Dr. Yaghoob Farnam has spent four years combining concrete and phase change material in various ways in a quest to develop a durable paving material that can melt ice and snow.
Farnam, an assistant professor in Drexel University’s College of Engineering, has previously measured the effectiveness of paraffin oil and methyl laurate, materials that have relatively high heat storage capacity (about 130 to 170 joules per gram) and a suitable phase transformation temperature (about 2 to 3 degrees Celsius). Both showed promise when contained in plastic tubes embedded in concrete. But the paraffin oil proved to be far more effective than methyl laurate in concrete made of lightweight aggregate infused with the PCMs. Chemical reactions between the methyl laurate and materials in the cement rendered that PCM ineffective and also appeared to cause cracks in the concrete.
In his latest paper
, published in Cement and Concrete Composites, Farnam's research team focused on paraffin oil. The team used concrete slabs to compare two methods of deploying the PCM. Steel pipes filled with paraffin and sealed with PVC caps were embedded in one slab. A second slab was made of porous lightweight aggregate infused with paraffin. A third slab, containing no paraffin, served as a reference point. Each slab was sealed in an insulated container and then covered with about 5 inches of shaved ice.
DrexelNow describes what happened in two tests:
"With temperatures inside the boxes held between 35-44 degrees Fahrenheit, both of the paraffin-treated slabs were able to completely melt the snow within the first 25 hours of testing, while the snow on the reference sample remained frozen. The slab with the paraffin-filled tubes melted the snow slightly faster than the one composed of paraffin-treated aggregate. Farnam suggests that this is because the paraffin inside the tubes is able to solidify more quickly — thus releasing its energy — because of the regular diameter of the pipes. While the diameter of the pores of the aggregate vary in size.
"But in the group’s second experiment, in which the ambient air temperature in the box was lowered to freezing before the snow was added, the paraffin-treated aggregate was more effective than the embedded pipes. This is because the capillary pore pressure delayed the freezing of the paraffin, thus allowing it to release its heat energy over a longer period of time."
The research is of particular importance to the airline industry, which has a keen interest in finding cost-effective and environmentally friendly ways to clear runways of ice and snow. The Federal Aviation Administration has supported Farnam's work with nearly $500,000 in grants through its PEGASAS program.
Farnam says additional research is needed to better understand how the addition of PCM affects pavement durability, skid resistance and long-term stability.