Building roads with wastes can deliver a heap of performance as well as environmental benefits – so long as they don’t become a dumping ground for discarded products. Cath O’Driscoll reports.
With an estimated value of around €16 trillion, Europe’s road network is its ‘most valuable asset’, according to the European Asphalt Pavement Association (EAPA). It’s also built on what many of us might consider a mountain of rubble.
‘Over the years, almost every conceivable waste material has been put into roads,’ said Fred Parrett, speaking at an SCI organised event at a University College London, UK in March 2018. The list includes everything from crushed glass and incinerator ash to cellulose fibres and crumb rubber from end-of-life tyres – or even discarded plastic wastes. Parrett himself recalled once using sulfur in a road in Greenwich, London, UK, which works fine at temperatures up to 160°C, when it reacts to produce hydrogen sulfide!
But while using wastes in asphalt can potentially deliver big environmental as well as performance benefits, road experts warn that is not the best option for all wastes. In the UK, a recent ALARM survey by the Asphalt Industry Alliance revealed that the length of roads in England and Wales that could fail if not maintained in the next 12 months would stretch almost around the world. Local authorities were cited as having a funding shortfall in 2017 alone of £556m needed to keep roads ‘in reasonable order’, while – after decades of underfunding – the survey authors say it would now take 14 years and more than £9bn to get UK roads back into a ‘reasonable steady state’.
Asphalt from which roads are made is a mixture of crushed rocks and stones and sand bound together with 3-7% sticky black bitumen. At the end of a road’s lifespan, it can be recycled to make new roads. ‘Asphalt itself is 100% recyclable,’ says Mike Southern, senior technical adviser for the European bitumen association, Eurobitume. ‘US statistics show it’s the most recycled product bar none. More asphalt gets recycled than glass, paper, steel or anything else’.
Any wastes or waste-derived materials that adversely affect road durability – or recyclability – are definitely ‘a bad idea,’ Southern says.
REOB controversy
Over in the US and Canada, one waste-derived product, in particular, has recently been ringing alarm bells. The bitumen additive REOB – re-refined engine oil bottoms – is produced from engine oil waste drained away after a car oil change. It’s also been linked – controversially – to premature cracking of roads and test roads in Canada and the US, respectively (see Box on page 29).
And in Europe, too, Southern says that REOB is ‘a highly sensitive issue at present’.
Under the European chemicals legislation REACH, all chemical substances must be registered and assigned a CAS number before their use is permitted. However, Southern says that Eurobitume has recently discovered that REOB manufacturers are placing their product onto the market using the CAS number for bitumen. ‘The detail of this is complex, but Eurobitume has been in dialogue with the European oil re-refining association and has made our position clear: that REOB is not bitumen and should not be using bitumen CAS numbers.’
Road guarantee schemes in Europe, together with differences in US and EU regulations, mean that many more bitumen and mixture additives are used in the US than in Europe, Southern says, pointing to some of the more unusual sources as including waste cooking oil, carbon black, and even products derived by pyrolysis of algae and pig manure. While, in the UK, REOB is used as a fuel and so unlikely to end up in roads, road experts say the case nevertheless underscores the need for tight specifications – and enforcement - not just for REOB, but also for other new bitumen substitutes too.
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The problem with any such [accelerated ageing] process is that the mechanisms of ageing might not be representative of what happens in the field. Think of trying to simulate ageing by boiling an egg; you don’t get the same result by speeding up six months of ambient ageing by boiling for 4 minutes!
- Mike Southern Eurobitume
Road repair
Roads start to deteriorate when the bitumen ‘glue’ that binds the aggregates together becomes harder and more brittle over time, whereupon potholes and cracks start to appear – a process accelerated by solar UV, oxygen, heat and cold, and particularly the freeze-thawing of water.
Bitumen additives or ‘modifiers’ help to slow this process down, but most of the traditional modifiers are expensive and derived from non-renewable fossil fuels. Bitumen substitutes made from end of life tyres (ELTs) and plastics wastes should potentially offer a cheaper, more sustainable option – but only if they improve rather than impair performance.
