# Durability and Safety Performance of Pavements with Added Photocatalysts
## 1 Introduction  
Photocatalytic oxidation has been proposed in the last few years as a possible solution to reduce the level of air pollution. This technique was first applied to energy generation in the 1970s by [PERSON] and [PERSON] [1] and [PERSON] et al. [2]. Given the huge area exposed to sunlight in construction materials, they have been one of the objects of choice of the technology for applications such as self-cleaning [3, 4, 5, 6, 7], decontamination [7, 8, 9, 10, 11, 12, 13, 14], self-sterisation [15, 16], anti-fogging [17], pollen and allergen degradation [18, 19, 20, 21] and soot elimination [22]. The photocatalytic functionality is implemented through the addition of a semi-conductor, usually nanoparticles of TiO\({}_{2}\). In light of the crucial importance of construction material durability, the use and effects of new components are usually extensively researched. Scientific portal Scopus reviewed for this study returned a total of 10,507 documents in a search using the keywords 'durability' and 'construction material'. Only 47 were identified when the word 'photocatalytic' was added. While some of those deal with the advantages and drawbacks of nanotechnology applied to construction materials [23, 24, 25], most address the durability and performance of photocatalytic coatings. Examples include studies of resistance to peeling and water under pressure in limestone coatings [26], leaching of the photocatalytic pavements [27], mechanical and chemical durability of photocatalysts on exposed aggregates [28], durability of self-cleaning TiO\({}_{2}\) coatings on fired clay bricks exposed to UV radiation and wet/dry cycles [29], TiO\({}_{2}\) coating abrasion and wear resistance and its effect on the environmental impact of the coatings [30, 31, 32], cyclic weathering in photocatalytic glass-reinforced concrete panels [33] bonding in a number of coatings [34], TiO\({}_{2}\)-cement mortar bonded to asphalt pavement with an epoxy resin [35] and durability of photocatalytic activity on different cement-based materials exposed to abrasion and freeze-thaw stress [36]. Papers have also been published on the effects of TiO\({}_{2}\) on the properties of hardened cement pastes, particularly when the photocatalyst is added during casting. Nano-TiO\({}_{2}\) has been reported to act as a catalyst in cement hydration reactions [37] and hydration and drying shrinkage [38], whilst its effect on the mechanical properties of cement mortar [39; 40] and microstructure [41] have likewise been studied. Other authors have explored the impact of cement chemistry [42; 43; 44] and the behaviour in expansive concretes [45]. Very little research has been conducted on the specific effects of photocatalytic admixtures or emulsions on the durability of the construction material itself. The areas studied include chloride penetration in TiO\({}_{2}\)-additional concretes [46] and the durability of photocatalytic concrete determined with accelerated carbonation [47]. A study of the effects of TiO2 nanoparticles on ordinary portland cement (OPC) and slag-blended mortar resistance to sulphate attack [48], in turn, concluded that the addition made the materials more vulnerable. Rheological properties, mechanical strength and carbonation have also been analysed [49]. All those studies were conducted on cementitious materials in which nano-TiO\({}_{2}\) was mixed in during casting and most dealt with particulars of the effects of the photocatalyst in isolation. No comprehensive papers were found on the effect of photocatalysts on the intrinsic behaviour of construction materials related to durability and safety parameters, as slip resistance is crucial in pavements. Both types of considerations are analysed here for different matrixes with different photocatalysts applied (porous asphalt intruded with photocatalytic slurries and sprayed with TiO\({}_{2}\)-based coatings, concrete paving tiles sprayed with emulsions and with built-in photocatalysts) in comparison with the same materials without the photocatalytic functionality.