5 Development of a robust methodology to examine surface texture changes
5.5 Paint erosion
Before detailed analysis was carried out, a simple version of the paint erosion indication technique described by Parslow et al. (1997) was used to verify that the stones chosen at random had indeed come into contact with the polishing rollers. Following completion of the polishing experiment (and the associated texture measurement of the two randomly chosen stones), the whole specimen was coated with a single layer of road marking paint and then polished in the W-S machine for another 90,000 polishing roller passes. The resulting surface is shown in Figure 5.19; the stones previously chosen and characterized are highlighted with a black outline.
Figure 5.19 Limestone test specimen after painting and polishing The areas which have not been in contact with the polishing rollers are bright yellow (including a central zone over which the rollers do not pass) with the grey areas where the
paint has been removed indicating that abrasion has occurred at these locations The centre of the specimen does not come into contact with the polishing rollers, which scribe an annulus 60 mm wide around the circumference of the specimen, but there are clearly parts of the ‘polished’ area that are still covered in paint. The aggregate particles are not completely flat and the polishing rollers are not flexible enough to conform to all the variations in the surface. The two photographs in Figure 5.20 show limestone particle 1 before and after painting/polishing, confirming that the majority of this
Figure 5.20 Photograph of the surface of Stone 1 before and after painting and second polishing
In principle, this technique could have been used throughout the experiment, and surface texture only measured on areas that were indicated as being in contact with the rollers. However, the presence of the paint may have influenced the polishing of the aggregate particle surfaces and thus altered the friction measurements.
10 mm
5.6 Aggregates
As in the work of Do et al. (2009), limestone was chosen as the first subject aggregate. Limestone is rarely used as road surface course aggregate in the UK because it typically has a low resistance to polishing.
As demonstrated, this is more evident when polished by traffic rather than in the laboratory (Figure 5.4). However, its low resistance to polishing, and therefore the wide range of friction levels observed when polished in the laboratory, makes limestone a good initial candidate for trial and development of the methodology.
Limestone is typically very homogenous, predominantly composed of a single mineral (calcite), with only small amounts of secondary minerals such as quartz or feldspars. A petrographic analysis of limestone taken from the same quarry as the limestone used in this experiment states that the “limestone is composed of robust fragments that resist breakage; pits and cavities are uncommon”. Accordingly, the surface texture of limestone should be amongst the simplest to interpret of any aggregate type. A photomicrograph of limestone from the same quarry is shown in Figure 5.21: further notes from the petrographic analysis state that the image, taken in plane polarised light, shows irregular margins of the aggregate and minor surface indentations.
The second aggregate studied, a gritstone from the Yorkshire Dales, is used widely in road surface courses because of its high resistance to polishing. It is also a sedimentary rock and it comprises particles of size between 4 and 30 microns. A petrographic analysis of aggregate from the same quarry indicates that the rock type should more properly be called siltstone, which might be considered a subset of gritstone; the specimen will be referred to as gritstone for the remainder of this thesis. The particle shape is described as “Angular to Subrounded” and the surface texture of particles is “Rough to Moderately Smooth”. This description is not particularly helpful for the purposes of this investigation but it is likely that the homogeneity of the structure and the particle size range will be useful information to assist with analysis of texture measurements.
For comparison with relative measurements of friction after polishing in the Wehner-Schulze machine, it is of interest to note that the approximate nominal PSV of the limestone is 40 while the nominal PSV of the gritstone is 65.
5.7 References
Do, M. -T., Tang, Z., Kane, M., & de Larrard, F. (2009). Evolution of road-surface skid-resistance and texture due to polishing. Wear 266, 574-577.
Dunford, A., & Roe, P. G. (2012). PPR604. Use of the Wehner-Schulze machine to explore better use of aggregates with low polishing resistance. 1: Capabilities of the Wehner-Schulze machine.
Crowthorne: TRL.
Flack, D., & Hannaford, J. (2005). Measurement good practice guide no.
80. Fundamental good practice in dimensional metrology.
Teddington: National Physical Laboratory.
Hosking, J. R. (1967). LR81, An experiment comparing the performance of road stones in different bituminous surfacings: A.30 Blackbushe, Hampshire. Crowthorne: RRL.
Hosking, J. R., & Woodford, G. C. (1976). LR738 Measurement of skidding resistance. Part II. Factors affecting the slipperiness of a road surface. Crowthorne: TRL Ltd.
Huschek, S. (2004). Experience with skid resistance prediction based on traffic simulation. 5th symposium of pavement surface
characteristics. Toronto.
Parslow, G. I., Stephenson, D. J., Strutt, J. E., & Tetlow, S. (1997). Paint layer erosion resistance behaviour for use in a multilayer paint erosion indication technique. Wear 212, 103-109.
Roe, P. G., & Dunford, A. (2012). PPR564. The skid resistance behaviour of thin surface course systems. HA/MPA/RBA collaborative programme 2008-11: topic 1 final report. Crowthorne: TRL.