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Cleaning Concrete Masonry

August 2018

Revised March 2005

INTRODUCTION

Concrete masonry buildings offer exceptional beauty, coupled with attributes such as structural strength, durability, fire resistance, acoustic performance and low maintenance. Proper cleaning after construction and throughout the life of the building will help preserve concrete masonry’s beauty.

Although the maintenance needs of a well-designed and constructed masonry wall are minimal, contaminants can detract from an otherwise attractive structure. Cleaning of mortar smears, construction dirt and possibly efflorescence from the construction phase is usually required. Subsequent cleaning may be required over the life of the building to address dirt and soot from the atmosphere or staining from specific sources. Appropriate cleaning can remove contaminants and help produce a more uniform appearance.

This TEK discusses several general cleaning methods, applicable to whole-wall cleaning. For information on removing stains or localized contaminants, refer to Removal of Stains From Concrete Masonry, TEK 08-02A (ref. 7).

SUBSTRATES

The ease of cleaning a masonry wall can be affected by the concrete masonry units and mortar used in the wall. Cleaning products and techniques applicable to one masonry wall may not be appropriate for others. In addition, special consideration should be given to walls incorporating more than one material, such as a concrete masonry wall with clay masonry banding.

Concrete Masonry Units

Conventional, or nonarchitectural, concrete masonry units (CMUs) have a relatively smooth surface, formed from a thin layer of cement paste resulting from the typical concrete masonry manufacturing process. Aggressive cleaning methods may remove this layer, exposing aggregate and altering the final appearance. For this reason, any aggressive cleaning should be performed consistently across the entire wall surface for a uniform appearance after cleaning. In some cases, abrasive cleaning or pressure chemical cleaning are specified with smooth faced units to produce a slightly mottled “stone washed” appearance.

Ground faced units (also called honed or burnished) are polished after manufacture to achieve a smooth finish with the appearance of polished natural stone. Coatings, which are often used to deepen the color, can also help cleaning efforts by preventing dirt and other contaminants from penetrating the surface. When using ground faced units, every effort should be made to keep the units clean and free from mortar smears and droppings during construction. When required, these units can be resealed after final cleaning.

Other architectural CMU, such as split faced, split fluted and tumbled units have a natural stone-like texture produced during manufacture. The rough texture tends to hide minor soiling and makes these units more forgiving of minor efflorescence or other discolorations. This texture is also more suited to cleaning with abrasives, if that technique is required.

Glazed concrete masonry units are manufactured by bonding a permanent colored facing (typically composed of polyester resins, silica sand and various other chemicals) to a concrete masonry unit. The result is a smooth impervious surface, highly resistant to staining and easily cleaned. During construction, mortar and grout smears and droppings should be cleaned off while still easily removable, before they harden completely to the surface.

Typical Sizes and Shapes of Concrete Masonry Units, CMUTEC-001 23 (ref. 5), contains more information on the various types of concrete masonry unit finishes.

Mortar

Choosing a mortar color close to that of the concrete masonry unit makes cleaning the wall of mortar smears easier, as the mortar tends to blend in. Mortar color should be chosen to match the unit color when smooth or ground faced units are used, as they can be difficult to clean without altering the appearance. Walls with contrasting mortar and masonry unit colors may require more aggressive cleaning to remove visible mortar.

In general, the lowest-strength mortar that will meet project requirements should be specified. Higher cement content mortars with higher compressive strengths should not be assumed to have better field performance, in fact the opposite is more often true. Mortars with lower compressive strengths tend to be easier to clean off the face of the wall than are stronger mortars. Lower strength mortars also exhibit better workability, which tends to increase quality of construction. Note that building codes may restrict the use of some mortars for particular applications. More detailed information on masonry mortars is available in TEK 09-01A, Mortars for Concrete Masonry (ref. 4).

CLEANING DURING CONSTRUCTION

Many stains can be prevented or minimized through proper design, construction and maintenance procedures. Construction practices can greatly impact the amount of cleaning required for a newly constructed wall. For example, proper grouting procedures can help prevent grout blowouts and the associated clean up. Keeping the masonry as clean and dry as possible during construction can allow for less aggressive cleaning methods when construction is complete.

Cleaning exposed concrete masonry during construction encompasses such issues as the control of efflorescence and of mortar and grout droppings and smears. Detailed information on construction practices that minimize efflorescence are discussed in Control and Removal of Efflorescence, TEK 08-03A (ref. 1). The following are recommended practices for minimizing mortar and grout stains during construction (refs. 3, 6):

Mortar squeezed out of mortar joints as units are placed should be cut off with the edge of the trowel, and care should be taken that the mortar doesn’t fall onto the wall surface or smear the surface as it falls off.
When mortar does land on or smear the surface of the concrete masonry unit, it should be removed after initial set. Walls should be cut and brushed clean before scaffolding is raised.
Do not cut mortar tags off until the mortar is thumbprint hard, particularly on split faced units. Similarly, joints should not be tooled until thumbprint hard.
Mortar droppings which adhere to the exposed face of the units can be removed with a trowel or chisel after being allowed to harden. Any remaining mortar can then be removed with a stiff fiber or bristle brush.
Excess mortar should be periodically removed from scaffolding.
Grout spills should be immediately removed by washing and brushing.
The base of the wall should be protected from splashing mud and mortar and grout droppings by spreading plastic sheets 3 to 4 ft (914 to 1,219 mm) on the ground adjacent to the wall and 2 to 3 ft (609 to 914 mm) up the face of the wall.

In addition to these recommendations, newly constructed masonry should be protected when adjacent construction procedures may splatter or otherwise stain the masonry. For example, plastic should be placed over masonry when concrete is poured nearby and when curing agent is sprayed.

PLANNING THE CLEANING PROCEDURE

The cleaning agent and procedure should be carefully planned, based on the type of contaminant and desired results. The cleaning method chosen should be the least aggressive that will effectively clean the wall. Before cleaning, ensure that mortar joints are cured, so the cleaning does not damage them.

Cleaning methods may alter the appearance of the finished masonry; typically, at least some cement paste is removed from the surface of the units. When this happens, more aggregate is exposed to view, which can alter the color. In general, the more aggressive the cleaning method, the more paste is removed and the greater the potential for altering the wall’s appearance. For example, sandblasting can be expected to alter the appearance to a greater degree than cleaning by hand with detergent and water. Note also that the same cleaning method may have different results based on the specific procedures used. Sandblasting at a lighter pressure will produce different results from sandblasting at a higher pressure. Again, the mildest cleaning method that will satisfy should be chosen.

