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Paraffin additive keeps the concrete warm

by Mark Cantrell
A US university is experimenting with concrete slabs that generate their own heat to keep them clear of ice and snow, with no need for grit or shovels

A US university is experimenting with concrete slabs that generate their own heat to keep them clear of ice and snow, with no need for grit or shovels

Pavement slabs deployed near a US university’s car park have proved an unlikely – but useful – laboratory setting for testing the real-world performance of self-heating concrete.

In freezing temperatures, snow, ice and concrete make for uncertain footing; shovelling, scraping and gritting becomes an annual chore. However, the concrete trialled by the researchers at Drexel University in Philadelphia, Pennsylvania, takes care of the problem all by itself.

By introducing phase-change materials to the concrete mixture, the Drexel team were able to create slabs that can warm themselves to melt snow and ice when temperatures approach freezing.

For the last three years, the two 30-by-30-inch test slabs have been fending for themselves, warding off snow, sleet, and freezing rain. According to the research team, their performance could herald a frost-free future for pavements and highways in the North East United States.

It’s not just about safety; there’s the economic impact too. According to the United States’ National Highway Administration, it costs an estimated $2.3bn a year to remove snow and ice, and millions more to repair damaged roadways.

Paraffin additive keeps the concrete warm
Dr Amir Farnam, associate professor, Drexel University’s College of Engineering

Dr Amir Farnam, an associate professor in the university’s College of Engineering, whose Advanced Infrastructure Materials Lab led the research, said: “One way to extend the service life of a concrete surfaces, like roadways, is to help them maintain a surface temperature above freezing during the winter.

“Preventing freezing and thawing and cutting back on the need for ploughing and salting are good ways to keep the surface from deteriorating. So, our work is looking at how we can incorporate special materials in the concrete that help it to maintain a higher surface temperature when the ambient temperature around it drops.”

The Drexel team has been developing its cold-weather-resilient concrete mix over the last five years with the goal of reducing the freezing, thawing and salting that eats away at roads and other concrete surfaces. Until now, the success of their self-heating concrete has only been in a controlled lab setting.

However, in a paper published last month in the American Society for Civil Engineering’s Journal of Materials in Civil Engineering, the group took the important step of proving its viability in the ‘wild’, as it were.

Farnham said: “We have demonstrated that our self-heating concrete is capable of melting snow on its own, using only the environmental daytime thermal energy – and doing it without the help of salt, shovelling or heating systems.

“This self-heating concrete is suitable for mountainous and Northern regions in the U.S., such as Northeast Pennsylvania and Philadelphia, where there are suitable heating and cooling cycles in winter.”

Warmth from within

The secret to the concrete’s warming is low-temperature liquid paraffin. This is a phase-change material, meaning it releases heat when it turns from its room-temperature state – as a liquid – to a solid, when temperatures drop.

In a previous paper, the group reported that incorporating liquid paraffin into the concrete triggers heating when temperatures drop. Their latest research looks at two methods for incorporating the phase-change material in concrete slabs and how each fares outside in the cold.

One method involves treating porous lightweight aggregate – the pebbles and small stone fragments that are ingredients in concrete – with the paraffin. The aggregate absorbs the liquid paraffin before being mixed into the concrete. The other strategy is mixing micro-capsules of paraffin directly into the concrete.

It’s cold outside

The researchers poured one slab using each method and a third without any phase-change material, as a control. All three have been outside in the elements since December 2021. In the first two years, they faced a total of 32 freeze-thaw events – instances where temperature dropped below freezing, regardless of precipitation – and five snow falls of an inch or more.

Using cameras and thermal sensors, the researchers monitored the temperature and snow and ice-melting behaviour of the slabs. They reported that the phase-change slabs maintained a surface temperature between 42- and 55-degrees Fahrenheit for up to 10 hours, when air temperatures dipped below freezing.

This heating is enough to melt a couple of inches of snow, at a rate of about a quarter of an inch of snow per hour. And while this may not be warm enough to melt a heavy snow event before ploughs are needed, the team say it can help de-ice the road surface and increase transportation safety, even in heavy snow events.

Simply preventing the surface from dropping below freezing also goes a long way when it comes to preventing deterioration, according to the researchers.

Robin Deb, a doctoral student in the College of Engineering, who helped to lead the research, said: “Freeze-thaw cycles, periods of extreme cooling – below freezing – and warming, can cause a surface to expand and contract in size, which puts a strain on its structural integrity and can cause damaging cracking and spalling over time.

“And while this alone may not degrade the structure to the point of failure, it creates a vulnerability that will lead to the problematic interior deterioration that we need to avoid. One of the promising findings is that the slabs with phase-change materials were able to stabilize their temperature above freezing when faced with dropping ambient temperatures.”

Aggregate results

Overall, the treated lightweight aggregate slab performed better at sustaining its heating, the research team found. This slab kept the temperature above freezing for up to 10 hours – while the slab with microencapsulated phase-change material was able to heat up more quickly, but only maintain the warming for half as long.

The researchers suggest this is due to the relative disbursal of the phase-change material within the pores of the aggregate, by comparison to the concentration of phase-change material inside the microcapsules.

Farnam said: “Our findings suggest that the phase-change material treated lightweight aggregate concrete was more suited for deicing applications at sub-zero temperatures due to its gradual heat release within wider range of temperature.”

More work remains to be done, of course. While both applications were able to raise the temperature of the concrete to between 53- and 55-degrees Fahrenheit, which is more than enough to melt snow, the research found their performance was affected by the ambient air temperature before a snowfall, and by the rate of snowfall.

They also noted that if the phase-change material does not have some time to ‘recharge’ by warming enough to return to its liquid state between freeze-thaw or snow events, then its performance may be diminished.

Deb said: “Conducting this research was an important step for us to understand how concrete incorporating phase-change material behaves in nature. With these findings, we will be able to continue to improve the system to one day optimise it for longer heating and greater melting.

“But it is encouraging to see evidence of significant reduction of freeze-thaw cycles, which demonstrates that [phase-change materials] concrete is more freeze-thaw durable compared to traditional concrete.”

The team plans to continue to collect data on the slabs to understand the long-term effectiveness of the phase-change materials and study how this method may extend the lifespan of concrete.

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