Retro-reflective plaster coatings after outdoor aging and soiling: an in-lab optical performance characterization

The potential benefits of highly reflective and retro-reflective (RR) materials on urban context have been performed by several studies through their optical characterization. During their lifetime, outdoor aging and soiling can affect the RR optical behaviour. To this aim, this study investigates the performance of RR plaster coatings after outdoor aging and soiling from May to October 2022 in Perugia, Italy. The same RR plaster coatings in pristine conditions were already characterized in lab in terms of optical performance. In particular, RR samples obtained by using glass beads with different diameters (i.e., 40 ÷ 70 μm and 70 ÷ 110 μm) distributed in three superficial density ranges (i.e., 0.30 ÷ 0.40 kg/m2, 0.20 ÷ 0.30 kg/m2 and 0.10 ÷ 0.20 kg/m2) have been investigated in this study through spectrophotometric and angular reflectivity analyses. In all cases, the post-aging RR samples exhibit lower global reflectance values with respect to the same RR samples in pristine condition. Concerning the angular reflection distribution analysis, a stronger relative RR component was found for the post-aging RR sample with an average diameter of 70 ÷ 110 µm and superficial density distribution equal to 0.10 ÷ 0.20 kg/m2 for each investigated incident direction.


Introduction
Researchers have given considerable focus to Urban Heat Islands (UHI) due to their noteworthy impact on urban thermal surroundings and human well-being.Urban population growing and consequent urbanization have led to an increase of city temperature and a worsening of UHI phenomenon [1].
In particular, a numerous cluster of buildings with enclosed spaces emerged due to urbanization, showcasing a variety of typical forms such as the canyon, semi-closure, and courtyard designs [2].The arrangement of buildings has a substantial influence on the urban climate caused by multiple reflections of solar radiation in comparison to more open structures or land [3].The consequence of this building configurations is that the urban surfaces absorb a huge amount of solar radiation, increasing urban temperature [4].
In order to solve this issue, numerous researches have been conducted towards investigating and creating effective mitigation approaches to mitigate the UHI effects, i.e. urban overheating [5][6][7].Creative solutions, including the application of high reflective and diffusive materials on roofs have been extensively studied.Additionally, measures aimed at enhancing the earth surface reflectivity have 2 been proposed for building implementation.These innovative strategies have demonstrated a notable ability to maintain significantly lower surface temperatures than traditional configurations due to their high albedo characteristics [8][9].
Another relevant technology is the use of retro-reflective (RR) materials, which have been proposed due to their ability to reflect sunlight primarily in the incidence direction [10].
Several studies have demonstrated the cooling benefits of RR materials, particularly in urban canyons where they are effective in reducing the energy absorbed by building surfaces [11][12].
However, the performance of RR materials may be diminished over time due to soiling and aging, which may lead to a material degradation.Many studies have tested the effect of aging on RR materials.In Paolini et al. [13], roofing membranes were exposed to natural aging for two years, resulting in an average reflectance decrease of 0.18.Yuan et al. [14] conducted extended outdoor tests on various glass bead RR samples spanning 368 days, and they found no substantial alterations in solar reflectance and angular reflection patterns post-outdoor aging.Meanwhile, Morini et al. [15] employed accelerated aging tests in a laboratory setting, following ASTM G154 standards, to evaluate the long-term viability of RR tiles and paints designed for building applications.The findings indicated that the RR properties remained consistent following the aging trials, with minimal alterations detected in directional reflectivity and overall reflectance.
The objective of this paper is to investigate the impact of natural aging and soiling exposure on the optical characteristics of RR coatings, which have been fabricated using glass beads of different diameters distributed according to three ranges of superficial density.In a previous work, the Authors [16] have characterized the optical performance of these same samples in their original state.In order to determine the optical performance of the aged samples, spectrophotometric and angular reflectivity measurements were conducted in this study.Finally, a comparative analysis of the optical properties between the post-aging and pre-aging samples was carried out.

