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.
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.
The extent of road ageing simulated by the US Pressure Ageing Vessel procedure, currently regarded as the best method
recyclable, asphalt is a mixture of crushed rocks and stones and sand bound together with 3-7% sticky black bitumen. Recyclable at the end of the road’s lifespan, it can be reclaimed and ‘recycled’ to make new roads
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
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.’
|Road rage and reob
REOB – re-refined engine oil bottoms (REOB) – is what’s left over after vacuum distillation of waste engine oil. Depending on who you talk to, it’s also referred to as Vacuum Tower Asphalt Extenders (VTAEs), Waste Engine Oil Residue (WEOR) and asphalt flux.
‘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.’
Fuente de la noticia: https://www.soci.org/chemistry-and-industry/cni-data/2018/6/asphalt-not-trashphalt