ABSTRACT Elastomeric wall coatings are an integral part of above-grade waterproofing and masonry-wall restoration. They are designed to provide functional performance and crack bridging over all types of masonry construction. A unique and novel test appa¬ ratus called the “Climate Drive Durability Tester” will be described; it analyzes weath¬ ering and membrane movement under in-service conditions. The need for coating per¬ meance and the phenomenon of carbonation (i.e., corrosion attack on metal rebars by atmospheric CO2 and moisture) will also be described. The attendee will gain a fun¬ damental understanding of the technology that underlies elastomeric wall coatings. Case histories will be used to exemplify the basic concepts. SPEAKER Bill Kirn is technical director and key accounts manager for National Coatings Corporation. He has been conducting research on roofing and construction materials for over 25 years, with emphasis on coatings and polymer applications. Bill is a Registered Roof Consultant (RRC) and was on the faculty of the Roofing Industry Educational Institute (RIEI). He is a recipient of the Richard M. Horowitz Award for excellence in writing for RCI’s Interface. Kirn currently chairs the technical committee and is on the board of directors of the Cool Roof Rating Council (CRRC). He is past president of the Reflective Roof Coating Institute (RRCI). He is on the board of directors of the Energy Coordinating Agency, a nonprofit organization that assists low-income seniors with energy related needs in Philadelphia, PA. Bill holds a bachelor’s degree in chemistry from Temple University, a master’s in organic chemistry from St. Joseph’s University, and an MBA from Temple University. CONTACT INFO: bkirn@nationalcoatings.com or 805-388-7112 Kirn -110 Proceedings of the RCl 24th International Convention
ABSTRACT
Elastomeric wall coatings are an integral part of above-grade waterproofing and masonry wall restoration. This paper will begin with a review of the fundamental research on elastomeric wall coat¬ ings. A novel mathematical model for determining the crack-bridg¬ ing requirements for elastomeric coatings as a function of a build¬ ing’s geographic location will be elucidated. A unique and novel test apparatus, called the “Cli¬ mate Drive Durability Tester,” will be described; the apparatus ana¬ lyzes which provides weathering and membrane movement under in-service conditions. The need for coating permeance and the phe¬ nomenon of carbonation, (i.e., cor¬ rosion attack on metal rebars by atmospheric CO2 and moisture) will also be described. This paper will provide the reader with a fun¬ damental understanding of the technology that underlies elas¬ tomeric wall coatings and will pro¬ vide solutions to above-grade waterproofing problems. Case his¬ tories are included to exemplify the basic concepts. ELASTOMERIC WALL COATINGS: THE BASICS Elastomeric wall coatings (EWCs) are designed to coat verti¬ cal masonry surfaces such as concrete masonry unit (CMU), tiltup concrete, block, EIFS, and stucco. While they are classified as “coatings”, they have attributes of both coatings and sealants. EWCs can be classified into two categories: penetrating sealers and barrier coatings. Penetrating sealers are usually silicone- or siloxane-based and employ an organic solvent as the carrier. These materials have high surface tension and cause water to “bead” when in contact. However, these sealers have surprisingly high water permeance and will easily allow the passage of water through the sealer and into the underlying substrate. Because these materials are siliconebased, it is difficult to achieve adhesion to them when a nonsili¬ cone coating is applied. The remaining portion of this paper will be devoted to barrier coatings. Barrier coating types are exemplified by either acrylic or sil¬ icone. Most acrylic EWCs are waterborne, while silicones can either be water- or solvent-borne. These coatings are composed of several classes of key ingredients. The first is the binder or polymer. PAINTS VERSUS ELASTOMERIC COATINGS There are key performance requirements that differentiate these two coating classes. The most notable is the ability of an elastomeric wall coating to toler¬ ate substrate movement and crack bridging at low service tem¬ peratures. Typically traditional architectural coatings or house paints have polymer Tg’s between 0° and 25°C. By contrast, EWCs have Tg’s between -40° and -10°C. The polymer Tg is an approximate temperature at which the coating becomes glassy or brittle. There are other key distinctions that dif¬ ferentiate these uniquely different coatings. This provides the adhesion to the coating substrate and also binds the individual discrete pigment particles to the coating matrix. The second is the pigment. This may be any one of several types of pigments, such as high-hiding pigments (e.g. titanium dioxide and zinc oxide), and extenders such as silica or calcium carbon¬ ate. Other addi¬ tives are in¬ cluded that pro¬ vide in-can sta¬ bility, coating rheology (does¬ n’t slump when applied in thick films), and mildewcides to pre¬ vent mildew growth. The polymer can be described physically as a spring with its ends attached to individual pig¬ ment particles. The polymer “spring” provides the elastomeric properties and also adheres to the pigment to the coating matrix and to the substrate, preventing the particle from becoming dislodged. The higher the polymer level, the Exterior Coatings: Traditional Fgl vs Elastomers • Traditional . Tg = 0-25 degrees C Typical substrates: wood, concrete, stucco masonry ,.. Dirt Resistant . Doesn’t tolerate Movement Not a water barrier – Nq acid rain or carbonation protection .