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THE ROAD AHEAD

28th June 1986, Page 74
28th June 1986
Page 74
Page 75
Page 74, 28th June 1986 — THE ROAD AHEAD
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Which of the following most accurately describes the problem?

Richard Turner, the Freight Transport Association's director of planning, explains why the make-do-andmend approach to road maintenance is short-sighted • Reliable roads are an integral part of industrial efficiency. In the past 10 years the work done by all lorries moving freight has increased by 19% from 89.9 to 106.9 billion tonne-kilometres, but users' expenditure (or the cost) over the same period has declined by 22%, representing a total improvement of cost effectiveness of 34%. At the same time haulage rates have moved downwards, emphasising how the intense competition in the road freight sector has effectively reduced rates by 27% in real terms.

Modern roads are often far more than strategic traffic routes; they can also be a shopping street, a parking area, a loading area, a pedestrian area, a market place and even a play area and right of way for telephone, water, sewage and gas pipes.

To cope with that pressure, roads are made up of two parts: the wearing surface, which to some extent is expendible, is there to provide skid resistance, dispel the water from the surface of the road and keep the rest of the road watertight; the second part is the structural elements which are designed to carry the imposed loads from wheels and dispel their intensity so that they can be safely carried, with only temporary deformation, by the natural soil underlying the road — the sub-grade.

Failure occurs when the wheel load is either excessive, or fatigue has weakened the pavement, and plastic deformation breaks up the monolithic nature of the road, often destroying the waterproofness and seeding decay.

The ultimate strength of a road is measured and designed according to the number of standard-weight axles that it can carry — the standard axle in Europe is 10 tonnes. This unit of road wear is now so important in road design and lorry taxation it justifies some understanding. In the 1950s, the American Association of State Highway Officials (AASHO) carried out some long term wearing tests on different roads using different axle weights. The outcome of that work was the conclusion that structural road wear was proportional to the fourth power of the axle weight. Therefore, a six-tonne axle (6 x6x6x6=1,296) is 16 times more wearing than a three-tonne axle (3x3x3x3=81).

Concrete and black-top road construction react differently to the struggle on them. Concrete roads have excellent load carrying properties and last much longer than black-top or bitumen roads,

but there are fundamental structural differences. A concrete road is a rigid structure which functions as a bridge, taking the weight of the wheels over a wide area and protecting the sub-grade from overload. However, such rigid structures are not suitable if the underlying ground is not fairly sound.

Over the years, concrete pavements have given difficulty with skid resistance. This is basically determined by the roughness of the surface of the concrete [fit is not correct, or wears off prematu. rely, often the only way to restore it is to recut the surface with diamond cutters. This gives rise to tyre noise. Other techniques have been tried using bituminous surfacing stuck to the concrete, but only with limited success.

Concrete roads are fine provided they are well-made, and do not need to be disturbed (to get to underground surfaces). For these reasons, and because concrete roads need curing times of up to four weeks, more often roads are made from bitumen mixtures which set to full strength quickly can be easily repaired and overlaid. Bitumen roads are also basically flexible and capable of accommodating movements in the subgrade.

The most frustrating aspect of road building, though, can be planning. It can take between 10 and 15 years to complete the planning of a new route before the contractor can start building it. The process includes a large amount of publii participation and consultation. This gene ally prompts objections from a wide rani of interests and some inquiries get bogged down at this stage. It is now be corning increasingly important that supporters of individual schemes should alsi get involved at the public inquiries.

Unfortunately, major problems rebuik mg our main road network are not goini to be a rarity. Why is this happening? There are two basic reasons. The first i that our roads are designed to have a 1: 20 year life in traffic capacity terms and also a 15-20 year life structurally — therefore they wear out when they are full. Second, there are no surplus links i the network which can be used to take the strain while one link is repaired.

With a 20-year life at best (some motorways have lasted only eight or 10 years before major repair) we can alway Kpect that 5% of the network will be in ..ed of repair each year.

It is only a matter of time before iort-term solutions fail to contain the tuation, and the consequences have to faced. The strength of a road sudmly falls off at the end of its life and it .Tomes costly to repair a road once it is started that rapid deterioration. If a lad is resurfaced at the right time oney and inconvenience to road users saved.

The effect of axle weights on road ear is obviously important for future !hide design. In terms of standard axs per 100 tonnes of freight carried the .5-tonne lorry causes nearly 25% ore road wear than the 38-tonne rry. Vehicle manufacturers are now sponding with new designs that would even kinder to the roads.

All things being equal, the taxation vings by running with an extra axle are ilikely in themselves to justify the extra pital cost, and loss of payload. But all ings are rarely equal and more often lw operators are finding that a combinam of factors, of which the tax savings e only one, prompts a different decim.


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