The cleaning agent and procedures should first be used on a sample panel or inconspicuous location to assess: their effectiveness for the type of contaminant being removed; their effect, if any, on the finished masonry appearance; as well as the agreed upon level of cleanliness. After cleaning, the sample panel should be viewed from a distance of 20 ft (6,096 mm) under diffused lighting to evaluate the results.

Whichever cleaning method is chosen, it is important that all of the masonry be cleaned in the exact same manner (including dilution rate, brushing/scraping method, dwell time, reapplication, rinse procedure, etc.) to maintain a uniform appearance. Similarly, care should be taken to avoid overlap of areas being cleaned, as this may lead also to a nonuniform appearance.

Materials such as glass, metal, wood, architectural concrete or concrete masonry and any landscaping adjacent to the area to be cleaned should be adequately protected, since they may be damaged by contact with some stain removers or by physical cleaning methods. The level of protection and area requiring protection vary with the cleaning method, so the cleaning agent manufacturer’s recommendations should be followed.

If a surface water repellent is specified for the wall after cleaning, it should be applied as soon as conditions allow to minimize further moisture absorption or soiling.

CLEANING METHODS

The methods of cleaning concrete masonry can generally be divided into four categories: hand cleaning, water cleaning, abrasive cleaning and chemical cleaning (ref. 2). Cleaning by any method should be performed on an inconspicuous section of the building or a sample panel to ascertain its effect.

Hand Cleaning

Simple hand tools such as a trowel, chisel, stiff bristle or fiber brush, abrasive block or broken piece of masonry are first used for cleaning during construction. Steel-wire brushes should not be used because they can leave behind metal particles that may rust and stain the masonry.

Water Cleaning

Water cleaning involves scrubbing with water and detergent, water soaking, steam cleaning or pressure washing. When using water cleaning methods, the amount of water used should be limited to the least amount that will effectively clean the wall, as any water that enters the wall may promote efflorescence. See Control and Removal of Efflorescence, TEK 08-03A (ref. 1), for more detail.

Unpainted walls can usually be cleaned by scrubbing with water and a small amount of detergent. This is a nonaggressive cleaning method that generally does not alter the masonry appearance. It may not be cost-effective for large areas, however, due to the labor involved.

Clay or dirt should first be removed with a dry brush. Steel-wire brushes should not be used because any metal particles left on the masonry surface may rust and stain the masonry. Nonmetal brushes such as stiff fiber or nylon are preferred. Soaking with water causes dirt deposits to swell, loosening their grip on the underlying masonry and allowing them to be flushed away with water. Again, this method may not be appropriate if efflorescence is the primary concern.

Heated water is useful on greasy surfaces or during cold weather. However, when used with alkaline chemicals, warm water should not exceed 160° F (71° C). There is no significant advantage to using hot water with acid cleaners (ref. 2).

Pressure washing equipment can be effective for surface cleaning, and is often specified for masonry restoration work to avoid the use of harsh chemicals. Water pressure should be kept to a minimum to avoid driving water into the wall which can cause efflorescence. Note that high pressures can damage masonry or alter the final appearance. Using a consistent pressure and maintaining a set distance from the wall will produce the most uniform results. If high pressure cleaning is used, it is recommended that:

a) the pressure be limited to 400 to 600 psi (2.76 – 4.14 MPa),
b) a wide flange tip be used, never a pointed tip,
c) the tip be kept at least 12 in. (305 mm) from the masonry surface, and
d) the spray be directed at a 45o angle to the wall (never perpendicular to the wall). Pressure washing can also be used as an adjunct to scrubbing. The mild agitation created by brush application improves the overall cleaning results and enables the rinsing pressure to be kept to a minimum.

Steam cleaning has been virtually supplanted by pressure washing. However, by supplementing heat to the water, the action of loosening and softening dirt particles and grease is improved, allowing them to be more easily rinsed away. Steam is normally generated in a flash boiler and directed toward the wall using a wand at a pressure of 10 to 80 psi (69 to 552 kPa), depending on the equipment used. Although steam cleaning is less aggressive than pressure washing, it is also slower.

Chemical Cleaning

Many proprietary cleansing agents are available for concrete masonry; the concrete masonry manufacturer can be consulted for recommended compatible products. Premixed chemicals eliminate many potential problems, such as those associated with mixing reactive chemicals. They are also mixed in the proper proportions to be safely used on masonry. Strict adherence to the manufacturer’s directions is required, to protect both the user and the masonry, and to avoid any potentially harmful runoff.

When used in conjunction with water washing techniques, chemical surfectants help dissolve contaminants and allow them to be washed away during the final rinsing process. If chemical cleaning agents are used, the surfaces to be cleaned must be thoroughly prewetted with low water pressure (maximum 30 to 50 psi, 207 to 345 kPa), cleansing agents must be diluted as directed by the manufacturer and the application pressures should be kept to a minimum. After application of the cleansing agent, the wall should be thoroughly rinsed with fresh water (preferably at low pressure), or if necessary at high pressure using the precautions discussed in the Water Cleaning section.

Chemical cleaning can be a more aggressive method than pressure washing and is often more efficient and cost effective. With proper technique, the results are uniform across the wall, although the wall’s final appearance can be changed by using this method. Apply chemical cleaning solutions with low pressure spray (less than 50 psi, 345 kPa) or soft-fibered brushes.

Chemical cleaning solutions can be used to clean concrete masonry without damaging the surface; avoid using raw or undiluted acids. Even diluted acids should be used with caution, and only after thoroughly prewetting the wall, as acids dissolve the cement matrix at the masonry surface and can also damage any integral water repellent at the surface. This leaves the face more porous and exposes more aggregate, thereby changing the color and texture of the masonry. In the case of masonry with an integral water repellent, acids can also reduce the water repellency at the surface. Acids should never be applied under pressure. As a guideline, any cleaner with a pH below 4 or 5 should be considered to be acidic in nature. In addition, highly alkaline products require an acidic neutralizing afterwash as well as thorough rinsing; efflorescence can be an unwanted result if there is residual alkali.

