by David H. Nicastro, PE, F.ASTM, and Patrick D. Gorman
Durability of exterior coatings is usually considered as a function of weathering, but an elastomeric wall coating (EWC) may fail long before environmental exposure can cause deterioration. As the name implies, these products are selected for their ability to stretch, especially over a crack in the substrate. If the crack propagates through the coating, the system can fail during its first year of service.
Many exterior cladding materials are porous, such as brick masonry, stucco, precast concrete panels, and some types of stone. Water is allowed to percolate through these claddings in a typical ‘drainage wall’ design; the water is collected inside the wall cavity on a water-resistive barrier (WRB) and drained out through weep systems. It is unusual to coat these cladding materials during original construction, except there appears to be a trend to specify an elastomeric finish on stucco—perhaps as a solution to another stucco trend, cracking.1 Concrete masonry unit (CMU) walls are often constructed with an elastomeric coating because they are so porous and usually lack a drainage system.
When water leaks occur because of breaches in the concealed WRB inside a wall system, it is common to apply an elastomeric coating as part of a ‘barrier’ remedy, with the intent of keeping water from penetrating through the building envelope’s outermost surface. The coating system may involve several coats and a primer. They are often integrated with remedial sealant installation or wet-sealing windows to complete the barrier.
Design life and service life
Initial success of an elastomeric coating project is judged on aesthetic appearance and waterproofing effectiveness. Similarly, the coating’s durability would be judged by its ability to continue to serve these functions for many years—that is, whether its actual performance (i.e. service life) meets or exceeds the expected useful life (i.e. design life). However, the authors have found it difficult to predict the durability of coatings based on manufacturer’s literature or published test results; their own lab tests had surprising results, which are discussed later.
A coating’s design life is dominated by weather resistance chemistry, ranging from a few years to decades. Eventually, coatings deteriorate due to weather exposure, and this design life can be predicted by laboratory-accelerated tests. Manufacturers typically publish the results of standard tests, but those industry standards have minimal weathering requirements. A designer would have to perform additional research to select a durable product.
A coating’s actual service life depends on more variables, including the quality of the application and the suitability of the product for the substrate and environment. If the coating is durable (i.e. the service life matches the design life), an additional topcoat usually can be applied at the end of its service for a fraction of the initial coating system project’s cost. This helps preserve functionality for another lifecycle. If allowed to deteriorate too far, complete removal and replacement of the coating may become necessary—possibly at a greater cost than the original coating project. The wall system’s water vapor transmission should be reevaluated when adding coating thickness.
Cracks undermine durability
However, weathering is not the only durability concern for elastomeric wall coatings. The service life can be cut short by cracks in the substrate telegraphing through the coating (Figure 1). Clearly, it would be a functional failure of an elastomeric coating if water penetrated through cracks—sealing is a fundamental property of these materials.
Additionally, elastomeric coatings must stretch over new cracks that form or existing ones that widen. Crack-bridging ability is the primary distinction between conventional paint and high-performance elastomeric wall coatings (EWCs). Generally, EWCs are more elastic than paint and applied thicker; together, these characteristics allow an EWC to absorb the crack-opening energy within the coating body, preventing the crack from propagating through to the surface. (Clear water-repellents are also related to paints and coatings, but generally have no crack-bridging ability.)
During cold weather, new cracks in wall cladding are more likely to form, and existing cracks are more likely to open; as the substrate volume shrinks in response to a temperature drop, the cladding experiences tension. In a water infiltration study conducted by one of the co-authors, no leaks occurred during water testing of cladding panels in afternoon sunlight, but the same panels leaked when tested the next morning in the shade—the cracks opened up with the cooler temperature. Given this fundamental physical behavior, it is surprising there are no industry standards for cold-temperature crack-bridging ability of these products. Before proposing such a test method, it is important to review commonly cited industry standards.
