Ensuring safe walking surfaces with exposed concrete

A GS-1 tribometer w/surfactant. A surfactant is used to simulate a soapy or greasy floor when measuring for wet dynamic coefficient of friction (WDCOF).

According to NFSI, high-traction floors have been clinically proven to reduce up to 90 percent of slip and fall claims. ANSI/NFSI B101.3 grades a walking surface under the assumption the person is moving on a contaminated surface (e.g. water and grease). ANSI/NFSI B101.3 was also specifically created with refined, polished concrete floors in mind.

Unlike other testing methods in which small, prefabricated material samples are sent to a laboratory to be tested and then conclusions are drawn on how a concrete floor (possibly yet to be poured) might perform in regards to safety, both NFSI and ISCS guidelines are built specifically to measure the actual floor. Field testing for floor safety consists of using an important piece of portable technology—a walkway slip-resistance tester known as tribometer, which measures coefficient of friction. Acceptable tribometers have gone through a battery of testing to get NFSI approval to ensure calibration, repair, maintenance, and field performance verification is possible to ensure device reliability. According to NFSI, any tribometer manufacturer should also provide validation and calibration reports as per the ASTM F2509-2011, Standard Practice for Validation and Calibration of Walkway Tribometer using Reference Surfaces. These types of tribometers are used in testing to discover whether a floor has in fact achieved acceptable DCOF as per ANSI/NFSI B101.3 The test itself consists of placing the tribometer on the ground and allowing the device to take multiple measurements in all cardinal directions to take a measure of the floors’ COF. Water and surfactants are also used in the test to lower the surface tension between the floor and the instrument contact point to approximate a foot coming in contact with a contaminated or wet surface.

Average roughness measurements
Architectural exposed concrete floors like polished finishes are different from tile and other floors because they are fabricated and installed onsite. The final floor’s safety depends on how one processes and refines the concrete surface while the floor is being created. How can the process of floor safety be quantified before testing it with a tribometer? Many design professionals may already be familiar with profilometer, a measurement device used in determining refinement benchmarks in concrete floor finishes. This portable tool can also aid in achieving a safe COF.

A profilometer measures the micro surface of the concrete floor to determine its level of refinement and quantify the surface micro texture. Average roughness (Ra) measurements, taken in either microinches (µin) or micrometers (µm) describe the surface profile of the concrete. American Society of Mechanical Engineers (ASME) B46.1, Surface Texture (Surface Roughness, Waviness, and Lay), describes Ra as the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the boundaries of the evaluation length. It is a set of separate measurements of peaks and valleys on a surface. These mathematical descriptions of the floor surface correlate directly with COF. Using prescribed ISCS methods, one can measure Ra during the process of installation to ensure the floor being created is safe.

Figure 1: Sample averages to showcase the correlation between coefficient of friction (COF) and average roughness (Ra). Readings were taken by a certified National Floor Safety Institute (NFSI) walkway auditor.
Image courtesy International Society of Construction Sciences

In Figure 1, one can see the average Ra value of the floor ‘400 grit’ is 0.47 µm (18.7 µin). Measurements taken by an approved NFSI tribometer on the same space show the average WDCOF is 0.53. At ‘800 grit,’ the averages were 0.35 µm (13.78 µin) and an average WDCOF reading of 0.45. It is a simple correlation the ISCS developed using NFSI B101.3 as the official safety benchmark. Just like in Figure 1, one can specify an Ra value of 0.41 µm (16 µin), and know that even when wet or contaminated, the floor meets safety guidelines. Using Ra as a guideline onsite, a constructor can make adjustments to floor refinement if COF is not being met prior to handing over a completed floor to the client and without need for additional testing or RFIs.

Maintenance of walking surfaces is the other half of slip resistance and should also be a part of the specification. This Ra number is of value to the owner while maintaining the floors as well—they are able to select maintenance pads and materials that retain the initial Ra value and preserve not only the floor aesthetic but also its COF throughout the facility’s lifecycle. In short, Paul Philippe Cret would be inclined to agree both form and function have been achieved.

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