Care needed when choosing a polymer modified binder

Not all polymer modified binders are the same and clients have to make careful, informed choices if the asphalt surfacings they specify are to achieve maximum durability, says Total Bitumen.

An example of an elastomeric PMB asphalt, applied as an overlay to a motorway in area 10.

There is no such thing as a single methodology for carriageway construction and there are numerous types of failure that can affect an asphalt surfacing. So it should come as no surprise to learn there is a wide range of polymer modified binders (PMBs) on the market, each designed to suit a particular application.

But according to Total Bitumen some highway maintenance professionals fail to understand such differences and do not adequately investigate the type of PMB they require. This can lead to asphalts not performing in line with expectations. The company is keen to make sure that the most appropriate grade of PMB is used in each application and it has begun discussions with clients and key decision makers about the full suite of modified binders in its Styrelf range.

“There are as many PMBs available as there are makes of car on the road,” says Total Bitumen’s Market Development Manager Rick Ashton. “And there is a perception among some end clients that if they just specify a ‘PMB’ then performance requirements will be delivered. But PMB procurement should take into account more than just unit price.”

It is not uncommon, he adds, to find that a ‘cut and paste’ approach has been taken to binder specification. Contract documents sometimes specify the generic term ‘binder = PMB’, without attention being applied to specific engineering requirements.

“This approach to binder procurement and selection could lead to either under specification – resulting in premature failure – or over specification, which increases risk and restricts value engineering that is crucial in the current economic climate,” says Mr Ashton. At one end of the PMB spectrum are very stiff products known as plastomers which are useful in resisting deformation and heavy point loads. At the other end of the scale are elastomers that allow greater flexibility to the asphalt, which can help to resist reflective cracking when asphalt is used to overlay a concrete base for example. Total Bitumen’s PMB range Styrelf can be described as ‘cross linked’ products that combine both elements of stiffness and flexibility.

Three commonly used polymers in PMBs are EVA (ethylene vinyl acetate), SBS (styrene butadiene styrene) and SBR (styrene butadiene rubber). These polymers can be blended into bitumen of varying penetration grades to provide characteristic levels of performance, across a spectrum of ‘flexible’ to ‘stiff’. “There are more PMB grades available than standard bitumen grades,” adds Mr Ashton, “so it is important to choose the right one.”

Clients can now consult document EN14023 for indication of performance classes for PMBs or should ask for guidance from a specialist bitumen supplier such as Total Bitumen, he adds.

Asphalt producers may be forgiven for thinking that with so many variations of PMB available they would not have sufficient storage capacity to accommodate modified binders. Total Bitumen’s Styrelf range, for instance, has several grades. But Mr Ashton points out there are one or two grades of Styrelf that can effectively be used in most applications. “An asphalt producer will generally use a grade that suits the bulk of its product portfolio and consult Total Bitumen when specialist applications are necessary.”

He adds that Styrelf should be viewed as an environmentally sustainable material, despite the fact that the binder is usually mixed hot and more often than not is used to coat high quality virgin aggregate. “Deciding whether a material is sustainable should not just consider how it is manufactured. That is just one piece of the jigsaw puzzle,” he goes on. “There is more to sustainability than producing lower temperature asphalt or simply adding recycled ingredients.

“Polymer modified products may consume more resources at the outset, but can often help a road to exceed its expected design life, requiring less maintenance over the longer term.” Real sustainability means a whole life cycle approach and durability is a major consideration, he adds.

Total Bitumen works closely with a number of asphalt manufacturers and contractors to develop PMBs including Aggregate Industries which uses Styrelf in both its thin surfacing materials and proprietary products like Bardon Superflex.

The company’s Head of Research & Development Bob Allen says that PMBs help to resist rutting in a surface and makes asphalt more durable when temperatures fluctuate. He adds that performance of asphalt materials both in very hot summers and extremely cold winters is set to become more of a pressing issue if changes with the climate persist.

But the main hurdle to selling more of the enhanced products is, as ever, price. “We try to explain the whole life cost benefits of polymer modified material and convince clients to spend just a little more but it can be very difficult,” he says.

Sites that demand more enhanced products should certainly invest in a robust surface course, Mr Allen adds. PMBs can also be of benefit to the binder course because of the damaging effects water ingress and tree roots can cause a road. “The increasing use of thin asphalt layers means it is more important than ever to consider durable materials.”

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