Barrett Byrd Associates

Advocates of surface dressing are beginning to specify highly durable materials that can be relied upon to perform well in changeable weather, reports Mike Walter

Summertime in Britain can be just as frustrating for surface dressing contractors as it is for sun worshippers. Both want warm weather and for the rain to hold off, but all too often the high season can seem to end before it has even begun.

Poor spells of weather can be disastrous for surface dressing crews whose planned maintenance schedules can be thrown into disarray. Surface dressing contractors therefore have to make the most of any spells of pleasant weather and are increasingly specifying durable bitumen binders that can be relied upon to hold aggregate chippings firmly in place first time around.

Once such contractor is Ringway Specialist Treatments, which carries out surface dressing in eight counties throughout England. It uses a pair of polymer modified emulsion binders from materials specialist Nynas Bitumen, which are known as Fleximuls and Duramuls.

This summer, the two binders received a unique endorsement from the British Board of Agrément. Both Fleximuls and Duramuls became the first two emulsions to be approved for use in surface dressing by HAPAS, the Highway Authorities Product Approval Scheme.

Award of a HAPAS certificate to the pair of Nynas Bitumen emulsions means they now meet the Specification for Highway Works Clause 922 and can be used in the surface dressing of the most onerous of sites and trunk roads. Award of the certificate is the culmination of a six stage approvals process that included the laboratory testing of materials, site inspection and road trials.

Ringway’s senior contracts supervisor Nick Holder says: “These modified binders allow us to surface dress a road with confidence during times of changeable weather. We may only have a small window of opportunity between periods of rainfall in which to carry out the treatment, so we need to specify binders that will perform well.”

The Duramuls binder is a premium grade material suitable for heavily trafficked roads and the Fleximuls binder is suitable for sites with less traffic. Both materials provide excellent grip on chippings and offer good elasticity, flexibility and tensile strength.

The two binders can be used in one of three surface dressing applications, the choice of which depends on the condition of the road which is to be renewed. The most elementary – a ‘single surface dressing’ – involves application of a binder followed by the laying of chippings.

A second method is known as a ‘racked in’ surface dressing whereby an application of bitumen and chippings is followed by a further layer of a smaller stone. A third type of treatment is known as ‘double surface dressing’. Binder and stone is followed by a second layer of binder and a smaller stone, to form a ‘sandwich’ that gives a stronger finish to the road.

Nynas Bitumen’s surface dressing consultant John Dennis says that it is not uncommon for one stretch of road to receive three different surface dressing applications. He adds that the latest plant used to distribute bitumen and chippings provides a uniform finish across a carriageway. “Our polymer modified bitumens are applied at 85ºC using the latest spray bar that extends from a 2.8m width to 4m. This allows for binder to be applied to the narrowest of roads and at ‘bell mouth’ junctions where the carriageway widens significantly. Certain jets on the spray bar can also be closed at the flick of a switch to ensure bitumen is only sprayed onto the road and not a verge.”

Chippings are applied using a Phoenix spreader that follows directly behind the bitumen tanker and a road roller compacts the stone into the bitumen. “Traffic can drive over the surface dressed road virtually straight away and vehicles effectively become a secondary roller to help bed the chippings into the binder,” he says.

Surface dressing is widely regarded a cost effective means of repairing worn carriageway or restoring texture to a stretch of road. Ringway’s Nick Holder says: “Compared to other road renewal treatments, surface dressing is a relatively inexpensive way of correcting cracked and crazed asphalt and giving added skid resistance.”

He adds that the popularity of surface dressing is largely on the increase. His company completed around 7Mm² of surface dressing two summers ago and around 10Mm² in 2004. This year looks set to be even more successful, with around 11Mm² of treatment due to be laid by Ringway on rural routes throughout Gloucestershire, Somerset, Wiltshire, Hampshire and Cambridgeshire.

The popularity of surface dressing in rural areas is, however, at the expense of its use in urban situations, such as on housing estates. Here, surface dressing is tending to lose ground to other forms of highway treatment and has attracted some criticism by residents who generally do not favour the spreading of loose chippings which can take time to bed down.

Ringway has recognised that the technique is not suitable on every type of road. It has focused its attention on developing long term partnerships with county council roads departments to provide technical advice and guidance as to where surface dressing can best be applied. One such arrangement to have started last year is a three year trial with Gloucestershire County Council, where Ringway provides surface dressing advice to council staff and carries out associated services such as white lining and patching.

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