Realistic and cost-effective roads

The report that an additional €6 million are to be spent on roads is welcome indeed as the state of our roads is certainly bad, but it is hoped that the money - even if it comes from the EU - will be spent wisely and efficiently. Our roads got to the state they are in largely because of lack of maintenance, weakness in their surfacing and unrestricted trenching works, usually very poorly reinstated.

I get very annoyed to hear that the only proper way to deal with our roads is to rebuild them from scratch. I consider this to be defeatist, unrealistic, wasteful and only applicable in a few extreme cases. Unless the geometry is seriously wrong, the condition of the underground services is very precarious or the foundations are very inadequate, I would favour rehabilitation any time. Ideally, this could give us double the mileage for half the cost and in a quarter of the time.

To start with, even if money were no problem, with our full labour force and machinery, we could only hope to completely rebuild about one per cent of our road network every year. In any case, it would be logistically impracticable to decommission a substantial part of our road network at any one time. So it would take us all of 100 years to complete the job. As the useful life of a road is some 20-30 years, the first roads to be taken on would have had to be redone three times! The mind boggles.

When a major road is reconstructed, the traffic that it serves has somehow to be diverted to other routes, usually with calamitous results. The additional social costs of congestion, time and fuel, to say nothing of accidents, CO2 emissions and frayed nerves, have to be factored into the equation. If there are delays - and recent experience has been that there are always massive delays - the outlook becomes much worse. So taking an established major road out of the network for any length of time should be considered as a desperate remedy and always with work going around the clock.

Recent experience with rebuilding roads from scratch has not been an unqualified success. I would not like to get too technical, but layers at a certain depth below the surface would have had the compaction and consolidation of many years and it would take some very good work to replace them with layers of equivalent strength. The original surfacing was insufficient in thickness and in a poor state but may have been incorporated in the road base below the new surfacing, which, in my humble opinion, should have been thicker and with superior aggregate in the wearing courses.

In UK practice, the pavement design (layered construction) of flexible roads - as against rigid or concrete roads - is usually based on the traffic loading of the road (in MSA or millions of standard axles for the design year) and the CBR (California Bearing Ratio) value of the various layers supporting the surfacing. In the more heavily trafficked roads this would imply a surfacing thickness of 100 mm on top of a road base thickness (also black top) of 60-100 mm. A CBR of 10-20 (achievable with local materials and proper techniques) would be aimed for at the top of the sub-base.

This total black-top thickness is unusual in local practice but it should be looked upon as an investment in the long-term life of the road. The surfacing not only provides the smooth riding quality of the road but it also seals it against the ingress of water and helps spread the traffic load uniformly on to the sub-grade, protecting it from deterioration. A bumpy, deformed or corrugated surfacing introduces a dynamic component (impact) in the loading, leading to the long-term disintegration of the road.

Superior (imported) aggregate in the wearing course is necessary as our best limestone aggregate is weak in polishing and abrasion.

It also soaks up too much bitumen, which is the most expensive component of the hot asphalt carpet.

Research on the improvement of wearing courses is ongoing. Last March's issue of Transportation Professional (the journal of the Institution of Highways & Transportation) has a very interesting article on reinforcing cracked surfacing with the use of a Gridseal fibreglass textile laid under tension beneath an asphalt overlay on the M69 and the A45. And what about mastic asphalt or epoxy resin around the manhole covers?

As part of the roads refurbishment programme I would also like to point out some incorrect cambers, super-elevations and acceleration lanes. There are also a number of roundabouts that are the wrong way round because they were designed and built on continental models where the traffic keeps to the right.

The author, a UK chartered civil engineer who specialised in highway engineering in England and Italy, served as roads engineer at the Public Works Department for many years and lectured at the university.