"Reflective and permeable pavement could be used in walkways, parking lots, playgrounds and commercial areas within urban areas," Hui Li of the University of California Davis, US, told environmentalresearchweb. "Ultimately, it is desired to fully implement the reflective FDP [full depth permeable] pavements on streets, highway shoulders and major urban roads to carry both light-duty traffic and, if our structural analyses are validated, certain heavy-duty vehicles while capturing stormwater run-off and mitigating the heat-island effect."

Li and colleagues tested the performance of six permeable and three non-permeable sections of pavement, using four-metre-square areas. The asphalt pieces, with an albedo of 0.08 or 0.09, reached temperatures of 70 °C. This was 10–25 °C hotter than the concrete pavements, which had albedos of 0.18–0.29.

"Reflective and permeable pavements are the most promising practices to help address the built-environment issues related to heat-island mitigation, and permeable pavements are a promising strategy for stormwater run-off management," said Li. "This innovative approach will potentially solve both problems, especially in urban areas."

Under wet conditions, the permeable pavements could also give lower surface and near-surface air temperatures than impermeable pavements. The team found that watering its test sections caused a peak cooling of 15–35 °C in the early afternoon during the central Californian summer.

"Most existing cool-pavement studies focus only on the reflective cooling from increasing the albedo," said Li. "Our research study extends the pavement-cooling effect to evaporative cooling and thermal resistance. It was found that to be effective for evaporative cooling, water must be near the surface in a permeable pavement. A significant effect on the thermal performance of thermal conductivity was also found, along with a difference in thermal conductivity of permeable and impermeable materials."

Permeable interlocking concrete pavers allowed the most water flow, with an infiltration rate of around 0.5 cm per second. But even the two permeable asphalt surfaces, the worst-performing when it came to permeability, had an infiltration rate of roughly 0.1 cm per second. This is adequate to drain rainwater without generating surface run-off during most typical rain events in central California, said Li.

To date, permeable pavements have mainly been used in parking lots and alleys as there have been doubts about their structural performance under loading from heavy vehicles such as buses.

"The current research is the continuation of our previous work at the University of California Pavement Research Center (UCPRC) related to the potential of adapting FDP pavement shoulder as a best-management practice for highway stormwater run-off management," said Li. "The hydraulic and structural performance of FDP pavement was simulated and found to be technically feasible, although needing full-scale validation."

A number of topics require additional investigation, according to Li, including hydraulic and structural performance under loading in the field, practical investigation of cooling technologies such as reflective coatings and high-evaporation materials and their impacts on human thermal comfort and building energy use, life-cycle assessment of the pavements' environmental impacts, and specifications for design and performance evaluation of cool pavement under different environmental and site-specific conditions.

Li and colleagues reported their study in Environmental Research Letters (ERL) , as part of the ERL Focus on Environmental Assessments in the Built Environment.

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