Materials and Methods
The initial part of this section entails a description of the samples under investigation, emphasizing their optical attributes.Subsequently, the section outlines the methodology implemented to assess the optical efficacy of the samples.

Samples
The analysed samples were manufactured and partially investigated in a previous work [16].Six RR samples were produced by painting a 10 x 10 cm mortar square with white high-reflective paint and embedding RR glass beads with different diameters and superficial densities, as shown in Table 1

Methodology
From 24 th May 2022, the rooftop of CIRIAF building at the University of Perugia (located at 43° 7' 9.449'' N, 12° 21' 27.451'' E) was used to test the samples to various aging climate drivers such as relative humidity (RH), air temperature (T), UV radiation, rain precipitation, and dust soiling (Figure 1).The aging process ended on 26 th October 2022.After that, the samples were analysed by spectrophotometric and angular reflectivity measurements.The obtained results were compared with those of the pre-aging samples.

Spectrophotometric and angular reflectivity analysis
A PerkinElmer UV/Vis/NIR Spectrophotometer LAMBDA™ 1050+ [17], equipped with a 150 mm InGaAs Integrating Sphere, was employed for spectrophotometric analysis.To ensure device accuracy, calibration was performed using a Spectralon™ [18] certified reflectance standard from Labsphere.Three measurements were conducted for each sample to determine the mean and standard deviation of their spectral reflectance.The wavelength range analyzed spanned from 300 to 2500 nm, with a data interval of 5 nm.Data collection was performed using UV WinLab Software, and reflectance measurements were executed in accordance with the ASTM E 903-20 method.Additionally, ASTM G 173-03 was employed to obtain terrestrial solar irradiance values, enabling the calculation of Solar Reflectance (SR) for each sample.
To assess the angular distribution of reflected radiation by each sample for different incident radiation angles, a directional reflectivity analysis was carried out.However, the conventional testing equipment available in the laboratory or on-site was not suitable for investigating the angular reflectivity characteristics of the samples.Consequently, the Authors implemented a specialized experimental setup previously introduced in their prior research [19][20].This setup comprises a semi-circular structure, graduated at 10° intervals, and a LED light source.Irradiance measurements at each angular position along the semi-circular structure, i.e., every 10°, were obtained using a Delta-OHM HD 9221 photo radiometer equipped with an LP9221/RAD probe [21].Two consecutive irradiance measurements were taken exclusively at the position where the light source was fixed, i.e., one on the right and one on the left of the light source.As such, the average irradiance value was considered representative of the reflected radiation towards the incident direction.

Results and discussion
In this paragraph, results of the investigation of the samples' optical characterization are presented and discussed.The obtained data are analysed and then compared with data of the same samples in pristine conditions already discussed in a previous work by the Authors [16].

Spectrophotometric characterization
As previously outlined, the spectral reflectance of the samples was determined by conducting three spectrophotometric measurements for each sample, allowing the calculation of the average spectral reflectance.
Figure 2 shows a comparison of the average spectral reflectance of all RR samples in pre-and postaging conditions.It can be observed that in the visible (VIS) range, all RR post-aging samples exhibit lower reflectance compared to the same RR samples in pristine condition.
Moving to the near-infrared (NIR) region, a lower reflectance difference is observed for all RR post-aging samples compared to the same RR pre-aging samples.2. The Δ values (%) are calculated as the difference between the RR samples in pristine condition and the same RR samples after aging in terms of SR values.
The aging process seems to negatively affect the total SR in all RR samples.The maximum Δ value is 13.8% for RR sample characterized by glass bead diameter equal to 70 ÷ 110 m and superficial density equal to 0.2 ÷ 0.3 kg/m 2 (RRϕ2, ρs2).

