Lafarge asphalts on the ‘Magic Roundabout’

Lafarge’s specialized asphalts products AxoShield and AxoAltoflex proved to be the materials of choice for a major revamp project at the award-winning yet notorious Plough roundabout in Hemel Hempstead, Hertfordshire. 

The gyratory, known locally as the ‘Magic Roundabout’ because of its complexity, is a traffic hotspot and features a bewildering combination of one large roundabout surrounded by six mini-roundabouts with drivers able to turn both left and right.

Over the years, ever-increasing volumes of traffic had taken their toll on the roundabout, which was looking dated and showing significant wear and tear. The asphalt surfacing had deteriorated noticeably with reflective cracking caused by a damaged concrete substrate adding to the decline.

Mindful of the need to repair the roundabout, Hertfordshire County Council (HCC) originally planned refurbishment in May, just after Plough had been voted Roundabout of Year 2011 in a national competition. However, work was delayed until the summer and a five-week schedule of renovation started in August.

Carried out by Hertfordshire Highways, a partnership between contractors Lafarge and Amey, designer Mouchel and client HCC, the work took place in the evenings to cause minimal daytime disruption.

Lafarge’s AxoShield was seen as the most suitable solution for part of the reconstruction of the existing pavement, particularly as problems with the underlying concrete had compounded the situation. Lafarge worked closely with Shell Bitumen to develop AxoShield with the aim of creating a resilient, low-maintenance road surface suitable for replacing poor-quality pavements, particularly where project times are tight and full reconstruction is not viable.

AxoShield contains carefully selected fine aggregate bound together using Shell’s highly elastomeric Cariphalte dense mixture (DM) binder – a material widely used in bridge decks and crack-relief systems where tensions need to be cushioned to prevent fractures.

The combination results in a durable, yet flexible, crack-inhibiting asphalt membrane which can absorb both the stresses of heavy traffic above and small movements in the concrete substrate below, thereby reducing the need for future maintenance and helping prolong the life of the pavement. This flexible layer also means complete remediation is not required on the damaged underlying concrete, saving both time and money.

Site agent Simon Ditton, who was responsible for Lafarge's surfacing operation and site team, explained: ‘The use of AxoShield as a crack inhibitor removes the need for specialist reinforcement grid systems and allows work to progress quickly without having to bring in specialist installation equipment and contractors.

‘It offers a straightforward installation solution that means less disruption to motorists and time and money gains for the client. An added benefit is the long-term low-maintenance properties of AxoShield, which is designed to reduce the need for future repairs.’

Reconstruction of the Plough roundabout started with carefully sequenced work each evening in line with a detailed traffic-management plan and temporary diversions. For each area of the work site, worn asphalt material was planed out to reveal the concrete below where repairs were made where necessary.

AxoShield was installed on top of the cleaned concrete before the addition of a binder course. A polymer-modified bond coat was used between each layer of the system to provide extra adhesion. These layers were completed in quick succession on each section of the carriageway, followed by the painting of temporary white lines to allow the surface course, Lafarge’s AxoAltoflex, to be installed later in a single operation.

AxoAltoflex is dense stone-mastic asphalt developed using Shell Cariphalte high-performance (HP) polymer-modified bitumen. The material – an alternative to conventional hot-rolled asphalt – offers a good level of skid resistance, making it suitable for heavily trafficked areas such as roundabouts.

Fully recyclable and with high density, low permeability and strong resistance to deformation, AxoAltoflex also offers the benefit of noise reduction. An application of high-PSV fine grit was placed on top of the hot surface course to provide a degree of early life skid resistance.

Mr Ditton said: ‘Everyone was pleased with how the job went. We were able to keep the client informed all the work being undertaken as we kept the work to programme and finished on time. We resolved issues quickly and successfully and handed the road back to the client as planned, despite delays due to bad weather.’

AxoShield was first installed as part of a trial in 2008 on the A45 at Great Billing, in Northamptonshire, in conjunction with the Highways Agency, and monitored by the Transport Research Laboratory (TRL). TRL returned to the site in 2010 and reported no signs of reflective cracking on the surface after two years in service.

A second application soon followed at Hall Quay in Great Yarmouth, for Norfolk County Council. Here, the road had exhibited cracking and crazing but removing the concrete foundation and reconstructing the road would have proved too expensive. The solution had to resist fractures from the concrete through the asphalt overlay for as long as possible.

Norfolk County Council’s highways team turned to AxoShield as the most viable option as part of the pavement rehabilitation for this busy town centre location, helping to minimize disruption for road users.

Mr Ditton commented: ‘We are extremely pleased with the performance of AxoShield in the county and are delighted it offers an innovative yet practical solution for our customers.’

For further information visit: www.lafarge.co.uk