# Durability and Safety Performance of Pavements with Added Photocatalysts
## 4 Discussion  
Table 6 summarises the major findings of this study, expressed as the percentage variation in each photocatalytic-treated material relative to its reference. The beneficial effects for durability or safety of adding TiO\({}_{2}\) to the materials are shaded in green and un  
Figure 14: Slip resistance (\(S_{R}\)) measured by a pendulum tester (standard EN-14231) versus surface roughness, \(Ra\). Figure 13: Surface roughness, \(Ra\) (pixels), for all the materials studied. derlined, whereas the adverse effects are shaded in red with a bold typeface. Even though this a rough simplification, shorter ultrasound transmission time, less effective porosity and a smaller capillary absorption coefficient were deemed to be beneficial, whilst lower slip resistance and roughness values were regarded as adverse. No shading means that the experimental material and the reference exhibited approximately the same behaviour (difference = \(\pm\)10%). These findings preclude any sweeping statement on the variation in durability and slip-safety of construction materials after photocatalytic treatment. The effect of adding TiO\({}_{2}\)-based photocatalysts on the durability of the substrates analysed here does not seem to be related to the photocatalytic properties themselves but derived mainly from the physical effect of the addition of a coating to the material, the inclusion of a filler in the mass of the tiles, or to the effect of compaction of the asphalt in the case cementitious slurries, with the resulting changes in the pore structure and surfaces of the matrixes. Pore structure is related to liquid and gas transport in porous solids. In damp environments, one of the forces driving aggressive agent ingress is the capillary absorption resulting from the surface tension generated in capillary pores. Absorption, which is related not only to pore structure but also to sample moisture, is a significant durability indicator. The amount of water that can be absorbed is determined by effective porosity (capillary and gel pores). Air voids, in turn, which adsorb the increase in volume induced by freezing water, are beneficial in the event of frost. Indeed, entrained air is one of the approaches for enhancing concrete freeze-thaw resistance. Unlike capillary pores, large air voids do not contribute to capillary absorption. In the materials containing slurries, the ultrasound transmission time was shorter in both the slabs and the cores than in the respective references, signifying greater compaction and mechanical strength, for the slurries filled the air voids in the asphalt. As a consequence, instead of drain water, it remains retained in the capillary porous structure of the slurry, increasing uptake of water and therefore, the effective porosity and coefficient of capillary absorption. Due to filling of the voids, roughness decreases greatly, which is not detected by the slip resistance measured by the pendulum in the case of S1-A. Whilst effect on freeze-thaw performance varied with the material: S2-A lay in the 'good' range and S1-A in the next lower category, in keeping with its greater water intake. Emulsion-sprayed asphalt resisted frost well, as expected, although its mechanical strength was not enhanced leading to similar ultrasound times and slip resistance. Similarly, the smaller mean pore diameter induced by the emulsions raised water intake to the detriment of durability. In the paving tiles, the TiO\({}_{2}\)-based emulsions generated a more refined pore system on the tile surface, with a predominance of mesopores. The direction of the variation in ultrasound transmission time relative to the reference differed in the paving tile slabs and the cores in all three emulsions. In the cores, where the readings were taken with