Macro-Texture, Skewness, Etc. and the Skid Resistance – Texture Link
In the effort to relate Skid Resistance to the surface properties of pavement, the measurement of macro-texture is an important step. We now have effective methods of estimating the macro-texture of a series of pavement segments even while traveling at near traffic speeds. While the probable relationship seems obvious and data supports that a positive relationship does exist, the actual relationship between high speed skid resistance and the traditional metrics for macro-texture is not the same for different pavement types and installations. I believe much of this difference can be explained by the skewness in the elevation profile. Below are two graphs that illustrate in a very stylized fashion what I am attempting to explain. While both shapes have the same Mean Profile Depth (MPD), they have very different Skews and will definitely impact the tire very differently. There are also many other metrics for describing the prevailing macro-shape of the pavement surface such as Kurtosis most of which can be calculated relatively easily and should certainly be considered as we attempt to link surface properties to skid resistance. Including Skewness in evaluating the contribution of macro-texture to skid resistance seems essential and relatively easy to do.
There have been numerous research studies that have attempted to establish an algorithm for the relationship between multiple pavement surface characteristics and skid resistance on active pavements. All the studies that I am familiar with are limited by one or more of the following circumstances:
1. The resolution of the texture measuring equipment is insufficient for the metric(s) being analyzed.
2. The varying impact of contamination (mostly water) that occurs in testing skid resistance.
3. The influence of a particular feature or metric on skid resistance does not completely stop or start at a particular wavelength.
4. The influence of a particular feature of the measured skid resistance likewise does not completely stop or begin at a particular wavelength.
5. The only “standard” available does not separate the impact of different texture wavelengths.
6. Profile wavelengths impact the tire-pavement interface differently at different travel speeds.
People smarter than me have established that the break point between macro and micro texture occurs at 0.5 mm. The shorter wavelengths are considered to be about aggregate shape and surface. Wavelengths longer are about aggregate size, shape and exposure. Even from this very limited description, it is clear that the shape of the aggregate can influence both macro and micro texture as they are currently defined. This problem is compounded by the reference measurement challenges. The most broadly accepted device for establishing the skid resistance of significant pavement segments is the locked rotor device described in ASTM E-274. It is a summation device measuring the total resistance supplied by all components of the pavement surface and modified by the applied water contamination. The test is only valid at a specified speed and under sliding conditions. While the ribbed tire focuses more on the micro-texture based component, all surface characteristics and test conditions still play a significant role.
Since the “Gold Standard” we have available today measures the total Sliding Skid Resistance that is a blend of all contributing factors, we must wait until there is a device that can measure the surface profile including micro-texture wavelengths before we can attempt to develop a reliable algorithm that relates pavement surface texture to the “standard” skid resistance as we measure it today. The algorithm will be very useful for comparing pavement surfaces but will only apply directly under a very limited set of conditions including, at a specific speed, with a specific tire compound and tread pattern. Initially, for algorithm development purposes, the profile measurements including micro-texture need not be at or near traffic speeds but to be usable on a network basis, they must be capable of collection at least 40 mph. Current attempts to develop algorithms that link Skid Resistance to the surface profile measurements we have available today face insurmountable challenges. I expect that the equipment community will continue moving towards the objective of ultra-high resolution and ultra-high speed surface measurements that are an order of magnitude better that anything we have today. Until then, if we are truly concerned about safety, the proactive testing of our pavements is essential especially in critical areas such as ramps and curves. The development of a high resolution skid resistance measurement device and method that can be effectively applied to a complete network should be and can be rapidly developed because the technology currently exists. To be effective, it should resemble as closely as practical, a typical vehicle travelling down the pavement and be able to reliably measure skid resistance every 0.01 mile (16 meters). It need not totally lock the wheel and will create a new metric which closely resembles the values of the E-274 method but will better relate to the vehicles currently on our highways.