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Pavement friction testing originated in the 1950s and has been studied internationally. There are four major types of friction-testing methods: lock-wheel test (LWT), fixed slip (FXSL), variable slip, and side force. The Federal Highway Administrator solicited the Louisiana Transportation Research Center (LTRC) to participate in a national study to evaluate fixed-slip testers that it had purchased from Dynatest International (FXSLD) and Findlay Irvine (FXSLG). LTRC developed a testing program where the fixed-slip testers were correlated to its LWT. Eight roadways were assessed with each device at 80.5 kph within a 2-h interval. Approximately 2286 m were assessed on each roadway. Testing with the LWT included both the smooth (LWTS) and ribbed tires (LWTR). The regression analysis between fixed-slip testers and a lock-wheel tester (ribbed and smooth tire) indicated that statistically viable correlations exist for the roadway surfaces and friction number (FN) ranges assessed in this study. More comprehensive testing should be conducted to validate the trend found in this study prior to establishing equations to be utilized on a statewide basis. The one-way analysis of variance (ANOVA) results indicated that a statistical difference exists between the FN maximum (μmax) values obtained by all devices used in this study. In the authors' opinion, this is a significant issue that must be addressed by the friction-testing community and regulation industry. Though both FXSL devices were calibrated in accordance with their manufacturer's recommendations, different FN maximums (μmax), as well as precisions (variances), were discovered. Such a scenario would make it unfeasible to establish investigatory or intervention friction criteria for a roadway network using FXSL testers under these conditions.
friction testing, rado model, lock-wheel friction tester, fixed-slip friction tester, variable-slip tester, side-friction tester, antilock brakes
Senior Pavement Research Engineer, Louisiana Transportation Research Center, Baton Rouge, LA
Research Administrator, Louisiana Transportation Research Center, Baton Rouge, LA