< Elastomeric . Tg – -13 -10 degrees Typical Substrates: Concrete, metal, SPF. roofing . Dirt Resistant Tolerates Movement Acid rain and carbonation resistant Figure 1 Proceedings of the RCI 24th International Convention Kirn -111 more flexible or elastomeric the coating and the better the adhe¬ sion. The higher the pigment level, the better the hiding or coating opacity, the poorer the adhesion, but the higher the tensile strength of the coating. Figure 1 compares some of these properties in more detail. These properties will be dis¬ cussed in greater detail through¬ out this paper. PRODUCT SELECTION: TRADITIONAL PAINT VS. EWC It is important to understand the distinctions and proper uses for traditional coatings (paints), elastomeric coatings, and sealants. Figure 2 compares the recom¬ mended film thickness and crack¬ bridging ability of these coatings. Because EWCs must have the ability to expand and contract as hairline cracks move, they are applied at dry-film thicknesses at 3-5 times the thickness of paint. Figure 2 HOW WIDE IS A “HAIRLINE CRACK”? While this question sounds rather simplistic and almost silly, it underscores the need for seal¬ ing even these very narrow open¬ ings in the wall system. Consider the following example: A 16-fthigh wall that has a hairline crack. The crack is 1/16-in wide in the win¬ ter and smaller in the sum¬ mer. Question: How large is that opening? Since the crack width is governed by the physical property of the coeffi¬ cient of ex¬ pansion and contraction, the theoreti¬ cal crack movement can be cal¬ culated. 1 Even if a wall is properly designed with sufficient expansion joints, cracks often develop. As the wall is continually exposed to natural weathering, the size and depth of the cracks continue to propagate and grow. The diagram in Figure 3 describes the evolution of these cracks. 1. Figure 3 – Dynamics of crack-bridging, an effect of weathering. 4. Answer: It is the same as a 3- in x 4-in hole in the wall: Twelve square inches! A hole this size in the wall would immediately be observed, and proper maintenance tech¬ niques using sealants or cementi¬ tious materials would be used to close the opening. This further validates the need to use these coatings to seal and protect the interior of the building to prevent moisture penetration, air move¬ ment, and insect infestation. FIELD EVALUATION AND DYNAMICS OF CRACK DYNAMICS OF CRACK BRIDGING Any masonry construction begins as a continuous, crackfree substrate. During its life, the cementitious material develops cracks. These cracks are the result of stress relief caused by the thermal and/or seismically induced expansion and contrac¬ tion of the masonry substrate. a=l/L dL/dT where: a = coefficient of linear expan¬ sion of the construction material (concrete) L = length T = temperature BRIDGING A comprehensive series of measurements were taken in the expansion joint on the south-fac¬ ing exterior wall on this project in Pennsylvania. Crack width and temperature measurements were taken daily for one year. A photo of the wall is shown in Figure 4. The data were plotted on the graph in Figure 5, which shows Kirn – 112 Proceedings of the RCI 24th International Convention the crack movement for the cold¬ est days in the winter and the warmest days in the summer. The data were then compared to the calculations using the for¬ mula above. Note the excellent correlation of the experimental to theoretical data. Using this model, the evaluat¬ ed wall with its dynamic crack could be “mathematically relocat¬ ed” to any geographic location in the world. Using weather data, the crack movement could be pre¬ dicted. These calculations were carried out for Miami, FL; San Juan, PR; Minneapolis, MN; Denver, CO; Phoenix, AZ; and Seattle, WA. TESTING EWCS Bench performance testing of these coatings is considerably dif¬ ferent from that of architectural coatings or paints, as these coat¬ ings actually create a fluidapplied membrane formed in situ on the masonry substrate. These tests include • Mechanical properties/tensile strength and elongation • Room and low-temperature environments • Hairline crack bridging • Wind-driven rain TTC-555B • Alkali resistance • Low-temperature flexibility coating to span dynamic cracks in the wall. Typical gauge length of 0. 