Barrett Byrd Associates

First motorway use of an innovative concrete overlay technique in combination with hi-tech bitumen binders has helped fast track recent resurfacing work on the M5 in Devon. Claire Symes reports.

Concrete carriageways can sometimes generate excessive road traffic noise and the Government is planning to resurface all concrete UK trunk roads and motorways with quieter materials by 2010. Removing concrete carriageways can be costly and disruptive and, until now, ‘crack and seating’ the concrete and overlaying it with a thick asphalt layer has been the preferred surfacing option. However, Stress Absorbing Membrane Interlayers (SAMI) offer an alterntive.

One type of SAMI features an innovative spray applied stress membrane, which has been used on the continent for over a decade, but has just made its UK motorway debut on the M5 near Tiverton in Devon. Aggregate Industries, trading as Bardon Contracting, has successfully completed resurfacing work on both carriageways two weeks early thanks to use of the newly introduced system, some innovative binders from Nynas Bitumen and unseasonably good weather.

“The concrete surface between junction 27 and the Willand overbridge to the south was laid in 1976 – I remember it well because I worked on the contract,” says Bardon Contracting surfacing manager John Rae. “Although the surface is 27 years old, it still has some residual life left and ‘crack and seat’ techniques are generally only applied to concrete surfaces which have reached the end of their design life.”

This section of the M5 is spanned by three overbridges. Existing headroom beneath the structures meant that the 150mm minimum layer of asphalt needed by crack and seat schemes was not viable. Also, the Highways Agency (HA) and its Area 1 term maintenance consultant Parsons Brinckerhoff (PB) was keen to keep traffic flowing at all times and avoid long term lane restrictions necessary for removing the concrete. Instead PB, with approval from the HA, opted to trial the continental SAMI overlay system.

The M5 in Devon carries around around 45,000 vehicles per day and the volume of traffic is at its peak during the summer. To help holidaymakers travelling to and from the south west, the HA does not allow any major road works on the M5 during July or August and restricts work carried out in May or June.

“We planned the work to begin in early September, to miss the main holiday season, but still early enough to catch the good autumn weather,” says Parsons Brinckerhoff supervisor Paul Harding.

The contract involved applying an initial 3mm thick polymer modified bitumen (PMB) binder ‘stress membrane’ to both the north and south bound carriageways. The membrane – designed to seal the road surface while preventing reflective cracking in subsequently laid asphalt layers – was sprayed directly onto the concrete carriageway. The membrane was then overlaid with a thin layer of microsurfacing.

An SMA binder course and Bardon Thinpave thin surfacing, both of which were bound using a PMB binder – Nypol TS – supplied by Nynas was laid over the microsurfacing. The contract also included drainage improvements and installation of new safety barriers.

“In effect SAMI’s flexible membrane works by decoupling movement of the concrete from the asphalt above,” says Harding. “Concrete moves continuously through daily and seasonal thermal changes. While the ‘crack and seat’ method effectively breaks each slab to minimise such movement, flexible overlays with a stress membrane are sufficiently elastic to absorb the movement and maintain a waterproof layer.”

Bardon Contracting selected to use Nynas’ Nypol TS binder for the binder course and final surfacing because the material’s PMB characteristics enabled the contractor to get maximum performance from a minimum surfacing thickness. “This must be one of the first surfacing contracts in the UK to use PMB binders in every asphalt layer,” says Rae.

Nypol TS also has a number of production and environmental advantages which made it even more ideal for the work. “Nypol TS is a thixotropic polymer modified bitumen binder, which means that it has shear-thinning properties that allow it to become thinner when mechanical energy is applied, and viscosity to increase when energy levels reduce,” says Nynas Bitumen southern area bitumen sales manager Miles Williamson.

“This means that during asphalt mixing the binder becomes less viscous to ease the process, then increases in viscosity when the product is placed into the vehicle, which prevents the binder from draining off the aggregate. The laying process is eased by the binder viscosity being reduced by the action of placing the asphalt into the paver and the viscosity increases when it is laid.”

Around 15,000t of asphalt, including approximately 1200t of Nypol TS, was used during the contract to form the binder and surface courses. The material was produced at Bardon Aggregates’ nearby Westleigh Quarry where the processes were overseen by the company’s area operations manager Scott Ford. “One of the main benefits of Nypol TS for us was the lower binder drainage during transportation,” he says. “It meant our drivers did not have to clean their wagons between loads which is not only safer but also speeds up delivery times and avoids waste.

“At the height of surfacing work, Nynas was supplying up to four tanker loads of Nypol TS every day from its plant on the Wirral. These had to be carefully programmed to arrive at the quarry at regular intervals and to tie in with Bardon Contracting’s surfacing schedule to prevent us from running low or having to stockpile supplies.”

Before any surfacing could begin, Bardon Contracting carried out vacuum grouting below selected joints in the concrete pavement to ensure the concrete’s integrity and prolong its design life. “Falling Weight Deflectometer and Ground Probing Radar surveys were carried out to identify areas which needed to be grouted,” says Rae.

Parsons Brinckerhoff specified Jean Lefebvre’s Flexiplast for the initial ‘stress membrane’. This was sprayed directly onto the concrete and, once set, was followed by a 10mm protective layer of Gripfibre microsurfacing.

Bardon Contracting then overlaid the Gripfibre with a 30mm thick layer of SMA binder course formed by a nominal 10mm sized limestone aggregate, sourced from Westleigh Quarry. This was followed by a 25mm layer of Bardon Thinpave which used a 10mm gritstone aggregate, again bound by Nypol TS, to create a highly skid resistant surface and a total overlay thickness of less than 70mm.

Even though much of the M5 contract focused on using hi-tech materials and getting the job done quickly and safely, environmental considerations were not forgotten. “All of the old drainage material, concrete and asphalt planings removed from the motorway were taken to Westleigh Quarry for recycling,” says Bardon Aggregates general manager Roger Full. “The material was reprocessed for reuse on the site and the remainder has been stockpiled for future use on other schemes in the area.”

Summer would have been the ideal time to carry out this kind of work, according to Harding, but despite the autumn start date the work went well. “Laying of both Flexiplast and Gripfibre is very weather dependent but we were fortunate and conditions were much better than hoped for,” says Harding. “The good weather, along with Bardon’s careful planning, has been a major factor in the early completion of this scheme.”

Rae is also pleased with the finished job. “Using any new technique for the first time, particularly on such a major job, is always a challenge and there is a steep learning curve for all involved,” he says. “But, thanks to the good relationship we have developed with Parsons Brinckerhoff on previous contracts and well planned logistics, everything went fairly smoothly.”

HA project sponsor Barry Scruby adds: “This contract has effectively strengthened a worn section of motorway while improving road safety and drainage. Use of quieter road surfacing materials on the scheme is an added bonus for local residents.”

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