In the fast lane

C&I Issue 3, 2014

Since it opened in 1909, the Indianapolis Motor Speedway racetrack in the US has hosted Indy Car racing, motorcycle races and MotoGP. With such sustained and brutal treatment, the surface is replaced at least every eight or nine years. But clever use of chemistry in 2004 – when it was last re-laid – has meant that the latest track may have twice the lifespan of its predecessors. ‘I think we’ll see 15 or 20 years from it,’ says Tony Kriech, director of research for US-based Heritage Research Group, which created the material for the track surface in collaboration with Firestone Polymers.

At the 2013 annual meeting of the American Chemical Society in Indianapolis, Kriech described some of the chemistry that helped to transform the performance – and longevity – of the track. It all came down to being able to link the two main phases of the road surface – the bitumen or asphalt (as it is referred to in the US) ‘matrix’, and the polymer phase. ‘Usually, these two phases are separate,’ says Kriech. ‘What’s unique is that we’ve tied them together.’

In simple terms, a road surface is a mixture of a few key substances – asphalt, the matrix that binds everything together; a rubber phase, which improves high temperature performance; and an aggregate, which improves the friction (grip) of the surface.

Ordinarily, says Kriech, the rubber phase – usually styrene block copolymer (SBC) – is cross-linked with itself, using a process similar to vulcanisation: sulfur acts as a cross-linking agent, binding butadiene groups to one another and increasing the molecular weight of the polymer. Kriech and his team reasoned that cross-linking the rubber with the asphalt phase would improve the modulus of the material. But there was a problem.

Asphalt or bitumen is not a single chemical entity. It’s a thick, glutinous substance – the non-distillable residue from crude oil processing – that contains more than 50,000 compounds. The majority are aliphatic and aromatic hydrocarbons, but there are also low levels of metals like iron, nickel and vanadium. As Kriech explains: ‘Sulfur does not react with the asphaltenes [the colloidal particles within the bitumen]. So we had to find new end groups [for the polymer] to help it “wet” into the asphalt.’

The team tried a number of different end groups, in an attempt to build a bridge between polymer and asphaltenes. Finally, they discovered a new cross-linking agent, which uses a sulfur–phosphorus bond. The sulfur continues to bind the polymer elements together, while the more reactive phosphorus links with the asphaltenes. ‘The structural components of asphaltene and polymer then work together, rather than being individual,’ says Kriech.

The researchers created a material with higher modulus (toughness), as well as better elastic properties – allowing the track to bend repeatedly without breaking. ‘The track is stronger, and has more flexibility,’ says Kriech.

On the turn

On first inspection, there seems little difference between a motorway, a runway and a racetrack. But each has to withstand varying amounts of load, stress or weathering. Racetracks, for example, must stand up to enormous amounts of shear stress – especially on the curved sections. ‘There are many examples of tracks that have failed in this respect,’ says Kriech. ‘In order to avoid it, your material must be above a certain modulus – the polymer network helps to do this.’ This is one area where the new material has been very successful.

Fluctuating temperatures can also be a problem for the different surfaces. Here, careful choice of the asphalt matrix can help. According to Kriech, naphthenic oils,  which are often used in lubricants and flow down to -40°C, ensure good low-temperature performance (especially in Indiana’s cold winters); while the polymer phase means that the track can be used at temperatures as high as 82°C, some way above the top track temperature of 70°C. ‘We are trying to fix the chemistry of asphalt so that the track gets the right properties,’ says Kriech. ‘It’s a collaboration between chemists and engineers.’

As with many chemical formulations, it is the ‘minor’ additives that tweak performance: just as the performance of plastics is transformed by the addition of carbon fibres, so the physical properties of asphalt can be modified by different aggregates. The aggregate is usually a mineral – like basalt, dolomite or limestone – and what is used depends very much on what is available locally. ‘We didn’t have basalt, so we picked steel slag,’ says Bill Pine, quality control director of asphalt technology at Heritage Construction and Materials. ‘It has wonderful durability, and is very similar to basalt.’

There is a large steel industry in Indiana, and a ready supply of steel slag. But as well as its availability, the material turned out to have a number of advantages. First, it is less ‘absorbent’ than other aggregates. Previous aggregates – for example, slag from a blast furnace – would soak up the asphalt ‘like a sponge’, explained Pine, which causes the road surface to dry out and crack.

Steel slag also adds ‘micro-texture’ to the surface, improving the friction and grip, but is also resistant to ‘polishing’ so that, it does not wear down and become ‘smooth’. The extra abrasiveness of the steel slag means that racing teams must plan for this, adds Pine. Firestone, which makes tyres for all the Indy teams, changes its composition for this track, says Pine. ‘Nobody goes into a race blind,’ he says. ‘The tyres are “tuned” for each track: there’s a lot of chemistry going on in these discussions.’

