Implementation Actions

B1. Study into the influence of determinant variables of photocatalytic activity in controlled conditions and regeneration trials

Action B1 Part A: Study into the influence of determinant variables of photocatalytic activity in controlled conditions and regeneration trials. Part A) Accelerated full-scale road surface trial

CEDEX is responsible for studying the influence of environmental and mechanical parameters (traffic loads) on the photocatalytic products applied to the bituminous mixes in road surfaces. This study was carried out in a controlled environment prior to the product’s application to streets in Alcobendas.

This was undertaken simultaneously with Phase I of Action C3 (the monitoring of the project’s impact on the materials trialled in the laboratory and on the test track).

Two photocatalytic additives were applied to two different substrates, a dense graded bituminous mix and an open mix, at the CEDEX test track. Real traffic was simulated by the controlled application of loads in order to determine the structural and superficial response and the material’s behaviour under an accelerated process of damage accumulation. This was applied to a sufficient extent that there was one section that had been subjected to vehicle rolling and another that had not. The purpose of this was to be able to determine the effect of climactic variables from the influence of traffic in the 6 sections studied. The stability of the different additives against wear from rolling and water was also studied through the analysis of photographs.

The modifications caused by the incorporation of additives into bituminous mixes were evaluated through laboratory tests on the 6 sections in Action C3. In addition to this, core samples were tested to study the effect of traffic and environmental variables on the bituminous mixes.

The conclusions drawn are that all the additives applied have some effect on the substrate. In general, these consist in a slight loss of the characteristics of the substrate; moreover, low durability was observed for all products in the accelerated wear test with vehicles simulating traffic.

  • Closed-porosity bituminous mix: The photocatalytic products in the area exposed to rolling wear have low durability, both additives caused a reduction in the surface characteristics of the mix (slip resistance) and a slight negative effect on some mechanical properties (rigidity and sensitivity to water).
  • Open-porosity mix: Low durability against rolling wear (greater durability for the grouted mix) was again observed for both the additives studied. There was a greater effect on the mechanical properties of the grouted mix, with reduced drainage capacity; for the other product applied, there was a slight reduction in the mix’s surface characteristics (slip resistance).

Action B1 Parts B and C: Part B) Trial into the photocatalytic activity of the selected materials. Part C) Trial into the regeneration of the material’s photocatalytic activity when subjected to washing.

A multivariable correlation parametric study was carried out, analysing the effect of flow, NO concentration, irradiance and relative humidity on photocatalytic activity. This employed the same experimental system that was developed to run the experiments required by standard ISO 22197-1.

Trials were also run to study the regeneration of the photocatalyst through gentle and heavy washing processes following prolonged exposure of the samples to outdoor conditions. Temperatura, pH, wash time and movement were some of the variables analysed. Photocatalytic activity was evaluated after the regeneration process was carried out for different test specimens both from building facades and from concrete and asphalt paving materials. This study was hindered by the loss of photocatalytic activity caused by the loss of the photocatalytic product both during the exposure period and during the washing process. Therefore, the intended method in for performing this task is to saturate the photocatalytic surface of the material in accelerated conditions with high NO concentration in the laboratory, throughout the execution timeframe for Action B1. A better correlation will thereby be established for the effect of the regeneration process without interference from other parameters, such as the loss of the photoactive material or the accumulation of other elements present in the air outdoors.

The following conclusions were reached:

  • The asphalt mix type (open or closed porosity) and the ageing of the mix strongly influence the behaviour of the samples. In the materials deposited both on closed and open porosity bituminous mixes, there is a significant loss of photocatalytic activity with the number of loads; this effect is stronger for open-porosity mixes and unaged substrates.
  • The multivariable correlation studies show the same behaviour for all the materials studied. There is a set of optimum conditions for NO concentration, flow, irradiance and relative humidity, for which there is an improved performance of the photocatalyst.
  • For concrete paving surfaces, photocatalytic efficiency can be recovered following prolonged exposure to outdoor conditions, through a process of gentle washing with ambient-temperature, neutral-pH water. The study has verified that rainwater is sufficient for the recovery of photocatalytic activity, provided that there is no leaching of the photoactive covering which may lead to the product moving to the aqueous phase. The test duration required by standard ISO 22197-1 is not sufficient to saturate the active sites of the photocatalyst. Saturation requires prolonged exposure to elevated NO concentrations; further study is therefore needed, avoiding the loss of photoactive material during the washing process.

Part A

Sections of the CEDEX test track treated with photocatalytic products. Measurement of environmental variables and road temperature Initial extraction of test samples at the CEDEX test track

Part B

Photographs of the change in appearance of the samples after 40,000 and 80,000 load cycles. SEM/EDX analysis of one of the samples following ageing by rolling traffic.