Value in the long run

The funding of highway maintenance in the UK makes it difficult to choose solutions that will save money in the long run but may cost more up front. But expert advice at an early stage can help local authorities understand how premium products provide the answer, as Norfolk County Council under its partnership with Mott MacDonald and May Gurney has discovered.

Highway maintenance engineers face the dilemma of working within short term budgetary constraints while aiming to give roads the longest service life they can. Premium products that can extend pavement life considerably – like polymer modified bitumens – are often considered too expensive because the initial cost is higher than traditional asphalt products. And yet, over the longer term, they can reduce maintenance costs considerably.

Norfolk County Council working innovatively in partnership with private sector expertise is bucking the trend. It is opting for a variety of maintenance solutions that will extend pavement life, including polymer modified binder courses, having taken advice from one of the leading suppliers in this field.

NCC’s approach to maintenance on the county’s roads is to treat each site individually and to find the best value solution at each location, based on the condition, location and network constraints – even if that means spending a bit more money. “We’ve learnt from early life failures in the past that the traditional solutions of “40mm out, 40mm back” or straight overlay don’t necessarily give you longevity,” explains NCC highways project manager David Allfrey. “The process we follow now is about trying to find the optimum solution.”

As part of that process, the council’s maintenance experts called on representatives from specialist bitumen supplier Nynas to discuss the uses and available options of polymer modified binders and the potential for using them in different layers within the pavement. “When we talk with local authorities, we are able to get a clear understanding about their specific issues, and identify how our products may be able to help them overcome long term, persistent maintenance problems,” explains Nynas’ regional sales manager Miles Williamson.

“For sustainability and long life pavements, the use of polymer modified bitumens against penetration grade bitumens provides improved longevity and definite whole life cost benefits.

“This has been demonstrated at Norfolk County Council, where they have been prepared to spend a little more to rebuild pavements that will cope with the increasing amounts of traffic and the stresses these put on Britain’s weakening and fragile infrastructure.”

The county council’s Planning & Transportation department is three years into a 10 year partnership with Mott MacDonald and May Gurney, which includes routine maintenance. Design work is split between Allfrey’s team and Mott MacDonald’s designers, while surfacing is carried out by three specialist contractors – Tarmac, Lafarge and Ennstone – under the coordination of May Gurney.

A vital ingredient is NCC’s highways laboratory, now in partnership with May Gurney’s lab activities, which plays an important role in deciding on the best strengthening or resurfacing option for every site. Typically, the first thing that happens after a brief is allocated to one of the partnership’s designers is that pavement engineering specialists from the laboratory look at all the data available for the site from deflectograph, scanner and SCRIM surveys, and then walk the job with the engineer. Cores or samples of the road are taken to confirm what is wrong with the road and what the best solution might be.

If the answer is likely to be a simple overlay, the road will be cored but when matters are more complex, window samples are taken in order for the properties of the unbound layers and subgrade to be used for a reconstruction solution. If the pavement is badly cracked, cores will be taken through the cracks to see how deep they are. From there, the pavement engineering team recommends the best option, and the debate starts about achieving the optimum solution for the money available.

Allfrey says the lab staff add an important dimension to the process: “They can talk in great detail about the technical issues with the supply chain. That expertise gives us a real advantage in terms of being able to look at best practice.”

Lab manager Bob Noakes is a recognised expert in asphalt, and is currently chairman of the British Standards Committee for asphalt products and leader of the UK delegation on the European Standards working group. He says: "My experience at national and international levels helps me in determining that novel solutions offered by the supply chain are robust and reliable, and in providing NCC with confidence that the latest techniques will deliver benefits.”

This translates into a willingness on the part of the highways department to try new materials and techniques if they are likely to lead to improved pavement life. Having three surfacing contractors on board also helps. “Because we’re looking so far ahead, we can talk to the supply chain and see who’s got the products that are best for the particular condition,” Allfrey explains.

Bob Noakes has been responsible for introducing a range of products and processes over the past five years, including binder courses made with polymer modified bitumen, asphalt reinforcement and both in- and ex-situ recycling.

Recycling is typically used for replacing thin pavements in rural areas, but for thicker pavements with extensive cracking, a more substantial treatment is required, and NCC currently opts for polymer modified binder courses as a strengthening treatment for between 25% and 30% of problem sites. These tend to be urban roads that are heavily cracked. Usually the top 100mm is planed off and replaced with a 60mm polymer modified binder course and 40mm surface course.

“We are trying to invest in the binder course,” explains Allfrey. “The surface course can easily be replaced, but we don’t want to be coming back in five years’ time because the road’s cracked.”

Materials engineer Mick Drury adds: “The most common use we have for polymer modified binder courses is where we’ve got urban roads where the bound layers are quite thick and they’re starting to suffer from fatigue cracking and/or reflection cracking. If you do “40mm out and 40mm back”, you’ll start to see reflection cracking within a year, and it will fail very quickly. We put in a polymer modified binder course to inhibit the reflection cracking, and then use a fairly modest specification surface course. It’s the binder course that’s providing most of the longevity to the pavement.”

He says he anticipates the binder course lasting for at least 20 years, while the “sacrificial” surface course can be replaced more frequently.

NCC – through its surfacing contractors – is using three of Nynas’ polymer modified bitumen products, Nyguard HR, Nypol 200 and Nypol TS, in different pavement layers. Specifying small amounts of polymer modified binders can add cost if the surfacing contractor is using an asphalt plant that is geared up for penetration grade bitumens, but the council is getting around this by grouping a number of jobs together to gain economies of scale. “The materials they are using help with problems of high stress on the surface, point loading and rut resistance, weak bases (flexible and unstable) and reflective cracking from existing concrete under layers,” explains Williamson.

The county council’s use of polymer modified bitumen is not limited to binder courses. In highly stressed areas – such as roundabouts – it is also used in the surface course. So far two of its surfacing contractors have successfully incorporated Nynas’ Nypol HR into their surfacing products on the county’s roundabouts. “The material is very tough and we don’t see any loose aggregate,” says Drury.

Bob Noakes adds: “The use of finite element design tools in assessment of pavements resistant to cracking demonstrates clearly the potential for increased life to onset of crack propagation by using polymer modified binders in asphalt layers. We work on double the life compared to straight run bitumen for an equivalent mixture. In most circumstances the materials are not double the price, we revisit the site less, to the investment is demonstrably cost effective.”

Notwithstanding the theory of finite element analysis, the engineers and materials specialists at Norfolk County Council acknowledge that time will tell if they are right in choosing premium products to provide long term value. They are currently instigating a system of long term monitoring to assemble real data on which to base future decision making. That said, they already have a strong feeling it is worth the investment in what can be higher initial cost materials for the long term cost benefit of reduced maintenance.

February 2008