Abrasive Cleaning

Abrasive cleaning is the most aggressive cleaning method, as the objective is not to wash away surface contaminants, but to remove the outer portion of the masonry in which the stain is deposited. For this reason, it should not be used on ground faced units, where the surface is smooth and polished. Although abrasive cleaning includes methods such as grinding wheels, sanding discs and sanding belts, it typically refers to grit blasting, also called sandblasting. Note that the use of silica sand is restricted in some areas due to its classification as an irritant, but many other blasting media are available.

Because it is a dry process, sandblasting will not promote efflorescence and can be performed in cold weather. As with pressure chemical cleaning, the cleaning method produces a consistent result across the wall with proper technique.

Care must be exercised when using abrasive cleaning techniques since overzealous applications can cause drastic changes to the appearance, durability and water tightness of the masonry. Sandblasting can alter the appearance of the masonry by roughening the surface or exposing aggregate. This is less of a concern with split faced units. In some cases, sandblasting can accelerate deterioration by increasing surface porosity. Pretesting using a sample panel is critical when sandblasting is considered.

To minimize potential damage, softer abrasives such as crushed corn husks, walnut shells or glass or plastic beads can be used. This process, sometimes called micro-peening, is slower and more costly and generally is not applicable to large scale cleaning operations.

Protective equipment and clothing must be used, including an approved respirator under a hood. Most of the dust that accompanies a dry sandblasting process can be eliminated by introducing water into the air-grit stream at the nozzle. However, the smaller particles remain a health hazard, so the same protective equipment and clothing are needed as for the dry process. The wet process requires the extra step of rinsing down the cleaned surface after blasting.

Sandblasting removes any previously applied water-resistant surface coatings, so these will need to be reapplied after abrasive cleaning.

CONCLUSION

Concrete masonry units are available in a variety of finishes, including ground faced, split faced and glazed. Contaminants from construction, such as mortar smears, and from the atmosphere after years of exposure can mar the otherwise attractive appearance of concrete masonry buildings. Cleaning methods that have been effective include hand cleaning and the use of water, chemical solutions and abrasive blasting. Some CMU manufacturers provide cleaning recommendations; in other cases, a knowledgeable professional may help determine how cleaning should best be accomplished. Field testing of cleaning materials and techniques helps ensure the desired results.

REFERENCES

Control and Removal of Efflorescence, TEK 08-03A, Concrete Masonry & Hardscapes Association, 2003.
Grimm, Clayford T., Cleaning Masonry – A Review of the Literature, Construction Research Center, University of Texas at Arlington, November 1988.
Masonry Cleaning Guide. Rocky Mountain Masonry Institute, 2001.
Mortars for Concrete Masonry, TEK 09-01A, Concrete Masonry & Hardscapes Association, 2004.
Concrete Masonry Unit Shapes, Sizes, Properties and Specifications, CMU-TEC-001-23, Concrete Masonry & Hardscapes Association, 2023.
Concrete Masonry Construction, TEK 03-08A, Concrete Masonry & Hardscapes Association, 2001.
Removal of Stains From Concrete Masonry, TEK 08-02A, Concrete Masonry & Hardscapes Association, 1998.

Removal of Stains From Concrete Masonry

August 2018

INTRODUCTION

With the continued use and expanding applications of architectural concrete masonry, segmental retaining wall units, and concrete pavers, exposed concrete masonry is becoming common across the country. Although maintenance of a well designed and constructed masonry wall is minimal, inadvertent staining from oil, grease, or other foreign substances can destroy the appearance of an otherwise attractive unpainted masonry structure. This publication provides information on effective methods for removing some of the most common stains.

STAIN PREVENTION

Many stains can be prevented or minimized through proper design, construction, and maintenance procedures. For instance design details that prevent or reduce water intrusion reduce the chance that efflorescence will occur – see Maintenance of Concrete Masonry Walls, TEK 08-01A (ref. 1).

During construction of exposed concrete masonry, minimize mortar and grout smears on the face of the units. Mortar droppings which adhere to the exposed face of the units can be removed with a trowel or chisel after being allowed to harden. Any remaining mortar can then be removed with a stiff fiber brush. Also, the base of the wall should be protected from splashing mud and mortar droppings by spreading plastic sheets 3 to 4 feet on the ground and 2 to 3 feet up the wall. Covering the tops of unfinished walls at the end of the workday prevents rain from entering the wall and thus reduces the chance of efflorescence forming on the wall. Covers should be draped at least two feet down each side of the wall and a method provided to hold them in place. See Cleaning Concrete Masonry, TEK 08-04A (ref. 6) for more information on cleaning concrete masonry during construction and further information on cleaning concrete masonry.

PLANNING AND PRECAUTIONS

The cleaning procedure should be carefully planned. No attempt should be made to remove a stain until it is identified and its removal agent determined. If the staining substance cannot be identified, it is necessary to experiment with different methods on an inconspicuous area. The indiscriminate use of an inappropriate product or the improper application of a product may result in spreading the stain over a larger area or in causing a more unsightly, difficult to remove stain. Removing stains from concrete masonry sometimes can leave the treated area lighter in color than the surrounding area because surface dirt has been removed along with the stain or the surface has become slightly bleached. This is particularly true for buildings that are several years old. This may necessitate treating the entire wall. Materials such as glass, metal, wood or architectural concrete or concrete masonry adjacent to the area to be cleaned should be adequately protected since they may be damaged by contact with some stain removers or by physical cleaning methods.

Many chemicals can be applied to concrete masonry without appreciable injury to the surface, but strong acids or chemicals with a strong acid reaction definitely should be avoided. Even weak acids should be used only as a last resort as it dissolves the cement matrix of the masonry beginning at the surface. This leaves the face more porous so that it absorbs more water and exposes more aggregate thereby changing the color and texture of the masonry.

CLEANING METHODS

The methods of cleaning concrete masonry can generally be divided into three categories water cleaning, abrasive cleaning, and chemical cleaning (ref. 2).

Water Cleaning

Water cleaning includes the use of water soaking, steam cleaning and pressure washing. Cleaning of unpainted walls can usually be accomplished by scrubbing with water and a small amount of detergent. Clay or dirt first should be removed with a dry brush. Steel wire brushes should not be used because they can leave metal particles on the surface of the masonry that later may rust and stain the masonry. Nonmetal brushes such as stiff fiber or nylon are preferred. Soaking with water causes dirt deposits to swell, loosening their grip on the underlying masonry and then allowing them to be flushed away with water. Some efflorescence can be removed when it first appears by dry brushing followed by flushing with water. More extensive efflorescence may require brushing with acid see the section on chemical cleaning or Control and Removal of Efflorescence, TEK 08-03A (ref. 3).