Current industry standards and coating thicknesses
Coating manufacturers cite standards inconsistently, so it is impossible to compare products ‘apples to apples’ based on data sheets. Properties tested by some, but not all, manufacturers include adhesion, mold/mildew resistance, dirt pick-up, chalking, freeze-thaw resistance, tensile strength, accelerated weathering, and water vapor transmission (i.e. breathability).
The range of standards can be narrowed by focusing on those specified in the “Product Validation Program” for elastomeric wall coatings created by the Sealant, Waterproofing & Restoration Institute (SWRInstitute).2 The program validates products by independently verifying they comply with the manufacturers’ claimed performance in accordance with the following ASTM International standards:
- ASTM D2697, Standard Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings (for solids content by volume and density);
- ASTM D6904, Standard Practice for Resistance to Wind-driven Rain for Exterior Coatings Applied on Masonry;
- ASTM D1653, Standard Test Methods for Water Vapor Transmission of Organic Coating Films;
- ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers–Tension, or ASTM D2370, Standard Test Method for Tensile Properties of Organic Coatings (tensile strength/elongation at break); and
- ASTM C1305, Standard Test Method for Crack-bridging Ability of Liquid-applied Waterproofing Membrane.
It is important to remember no standards were developed for elastomeric wall coatings—rather, they are all ‘borrowed’ from other industries, leading to varied interpretations of how to modify the tests or prepare specimens to work with the typical dry film thickness (DFT) of the products in question. For example, ASTM D412, one of the SWRInstitute options for validation of tensile strength/elongation at break, requires testing samples at a DFT of 3.0 ± 0.3 mm (118 ± 12 mils). This is far in excess of the average DFT of 0.33 mm (13 mils) recommended by EWC manufacturers. The other option, ASTM D2370, does not even specify a coating thickness for testing.
Compounding the testing problem, there is a wide range of manufacturer-specified DFTs. For example, most elastomeric coatings claim to bridge crack widths of 1.6 mm (1/16 in.), but two of the tested products had specified DFTs of 0.18 mm (7 mils) and 0.51 mm (20 mils) respectively—a difference of almost 200 percent in coating thickness for the same test.
ASTM C1305, the crack-bridging test included in the SWRInstitute validation, requires testing samples at a DFT of 1.50 ± 0.10 mm (60 ± 5 mils). This is closer to the average EWC thickness, but is still eight times thicker than one of the tested products. As crack-bridging ability depends to some degree on thickness (as mentioned previously on a theoretical basis, and confirmed later in this article), using the manufacturer-specified DFT would be a better indication of field performance. Most importantly, this test is performed at room temperature, which does not adequately predict low-temperature crack-bridging ability.
Test for low-temperature crack-bridging ability
An economical and practical method was developed to test the crack-bridging ability of coatings at a low temperature.3 Refinement is needed before this method could be considered as an industry standard, but the following summarizes the prototype equipment and testing:
- 1. As shown in Figure 2, a bench vise was welded to a steel frame measuring 360 x 760 mm (14 x 30 in.). The dimensions were selected so the entire frame can be set into a common chest freezer, but a larger environmental chamber is used in the lab (Figure 3).
- The vise handle was replaced with a welded socket to allow ratcheting within the freezer. Bars were added to clamp specimens to the vise jaws.
- An industrial vinyl sheet was selected as a standard substrate; the material was 0.25 mm (0.01 in.) thick, and came in panels 165 x 432 mm (6.5 x 17 in.) that could be cut into pieces for multiple specimens. During testing, no adhesion failure was observed between the coating and the vinyl substrate except near the induced crack.
- The vinyl sheet was scored at its center, and then folded in half to initiate a crack almost (but not completely) through the panel.
- An eight-path wet-film-applicator was used to apply the coatings to the non-cracked side of the vinyl substrate at various wet film thickness (WFT), as shown in Figure 4.
- After the specimens were cured for 21 days at room temperature, the DFT was verified by measuring the coating’s thickness with digital calipers.
