Research into pavement design has been historically carried out for applications under conditions very different from those experienced in hot arid environments. For this reason, it is not always the case that optimal designs are used in such regions, resulting in a distinctive form of deterioration. It is common for countries with hot arid climate to favour asphalt pavements and use road marking materials that are largely inspired by those specified for use in European countries, where the weather is significantly different. This choice of materials, combined with high maximum temperatures, low precipitation rates and large temperature changes each day leads to a specific form of surface deterioration seen in hot arid regions. The surface deterioration is characterized by cracks originating at the boundary of road markings. The cracks propagate by closely following the edge of the marking and from there spread across the rest of the carriageway. The cause of this rather unique form of surface cracking is believed to be due to the contrasting optical properties of asphalt and standard road marking paints, which cause vastly different rates of radiative heating. The present study was undertaken to examine the properties of existing road marking materials and possible alternatives to evaluate how the localised temperature gradients observed at the boundaries of road markings could be minimised. The study examined how modifications could be made to the absorbance, transmittance and reflectance of existing road markings and to their application methods. It discussed the use of specialist IR (infrared) pigments, silica (glass) beads, pigment extenders, infrared transparent glasses and phosphorescent pigments and provided strong evidence that with such materials a solution could be achieved. The specialist IR pigments were found to be particularly appropriate for this application. A grey pigment (Minatec 230 A-IR by Merck Chemicals) with a high infrared absorbance and designed to replace a percentage of the white pigment in a coating was found to significantly increase the temperature of a white road marking. Another specialist IR pigment designed to be a ”cool” black (Black 30C940 by Shepherd Color) was considered as an alternative to the current black paint sometimes used for so-called contrast strips on exposed concrete roads. It was thought that these strips could be engineered to reach a temperature somewhere between that of the white road marking and that of the asphalt pavement. This way the localised temperature gradient currently experienced would be dispersed across a wider area. It was expected that for this application the presence of silica (glass) beads would result in a net cooling effect due to their intrinsic retro-reflective properties, though it was appreciated that there would be an element of increased absorption related to the absorbance of the silica. In fact, when trialled, silica beads caused a considerable increase in temperature, suggesting the use of silica beads to be beneficial. The study suggests that there is potential for using a range of materials and application methods to minimise the temperature gradients observed at the boundary of the road markings. However further research would be required to investigate whether minimizing the temperature gradient does, as was assumed, actually achieve a reduction in surface cracking. (Author/publisher)
Abstract