At the University of Nottingham, for example, Davide Lo Presti points out that thousands of miles of roads have already been made with asphalt incorporating ELTs. The group’s research has shown that rubberised asphalt – incorporating crumb rubber from ELTs – performs just as well as asphalt mixes with synthetic binder additives such as the polymer styrene-butadiene-styrene (SBS), he says. As well as making quieter, smoother, road surfaces, importantly, he adds that incorporating ELTs in asphalt also extends road lifespan (see Box on page 31).
Scottish firm MacRebur’s MR products, which mix waste plastics with bitumen, are also claimed to offer big performance benefits. ‘[They] allow for less bitumen to be added to the mix giving the manufacturer instant increased profits,’ the company brochure states. ‘In the UK alone, 20m t of asphalt are produced annually – If MacRebur’s MR products were used in every ton, 60,000 t of plastic would be saved from landfill every year.’
In February 2017, MacRebur raised over £1.2m through the crowdfunding platform Seedrs. The technology is also backed by Sir Richard Branson, and is already being trialled in local authority roads around the UK and in countries as far flung as Bahrain. Asphalt made with MR products is claimed by the company to ‘outperform’ regular British standard asphalt by 60%.
Nevertheless, some in the industry are sceptical. Plastics additives – both virgin, or waste – have been tried over several decades, Southern says. A far better use of waste plastics, in his view, would be to make new plastic products – in accord with the European Commission’s ‘Waste Hierarchy’ guidance, which states that the order of preference is to reduce, re-use or lastly, recycle wastes.
Particularly for many newer wastes incorporated into asphalt, meanwhile, there is criticism that not enough hard evidence of performance is available over the full road lifecycle, including end of life and re-use/recycling. In the US, many in the sector privately believe that, rather than being low cost, using wastes can ultimately end up costing more over the longer term, according to Terry Arnold, the US Federal Highways researcher who headed up the REOB research.
‘TRB, one arm of the AFK20 asphalt binder committee, came to the conclusion three years ago: recycle costs more,’ he says. ‘And it does; every time you use recycled stuff it’s going to cost you more money. It’s one of the arguments with REOB: pavements have to take their share of waste material and for many years roads over here have been described as ‘linear landfills’. You want to get rid of broken glass – break it up and put it in the pavement. That’s been done. There are all sorts of things and REOB is just another one. Nobody cares – well they do because the road may have a short half-life.’
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Road rage and reob ‘There are a number of names, but when you say VTAE that suggests it’s what it was made for. It’s purely a marketing gimmick,’ says Terry Arnold, who led the research by the US Federal Highway Administration Research and Technology agency in Washington: ‘I prefer to call it what it is: REOB.’ In 2015, Arnold and his federal highways colleagues reported an alarming discovery - that ‘clandestine’ use of REOB was widespread in samples of liquid bitumen tested from states across the US, and in Canada. Of 1532 samples of bitumen samples tested – from 40 US states, one Canadian province and two Federal Lands Highway Divisions – the survey revealed that 12% contained REOB. Levels in some cases were 10-20% of the bitumen content, while the highest level of 34% was seen in a sample from Texas. The survey results caused ‘a huge emotional reaction,’ Arnold says. Several states that sent samples were completely unaware of their REOB content, and many had previously effectively banned its use or stipulated permissible levels well below those found. ‘People were totally bent out of shape, especially in New England where it’s really cold. One guy told me: “I spent two years repaving that road and two years later it fell apart. Now I don’t have the money to repair it”.’ Use of REOB as a bitumen additive was first flagged by researchers in 2010, after US asphalt specifications had allowed it to go under the radar for decades. The US Superpave testing protocol was designed to be blind to additives and relies on the measurement of purely engineering properties, so the presence of REOB was never detected. No tests were done to determine the effect of REOB on pavement life and it use was not disclosed, Arnold points out. With huge profits to be made from substituting low cost REOB for increasingly pricey bitumen, however, many questions are now being asked about its performance – particularly after its presence has been linked to cases of premature road cracking in Canada, and on several US test roads. ‘The Ministry of Transportation in Ontario is currently at war with REOB,’ Arnold says, referring to a photo of a segment of Highway 655 in Ontario used in a feature in Public Roads magazine (vol 81, no 2, September 2017), which shows significant cracking after just nine years of service. A further stretch of the same highway just 0.6 miles away – which does not contain REOB – reportedly remained crack-free after the same period. Asphalt manufacturers and their supporters have been quick to defend ‘VTAE’, pointing to a lack of hard evidence linking it to any problems. In a statement back in 2015, NORA, formerly the National Oil Recyclers Association, emphasised that VTAEs ‘have been successfully used for more than three decades for both asphalt paving and asphalt roofing materials. [They provide] essential components to asphalt products including moisture resistance and viscosity.’ Many other factors may be responsible for the adverse effects reported, say VTAE proponents. Much criticism for the present problems, meanwhile, has been levelled at regulators. Current US AASHTO (American Association of State Highway and Transportation) tests have allowed use of REOB to go unchecked as they look only at the physical properties/performance and not at composition, Arnold believes, potentially offering some suppliers a loophole to substitute expensive bitumen with cheaper additives. ‘All of these tests on asphalt [bitumen] were designed by engineers who are interested in what happens when a truck drives over it. They don’t care much what’s in it,’ he explains. ‘The tests are designed to be blind to additives. So as long as you meet the test requirements there’s no regulations about how you did that.’ It’s a loophole that US Federal Highways would like to see closed. In the 2015 survey, Arnold and colleagues used X-Ray Fluorescence spectroscopy (XRF) to look for the presence of calcium, copper, zinc and molybdenum – elements used in wear additives in engine oil. Thanks to its success, the team is now in the process of calibrating and validating a hand-held XRF protocol for submission as part of AASHTO’s required testing protocol. ‘The idea is instead of using physical properties, you can look at the chemistry,’ Arnold says. ‘When people selling you asphalt know you have that capability, they might think twice about trying to mislead you.’ However, it could be a long wait before it gets accepted; another test the team developed over a decade ago for lime/calcium hydroxide only finally reached the status of a permanent standard 18 months ago, Arnold says. As for the true extent of the REOB problem, he adds that a lack of hard data continues to hamper progress. When state officials were asked for feedback about the state of their roads after the 2015 study, for example: ‘For whatever the reason,’ he says, ‘maybe people didn’t have enough staff or didn’t know if the road had REOB, or didn’t know where the asphalt [bitumen] went - but we never got back a single reply.’ |
Road chemistry
Image: Getty
Road ageing, however, is notoriously difficult to predict. Current EU binder specifications only address short-term ageing, Southern says, from supply of the virgin material through to application on the road. The most established procedure for long-term bitumen binder ageing is the Pressure Ageing Vessel (PAV), developed as part the US ‘Superpave’ specification, and increasingly used in the EU, which is claimed to simulate up to seven years ageing at best.
While the next generation of EU binder specifications is likely to include long-term ageing, ‘the problem with any such process is that the mechanisms of ageing might not be representative of what happens in the field,’ Southern says. ‘Think of trying to simulate ageing by boiling an egg; you don’t get the same result by speeding up six months of ambient ageing by boiling for 4 minutes!’
At the Western Research Institute (WRI) in Laramie, Wyoming, US, Jean-Pascal Planche believes new tools to better analyse and understand how the chemical composition of bitumen relates to road performance are urgently needed. The variability of bitumen binders is the major issue affecting premature potholing and cracking of roads after poor road construction or design – and the subject of an ongoing WRI investigation that will report its findings in 2019.
In the EU, bitumen supplies are nearly all delivered direct from the refineries and the exact composition can vary considerably from production batch to batch, Planche points out, while regulatory specifications in the US and EU have also failed to keep pace with the development of new types of bitumen modifiers - particularly when it comes to performance, he says.
Many of the problems can often stem from a lack of compatibility between the source bitumen and its various additives, Planche explains – highlighted, for example, by the group’s recent study of REOB (see Box on page 32).
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Roads start to deteriorate when the bitumen ‘glue’ that binds the aggregates together becomes harder and more brittle over time, whereupon potholes and cracks start to appear – a process accelerated by solar UV, oxygen, heat and cold, and particularly the freeze-thawing of water. |
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High molecular weight plastics such as PET could also potentially accelerate cracking, he believes – as could plastics with solubility parameters very different from ‘asphalt maltenes’ found in bitumen. ‘When a plastic is incompatible it can create lots of problems during storage or application linked to separation when added to the bitumen,’ Planche says, adding that as always, the effects depend on the dose. ‘Case by case studies are recommended. This is also the case for ground tire rubber (GTR) but to a lower extent. Much more work has been done on GTR to improve their compatibility, with more success.’