Angular reflectivity characterization
Within this section, the angular reflectivity distribution for all samples under investigation is presented.It delved into the radiation reflected (W/m 2 ) by each sample across incident angles ranging from 0° to -70°, at 10° intervals, along the designed semi-circular gonio-reflectometer array.In Figure 3, a visual comparison of the angular reflectivity distribution for both pre-and post-aging RR samples at incident angles of 0° and -50° is shown.The direction of the incoming radiation striking the test sample is indicated by the black arrow.The presentation centers on the relative angular reflection distribution, denoting the percentage ratio between the radiation reflected by each sample in a specific direction, relative to the total reflection contributions across all directions by the same sample for each examined incident radiation angle.Consequently, the comparison of reflection percentages is feasible exclusively within the context of the same sample's angular reflection distribution and is not applicable for different samples.All RR samples in pristine conditions exhibit a good RR capability, as described in [16].RRϕ1, ρs2 exhibited the highest RR capability among the RR samples in pristine conditions for each incident radiation angles investigated.
Considering the pre-aging RR samples characterized by glass bead diameter of 40 ÷ 70 μm, RRϕ1, ρs2 provided the highest RR capability with a RR percentage equal to 15.9% and 12.9% for 0° and -50° incident radiation angles, respectively.Otherwise, among the RR samples in pristine conditions with glass bead diameter of 70 ÷ 110 μm, RR with a superficial density equal to 0.30 ÷ 0.40 kg/m 2 (ρs1) exhibited the highest RR component values, equal to 15.6% and 12.7% for 0° and -50° incident radiation angles, respectively.
The RR behaviour was confirmed for all post-aging RR samples.The RRϕ1, ρs1 pre-and post-aging exhibited the same RR behaviour at 0°C with a RR percentage of 14.7%, whilst the RRϕ1, ρs1 post-aging showed the highest RR capability for lower angle of incidence (i.e.-50°) with a RR percentage of 12.9%.
Concerning the RRϕ1, ρs2 post-aging, it exhibited the lowest RR performance at 0° and -50° with respect to the same RR sample in pristine condition.
The RRϕ1, ρs3 post-aging exhibited the best RR performance both at 0° and -50° angles of incidence, equal to 14.8% and 12.7%, respectively, with respect to the pre-aging sample.
Among the RR characterized by glass bead diameter of 70 ÷ 110 μm (ϕ2) in post-aging conditions, it is clear that: RRϕ2, ρs1 post-aging showed the highest RR capability only for lower angle of incidence (i.e.-50°) with a RR percentage of 13.1% compared to the same pre-aging RR sample; RRϕ2, ρs2 postaging as well as RRϕ1, ρs2 exhibited the lowest RR capability for each incident radiation angle compared to the same RR sample in pristine condition; RRϕ2, ρs3 post-aging as well as RRϕ1, ρs3 exhibited the highest RR capability at 0° and -50° with a RR percentage equal to 15.6% and 13.1%, respectively.
Thus, among the post-aging RR samples, RRϕ2, ρs3 performed the highest RR capability for each incident radiation angles investigated.
The effect of the aging processes on the RR optical behaviour was assessed comparing the results with those obtained for the same RR materials in pristine conditions.The main findings are: • Concerning spectrophotometric analysis, all post-aging RR samples exhibited lower spectral reflectance values both in VIS and NIR ranges compared to the same RR samples in pristine conditions; • Concerning the angular reflectivity analysis, among the post-aging RR samples, RRϕ2, ρs3 performed the highest RR capability, with a RR percentage of 15.6% at 0° and 13.1% at -50°.Future developments may involve the application of a protective layer to investigate its effect on the optical performances of RR materials after aging processes.

Figure 1 .
Figure 1.Samples during the outdoor aging and soiling in summer.

Figure 2 .
Figure 2. Comparison between the pre-and post-aging RR samples in terms of average spectral reflectance over three measurements.

Figure 3 .
Figure 3.Comparison in terms of angular reflection distribution for all pre-and post-aging RR samples for incident radiation angles of 0° and -50°.

Table 1 .
Characteristics of the RR samples that were tested, including data of glass bead diameters, the range of superficial densities, and the calculated weight of the embedded glass beads.

Table 2 .
Total SR [%] of pre-and post-aging RR samples.