Durability of a photocatalytic coating on a bituminous pavement

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  • M. Palacios (1), L. Núñez (1), M. Pujadas (1), S. Suárez (1), B. Sánchez (1), M. B. Gómez-Mancebo (1), J. Fernández-Pampillón (2), A. Mazarío-Fernández (3), A. Moral (4) and J. Pérez (4)
  • (1) CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas).
  • +34- 91 346 65 41, e-mail: magdalena.palacios@ciemat.es
  • (2) UNED (Universidad Nacional de Educación a Distancia).
  • (3) CNME (Centro Nacional de Microscopía Electrónica).
  • (4) CEDEX (Centro de Estudios y Experimentación de Obras Públicas).
  • A system composed of a tripod, Canon EOS 400 digital camera, photo-shooting tent and lights was designed in order to assess the photocatalytic material stability to wearing by obtaining digital images of section surfaces throughout the course of the accelerated test carried out at CEDEX ATT. In one section, where the photocatalytic product was applied, and in another control section, no tracking area and wheel path surfaces were photographed. Images were processed and luminance was determined against barium sulphate standard (I-Vision Assistant V8.6, National Instrument).
  • Applying TiO2-modified coatings onto the external covering of roads might be a supplement to conventional technologies for mitigating nitrogen oxides (NOx) air pollution. In spite of the promising benefits of some photocatalytic materials and although their air-purifying activity have been deeply studied in laboratory, the durability and resistance to wear of this technology in pavement application needs to be evaluated before large-scale implementation is undertaken 1.
  • In the framework of the LIFE MINOx-STREET European project, co-financed by the EU, a variety of commercial photocatalytic coatings, designed for use in bituminous mixtures, has been subjected to rigorous essays, testing both their depolluting capability as the operation-induced changes and durability.
  • Here, we present the effects of wearing on a TiO2 surface layer and NO removal efficiency for one photocatalytic coating applied on a close-graded bituminous pavement at a test track.
  • CEDEX Accelerated Test Track (ATT) facility was used to apply one selected photocatalytic coating on a close-graded bituminous mix similar to other mixes widely used in Spanish municipalities. The photocatalytic product was applied by rolling following manufacturer instructions. A coring of asphalt mix of two test sections (side and center) was carried out.
  • The distribution of titanium (Ti) on the coating surface before and after durability testing was investigated by Scanning Electron Microscopy (SEM) at CNME 3 . SEM images recorded by working at an acceleration voltage of 20 kV at different magnifications, in most cases at X3000, allowed a semi-quantitative analysis regarding the composition of the sample. Additionally, the cationic composition was analyzed by X-ray energy-dispersive spectroscopy (XEDS) at an acceleration voltage of 20 KV. Graphite was deposited on the surface of the specimens to ensure the conduction of electrons and prevent the collection of charged images. After that, image histograms from spectrum images (CorelDraw software) were used to compute the quantity of Ti as a function of the number of pixels within a pre-evaluated and selected tonal range.
  • CEDEX Accelerated Test Track in operation and detail of coring asphalt mix.
  • Scheme of the ISO 22197-1 test equipment and some photocatalytic concrete samples
  • The samples obtained were cut into 99 mm x 49 mm x 5 mm specimens. The photocatalytic activity of these photocatalytic concrete samples was then essayed under ISO 22197-1:2007 2 by using a bed flow photo-reactor before and after accelerated pavement testing (35000 and 80000 cycles). A test gas mixture flow (NO, air, H2O) (50% relative humidity) is passed over the rectangular sample and is irradiated by UV-A light (10 W m-2 irradiance) through a UV transparent window with a distance to the sample of 5 mm.
  • Scanning microscope image (by SEM) and spectrum image (XEDS map, where brightest areas denote Ti presence)
  • JEOL 6400-JSM scanning electron microscope and attached OXFORD INCA analyzer and specimens (10x10x10 mm3)
  • (10 x 10 x 10 mm3)
  • http://www.cnme.es/
  • A methodology, based on different techniques and essays, has been set up in order to characterize the changes induced by wearing over a commercial photocatalytic coating applied on a bituminous pavement and the corresponding potential decrease of photocatalytic activity.
  • The study showed that both the surface distribution as the total amount of Ti decreased over time and, coherently, the NO removal efficiency. The structural analysis carried out highlights the heavy dependence of the NO removal efficiency on the exposure time rolling. In fact, effects of wearing on the NO removal efficiency are dramatic in the studied operating conditions. Consequently, it is highly recommended to investigate the operation-induced changes and durability of photocatalytic coatings before their implementation at real scale.
  • Acknowledgements: With the contribution of LIFE financial instrument of the EU.
  • AXIOS XRF spectrometer
  • Aliquots of each TiO2-modified coating sample were ground to a particle size lower than 63 µm. Crystal structures were determined from powder X-ray diffraction (XRD) measurements using a CuKα radiation source by using a PANalytical X´Pert PRO diffractometer operating with Bragg-Brentano geometry. Data was collected from 20 -120º (2). Also, major and minor elements were analyzed by wavelength dispersive X-ray fluorescence spectrometry (XRF) using a PANAlytical, AXIOS automated XRF spectrometer (CIEMAT laboratories).
  • [1] M.M. Hassan, H. Dylla, L.N. Mohammad and T. Rupnow, Constr. Build. Mater., 24, (2010) p. 1456;
  • [2] International standard ISO 22197-1:2007, Geneve, 2007;
  • [3] http://www.cnme.es/;
  • [4] J.O Carneiro, S. Azevedo, V. Teixeira, F. Fernandes, E. Freitas, H. Silva and J. Oliveira, Constr. Buil. Mater., 38, (2013) p. 594. 
  • The samples characterized by XRD (M13, M15, M20) were obtained by scratching a part of the original samples surface. The crystalline characterization show a similar composition in the three samples . The most abundant crystalline phases were quartz and calcite but distributed in different proportions. TiO2 coating is composed of rutile-type crystalline phase, as other authors also found 4, with no TiO2 phase changes after wearing. The semi-cuantitative analysis showed that TiO2 coating amount decreases after wearing. While in sample M13 the concentration is 5 %, in sample M15 this amount is reduced to 4 % and in sample 20 it is below the limit of quantification.
  • In the no tracking surface, luminance was found to be independent of the time by which the load cycles were applied. In the wheel path surface, a high correlation between both variables, was found (R2= 93.9%), showing that the product was removed by the contact with the wheel. The luminance reduction was more intense in the first 30000 applied load cycles, stabilizing afterwards. The closed bituminous mixture provides a high number of contact points which facilitates entrainment thereof by the tire.
  • Luminance ratio dependence on number of applied load cycles
  • The NO removal capacity (as % NO removal, (NOinput-NOoutput)/NOinput*100) varied from 46% to 22% after 35000 load cycles and to 7% after 80000 load cycles. So, results from the ISO essay reveal a sharp decrease in the NO removal capability of the photocatalytic material under test (around 52 % and 85% after 35000 and 80000 cycles of wearing, respectively) as a reflect of the a substantial lessening in the amount of Ti distributed at the surface of the samples (XEDS) and within the samples (XRF) as the wearing progresses.
  • XRD spectrum of the analyzed samples (M13 –non road–, M15 and M20 –35000 and 80000 load cycles, respectively–)

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