July 26, 2019
by Casey Ball
Preparing a resinous flooring system specification for any facility requires some number crunching, and not just on the budget side. Specifiers need to review numerous performance data points to select the optimal flooring system for the given environment. Any ‘accounting’ mistake made during the review may sacrifice performance in the final floor coating selection.
A major specification error could mean years of lost durability, for example. This could easily happen when comparing the Shore D hardness of different resinous flooring systems. Simple logic would predict the harder the floor, the better its durability. However, harder coatings abrade easier than softer ones. By selecting a system with a lower hardness value, such as a thin-film urethane, a specifier can enhance the flooring’s abrasion resistance and esthetic performance. Of course, the flooring should not be so soft that it cannot meet the demands of its service environment, which may include handling heavy vehicle traffic. The specifier, therefore, needs to select a system striking the right balance to enable long-term performance.
To determine the optimal balance of flooring performance properties for a given service environment, specifiers need to understand how the various data points influencing product selection translate into real-world performance. Data sheet figures can be meaningless if they are not considered in the context of the actual service environment. Additionally, such figures may be misleading, as different labs may produce varied testing results, making it difficult to perform direct product comparisons. Further, certain performance data may lack relevance because the concrete substrate supporting the flooring system likely has lower performance characteristics than the flooring itself. Thus, the concrete will fail at lower applied forces than the flooring system’s ratings, making the data less relevant to actual service conditions.
By understanding testing protocols that influence product data reporting, specifiers will be able to optimize flooring system recommendations for specific environments.
Pay attention to data precision, bias, and variances
When developing facility flooring guidelines, specifiers need to understand the limitations of some testing methods used to generate data for product comparisons, including their precision, bias, and allowable variances. Some ASTM tests and data points enable reasonably accurate comparisons, while others offer less relevance due to potential discrepancies between labs and the personnel running tests. This is why ASTM does not intend for its tests to provide numerical comparisons of standalone data. Specifiers should only make direct product performance comparisons when similar systems are tested at the same time, in the same lab, using the same technician.
The overall relevance of using ASTM tests to evaluate physical and chemical characteristics of flooring systems relates to the precision—or repeatability and reproducibility—of each testing method, as well as the resulting acceptable levels of variance. Tests conducted mechanically, such as one using a pneumatic testing device, have lower allowable variances, while tests relying on human factors and subjective observations, such as manual adhesion tests, have greater allowable variances.
Per ASTM, the repeatability of a test “addresses variability between independent test results gathered from within a single laboratory” (intralaboratory testing). Reported repeatability figures measure the maximum difference calculated between multiple tests run on the same piece of equipment with the same technician in the same lab. For reproducibility, ASTM “addresses variability among single test results gathered from different laboratories” (interlaboratory testing). For more information read Pat Picariello’s “Fact vs. Fiction: The Truth about Precision and Bias,” published in ASTM Standardization News, March 2000.
Labs should perform round robin testing on samples to determine repeatability and reproducibility values. Coatings suppliers can then publish an average, using the greater and lesser results to determine an allowable variance. This variance itself can create some confusion when comparing products because it may be broad. For example, when comparing the Shore D hardness of flooring systems, ASTM D2240-15e1, Standard Test Method for Rubber Property—Durometer Hardness, allows for a 16-unit reproducibility variance between labs. Therefore, a Shore D value of 80 in one lab would be considered equivalent to a Shore D value of 64 reported in a different lab. This is a large gap that may reduce a specifier’s confidence when making a product selection.
When considering the precision of testing methods, intralab repeatability results are typically accurate since the conditions and test operator are the same. Thus, specifiers can make accurate comparisons when two or more products are tested in the same lab under identical conditions. However, variances between labs can make interlab reproducibility results suspect, especially if a coatings manufacturer’s lab has greater accreditation than a third-party lab performing the same test. The third-party lab’s results, while unbiased, may not be as accurate.
Speaking of bias, specifiers should review any bias statements listed in ASTM standards to assess the general accuracy of tests. Bias occurs when a systematic error exists “contributing to the difference between the mean of a large number of test results and an accepted reference value” (For more information read Pat Picariello’s “Fact vs. Fiction: The Truth about Precision and Bias,” published in ASTM Standardization News, March 2000.). Bias statements describe how labs correct test results to provide accurate figures for comparison.
Finally, specifiers need to carefully review reported results when comparing data, as manufacturers do not follow set reporting standards. For example, one manufacturer may use ASTM C579-18, Standard Test Methods for Compressive Strength of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings, and Polymer Concretes, while another uses ASTM D695-15, Standard Test Method for Compressive Properties of Rigid Plastics. The standards are similar, but reported results may not be directly comparable. It is also possible suppliers report data in different units of measurement, which can create confusion. For instance, one supplier may express abrasion resistance from ASTM D4060-14, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser, for its product as 30 mg (0.0011 oz) of material lost, while another supplier reports its product as 0.04 grams (0.0014 oz) lost. At a quick glance, the data point in grams may look superior.