- The specimens were gently bent until the vinyl cracked through, but without pulling the halves apart (thus, the initial crack width was still negligible).
- The specimens were then clamped into the vise and tested at room temperature or in the freezer after conditioning to a constant –9.4 C (15 F). This temperature can be changed to represent different climates; it is the lowest temperature typically experienced in El Paso, Texas, where the testing was performed.
- The vise jaws were slowly opened until failure of the coating occurred (as defined in ASTM C1305), recording the substrate crack width at failure.
Other test methods were evaluated, but they did not provide reliable, quantitative data for low-temperature crack-bridging ability of elastomeric wall coatings. For example, ASTM D522, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings, was adapted by performing the test at low temperature; even the coating with the lowest recommended DFT passed the test. A test method is not useful if all products pass or fail.
Using the test method described in this article, 14 elastomeric wall coating products (including acrylics and silicones) were tested for crack-bridging ability (Figure 5). Tests were run at room temperature (matching the requirements of current standards), as well as low temperature. The specimens were prepared with coating applied at the thickness recommended by each manufacturer. The results are shown in Figure 6. Product A is excluded from the graph because it is literally ‘off the charts;’ its crack-bridging ability was so large it dwarfs the other results.
Remarkably, five of the products failed the test at room temperature—the coating cracked before spanning a 1.6-mm (1/16-in.) gap manufacturers claim to bridge (the horizontal dashed line in Figure 6). It is important to remember those claims were based on a different test method, but it is still reasonable to be concerned about the performance of these products. All the tested products that have SWRInstitute validation passed this test, corroborating the value of that program’s mission to verify manufacturers’ claims.
Performing the test at low temperature, the same five products failed, along with three additional products in this more severe test. The range of crack-bridging ability measured shows the method can discriminate between products on the tested property—a key measure for consideration as an industry standard.
Additional tests were performed with the coating applied at several uniform wet and dry film thicknesses to normalize the manufacturers’ specified thicknesses and solids contents. There was a correlation between thickness and crack-bridging ability, but not enough to account for the pass/fail rankings—applying an inferior product thicker did not dramatically improve results.
Sunlight, including high temperature and ultraviolet (UV) radiation, is the primary cause of weathering deterioration that governs the design life of coatings. However, a lack of sunlight undermines elastomeric coatings’ service life: products fail prematurely if they have poor cold-temperature crack-bridging ability.
These two important aspects of durability—weathering and crack-bridging ability—are independent. The best products in accelerated weathering tests do not necessarily have good cold temperature crack-bridging ability, and vice versa. To select a durable elastomeric wall coating, specifiers require data from both types of tests, but that information is not readily available now. Better industry standards are needed.4
1 See the article, “Improving Stucco Durability Using Moist Curing,” by Nickie Ramm, EIT, in the January 2013 issue of The Construction Specifier. Visit www.constructionspecifier.com and select “Archives.” (back to top)
2 Visit www.swrionline.org/validation/products_wallcoatings.asp. (back to top)
3 This test method was developed by co-author Patrick D. Gorman, based on recommendations from Rohm and Haas Company (now a wholly owned subsidiary of The Dow Chemical Company), whose assistance is gratefully acknowledged. (back to top)
4 The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at the Durability Lab, a testing center at the University of Texas at Austin. (back to top)
David H. Nicastro, PE, F.ASTM, is the founder of Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes arising from them. He is a licensed professional engineer, and leads the research being performed at Building Diagnostics’ testing center, The Durability Lab, at The University of Texas at Austin. He can be reached by e-mail at firstname.lastname@example.org.
Patrick D. Gorman is president of Gorman Moisture Protection Inc. (El Paso, Texas), a specialty waterproofing contractor. He is on the board of directors of the Sealants Waterproofing and Restoration Institute (SWRInstitute), and a former officer of ASTM International’s Committee C24 on Building Seals and Sealants. He can be contacted at email@example.com.