Guide for Pavement Friction (2009) / Chapter Skim
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19 CHAPTER 4. PAVEMENT FRICTION AND SURFACE TEXTURE PAVEMENT FRICTION Definition Pavement friction is the force that resists the relative motion between a vehicle tire and a pavement surface.
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20 Longitudinal Frictional Forces Longitudinal frictional forces occur between a rolling pneumatic tire (in the longitudinal direction) and the road surface when operating in the free rolling or constant-braked mode.
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21 Figure 9. Rolling resistance force with a free-rolling tire at a constant speed on a bare, dry paved surface (Andresen and Wambold, 1999)
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22 The coefficient of friction between a tire and the pavement changes with varying slip, as shown in figure 11 (Henry, 2000)
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23 where: FS = Side friction. V = Vehicle speed, mi/hr.
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24 Figure 13. Brake (Fx)
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25 Figure 15. Key mechanisms of pavement–tire friction.
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26 Factors Affecting Available Pavement Friction The factors that influence pavement friction forces can be grouped into four categories -- pavement surface characteristics, vehicle operational parameters, tire properties, and environmental factors. Table 2 lists the various factors comprising each category.
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27 Figure 16. Micro-texture versus macro-texture (Flintsch et al., 2003)
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28 The effect is for the tire to trap water at the pavement surface rather than allow it to flow through the treads. As a consequence, hydroplaning speed is decreased.
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29 A very small amount of water can significantly reduce pavement friction. Test results from an FHWA-sponsored study (Harwood, 1987)
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30 PAVEMENT SURFACE TEXTURE Definition Pavement surface texture is defined as the deviations of the pavement surface from a true planar surface. These deviations occur at three distinct levels of scale, each defined by the wavelength (λ)
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31 Figure 18. Simplified illustration of the various texture ranges that exist for a given pavement surface (Sandburg, 1998)
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32 Figure 20. Effect of micro-texture and macro-texture on pavement–tire friction at different sliding speeds (Flintsch et al., 2002)
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33 • Layer Thickness -- Increased layer thickness for porous pavements provides a larger volume for water dispersal. On the other hand, increased thickness reduces the frequency of the peak sound absorption.
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34 For friction testing alone, there are several commercially produced devices that can operate at fixed or variable slip, at speeds up to 100 mi/hr (161 km/hr) , and under variable test tire conditions, such as load, size, tread design and construction, and inflation pressure.
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36 Variable-slip devices (ASTM E 1859) measure the frictional force, as the tire is taken through a predetermined set of slip ratios.
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37 inherent in the manually performed volumetric methods. Some of the applications of ROSANV include the following: • Texture measurements for pavement management systems (PMSs)
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38 Table 4. Overview of highway-speed pavement friction test methods.
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39 Table 5. Additional information on highway-speed pavement friction test methods.
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40 Table 6. Overview of pavement friction test methods requiring traffic control.
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41 Table 7. Additional information on pavement friction test methods requiring traffic control.
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42 Table 8. Overview of highway-speed pavement surface texture test methods.
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43 Table 10. Overview of pavement surface texture test methods requiring traffic control.
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44 Table 11. Additional information on pavement surface texture test methods requiring traffic control.
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45 FRICTION INDICES Friction indices have been in use for a long time. In 1965, ASTM started the use of the Skid Number (SN)
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46 drainage capability that macro-texture has for wet pavements during or immediately after a rainfall. This capability will also minimize the risk for hydroplaning.
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47 For example, a measurement made at low speed, say 20 mi/hr (32 km/hr) , or one made at a high speed of 60 mi/hr (96 km/hr)
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48 The Rado friction model was developed to complement the PIARC model by modeling the behavior of the maximum friction value. This model takes the following form: Eq.
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49 The technique is characterized by doing controlled wheel braking on the measuring tire, while maintaining a constant traveling speed. The measuring wheel is braked gradually from free rolling to locked state through the range of available slip speeds.
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50 For estimating MTD from profiler-derived measurements of MPD (ASTM E 1845) : Estimated MTD (or EMTD)
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