Beware of substrate bias in compressive, tensile, and flexural strength tests
To better understand ASTM testing data, it is critical to review five standards likely to appear in the mix of data comparisons for flooring specifications.
First, this article will focus on three test methods—for compressive, tensile, and flexural strength—which can be misleading due to the tests being performed on floor coating samples not adhered to a concrete substrate. Each test should yield results exceeding the properties of an average concrete substrate, which is certainly desirable. However, the reported test values will not match real-life performance because they do not consider the floor coating’s application to a weaker concrete substrate. Based on the reported data alone, a specifier may expect the flooring system to perform to its published ratings, not realizing the concrete underneath is likely to fail well before these levels.
Concrete flooring substrates are commonly poured at a thickness of 102 to 152 mm (4 to 6 in.) (Figure 1) and typically feature average performance values of:
Each property above can be enhanced by adding reinforcing elements, with the concrete slab ultimately needing to be designed so that it meets the performance requirements of the environment.
Resinous floor coatings are applied to concrete slabs at a thickness of only 3 to 6 mm (118 to 236 mils) (Figure 1). Even at this low thickness, the floor coating will have better compressive, tensile, and flexural strength than average concrete. However, these higher values may cause confusion for specifiers, who may assume they can realize this performance level from the entire floor. In reality, because the concrete is the lowest common denominator, the performance of the flooring system essentially becomes that of the concrete. For example, a floor coating with a higher compressive strength than concrete may hold up under a heavy load, while the concrete underneath may be subject to failure. While the coating may remain intact, the floor has failed, as coating adhesion issues are sure to follow.
ASTM testing results for compressive, tensile, and flexural strength can still be helpful for making general product comparisons despite their limited relevance to the concrete substrate. However, it is important for specifiers to understand some nuances for each test method that may affect their comparisons of interlab data.
ASTM C579-18 measures a material’s resistance to deforming or breaking under an applied compression force. Following the standard’s Test Method B protocol, a flooring manufacturer or lab will cast a 50-mm (2-in.) cube of a coating material. A technician will then determine how much force can be applied to the cube before the sample crushes (Figure 2).
Values obtained via this testing method may not correlate to the real-world application of the coating at its normal thickness. The applied coating’s strength will still be greater than that of average concrete. This is why the concrete could theoretically crush and crack before the coating, making a coating’s higher reported value almost irrelevant for product comparisons. Specifiers simply need to design the concrete to meet service demands and then ensure the specified floor coating’s compressive strength is higher than the concrete’s rating.
ASTM C579-18 provides a comparable value among test specimens, allowing specifiers to assess reasonably accurate testing values from product data sheets. Still, those values may not be precise, as this testing method has a 15 per cent allowable variance for reproducibility due to sample preparation variability.
To overcome the deficiencies of this test method, specifiers may want to look to the coating’s elongation properties to ensure it can bridge any cracks that may eventually form in the concrete substrate.
ASTM D638-14, Standard Test Method for Tensile Properties of Plastics, determines a floor coating’s resistance to breaking under the stresses of being pulled apart (tension). It rates a coating’s elongation capability (pulled apart), which is the opposite of its compressive strength (pushed together).
Using a prepared dumbbell-shaped test specimen of a floor coating, a lab will insert the sample into a testing apparatus. The machine pulls the sample in one plane to determine how much it stretches before deforming drastically (elongation at yield) or breaking (elongation at break).
When evaluating flooring systems based on tensile strength, specifiers need to ensure the coating will adhere tightly to the concrete substrate. As a resinous flooring system cures and its chemical crosslink tightens, the coating will exert a lateral (tensile) force on the concrete. A similar force occurs as vehicle traffic moves across the floor and transfers that lateral energy to the bond line between the concrete and flooring. If the tensile strength of the concrete is too low, the flooring system could delaminate from the concrete substrate in either the curing or vehicle traffic scenario.
To reduce delamination potential, specifiers should select concrete with a minimum tensile strength of 1.7 MPa (250 psi). This is usually easy to accommodate, as the tensile strength of average concrete is about 2 to 3 MPa. Tensile strength can be as high as 14 MPa (2000 psi) for resinous coatings. It is important to remember the actual performance of the applied floor coating will ultimately be much closer to that of the concrete substrate, not its lab-tested values, which are independent of concrete.
When comparing products’ tensile strength data, the data may not provide apples to apples comparisons, as ASTM D638-14 says, “Tensile properties are known to vary with specimen preparation and with [the] speed and environment of testing.” To make a truly direct comparison, specifiers need to review data from equally thick samples prepared and tested side by side-in-the same lab.
ASTM D790-17, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials, must also be reviewed within the context of the concrete substrate, as tests usually are performed independent of a substrate. Lab technicians prepare a rectangular cross-section specimen of coating material and then use a three- or four-point loading system to apply a bending load to the specimen (Figure 3). The sample’s measure of deflection before its outer edges rupture is its flexural strength.
