2.1 Wet-Pavement Skid Resistance
2.1.3 Pavement Skid Resistance Measurement
The classical friction theories are insufficient to completely understand the mechanisms in wet-pavement skid resistance, therefore research efforts were made to better understand skid resistance phenomenon and evaluate the pavement frictional performance. Various measurement devices and methodologies have been developed and used around the world. Each was specially designed in particular aspects. In the United States, standard test methods have been established by the American Society
21 for Testing and Materials (ASTM, 2004; 2006; 2008a-b; 2009a-d; 2011a-b; 2012b;
2013b-c). Similar specifications have also been issued by organizations in other countries, such as the British Standards Institution (2000a-b; BSI, 2009).
The skid resistance measurement methods can be broadly classified into two groups: direct and indirect approaches. The direct method produces an output in the form of friction coefficient. Lock-wheel method (ASTM, 2011a), slip method (ASTM, 2011b), side-force method (ASTM, 2009b) and British pendulum tester (ASTM, 2013b) are all direct approaches. The indirect method typically measures texture property of a pavement surface and the friction parameters are then deduced through empirical correlations. Sand patch test (BSI, 2000b; ASTM, 2006) and sensor- measured texture depth (ASTM, 2009a; 2009c) are typical indirect measurements of skid resistance. The representative direct skid resistance measurement methods are introduced in the following sub-sections, which are further classified into laboratory measurements and in-situ measurements.
2.1.3.1 Laboratory Measurement of Skid Resistance
British pendulum tester (BPT, see Figure 2.3) is widely used to measure the skid resistance of pavement materials in the laboratory. The procedures of measuring pavement friction using BPT are specified by ASTM E303 standard (ASTM, 2013b).
Skid resistance, related to the energy loss during rubber sliding on pavement surface, can be measured in BPT through the difference in pendulum height before and after the interactions between rubber slider and pavement sample (Henry, 2000). BPT can measure the friction level at very low speeds (about 10 km/h) and the resulted British pendulum number (BPN) is usually taken as an indicator of pavement microtexture.
A disadvantage of BPT is that it can only evaluate the skid resistance of discrete locations at a single speed, and as such, no continuous measurement is available.
Dynamic friction tester (DFT, see Figure 2.4) was designed to measure the speed dependency of pavement friction (Saito et al., 1996). This test is standardized
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in ASTM E1911 (ASTM, 2009d). DFT device consists of a horizontal spinning disk fitted with three spring loaded rubber sliders that contact the pavement surface as the disk rotational speed decreases due to friction. Water is applied to the tested surface.
The torque generated by the friction force measured during the spin down is then used to calculate the friction as a function of speed.
Polished stone value (PSV) test (BSI, 2009) is usually employed to evaluate the long term frictional performance of pavements. An accelerated polishing machine (see Figure 2.5) is used to polish the samples before their friction properties are tested by the BPT. The experiment is designed to simulate the polishing effects of traffic on pavement surface materials and to estimate the friction losses due to traffic polishing.
2.1.3.2 Field Measurement of Skid Resistance
The laboratory tests commonly cannot measure high speed skid resistance using the real tires. Therefore, in-field measurements are developed to evaluate the actual friction experienced by vehicles. The field measurements of pavement skid resistance can be generally divided into four classifications: lock-wheel method, slip method, side-force method and stopping distance method. A major advantage of field tests is that the measurement can be conducted at highway operation speeds. The selection of different methods depends on the test purpose and availability of devices.
The lock-wheel skid resistance trailer (see Figure 2.6) is preferred for field measurement of skid resistance by most highway agencies in the U.S. due to its ability to define and control most operational variables (such as water application, tire type, inflation pressure and wheel load). Lock-wheel test represents a critical braking status and the skid resistance measured in this condition is the lowest in the normal tire slip range. Skid number (SN) can be determined from the test procedures specified in ASTM E274 (ASTM, 2011a). The tests can be conducted at various speeds to establish the relationship between skid resistance and sliding speed. A disadvantage of this method lies in its discontinuous piece-wise measurements, which
23 may overlook some localized low friction spots within a pavement section. Other lock-wheel devices used across the world include Stuttgarter Reibungsmesser, Skiddometer BV8 (Zoeppritz, 1977) and front locked wheel car (Albert and Walker, 1968).
Unlike the lock-wheel approach, slip method is used to measure the braking performance of vehicles with anti-lock systems. Its output is brake slip number (BSN), which depends on travel speed and slip ratio. Both fixed slip and variable slip devices have been developed. The Griptester (see Figure 2.7), available since 1987, is one of the most commonly-used fixed slip devices and its test procedure is specified in the BS 7941-2 standard (BSI, 2000a). ASTM E1859 (ASTM, 2011b) provides a friction evaluation method using the variable slip technique. Slip methods provide continuous measurement without the undue wear on tire. The tests can be used to determine peak braking force coefficient and the relationship between skid resistance and slip ratio.
The sideway-force coefficient (SFC) is defined as the ratio between sideway force (force perpendicular to tire plane, see Figure 2.8) and vertical load. It indicates the ability of pavement surface to keep a vehicle under control when it travels on a horizontal curve. SFC depends not only on vehicle speed, but also on yaw angle, the angle between tire plane and vehicle motion direction (see Figure 2.8). The use of Mu-Meter (see Figure 2.9) in measuring pavement sideway friction is specified by ASTM E670 (ASTM, 2009b). Another famous sideway-force device widely used in Europe is the Sideway-Force Coefficient Routine Investigation Machine (SCRIM, see Figure 2.10). It is specified as the standard device to be used in monitoring the in- service skid resistance of UK trunk roads for maintenance purpose (The Highways Agency, 2004). These devices make it possible to measure wet friction continuously over pavement sections, with a higher probability in detecting the locations with poor skid resistance performance.
Stopping distance method measures the distance required by a given vehicle traveling at a specific speed to stop completely on a wet pavement, usually with all
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wheels locked. ASTM E445 (ASTM, 2013c) specifies the standard procedures for this approach. Coefficient of friction is expressed as a function of stopping distance, which is calculated from Equation (2.5) using the energy conservation principle.
D
V
f 2 30 (2.5)
where f is the average coefficient of friction; V is the initial speed in mph; and D is the stopping distance in feet. Although this method provides an estimation of actual stopping distance on roads, it poses a hazard to both traffic operations and test driver.
All these skid resistance measurement methods have been widely used in pavement friction studies (Wu and Nagi, 1995; Henry, 2000; PIARC, 2005).
Correctly selecting the appropriate method for a specific problem is crucial to the proper assessment of pavement frictional performance. The extensive research on skid resistance measurements have led to a comprehensive understanding on the mechanisms of wet-pavement skid resistance.