And many spectators at the Indianapolis racetrack, though blissfully unaware of it, get a chance to drive on this innovative surface themselves – the I65 highway, which runs north of Indianapolis, is now paved with the same material. This is a tangible transfer of the racetrack technology onto a public highway. Other examples are less direct, but draw on the experience of developing a ‘superior’ material in other areas.

‘We’ve done work in China where there are no weight restrictions,’ says Kriech. ‘It’s not odd to have 100t trucks going over the road.’ He points to other extreme examples: roads in Siberia that must function down to -50°C and roads in Algeria that work at the opposite end of the scale. ‘From these projects, we learn how to use the tools we have – such as measuring modulus and fatigue – to establish performance before we build the road,’ says Kriech. ‘It’s too late to do it afterwards.’

Small change

Elsewhere, nanoparticles are being considered as additives to improve asphalt formulations. Researchers at Michigan Technological University (MTU) in the US, for example, are investigating the use of nano-clays and nano-silicon as additives. ‘One thing we’re trying to do is to eliminate or reduce rutting potential,’ says Zhanping You, associate professor of civil and environmental engineering at MTU. Rutting is the ‘vertical deformation’ that roads suffer owing to heavy loads, weathering and constant traffic. This leads to long ‘grooves’ in the road – similar in appearance to ski tracks in the snow – which will, at some point, need to be repaired. According to You, adding nanoparticles to asphalt could cut the effect of rutting by half.

In the MTU study, asphalt formulations were tested under load for 8000 cycles to simulate heavy trucks on a road surface. ‘With each cycle, you generate some kind of rut,’ he says. For a formulation without nanomaterials, the depth of the rut was around 6mm – a formulation containing 2% by weight of nano-clay reduced the rutting to 3mm. Doubling the nano-clay to 4% reduced rutting slightly more, to 2mm, but You thinks that the 2% level is the most promising. ‘I would stick with 2%,’ he says. ‘It already has a marked effect – and doubling the loading to 4% would be expensive.’

Nanoparticle effects

Not all the nanoparticles had the same effect. Others, such as nano-silicon or nano-lime, were less effective: a 6% addition of nano-silicon, for example, reduced rutting from 6mm to 3.5mm.

The nanoparticles add extra stiffness – or ‘dynamic modulus’ – to the matrix to help it resist physical battering. According to You, the layered structure of the particles – and their huge surface area – helps to stiffen the entire matrix. However, the exact mechanism behind it is still unknown, as some anomalies exist. ‘All the nanomaterials increased the dynamic modulus of the asphalt – but not always the viscosity,’ he says. ‘And, in general, all of them decreased rutting to some extent.’

Dispersing the nanoparticles evenly throughout the thick asphalt matrix is not easy – but is vital to harness their full effect. You’s team has done a lot of work to achieve ‘reasonable’ levels of dispersion, and tested this using microscopy.

Other factors that affect road life include fatigue and cracking, and You is now studying how nanoparticles might reduce these types of failure. And, while the work is a nowhere near commercialisation, You says it could have a huge effect on the cost of road maintenance. ‘Usually, you would fix a road every eight years,’ he says. ‘This could mean you only need to fix it every 15.’

Over the past 10 years, there have been various attempts to make ‘greener’ roads by using sustainably sourced materials. A number of ‘bio-asphalts’ have been investigated, in which part of the asphalt matrix is derived from sustainable sources.

Researchers at Iowa State University, US, for example, have been working on ‘fast pyrolysis’ methods of converting waste plant material into ‘bio oil’. Crop residues, such as corn stalk and wood waste, are quickly heated in an oxygen-free environment to create the oil. Christopher Williams, associate professor of civil, construction and environmental engineering at the university, has used the oil to develop a form of ‘bio-asphalt’, which has been used to pave a cycle path in Des Moines, the state capital. The bio-asphalt content of the cycle path is around 5%, thanks to the addition of bio-oil fractions derived from oak.

Williams says that bio-asphalt offers a number of advantages – it can be mixed and paved at lower temperatures than conventional asphalt, for example. This could cut paving costs by 20%, and greenhouse gas emissions by 30%. On the strength of this, three graduate students in the department have launched a spin-out company, Avello Bioenergy, to commercialise the technique. The oil can also be used as fuel, and as a chemical feedstock.

For the time being there is no likelihood of this material being used on any racetracks – although there may be some occasional overtaking on the cycle path.

Lou Reade is a science writer based in Kent, UK

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