For the lab-tested resinous flooring samples, ASTM D790-17 results may be in the 14 to 28 MPa range, but the flexural strength of average concrete is only 4 to 6 MPa. Therefore, the concrete substrate will break under much lower flexural stresses than the flooring system. If the concrete cannot handle an applied load, then a thin coating on top will not make much of a difference, making the higher reported flexural strength results nearly irrelevant when comparing flooring systems. Specifiers should design the concrete for the facility’s requirements and then specify coatings with higher flexural strength than the concrete. Additionally, when reviewing the testing data, specifiers should understand reported data may not be accurately comparable among the products, as ASTM D790-17 says, “Experience has shown that flexural properties vary with specimen depth, temperature, atmospheric conditions, and strain rate.”
The variability in abrasion resistance testing
ASTM D4060-14 evaluates a floor coating’s abrasion resistance using an automated taber abraser with a weighted, abrasive wheel to scuff the surface of a rigid coated sample. Lab technicians pre- and post-weigh the sample to determine how many milligrams of coating are lost after a set number of cycles.
The reproducibility and repeatability of this test is highly subjective “due to changes in the abrasive characteristics of the wheel during testing,” per ASTM D4060-14. Further, the smoothness of the coating’s finish can cause varied results. Coatings with greater textures may also demonstrate higher abrasion potential simply because the abraser will grind off the coating’s ‘peaks’ before reaching a smoother area. Even if the smooth area of the coating has low abrasion potential, the weight of the lost original peaks suggests a lower overall abrasion resistance.
When comparing testing data among floor coatings, specifiers need to ensure the tests used the same wheel weights and the same number of cycles for more accurate comparisons. Additionally, as noted previously, they should ensure the data uses the same units of measurement.
Subjectivity in chemical resistance testing
ASTM D1308-02(2013), Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes, is a useful measure of a floor coating’s endurance when exposed to cleaning solutions or spills. Lab technicians conduct the test by soaking a cotton ball in a chemical reagent and resting it on top of a coated sample, covered or uncovered, for a specified duration under controlled temperature and humidity conditions. However, the reported results are subjective and are expressed via a visual, nonquantitative evaluation of the coating’s surface condition after chemical exposure. Hence, coatings manufacturers’ chemical resistance assessments are not directly comparable.
When evaluating chemical resistance, specifiers should consider the subjective nature of reported data and compare samples rated by the same lab at the same time, if possible, to ensure results are at least somewhat comparable. It is also important to understand chemical resistance and stain resistance is not the same. A floor coating with excellent chemical resistance against changes in gloss, blistering, softening, or loss of adhesion may still stain.
Optimizing product selections
Specifying an optimal floor coating system for a specific environment can lead to enhanced efficiencies, improved performance, reduced life-cycle costs, and other benefits. This is why it is important to make careful, informed coatings selections. Given the variability and subjectivity of certain ASTM tests, it can be difficult for specifiers to make sound selections. To help, here are a few tips.
Compare relevant side-by-side testing
Make sure the products being compared were tested in the same manner at the same time in the same lab and with the same technician. These parameters ensure objectivity and standardization for the reported data. Also, it is important to make sure the tests are relevant to the application.
Evaluate case histories
While lab tests are helpful, there is no substitute for real-world performance. Some products will perform poorly in accelerated testing environments and exceptionally in the field, or vice versa. It is beneficial to look to relevant application case histories to verify product performance.
Avoid sole-source specifications
Specifications listing a specific product—whether explicitly or by default because other materials cannot meet the published requirements—can artificially inflate the job costs without providing any extended service life benefit. It is advisable to expand the specification and entertain requests for viable alternatives if they enhance performance.
Explore the data
Be sure to question the relevance and accuracy of data points and ask a reputable coatings manufacturer to explain why certain data is important for specific applications.
Design the concrete for the application
It is important to remember the concrete substrate is the lowest common denominator in the resinous flooring equation. First, it is crucial to make sure the concrete’s compressive, tensile, and flexural strength properties meet the facility’s performance requirements. Then, ensure the specified coating offers better performance than the concrete in these categories, understanding the flooring will not perform significantly better, but that it should not fail before the concrete.
Following the guidelines above can help specifiers improve the validity and relevance of their specification requirements, ultimately leading to optimal floor coating selections that ensure long service lives and minimal life-cycle costs for asset owners.
|ASTM TESTING TERMS DEFINED|
|Precision: The closeness of agreement among test results obtained under prescribed conditions. A precision statement provides guidelines for allowable variances.
* For more information, read Pat Picariello’s “Fact vs. Fiction: The Truth about Precision and Bias,” published in ASTM Standardization News, March 2000.
Casey Ball is global market director – flooring, food & beverage and pharmaceutical for Sherwin-Williams Protective & Marine Coatings. Ball has specialized in the flooring and coatings market with the Sherwin-Williams Company for 17 years. He has certified coatings inspector accreditation from National Association of Corrosion Engineers (NACE) International and certified concrete coatings inspector accreditation from the Society for Protective Coatings (SSPC). He holds a bachelor’s degree in business from Wilmington College and earned his MBA in marketing from Franklin University. He can be reached at firstname.lastname@example.org.
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