The group’s work should help researchers to formulate bitumen bases with ‘the right additive at the right amount’, Planche says. ‘Bitumen may become like a lube oil in the future, he believes, with a base modified by a package of additives.’
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Road rubber, meep meep EOL rubber allows manufacturing premium quality asphalts for quieter and smoother road surfaces, Lo Presti says. It helps to make thicker coatings of mineral aggregates, which in turn allows for a higher binder content resulting in very flexible asphalts mixes. Premium asphalts containing recycled tyre rubber polymers produce more durable and longer lasting roads than with conventional bitumen, he says – and are generally less expensive than asphalt made with synthetic polymer modifiers. In the UK, Lo Presti believes crumb rubber roads have been held back both by a lack of government incentives as seen elsewhere in Europe, and by the cost of retrofitting asphalt plants, estimated at around £300,000/plant. But new process technology could be about to change all that. Standard ‘wet processes’ to use EOL rubber involve ‘cooking’ the rubber with liquid bitumen before combining it with mineral aggregates, Lo Presti explains, which requires an extra process step. However, alternative ‘dry’ technologies add EOL rubber directly to asphalt mixers – without this extra step. ‘Wet processes are wet because they are aimed specifically at wetting the rubber and modifying the bitumen,’ he says. ‘Dry processes instead involve adding EOL rubber directly into the mixer so we don’t need to modify the asphalt plants.’ But while the dry process is more practical, Lo Presti notes that it usually doesn’t provide a premium modification and is much more difficult to control. Without any pre-treatment, EOL rubber can continue absorbing the oily parts of the bitumen after asphalt manufacture or even after the road surface has been laid. The process therefore has to be carefully controlled to prevent the mixture from drying out as the rubber particles continue swelling, which could easily lead to premature de-bonding and cracks. Instead, Lo Presti believes, the solution is to pre-treat the tyre rubber before adding it to asphalt, to stop swelling from occurring and increase the interaction with bitumen. His group is now investigating two different pre-treatment approaches. The first involves ‘pre-cooking’ the rubber with bitumen before adding it to asphalt. This technology allows the rubber to swell until it absorbs bitumen up to a maximum absorption capacity. The pre-cooking is then finished with a mineral powder that blocks the reaction so there is no further swelling after adding it to asphalt, Lo Presti explains. The group is now ‘collaborating with colleagues in Spain and Italy and supporting partners in Switzerland who want to implement this technology,’ he says. The second pre-treatment method involves a very different approach, he continues: by chemically modifying the surface of the rubber not only to reduce swelling but also to enhance the interaction with bitumen through the creation of chemical bonds. ‘This is closer to what you have when you put synthetic polymers in bitumen,’ Lo Presti says. ‘You create an elastic network so the relative asphalt mixture is more flexible and resistant to damage.’ Pre-treated EOL rubber could one day replace expensive synthetic polymers completely, he believes. A European Green Procurement directive that incentivises recycling and re-use of tyres – along with other ‘alternative materials’ traditionally been regarded as wastes – should help to promote rubber technology, he believes. However, whether this will be implemented in the UK post-Brexit remains unclear. Alongside his role as principal research fellow at Nottingham, meanwhile, Lo Presti is also international research programmes coordinator for a number of collaborative projects aimed at improving road sustainability and performance. Two key topics under investigation include the ease of recycling of roads with new biomass-derived binders that may one day supersede bitumen and the eco-design of asphalt technologies for more sustainable management of road pavement assets by road authorities. |
Safety first
Designing the most appropriate asphalt mixture for a chosen application is a complex engineering and chemical challenge. ‘While chemistry is more of a bitumen topic, and asphalt more like engineering, the two are intrinsically linked,’ says Malcolm Simms, director of MPA (Mineral Products Association) Asphalt.
‘Asphalt mixture design and selection of ingredients seeks to result in a balanced outcome as designing for only one characteristic or property can have an impact on another property, eg increased stiffness = reduced flexibility/increased brittleness. Mechanical properties also need to be balanced against installed properties such as texture in the road surface in order to provide grip/skid resistance for vehicle tyres, whilst being ‘smooth’ in profile to help enhance fuel economy of vehicles.’
