Smart materials promise long term road repairs

Abir Al-Tabbaa is based at the University of Cambridge’s Engineering Department. She’s trying to find ways to solve two sets of problems: the short term issue of how to repair potholes right now and in a way that lasts for a reasonable length of time, by improving the materials we’re working with. She’s also got researchers working on ways to make surfaces more resilient in the future by embedding sensors into roads so they can self-report on their status; they’re even looking at implanting “radiators” to keep road surfaces warm in winter so they don’t crack in the first place, saving money on salt, grit and repair materials…

Abir - A pothole is a damage that is caused in asphalt pavements because it occurs when you get freezing and thawing. Normally, water will get into the asphalt, either through cracks or from the soil underneath, that will expand, the asphalt will also expand, and then the next morning when you get the thawing, it will move down or contract. That repeated process will eventually make the crack form into a pothole. Normally, you see the pothole growing quickly because of traffic.

Chris - And is that just because once you've broken down the integrity of the road surface, it's just weak and there's not so much binding things together?

Abir - Yes, that's the problem with asphalt, the weak bonding between the aggregates. The black stuff is bitumen, which comes from petroleum.

Chris - And that's the binder, that's what's sticking it together.

Abir - Correct.

Chris - Is asphalt the majority of what we use?

Abir - Correct. Most of our roads are asphalt pavements, particularly local roads. On the motorways, a number of these are concrete, but even at that national level, I believe only 4% of the strategic road network is concrete.

Chris - And are they equally susceptible to potholes or is one worse than the other?

Abir - Definitely one is worse than the other. Potholes only occur in asphalt pavements. Concrete is far more durable and resilient. Concrete is a lot more expensive, takes much longer to build, but mostly it is much noisier. Noise is a big problem on concrete roads.

Chris - So our challenge at the moment then is that the vast majority of our road stock is made of asphalt, is susceptible to potholing, has got bad potholing, and therefore is in dire need of repair.

Abir - Yes, that's correct.

Damian - My name is Damian Palin. I'm a senior researcher here at Cambridge working with Abir. We're looking at various strategies to enhance the performance of existing road materials on the road network.

Chris - Abir told us that, in asphalt, there is a binder, the tar, the bitumen that sticks everything together, and then stones, hard bits. What can you add then that makes that a better, more resilient road surface than what we have already?

Damian - One of the what you might call low hanging fruit options is to add fibres. By adding fibres to the bitumen, you can improve its fatigue performance.

Chris - When you say fibres, do you mean things like threads of cotton? That sort of scale?

Damian - Indeed. Could be. We're not using natural fibres or cotton, we're looking more so at using glass fibres, steel fibres. By adding something like glass fibres, you are taking over tensile loads and you're restricting the movements of the bitumen and therefore creating a stronger material. We're seeing 20% improvement of compressive strength and we're seeing a similar kind of level of improvement in terms of tensile strength.

Chris - The thing that really bothers the people who have to repair roads and, critically, the people who have to pay for people to repair roads, is the bottom line. What does this add in terms of cost and what do you recoup through not having to repair quite so often if this does what you think it's going to?

Damian - Indeed. Well this is a very interesting question. We're now thinking about whole life performance. The repair material itself is a small cost. It's actually the closing of the road that's the big cost. Some people project that it could be 10 times the cost to close the road in terms of economic value over not closing the road. That's what we're really trying to save on: better repair material, don't close the roads as much.

Chris - Let's hope you are right. One of the other things we can do more long term is to try to invest in ways that we can make roads better at telling us what's wrong with them in the first place and how they actually do go wrong so we know better how to fix them, how to perhaps make them more environmentally friendly when we fix them, and also make better materials that will last the distance in the first place.

Sripriya - Hi, I am Sripriya Rengaraju. I'm a senior researcher. I'm looking at implementing different kinds of sensors to understand what is really happening in the field in terms of road infrastructure. Longevity will help us to go for a longer duration without any intervention for repair and that's low carbon. One of our key motivations is to implement different kinds of sensors so we understand what is the threshold at which we need the intervention to happen.

Chris - When you say sensors, do you literally mean we put electronic devices into the road surface and they're telling us what condition the road is in? It's a bit like me implanting a device into my body to tell me what my heart's doing?

Sripriya - It is similar to that. We have different kinds of sensors. One, we can embed it in the infrastructure, and some can be on the surface. They can be mounted later for monitoring. Different kinds of deterioration happen on the road surface. It could be because of the environment and, now, with the advent of electric vehicles, we have different types of vehicles coming onto the road which the road was not actually designed for.

Chris - What sorts of information can the sensors give you and how do they actually work? What are they measuring?

Sripriya - One is monitoring temperature, humidity, the freeze thaw and even how much water stagnates on the road surface. Then, there are other kind of sensors which can monitor the stress development in the material, whether the vehicular movement is causing some stress and what kind of vehicular movement is causing that stress.

Chris - That would tell you a lot about how a road surface is coping with all the different challenges we are throwing at it. What about the other aspiration that Abir and Damian have both mentioned, which is making people who have to repair roads a bit less busy, roads that can repair themselves? We can do that for concrete. We can make materials that will heal a cracking concrete. Can we do that for asphalt and how can we do it better?

Sripriya - There is a lot of research now ongoing for self-healing asphalt. One of the things we can do is we can have heated ribbons. When it snows, what happens is the bitumen and the aggregates contract and, because of that, there will be a crack formation. But when you have a heated ribbon in place, this contraction will not happen.

Chris - It's almost immunising a road against cracks and potholes, isn't it? Because if the road stays warmer, the mechanism of: getting water in, ice formation, cracking, it doesn't happen in the first place. And the warmth presumably means that stuff stays more flexible.

Sripriya - Yes, exactly. That is the mechanism. What we are targeting at is, can we use the existing roads and make them last longer? That is the idea.