Dressing's back in fashion

Surface dressing has undergone both a technological transformation and – in Hertfordshire, England at least – a renaissance as the preferred means of making best use of road maintenance spend.

Perceptions are back in favour of surface dressing in Hertfordshire, with the county’s engineers swung by advances which make the process a very viable means of extending the life of the county’s roads. Problems encountered in the past are being avoided with better dressing designs, materials and plant, and with costs still lower than the alternatives, Hertfordshire Highways is getting good value.

The county’s road maintenance partnership of Hertfordshire County Council, Mouchel Parkman and Amey/Lafarge is now achieving serious coverage of its road network, in terms of areas resurfaced. With the help of additional funding from the county council, Hertfordshire Highways has surface dressed 200km of the county’s road network this year.

That represents a big win, given that even with an extra £10M from HCC, capital funds are still stretched to maintain all of Hertfordshire’s roads. But by widespread use of surface dressing, Hertfordshire Highways is now managing to stop further deterioration of a lot of them.

“Surface dressing is still unpopular to a degree with the public. Its reputation has not improved a great deal yet, but it was chosen because it is an excellent asset management treatment, keeping water out and restoring surfaces,” says Hertfordshire Highways’ principal strategy development manager Chris Allen-Smith. “Prior to this year, we had not made extensive use of surface dressing for some time, due to problems, which were actually often caused by people driving inappropriately on the new surface. Advances in binders and techniques convinced us to use it again in a big way this year and so far the phones have not lit up,” Allen-Smith says.

Hertfordshire Highways has surface dressed over 200 sites around the county this year, totaling 1.25 million square meters. “That’s over 4% of the total carriageway network surface dressed this year, not counting the other types of surfacing work that are being undertaken,” Allen-Smith says.

Hertfordshire Highways has had two surface dressing gangs working. One has been laying a ‘Premium’ dressing on heavily trafficked sites including stretches of “A” road dual carriageway and another has been doing more lightly trafficked roads, totalling around 130 sites. “We have carried out a very varied programme of surface dressing designs in Hertfordshire this year, but they can be separated into either Conventional or Premium,” says Allen-Smith.

Nynas Bitumen’s Fleximuls binder has been used for the Conventional dressings and its Duramuls for the Premium dressings, but even the Conventional designs are advanced from dressings of old. “The intermediate Fleximuls is ‘racked in’ beneath two different sizes of aggregate,” says Mr Allen-Smith. “The premium sites are primarily all double dressings, using Duramuls binder with a 10mm chipping, then a further layer of binder and 6mm chipping.”

The Nynas emulsion binders are approved for use on all roads by HAPAS (the Highway Authorities Product Approval Scheme) and are modified with polymers to make them perform better than conventional types. Compatability with the binders and high grading are the key criteria of stone selection. High PSV (Polished Stone Value) aggregates from specific quarries were chosen – with advanced plant also significant to surface dressing’s rebirth.

Rates of spread of both binder and chippings are vital elements of the whole process. Computer controlled Acmar sprayers are now commonly used for applying binders and advanced Phoenix Mk10 chippers for controlling the spread of the aggregate. Three quarters of the surface dressing plant in use in Hertfordshire is new this year, such is the sharp rise in surface dressing work and this looks set to continue.

Chris Allen-Smith says: “Over recent years our council members have really bought into the principles of Highway Asset Management, which shows the importance of arresting deterioration quickly to add life to roads in the long term. We are pursuing this approach, of making early intervention and preventative maintenance a priority with surface dressing.”

Hertfordshire Highways is a leading authority in the development of Transport Asset Management Plans (TAMPS), which are being introduced at the behest of the Department for Transport to make better use of limited resources. Hertfordshire County Council engineers were instrumental in putting together the national framework for TAMPS with the County Surveyors Society and Hertfordshire Highways has continued in the same vein.

“As far as possible, our spending is now on preventative works, adding life to road surfaces,” Allen-Smith says. “This helps us make the best us of the available funding. But this year our spend on road and footway surfacing has been virtually doubled with the additional money, allowing us to do an awful lot of preventative maintenance and clearing of problem areas.”

Some sceptics still had to be won over towards surface dressing however. “Presentations from the RSDA (Road Surface Dressing Association), including a one day seminar on the technical advances were helpful in making everyone aware of what’s now possible,” Allen-Smith says.

“We have shaken off a lot of the old perceptions, including that a lot of preparatory work is needed prior to surface dressing, which is no longer the case. More coverage can be gained than before and although TAMP principles show intervention has to be made earlier, the life cycle costs with surface dressing are much better.”

March 2008