Heated water is useful on greasy surfaces or during cold weather. However, warm water when used with alkaline chemicals, should not exceed 160° F (71° C). There is no significant advantage to using hot water with acid cleaners (ref. 2).

Steam cleaning virtually has been supplanted by improved and innovative pressure washing equipment. However, by supplementing heat to the soaking with water, the action of loosening and softening of dirt particles and grease is improved allowing them to be more easily rinsed away. The steam is normally generated in a flash boiler and directed toward the stain by means of a wand at a pressure of 10 to 80 psi depending on the equipment used. A drawback with steam cleaning is that is rather slow when compared to pressure washing. An advantage of steam cleaning is that it essentially leaves the concrete masonry surface intact.

High-pressure washing equipment can be extremely effective for restorative cleaning of older masonry; however, when improperly applied, it can cause severe damage. If pressure application of chemical cleaning agents is considered, the surfaces to be cleaned must be thoroughly prewetted, cleansing agents must be prediluted, and the application pressures should be kept to a minimum. High pressure washing, however, should not be mistaken as a total replacement for hand labor. The mild agitation created by brush application improves the overall cleaning results while enabling rinsing pressure to be kept to a minimum.

Abrasive Cleaning

The objective in abrasive cleaning is not to dissolve and wash away the stain, but to remove the outer portion of the masonry in which the stain is deposited. Included in this category are grinding wheels, sanding discs, sanding belts, and the more popular grit blasting. Silica sand in recent years has been replaced as the abrasive blasting material by other products such as crushed slag in the concern over health hazards posed by airborne silica dusts. Protective equipment and clothing must be used, including an approved respirator under a hood.

Care must be exercised when using abrasive cleaning techniques since over zealous applications can cause drastic changes to the appearance, durability, and water tightness of the masonry. To minimize this, softer, less damaging abrasives such as crushed cornhusks, walnut shells, glass beads, etc. can be used on more delicate surfaces. This process, sometimes called micro-peening, is slower and more costly and generally is not applicable to large scale cleaning operations.

Most of the dust that accompanies the dry process can be eliminated with wet abrasive cleaning by introducing water into the air-grit stream at the nozzle. However the smaller, harmful particles remain a health hazard so the same protective equipment and clothing are needed as for the dry process. The wet process requires the extra step of rinsing down the cleaned surface after blasting.

Needless to say, previously applied waterproofing agents are removed during the abrasive cleaning process. Therefore, they need to be reapplied after abrasive cleaning.

Chemical Cleaning

The popularity of chemical cleaning techniques has increased substantially in recent years. When used in conjunction with one of the water washing techniques previously described, chemical solvents dissolve staining materials and allow them to be washed away during the final rinsing process.

Many proprietary cleansing agents for removal of stains are available today. They are generally much safer for the user in that the chemicals are premixed so there virtually is no danger of mixing reactive chemicals and also for the masonry in that they are mixed in the proper proportions. Strict adherence to the manufacturer’s directions is still required, however, as improper use can still pose danger to both the user and the masonry. For the most part, products suitable for concrete are suitable for concrete masonry and can be found at most construction specialty and automotive supply centers and at hardware or paint stores.

Tables 1 and 2 provide information covering the removal of many common materials that stain. Table 1 describes the chemicals, detergents, or poultice materials recommended for a particular stain. Table 1 also provides letter keys which indicate steps to be followed in the removal of the stain identified in Table 2.

A poultice is a paste made with a solvent or reagent and a finely powdered, absorbent, inert material used to keep stains from penetrating deeper or spreading. It also tends to pull the stain out of the pores. Enough of the solvent or reagent is added to a small quantity of the inert material to make a smooth paste. The paste is spread in a ¼ in. to ½ in. (6 to 13 mm) thick layer onto the stained area and allowed to dry. The solvent dissolves the staining substance and absorbs it into the poultice and is left as a loose, dried powdery residue that can be scraped or brushed off (ref.4). This process frequently takes several applications to remove the stain.

CHEMICAL SUBSTANCES

The following text provides general information on the chemicals and cleaning agents referenced in Table 1 (ref. 5). As with any chemical, refer to the chemical’s Material Safety Data Sheet and always follow label directions.

Ammonium Chloride (Other names: Amchlor, chloride of ammonia, darammon, salammonite)
Odorless white crystalline substance used in some agricultural processes. Available from chemical and dry-cleaning supply centers and hardware stores.
Hazards: Toxic and corrosive.

Ammonium Citrate (Other names: Citric acid, diammonium salt) White odorless substance in either granular or crystalline form. Found at supermarkets and hardware stores. Hazards: Corrosive and flammable.

Ammonium Hydroxide (Other names: Ammonia solution, ammonia water, household ammonia) A colorless liquid with a strong irritating odor. Found at most supermarkets and hardware stores. Hazards: Toxic.

Ammonium Sulfamate (Other names: Amicide, ammonium amidosulphate) A white crystalline substance commonly used as a weed killer. Found at chemical and garden supply centers. Hazards: None.

Benzene (Other names: Benzol, benzole, coal naptha) An excellent solvent and colorless liquid with characteristic odor and burning taste. Found at automotive, chemical and dry cleaning supply centers and hardware and paint stores. Hazards: Violently flammable and carcinogenic

Calcium Hypochlorite (Other names: B-K Powder, losantin, pool chlorine) White in powder, granule, or pellet form used to kill algae, fungus, and bacteria. Found in pool chemical and garden supply centers. Hazards: Corrosive to flesh and flammable when in contact with organic solvents.

Carbon Tetrachloride (Other names: Perchloromethane, tetrachloromethane) A nonflammable, clear, poisonous liquid used in fire extinguishers and as a solvent. Available at chemical, dry cleaning, and pharmaceutical supply centers, and paint stores. Hazards: Toxic.

Glycerine (Other names: Glycerol, glycyl alcohol) An odorless, colorless, syrupy liquid prepared by the hydrolysis of fats and oils. Found at chemical, pharmaceutical, photographic, and printer supply centers. Hazards: Flammable.