5-1.0 inches (distance between the jaws of the Instron) is obvi¬ ously too large to simulate the hairline cracks associated with masonry construction. By con¬ trast, a dynamic crack may be coated in the summer when it is completely closed up. During the colder winter months, however, the crack will open. This situation is totally unlike a typical sealant installation, where there is a gap between the CMU sides, and the sealant is installed. The EWC problem is sometimes called the problem of infinite elongation, where in the equation: Percent elongation = x 100 • Dirt pickup resistance Figure 6 • Permeance While the measurement of tensile strength and elongation is a common test used to evaluate the mechanical pro¬ perties of mem¬ branes and coatings, this test does not simulate the ability of the The Lo is zero; hence, the name “infinite elongation.” A novel test method called “hairline crack bridging” has been developed. It utilizes a piece of rigid PVC sheet that is scored to create cracks in the film. The coating to be tested is applied at the proper film thickness over the cracked PVC. After curing/ drying, the composite is tested in an Instron-type tester. The results are reported as length-to-break, known as “B” value, and tensile strength. Figure 7 shows the com¬ posite under extension with the white EWC clearly visible. Proceedings of the RCI 24th International Convention Kirn – 1 1 3 Figure 8 – Crack-bridging ability of a representa¬ tive EWC. Figure 9 – Wind-driven rain tester Fed Spec TTC- 55SB. Figure 7 – Hairline crack bridging “B” value. This apparatus can serve as a valuable research tool for under¬ standing the performance limits of a specific EWC. If a series of coated PVC samples having increasing film thickness are pre¬ pared and tested, the ability of the coating to tolerate expansion can be elucidated and quantified. The graph in Figure 8 shows the per¬ formance profile of one represen¬ tative coating. Another important test is called “Wind-Driven Rain,” also known as Federal Specification TTC-555B. In this test, concrete blocks are coated and installed in an apparatus designed to simu¬ late rain at a velocity of 105 mph. The weight gain of the block is reported. If the back side of the block is wet, the coating has failed the test. Recently, this test method has been eliminated from the Federal Specifications and has been replaced by ASTM D6904 “Standard Practice for Resistance to Wind-Driven Rain for Exterior Coatings Applied to Masonry.” This method has no minimum performance requirements. Another important test unique to this type of coating is alkali resistance. It is not uncommon for an EWC to be applied to fresh concrete where the pH of the sub¬ strate is >12. Some coatings are susceptible to attack from these high pH environments. Typical tests are to immerse the coating in a saturated calcium hydroxide Ca(OH) 2 solution, measure weight gain, and observe any membrane deterioration. Low-temperature flexibility is another common test conducted on EWCs. This is a more expedi¬ ent and less costly test than mea¬ suring the mechanical properties at low temperature. While not as quantitative, it does provide valu¬ able information about the ability of the coating to tolerate move¬ ment at low service temperature. CLIMATE DRIVE DURABILITY TESTER … OR “MOVING WALL” INSTRON All laboratory bench tests are designed to measure some facet of the expected performance demands placed on any construc¬ tion material. Adhesion, low-tem¬ perature flexibility, and artificial weathering are some of the key properties that must be measured and proven. We have developed an innovative test methodology for measuring adhesion, tolerance for expansion and contraction of the substrate, alkali resistance, and weathering durability that is sim¬ ple, cost-effective, and virtually Kirn – 1 14 Proceedings of the RCI 24th International Convention labor-free. First, an expansion joint that is dynamic (i.e., actual¬ ly moves during the hot/cold, summer/ winter cycle) must be identified. Then a clamping device as shown in Figures 10 and 11 must be attached to each side of the joint. The device holds test specimens that are made from unglazed cement asbestos (UGCA) panels coated with the EWC under investigation. Like the PVC samples, the UGCA panels are butt-joined with coating applied to the top face. Figure 11 shows the basic design. The close-up in Figure 11 reveals the performance of vari¬ ous smooth and textured coatings tested. Note how some of the coat¬ ings are performing satisfactorily, while others have cracked. It is noteworthy that smooth EWCs demonstrate better tolerance for movement than their textured counterparts. year to all weather condi¬ tions; hence the name “Climate- Driven Durabil¬ ity Tester.” The perme¬ ance of the coat¬ ing is measured using one of the protocols de¬ scribed in ASTM E-96. Theoret¬ ically, perme¬ ance is defined as the passage of bulk water or Figure 12 water vapor through the coating film. Permeance < 1.0 is considered a vapor retarder and not a “breather.” However, for EWCs, a somewhat higher degree of permeance is needed to allow the wall to exhaust trapped mois¬ ture. REBAR CARBONATION Reinforcing bars (rebars) are incorporated into concrete con¬ struction to increase the flexural strength of the CMU. When the rebar is surrounded by concrete, it is