Keeping lane closure periods to a minimum during surface maintenance is a prime concern for the Highways Agency. Any surface treatment that can speed laying times and hence shorten the closure of motorway lanes and trunk roads is of interest.
'It is a bonus if the material will also reduce surface noise,' adds John Williams of the Highways Agency.
With this in mind, the Agency is testing a micro-surfacing product over a section of the trunk road between Kings Lynn and Thetford in Norfolk.
It will be two years before the Highways Agency can evaluate this micro- asphalt system. Unqualified approval will depend on the integrity of the surface texture at the end of the trial period.
'We would expect to see only a little loss of surface texture in wheel tracks over the test period to approve this system,' says Williams.
What interests the Agency and the county councils and local authorities responsible for the upkeep of the road network are the potential cost savings, long-term durability and noise reduction that a surface treatment can offer.
There is often a trade-off between these competing criteria. Road-related noise is proportional to road surface texture and traffic speed. If traffic speed is kept constant, then the more textured and indented the surface the greater the surface noise generated. In addition, a non-porous surface will generate more noise than a porous texture. Moreover, at low vehicle speeds, through towns and built-up areas, the road noise is significantly less than on trunk roads.
Surface dressings are the cheapest products, followed by multi-layered surface dressings and then the thin wearing course systems. Surface dressings are usually specified in towns, where there are low speed restrictions.
The micro-asphalt system that is being tested in Norfolk is a cold-applied, fibre-reinforced bitumen emulsion-based system called Ralumac 2000, developed by Colas. Ralumac 2000 incorporates a polymer-modified bitumen emulsion binder and fibres with small, high-quality aggregates.
Colas describes it is a specially formulated, cold-applied, wearing course which eliminates the need for heater boxes, prolonged curing times and the pungent aromas associated with hot applied systems.
'On price it comes between a multi-layered surface dressing and a hot paver-laid thin wearing course,' says Williams.
'It is applied to the road surface as a self-levelling screed and has the ability to fill wheel ruts and deformations in road surface in one pass,' says Joe Dinnen of Colas. 'Within 15 minutes of laying Ralumac 2000, traffic can be allowed to run on the surface.'
Local authorities have already been using the system. Wiltshire County Council highways maintenance manager John Dunn says: 'With tighter and tighter budgets to work with, we just cannot afford to plane the top of a road surface once it has rutted and been badly scarred by trenching and lay a new wearing course. Ralumac 2000 is the next best thing and it is a lot cheaper.'