Denatured Alcohol (Other names: Methylated Spirit) Found at pharmaceutical and printer supply centers and hardware stores. Hazards: Toxic and flammable.

Hydrochloric Acid (Other names: Muriatic acid) A strong, highly corrosive acid commonly used for cleaning metals and balancing the pH of swimming pools. It can be found at swimming pool supply centers, chemical supply centers and hardware stores. Hazards: Toxic, very corrosive to flesh and concrete materials. Reacts vigorously with ammonia and detergents containing ammonia. Use extreme caution when handling and applying. Never use full strength. Dilute by adding acid to water, never water to acid. Rinse thoroughly within 10 minutes after applying.

Hydrogen Peroxide (Peroxide of hydrogen) A colorless, syrupy liquid used as a bleaching and disinfectant in low concentrations and as a rocket fuel in higher concentrations. Available at chemical supply centers, drug stores, supermarkets, and hardware stores. Hazards: None in the normal 3% solution. Toxic, corrosive to flesh and flammable in higher concentrations.

Sodium Citrate (Other names: Citrate of soda, trisodium citrate) White odorless substance in crystalline, granular, or powder form. Commonly used as a neutralizing buffer in chemical research. Available from chemical supply centers and drug stores. Hazards: None.

Sodium Hydrosulfite (Other names: Hydrolin) White powder with little odor. Commonly used in industrial cleaners. Found at chemical supply centers. Hazards: Very toxic when in contact with moisture.

Sodium Hypochlorite (Other names: Clorox, hypochlorous acid, household bleach) Faint yellow to clear liquid with chlorine smell. Available at supermarkets. Hazards: Corrosive to flesh.

Sodium Perborate (Other names: Perboric acid, perborax, sodium salt) White, odorless, crystalline powder commonly found in “allinone” laundry detergents and some dishwashing powders. Available at chemical and pharmaceutical supply centers and supermarkets. Hazards: Toxic and flammable when in contact with organic solvents.

Trichloroethylene (Other names: TCE, ethynyl trichloride) Colorless liquid with chloroform smell found in common cleaning solvents. Available at automotive, chemical, dry cleaning, paint, photographic, and printer’s supply centers. Hazards: Highly toxic and can react with strong alkalies in fresh mortar or concrete to form dangerous gases.

Trisodium Phosphate (Other names: Sodium orthophosphate, TSP, phosphate of soda) A crystalline, white, odorless compound found in household cleaning detergents such as “Spic and Span”. Available at supermarkets and hardware stores. Hazards: Corrosive to flesh

MATERIALS FOR POULTICES

The main properties desired in the powdered materials used to make poultices are: 1) grains sufficiently fine so the paste will hold plenty of liquid; 2) enough range in particle size so they will make a smooth, readily moldable paste; and 3) chemical inertness to the chemicals with which the powdered material is used. The last precludes using portland cement in combination with water, although it can be used with organic liquids. For the same reason, if acids are to be used, the paste must not be made with whiting (calcium carbonate), ground limestone, hydrated lime, or portland cement. Otherwise, the finely divided materials are more or less interchangeable.

Diatomaceous Earth (Other name: Diatomite, filter media, fuller’s earth) Available at swimming pool supply centers.

Lime (Other names: Calcium hydroxide, caustic lime, mason’s lime, quicklime) Available at building material supply centers and nurseries.

Portland Cement (Other names: Cement) Found at building material supply centers and ready mixed concrete plants.

Talc (Other names: Talcum powder) A very soft mineral that is a basic silicate of magnesium, has a soapy feel, usually white in color, and is used especially in making talcum powder. Available at supermarkets and drug stores.

Whiting (Other names: Calcium carbonate, baking powder) Found in nature as calcite and aragonite and in plant ashes, bones and shells. Available at supermarkets and nurseries.

REFERENCES

Maintenance of Concrete Masonry Walls, TEK 08-01A, Concrete Masonry & Hardscapes Association, 2004
Grimm, Clayford T., Cleaning Masonry A Review of the Literature, Construction Research Center, University of Texas at Arlington, November 1988.
Control and Removal of Efflorescence, TEK 08-03A. Concrete Masonry & Hardscapes Association, 2003.
Removing Stains and Cleaning Concrete Surfaces, Portland Cement Association, 1988.
Removing Stains from Concrete, Concrete Construction Publications, Inc., May 1987.
Cleaning Concrete Masonry, TEK 08-04A, Concrete Masonry & Hardscapes Association, 2005.

Aesthetic Design With Concrete Masonry

August 2018

INTRODUCTION

One aspect of concrete masonry that has kept it at the forefront of building materials is its ability to incorporate and reflect a broad spectrum of existing architectural styles, as well as providing the designer with the ability to develop and present unique aesthetic affects and techniques. When skillfully designed, simple materials can provide unparalleled aesthetic enhancement. Inventive patterns, color choices (unit and mortar), unit sizes, and surface finishes (split face and standard) can be used in various concrete masonry bond patterns to evoke a sense of strength, modernity, tradition, or even whimsy.

Within the confines of meeting applicable building codes and specifications, concrete masonry’s modular sizes and range of colors, textures and patterns provide ample opportunity to demonstrate a design technique or overcome design challenges. In addition to the architectural finish, concrete masonry can provide the wall’s structure, fire resistance, acoustic insulation, and energy envelope.

This TEK addresses the proper application of architectural enhancements in concrete masonry wall systems. Where appropriate, related TEK and other documents are referenced to provide further information and detail.

Communication With Clients

Common dilemmas faced by designers are a client’s changing expectations and responses to the project’s changing appearance over time and under varying conditions. As discussed below, there are some basic requirements relative to aesthetics, but these are far from comprehensive. It is important to realize that code requirements primarily govern structural performance, not aesthetics. For example, code required construction tolerances are designed to ensure that masonry units are placed such that the completed wall can act structurally as an integrated unit.

These requirements assume an understanding of the techniques unique to the nature of masonry. The design and construction team should establish and consistently support ground rules affecting aesthetic interpretations of a project. It is also important for the client to realize the aesthetic standard that the project is based on, and that unusually high aesthetic standards can be more costly. In addition, certain high-profile areas, such as a building entrance, may require a custom level of quality, commensurate with an additional cost for the defined area. Several state and local masonry associations have developed guidelines for defining aesthetic requirements, and these can be a good resource for clarifying a project’s aesthetic standards.

