or, at the maximum, only a minor occurrence of efflorescence on the brick masonry to be treated. Walls less than one year old should not be treated because efflorescence, "new building bloom", may be entrapped by the coating. Walls with a history of efflorescence should be coated only after the source of moisture has been addressed.

6. The wall must be clean at the time of application. Heavy accumulations of atmospheric dirt will interfere with proper penetration or adhesion of the coating and result in poor performance and shorter life. See ASTM D 5703 Practice for Preparatory Surface Cleaning for Clay Brick Masonry for a discussion of cleaning techniques which may be required [2]. In addition, freshly repointed mortar and repaired sealant joints should cure a minimum of 72 hours before a coating can be applied [13].

7. The brickwork should have a moisture content consistent with that recommended by the coating manufacturer. Moisture content of the brick masonry should be checked at several locations by the method recommended by the coating manufacturer.

These steps must be taken in conjunction with the recommendations contained within the applicable sections of this Technical Notes. They cannot guarantee successful performance, but will greatly increase the likelihood that the colorless coating will perform as intended. The coating manufacturer will often have additional recommendations regarding coating selection, substrate preparation, curing, application methods and coverage rates. Failure to consider these items can result in poor performance of the coating and can cause severe harm to the masonry or surrounding elements.

This Technical Notes has discussed both the reasons for and the suitability of colorless coatings for brick masonry. For most exterior brick masonry, it is advisable not to use colorless coatings. Furthermore, clear water repellents are not necessary on properly designed and constructed brick masonry. However, under certain conditions clear water repellents and other colorless coatings may be beneficial.

The information and suggestions contained in this Technical Notes are based on the available data and the experience of the engineering staff of the Brick Industry Association. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of brick masonry. Final decisions on the use of the information contained in this Technical Notes are not within the purview of the Brick Industry Association and must rest with the project architect, engineer and owner.