face. Since the frictional force between the tire and
different minerals present in the aggregate (Trem-
the pavement surface is a function of the vehicle
blay et al. 1995).
speed, it is usually reported at a speed of 65 km/
The macroscopic roughness is provided by
hr. The frictional coefficient is multiplied by 100
spacing between the aggregate from 0.5 to 50 mm
and reported as the skid number (SN).
(horizontal) and 0.2 to 10 mm (vertical) (Donbav-
and 1989). This spacing provides channels for
rapid drainage of water from the surface and is
Roughness and texture
The microscopic and macroscopic roughness
important at vehicle speeds greater than 50 km/
are characterized by the texture of the aggregate
hr (Donbavand 1989). Therefore, at speeds higher
surface and by the overall roughness of the pave-
than 50 km/hr, the skid resistance depends pri-
ment surface due to protrusion of the aggregate
marily on the spacing provided by the coarse
surfaces, respectively. Macro- and microtextures are
aggregates (Beaton 1976).
illustrated in Figure 13. Macrotexture has the great-
The microtexture refers to the irregularities on
Microtexture
Macrotexture
BINDER
Figure 13. Road surfacing textural characteristics (Tremblay 1995).
GooodMacrrotextturre, Good Miicrrottextture
G od Macote x u e
M c o exure
PPoorMiccrotexturre,, Good Macrottextture
oor Mi rotext u e
cro exure
GooodMiccrotextturre,, Poor Macrottextture
G od Mi rotex u e
cro exure
PPoorMiccrotexturre,, Poor Macrottextture
oor Mi rotext u e Poor Macro exure
Speeded
Spe
Figure 14. Correlation between macrotexture, microtexture, skid
resistance, and speed (Tremblay 1995).
est influence on the change in friction with speed.
the surface of the coarse and fine aggregates. It is
Figure 14 illustrates the effect of the macro- and
also affected by the amount of fines in the HMA
mix (Tremblay et al. 1995). The effect of different
microtextures on skid resistance as a function of
types of pavement surfaces on the macro- and
speed. Clearly, to maintain a constant high skid
microtextures is illustrated in Figure 15. The
resistance value at various speed levels, the pave-
terms viscous and dynamic fluid pressure allevi-
ment surface has to have both good macro- and
ation in Figure 15 relate to the natural flow and
microtextures. The change in the texture depends
flow under the impact of a moving tire. The effect
on the aggregate resistance to fragmentation, wear,
of texture on skid resistance is shown in Figure
and polishing. Aggregate fragmentation and wear
16. Several observations can be made. First, the
depend on the toughness and hardness of the
microtexture can provide adequate skid resis-
aggregate minerals and the aggregate itself. Pol-
tance at lower speeds, such as in urban areas (line
ishing depends on the difference in hardness of the
10
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