Sample panels are a good means to communicate the minimum contract-based aesthetic standard to all parties. The sample panel is typically constructed prior to the project, and in some cases a portion of the work can serve as the sample panel. The sample panel remains in place or at least available until the finished work has been accepted, since it serves as a comparison for the finished work. The sample panel should contain the full acceptable range of unit and mortar color, as well as the minimum expected level of workmanship. Cleaning procedures, as well as application of any coatings or sealants, should also be demonstrated on the sample panel. See TEK 08-04A, Cleaning Concrete Masonry, (ref. 1) for more information on cleaning.

CONSIDERATIONS FOR CHOOSING CONCRETE MASONRY UNITS

Architectural Concrete Masonry Units

One of the most significant architectural benefits of designing with concrete masonry is its versatility—the finished appearance of a concrete masonry wall can be varied with the unit size and shape, color of units and mortar, bond pattern, and surface finish of the units. The term “architectural concrete masonry units” typically is used to describe units displaying any one of several surface finishes that affect the color or texture of the unit, allowing the structural wall and finished surface to be installed in a single step. CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, (ref. 2) provides an overview of some of the more common architectural units, although local manufacturers should be consulted for final unit selection.

Architectural concrete masonry units are used for interior and exterior walls, partitions, terrace walls and other enclosures. Some units are available with the same treatment or pattern on both faces, to serve as both exterior and interior wall finish material, increasing both the economic and aesthetic advantages. Architectural units comply with the same performance-based quality standards as conventional concrete masonry, such as Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90 (ref. 3). See Aesthetics in ASTM C90 (page 4) for more information.

Concrete Masonry Unit Color

Being produced from natural aggregates, concrete masonry has natural color variations from unit to unit. When a more monotone appearance is desired, there are various techniques that may be specified to increase the color uniformity in concrete masonry. Perhaps the best method is to specify the use of mineral pigments in the concrete mix, which are available in a wide range of colors. Pigments provide an integral color throughout the unit and minimize variations in color and texture found naturally in aggregate and sand deposits. Using several colors of integrally-colored concrete masonry units in the same wall is an effective technique for producing other visual impacts, such as two-tone banding or complementary color palates (see Figure 1).

Other methods are also used to improve color uniformity. One method is to specify the use of a post-applied stain, paint or coating on the units. With a paint or coating, the resulting film minimizes the texture of the masonry surface as well as the visual impact of the mortar joints. Paints and coatings for concrete masonry should be compatible with the masonry, and should in general allow for water vapor transmission. TEK 19-01, Water Repellents for Concrete Masonry Walls, (ref. 4) contains information on the applicability of different types of paints and coatings for concrete masonry walls.

A more laborious method to improve color uniformity is to arrange with the masonry contractor for a pre-sorting of on-site supplied block during certain stages of construction.

Interaction With Sunlight

Because it is produced from natural materials, concrete masonry walls often interact with changing sunlight in much the same way that natural stone does, appearing to change color as the light hits the wall at different angles. Figure 2 shows how even a conventional gray concrete masonry wall can interact with sunlight to present a range of color. This same attribute can be used to advantage with electric lighting, as well as on interior walls.

Fluted concrete masonry units provide a rich texture and tend to enhance the sound attenuating properties of concrete masonry.

The vertical flutes also provide an interesting interplay of light and shadow, which can be much more dramatic than smoothfaced units.

MORTAR JOINTS

While mortar generally comprises less than ten percent of a typical concrete masonry wall surface area, it can have a significant impact on the overall aesthetics of the completed structure. Mortar joint finishing, profiles and color can all impact the overall wall aesthetics. See also Concrete Masonry Handbook for Architects, Engineers, Builders (ref. 5) for information on mortar joints.

Mortar Joint Tooling

Tooling refers to finishing the mortar joints with a profiled tool that shapes and compacts the surface of the joint and provides a sharper, cleaner appearance for the wall. The surface shape of the tool determines the joint’s profile (discussed in more detail in the following section). Tooling mortar joints also helps seal the outer surface of the joint to the adjacent masonry unit, improving the joint’s weather resistance. For this reason, tooled joints that compact the mortar and do not create ledges to hold water are recommended for construction that will be exposed to weather.

Mortar joints should be tooled when the mortar is thumbprint hard (a clear thumbprint can be pressed into the mortar without leaving cement paste on the thumb). Tooling the joints before they reach this stage results in lighter colored joints, because more cement paste is brought to the surface of the joints. Joints tooled too early can also subsequently shrink away slightly from the adjacent concrete masonry unit. Tooling at the proper time allows this initial shrinkage to occur, then restores contact between the mortar and the unit producing a more weatherresistant joint. Conversely, later tooling can produce a darker joint. A consistent time of tooling will minimize variations in the final mortar color.

For the cleanest result, horizontal mortar joints should be tooled before vertical joints. For white and light-colored mortar, Plexiglas jointers can be used to avoid staining the joints during tooling. After all joints are tooled, any mortar burrs on the wall should be trimmed off with a trowel or other tool (a tool such as a plastic loop is easier to use on a split face wall than a trowel, for example). As a final step the joints are dressed using a brush, a piece of burlap, or similar material.

Mortar Joint Profiles

Traditional mortar joint profiles are illustrated in Figure 3. For walls not exposed to weather, the joint profile selection can be based on aesthetics and economics (as some joint profiles are more labor intensive to produce). For exterior exposures, however, the mortar joint profile can impact the wall’s weather resistance, as discussed above.

Unless otherwise specified, mortar joints should be tooled to a concave profile when the mortar is thumbprint hard (refs. 6, 7). For walls exposed to weather, concave joints (Figure 3a) improve water penetration resistance by directing water away from the wall surface. In addition, because of the shape of the tool, the mortar is compacted against the concrete masonry unit to seal the joint. V-shaped joints (Figure 3b) result in sharper shadow lines than concave joints.

Grapevine and weather joints (Figures 3c, 3d) provide a water shedding profile, but do not result in the same surface compaction as concave or V-shaped joints. Both are used in interior walls to provide strong horizontal lines.

Beaded joints (Figure 3e) are formed by tooling the extruded mortar into a protruding bead shape. Care must be taken to obtain a straight line with the bead. Although technically a tooled joint, the beaded tooler does not produce the same mortar surface compaction as a concave or V-shaped tool. In addition, the protruding bead can allow water, ice or snow to collect. Therefore, beaded joints are not recommended for weather-exposed construction.