considered “passivated” and the bar will not corrode due to the As can be seen, the apparatus creates expan¬ sion and con¬ traction de¬ mands on the coatings. The apparatus tests adhesion to a high pH cemen¬ titious sub¬ strate, and the samples are ex¬ posed 24/7, 365 days per high pH of the concrete. However, if the pH of the area surrounding the bar drops, the bar will cor¬ rode. The corrosion is caused by the presence of water and carbon dioxide. A solution of these two compounds forms carbonic acid, which effectively lowers the pH of the area surrounding the rebar and causes rust to form. The physical manifestation of carbonation is the rust-stained concrete that spalls from the affected area. The formation of the rust is approximately seven times greater than the volume of the steel (iron) from which it was formed. This expansion process causes the concrete to expand and crack. To be effective in preventing carbonation, the EWC must act as a barrier to carbon dioxide and water. Besides the standard test for water permeance described previously, there is a standard test for CO2 permeance used to qualify EWCs that are designed to prevent carbonation. These EWCs Proceedings of the RCI 24th International Convention Kirn – 1 15 Figure 13 • Pressure wash to remove salts, dirt, and chalk. • Repair wide cracks using elastomeric sealant. • If still chalky, use a pri¬ mer/ sealer. However, priming is no substitute for proper cleaning. Use the “masking tape test” when in doubt. Apply a piece of 2-in masking tape to the cleaned surface. If it adheres well, so will the coating. If it comes off easily and has “chalk” or dirt adare sometimes called “anti-carb coatings.” This problem is being exacerbated by the increasing lev¬ els of CO2 in the atmosphere. The levels of CO2 are tracked by the National Oceanographic and At¬ mospheric Administration (NOAA). Maps showing CO2 concentrations can be found at www.esrl.noaa.gov /gmd/ ccgg/ carbo ntracker /. This problem is further com¬ pounded when locally generated acid rain falls on a reinforced-concrete structure. DIRT PICKUP RESISTANCE The aesthetics of an EWC are an important property and must be considered when specifying products. Properties such as color, texture, and aggregate incorporation all drive the deci¬ sion to specify one coating versus another. One very important prop¬ erty is dirt pickup resistance. Although the coating is clean when it is newly applied, it may quickly pick up dirt and mildew. Mildew accumulation is greatest in warm, moist climates and on the north or shady sides of build¬ ings. Properly formulated EWCs have inherent dirt pickup resis¬ tance and are formulated with mildewcides to prevent discol¬ oration caused by mildew. Currently, there is an ASTM Task Group within the D08.06 Subcommittee to develop a lab¬ oratory method for measuring dirt pickup resistance. One method currently used is to “soil” coated panels with brown iron oxide pigment, rinse the sam¬ ples with water, and visually com¬ pare the dirt pickup resistance of the samples. Results of this test can be seen in Figure 14. APPLICATION CONSIDERATIONS OF EWCS While prudent selection of a quality EWC is vitally important for a successful wall-coating pro¬ ject, proper surface preparation is an equally important considera¬ tion. Key points regarding surface preparation and application include the following: hered to it, the coating will have unsatisfactory adhesion. Apply the coating in two coats via spray, roller,* or brush. * Roller may be required for irregular surfaces. Follow manufacturer’s direc¬ tions! 28-day cure may be required before the EWC* can be ap¬ plied to allow the pH to drop sufficiently. * Beware of “hot concrete.” Beware of efflorescence on interior or exterior walls.* * May be a roof/parapet leak. * Observed as water-filled coating blisters in coating. Kirn – 1 16 Proceedings of the RCI 24th International Convention WHAT CONSULTANTS NEED TO KNOW ABOUT SPECIFYING AND SELECTING EWCS One key performance criterion for EWCs is the ability of the coat¬ ing to tolerate movement at low service temperatures. The term “low service temperature” has a different meaning if the building is located in Minneapolis, MN, or in Miami, FL. This was made obvi¬ ous in the graphs in the preceding section of this paper. Thus, this key information must be obtained from the manufacturer. A product that has a successful track record in Miami, FL, may not perform satisfactorily in a much colder cli¬ mate, such as Minneapolis. In fact, some architectural coatings are repackaged and sold as “elas¬ tomeric” coatings in Florida and perform fully satisfactorily. How¬ ever, these coatings will crack if applied on a moving hairline crack in a colder climate. Sufficient film thickness is a key requirement for an EWC. Obviously, the thicker the coating applied over a dynamic crack, the better able the coating is to toler¬ ate repeated cyclic movement of the underlying crack. While the manufacturer’s recommended film thickness provides a proxy for film thickness recommenda¬ tions, there is a more accurate method for determining the