While incorporating wastes in asphalt may seem like a good idea, Simms urges caution. Manufacturers generating waste have a legal Duty of Care to ensure that any potentially dangerous wastes are not included in other, recycled, product streams, Simms says. ‘In some instances, however, products or wastes have only subsequently been re-classified as hazardous, increasing the burden of the responsibility for managing it to a new level at which some “owners” would wish to defer their responsibility by seeking to have it “recycled” within other products.’
One example of a reclassified waste is coal tar, which was formerly used as a binder in the same way as bitumen, but is now listed as a category 1 human carcinogen.
‘Other industries seem to view asphalt as a potential dumping ground for their wastes,’ warns Simms. ‘But we still have to produce a safe end product that will be exposed to the climate and traffic for several decades with due consideration of the whole life cycle. Does it actually make sense to take a potentially incompatible waste out of one environment and then re-introduce into another more harsh environment? It is not always as simple as “cover it in something black and sticky and forget about it”. We strive to produce asphalt, not “trashphalt”.’
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Bitumen chemistry unstuck Bitumen is the ‘the glue’ that holds together the aggregate skeleton in an asphalt, Planche says. Made up of hundreds of thousands of hydrocarbon molecules, however, bitumen is far from inert. Over time, some of these various hydrocarbons react with the oxygen in air to produce new structures that can ultimately cause it to become brittle and moisture sensitive – leading to the all-too familiar cracking and potholes. Over the past decade, Planche and his research group at WRI have been developing new tools to better understand the properties of bitumen. The group’s SAR-AD technology, which was granted a USPTO patent in 2016, allows them to separate bitumen into different chemical fractions, he explains – saturates, aromatics, resins and asphaltenes – according to their aromaticity, polarity, and their adsorption affinity to chromatography columns. Combined with chemometrics analysis, this information allows them to correlate bitumen chemistry with behaviour. ‘[The software] can work for any kind of data, but more particularly for relating bitumen chemical parameters to physical characteristics,’ Planche says. ‘It is also able to create its own artificial data points based on the known precision of the measurements.’ The group recently used the technique to study the bitumen additive REOB (see Box on page 29). ‘The composition of REOB is very different from regular bitumen: it is mainly saturated with aliphatic hydrocarbons, linear or naphthenic, with some resins and very little if any aromatics and asphaltenes,’ Planche says. ‘Due to this composition we believe that the main issue is the compatibility of REOB with some bitumens, particularly the asphaltene rich ones. The effect becomes even more crucial as bitumen oxidises in the field.’ Road cracking typically occurs as bitumen oxidises over time, leading to increased hydrogen bonding and aromatic stacking that makes it stiffer and more brittle. This bitumen oxidation process also makes it more asphaltenic and lowers the solubility of the asphaltenes so that they can phase separate, making it even less compatible with REOB – which could lead to more cracking, he explains. Ageing behaviour not only depends on the composition of bitumen, but also on its structure, he says. ‘A little bit of oxidation on a highly structured bitumen has more impact than a lot of oxidation on a weakly structured one.’ The quantity of REOB included in the formulation is critical. ‘At the right amount, REOB can improve the low temperature properties of bitumen,’ Planche says. ‘By the same token it can make it brittle, particularly on ageing, if added in excess or with the wrong kind of asphalt.’ WRI is currently involved in two European projects BioRepavation and Alterpave aimed at improving the recycling rate of recycled asphalt pavement (RAP) by adding other waste derived materials and rejuvenating agents including biobased additives. ‘Our findings are as always: adding the proper amount is not detrimental, adding too much can become problematic,’ he says, adding that ‘the main issues are excessive stiffening and loss of compatibility that can lead to early cracking’. One of the biggest contributors to premature cracking, or potholing of roads, in Planche’s view, is poor construction. ‘When construction is done right, the binder plays a big role but it is not straightforward. Bitumen/aggregate interaction is also very important. The NCHRP 9-60 project led by WRI is dedicated to address the premature cracking outside construction concerns. Stay tuned, the outcome is expected mid-2019. In brief, binder variability in composition is at the heart of the project.’ SAR-AD is now routinely used by the WRI and dozens of their clients around the globe. However, Planche says WRI is now working on a second generation version that will provide even more insights on bitumen composition, particularly for modified binders. |
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