Flush joints (Figure 3f) are typically specified when a wall will be plastered. Excess mortar is simply struck off the face of the wall with the trowel, then dressed with a brush or other tool.

Squeezed or extruded joints (Figure 3g) are made using excess mortar that is squeezed out as units are laid. They may be specified for interior walls.

Struck joints (Figure 3h) provide a strong horizontal line, similar to weather joints, however because the shape provides a ledge for rain, ice or snow, they are not recommended for walls that will be exposed to weather. Raked joints (Figure 3i) provide a dramatic contrast between the units and mortar joints. They are formed using a joint raker, which removes the mortar to a maximum depth of 1/2 in. (13 mm). With raked joints, small imperfections on unit edges can be more noticeable, because the mortar is not compacted against the unit (the compaction tends to fill in small surface irregularities along the unit edge). The resulting joint is not weather-resistant, and may not leave enough mortar cover over horizontal joint reinforcement (joint reinforcement is required to have 5/8 in. (16 mm) mortar cover in walls exposed to weather or earth (refs. 6, 7)). A better option for exterior surfaces is to specify an integrally colored mortar to provide the visual contrast.

Mortar Joint Color

Choosing a specific mortar color allows additional creativity by specifying integral color to either provide a visual contrast or to match the unit color, as shown in Figure 4. Note that using a mortar color that matches the surrounding units minimizes the effects of minor mortar staining; i.e., with a contrasting mortar color, greater care should be used to remove mortar droppings and splatters from the masonry units.

Because foreign material in mortar sand can affect the mortar quality, as well as appearance, ASTM C144, Standard Specification for Aggregate for Masonry Mortar (ref. 8), limits deleterious substances in aggregates for masonry mortars. Sand can also affect mortar color: sands from different natural sources may have different hues. Therefore, all of the sand for a particular project should come from the same source. Silica sand, which is more expensive than typical masonry sand, is often specified for white mortar. Consistent batching and mixing procedures also help produce uniform mortar color from batch to batch. See TEK 03-08A, Concrete Masonry Construction (ref. 9), for further information.

Using a consistent amount of mix water is important to maintain color uniformity for all mortars and especially when using integrally colored mortar. Changing the amount of water can significantly change the resulting mortar color intensity. For this reason there are special methods and equipment, such as shading materials and equipment from direct sunlight, the use of cooled water, and the use of damp, loose sand piles to reduce excessive retempering. Mortar that is too stiff or older than 2 1/2 hours after initial mixing is to be discarded.

EXPECTATIONS FOR UNITS AND CONSTRUCTION

Aesthetics in ASTM C90

ASTM C90 provides minimum requirements for concrete masonry units that assure properties necessary for quality performance. The specification includes requirements for materials, as well as dimensional and physical requirements such as minimum compressive strength, maximum water absorption, maximum dimensional tolerances, and maximum linear drying shrinkage. It also includes finish and appearance criteria for concrete masonry units.

It should be noted that the requirements in ASTM C90 are intended to address the performance of the masonry units when installed, not the aesthetics of the units nor of the constructed masonry. The time for product inspection is before placement. As such, the finish and appearance criteria, for example, prohibits defects that would impair the strength or permanence of the construction, but permit minor cracks or chips incidental to usual manufacturing, shipping and handling methods.

Qualities that are not included in C90 include color, surface texture, surface features such as scores or flutes, density choice, water repellency, fire resistance rating, thermal properties and acoustic properties. If required, these properties must be addressed in project contract documents. ASTM C90 does, however, include acceptance criteria for unit color and surface texture: namely, that the finished unit surfaces that will be exposed in the final structure conform to an approved sample of at least four units. The sample should represent the range of color and texture permitted on the job. As a practical matter, color and texture should be expected to vary somewhat due to the nature of the material.

The ASTM C90 specification is described in more detail in CMU-TEC 001-23, (ref. 2).

Considerations for Integrally Colored Smooth-Faced Units

Integrally-colored concrete masonry units are available in a wide variety of colors and shades. The mineral oxide pigments are evenly dispersed throughout the concrete mix, producing a low-maintenance enhancement that lasts the life of the structure.

During unit manufacture, the integrally-colored concrete mix is placed into a steel mold, which is stripped off while the concrete is still plastic. This stripping of the mold draws moisture and coloring pigment to the unit surface, which impacts the surface appearance. On split-faced or ground-faced units, this surface is either ground away or not exposed (in the case of split-faced units). Because the formed surface is the final exposed surface on smooth-faced units, however, these units will have a wider color variation than is seen with split-faced or ground-faced units. Understanding this color variation will help avoid possible disappointment that the finished wall does not have the color uniformity of a painted or stained wall.

Construction Tolerances

The International Building Code and Specification for Masonry Structures (refs. 6, 7) contain site tolerances for masonry construction which allow for deviations in the construction. The permissible tolerances are intended to ensure that misalignment of units or structural elements does not impede the structural performance of the wall. Although the tolerances are not intended for the purpose of producing an aesthetically pleasing wall, these tolerances are generally adequate for most aesthetic applications as well. If tighter tolerances are desired, they must be specified in the project documents.

As an example, unless otherwise specified, the actual location of a masonry element is required to be within a certain tolerance of where the element is shown on the construction drawings: + 1/2 in. in 20 ft, + 3/4 in. max (+ 13 mm in 6.2 m, + 19 mm max). More precise placement dimensions can be specified, typically at a higher cost.

Tolerances apply to: plumb, alignment, levelness and dimensions of constructed masonry elements, location of elements, levelness of bed joints, mortar joint thickness, and width of collar joints, grout spaces and cavities. A full discussion of code-required masonry construction tolerances is presented in TEK 03-08A, Concrete Masonry Construction (ref 9).

MODULAR COORDINATION

Concrete masonry structures can be constructed using virtually any layout dimension. However, for maximum construction efficiency, economy, and aesthetic benefit, concrete masonry elements should be designed and constructed with modular coordination in mind. Modular coordination is the practice of laying out and dimensioning structures to standard lengths and heights to accommodate modularly-sized building materials.