required film thickness for a coat¬ ing. This will be described in greater detail in the following sec¬ tion. Dirt pickup resistance is a key performance requirement for any exterior coating. This term en¬ compasses not only dirt pickup but also mildew and algae growth that may propagate on the wall coating. Properly formulated EWCs are designed to prevent or retard dirt accumulation, and mildewcides to prevent bacterial growth. Adhesion to the underlying substrate is another obvious Figure 15 CRACK WIDTH AT BREAK (mm 3.5j 3.0 IM 2.0 1.6 1.0 as TRADmONAL COATING 0.40 0.60 DRY FILM THIC KNESS ( 0.0 0.00 . _ CRACK WIDTH AT BREAK Figure 16 requirement for an EWC. However, the specifics of the underlying substrate must be fully understood. Is the surface stucco? block? tilt-up? acrylicmodified stucco? Is the surface coated with an architectural coat¬ ing? Is the coating acrylic? sili¬ cone? urethane? epoxy? It is important that the EWC has acceptable adhesion to that spe¬ cific substrate. Underlying all these require¬ ments is a fundamental question that can be posed by the consul¬ tant to the manufacturer: What is the performance track record of this coating in the same geo¬ graphic location and over the same substrate as my project? There is no substitute for suc¬ cessful “proof statements” or case histories. The term “semielastomeric” has been recently spotted on some manufacturers’ Web sites and product data sheets. (Au¬ thor’s note: I have absolutely no idea what this means!) However, the key to proper EWC selection is to work closely with the coating manufacturer and establish what the key performance criteria and product requirements are, and allow the manufacturer to assist in coating selection, wall surface preparation, and application rec¬ ommendations. This will ensure a successful project. Proceedings of the RC1 24th International Convention Kirn – 1 1 7 HOW THICK SHOULD THE EWC BE? MANUFACTURER’S DIRECTION? …OR IS THERE A BETTER WAY? Coating application film thick¬ ness requirements are usually included on the manufacturer’s product data sheets. These rec¬ ommendations are based on the EWC’s performance history. How¬ ever, if we consider the functional (elongation and compression) requirements imposed on the coating, simply reading the PDS may not be sufficient. Previously in this paper, it was shown that the size of any crack is a function of geography (summer and winter temperatures). The graph in Figure 15 shows the crack history for the building in Spring House, PA. Figure 15, together with Figure 16, showing the crack-bridging ability for a specific coating, will provide the recommended film thickness for the EWC. The required film thickness is around 40 dry mils for this prod¬ uct. Other products may require more or less coating, depending on their mechanical (tensile strength and elongation) proper¬ ties. WHEN NOT TO USE EWCS Consider the photo in Figure 17. There is evidence of effloresence (presence of water) on the wall. An immediate solution to the problem would be to apply an EWC to prevent moisture intru¬ sion into the block and effloresence discoloration on the wall. However, upon further investiga¬ tion, the problem is more com¬ plex. Figure 18 shows the problem in better detail. The efflorescence begins below the weep holes. Ideally, by design, any water trapped in this cavity wall is diverted using a drainage system to the weep holes and out of the building. However, in this case, the water flows beneath the Figure 18 diverters and slowly migrates through the CMU below the weep holes and is evidenced as the white efflorescent stain. The root cause and the severe effloresence may be related to factors such as lack of a watertight transition between the roof and wall, improper flashing within the wall cavity, or blocked weep holes. Obviously, in each case, EWC application would not be the proper repair for this problem. The root cause must be identified before any coating option is con¬ sidered. Kirn – 11 8 Proceedings of the RCI 24th International Convention Proceedings of the RCI 24th International Convention Kirn – 1 19 Elastomeric wall coatings are different from traditional architec¬ tural coatings. They not only serve aesthetic functions but also have functional properties and the abil¬ ity to tolerate dimensional sub¬ strate movement at low service temperatures. They are typically applied three to five times thicker than a regular house paint to achieve these properties. They must have the ability to bridge hairline cracks in the wall under repeated cyclic movement. They must also be alkali-resistant and have the ability to “breathe” and allow moisture vapor to transpire. They also have dirt-pickup and mildew-resistant requirements. The successful installation of an EWC is the result of using the correct product for the unique cli¬ matic environment and installing it properly. This is the recipe for success when selecting any prod¬ uct for construction applications. BIBLIOGRAPHY 1. University Physics, Sears and Zemansky, 1964, 3rd Edition. CASE HISTORIES Kirn- 120 Proceedings of the RCI 24th International Convention