Standard concrete masonry modules are typically 8 in. (203 mm) vertically and horizontally, but may also include 4-in. (102 mm) modules for some applications. These modules provide the best overall design flexibility and coordination with other building products such as windows and doors. Designing a concrete masonry building to a 4- or 8-in. (102- or 203-mm) module will minimize the number of units that need to be cut, providing a more harmonious looking masonry structure. TEK 05-12, Modular Layout of Concrete Masonry (ref. 10) provides details of modular wall layouts and openings.

CONTROL JOINTS

Control joints, a type of movement joint, are one method used to relieve horizontal tensile stresses due to shrinkage of concrete products and materials. They are essentially vertical planes of weakness built into the wall to reduce restraint and permit longitudinal movement due to anticipated shrinkage. When control joints are required, concrete masonry requires only vertical control joints. When materials with different movement properties are used in the same wythe (such as clay masonry and concrete masonry), this movement difference needs to be accommodated, and may require horizontal movement joints as well (see the Banding section, below). Recommendations for band in a split-faced wall (see Figure 5); with different unit sizes, such as the use of a 4-in. (102-mm) high band in a wall of 8-in. (203-mm) units; or with a combination of these techniques. Combining masonry units of different size, color and finish provides a virtually limitless palette.

The use of concrete masonry bands in clay brick veneer has also become very popular. The architectural effect is very pleasing; however, proper detailing must be provided to accommodate the different movement properties of the two materials to prevent racking. The detail shown in Figure 6 has demonstrated good performance in many areas of the United States and is the preferred detail, as it is economical and maintenance free. Horizontal joint reinforcement is placed in the mortar joints above and below the band, as well as in the band itself if it is more than two courses high. In addition, lateral support (wall ties) are provided within 12 in. (305 mm) of the top and bottom of the band and the band itself must contain at least one row of ties. Some designers prefer placing joint reinforcement in every bed joint of the concrete masonry band. In this case, a tie which accommodates both the tie and reinforcement in the same joint (such as seismic clips) should be used. Another, but less recommended, option is to use horizontal slip planes between clay masonry and the concrete masonry band (see TEK 05-02A, Clay and Concrete Masonry Banding Details, Reference 12).

The maximum spacing of expansion joints in the clay masonry wall should be reduced to no more than 20 ft (6.1 m) when concrete masonry banding is used. When the clay masonry expansion joint spacing exceeds 20 ft (6.1 m), an additional control joint should be placed near mid-panel in the concrete masonry band, although the joint reinforcement should not be cut in this location. At locations control joint spacing, locations and construction details can be found in CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (ref. 11).

Aesthetically, control joints typically appear as continuous vertical lines in the field of the masonry walls, and perhaps at other areas of stress concentration, such as adjacent to openings, at changes in wall height, etc. Several strategies can be used to make control joints less noticeable. Perhaps the simplest approach is to align the control joint with another architectural feature, such as a pilaster or recess in the wall. In this case, the vertical shadow line provided by the architectural feature provides an inconspicuous control joint location.

BANDING

Concrete masonry banding is successfully used in many architectural applications. Banding can be accomplished with different colors of block; with different textures, for example a smooth-faced of expansion joints in the clay masonry, joints should be continued through the concrete masonry band and the joint reinforcement cut at these locations. TEK 05-02A provides a fuller discussion and additional details for combining these two materials, including details for incorporating clay masonry bands into concrete masonry walls.

LIGHTING DESIGN CONSIDERATIONS FOR CONCRETE MASONRY WALLS

Masonry has historically been associated with diffuse illumination located on or recessed into ceilings, as step (walkway) fixtures located below the waist, or generally placed at a distance from the masonry wall assembly. Diffuse lighting does not concentrate a focused beam but rather spreads the light to provide soft illumination. Although this is sometimes accomplished using an array of many individual light sources at a distance, it is more typically accomplished with fixtures and devices made for this purpose. When wall-mounted light sources are necessary, there are specialized fixtures adapted for masonry that internally refract, reflect, deflect, partially block, diffuse, and/or shade light from directly impinging on the wall surface. Often, the fixture includes additional light diffusers facing away from the wall surface to assist in softly lighting the adjacent area. No noticeable shadows are cast onto the wall, because the shadow is intentionally located away from the wall surface, thus masonry aesthetics are enhanced with a lower lighting intensity and more graceful illumination. These concepts are illustrated in Figure 7.

Non-diffuse light shining onto a concrete masonry wall from a surface mounted light fixture or sconce can sometimes cast unwanted long shadows, giving the erroneous visual appearance of unacceptably poor materials or workmanship (see Figure 7). With non-diffuse light, glossy surface treatments and coatings could also inadvertently magnify this problem. Well-designed diffuse light can eliminate such concerns.

Certain concrete masonry units, such as ground face (also called honed or burnished), can be highly reflective. Figure 8 shows a residential project using a custom-fabricated white ground face block. The designer used a complementary balance of several lighting fixtures with what might have otherwise been a challenging masonry reflective finish. The harmonious use of interior lighting combined with exterior overhead (recessed trim) and step lighting is an effective way of solving this challenge.

REFERENCES

Cleaning Concrete Masonry, TEK 08-04A. Concrete Masonry & Hardscapes Association, 2005.
Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, CMU-TEC-001-23, Concrete Masonry &
Hardscapes Association, 2023.
Standard Specification for Loadbearing Concrete Masonry Units, ASTM C90-09. ASTM International, 2009.
Water Repellents for Concrete Masonry Walls, TEK 19-01.
Concrete Masonry & Hardscapes Association, 2006.
J. A. Farney, Melander, J. M., and Panarese, W. C., Concrete Masonry Handbook for Architects, Engineers, Builders, Sixth Edition, Engineering Bulletin 008. Portland Cement Association, 2008.
International Building Code, International Code Council, 2009.
Specification for Masonry Structures, TMS 602/ACI 530.1/ASCE 6. Reported by the Masonry Standards Joint Committee, 2008.
Standard Specification for Aggregate for Masonry Mortar, ASTM C144-04. ASTM International, 2004.
Concrete Masonry Construction, TEK 03-08A. Concrete Masonry & Hardscapes Association, 2001.
Modular Layout of Concrete Masonry, TEK 05-12. Concrete Masonry & Hardscapes Association, 2008.
Crack Control Strategies for Concrete Masonry Construction, CMU-TEC-009-23, Concrete Masonry & Hardscapes Association, 2023.
Clay and Concrete Masonry Banding Details, TEK 05-02A.
Concrete Masonry & Hardscapes Association, 2002.