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Road loader

On first appearance the Heavy Vehicle Simulator is quite a strange sight that has, not surprisingly, attracted attention on State Route 14, in Palmdale, California. It looks something like a mobile crane rig with lots of wheels but no jib.

The HVS is a £1.1M mobile test machine which is used to subject roads to accelerated trafficking. The rig contains a wheel which is rolled up and down the pavement to simulate truck traffic. It can apply wheel loads of up to 100kN, which is two and a half times greater than the standard load applied by a truck.

By overloading the pavement in this way, the effect of many millions of standard axle loads can be reproduced in a short period: 20 years of road deterioration in as little as three months. This allows engineers to investigate pavement deterioration in situ, using an array of instrumentation to measure strains, rutting and cracking.

Variations in newbuild pavement and overlay design can be thoroughly tested, and optimum designs identified.

'Cherries just drop into your lap, ' says Dr John Harvey of the University of California at Berkeley, who has carried out extensive testing with HVS on concrete and asphalt pavements. 'We have gained a lot more than we expected to from the HVS.'

A large proportion of the state roads and freeways in California were built in the 1960s and 70s with a 20 year design life, which is now coming to an end. Asphalt pavement design in California has traditionally used a recipe approach - not ideal for the state's climatic extremes which vary from desert to alpine.

Caltrans, California's department of transport, is developing a new analytical based pavement design, in which HVS is playing a key role.

Early recommendations from the study include improving the compaction of the asphalt, and applying a bituminous tack coat between all the asphalt layers. Caltrans estimates it could save $50M-$70M by improving compaction, and $6M-$12M if it adopts tack coating. The HVS tests on concrete pavements carried out at Palmdale, near Los Angeles, have revealed the importance of concrete shrinkage tests and the use of dowel joints.

The HVS was originally developed in South Africa at the Council for Scientific & Industrial Research in the late 1960s, and has played a major part in the development of economic pavement specifications in South Africa.

With the ending of sanctions against South Africa the HVS, marketed by pavement specialist Dynatest, has spread to the US, where there are now five machines, including two at Berkeley.

Pavement testing in the UK is conducted in lab conditions using wheel track machines, and on site with falling weight deflectometers. The next step may be to bring an HVS onto 'as-built' road pavements here.

The UK spends £200M a year on trunk road pavement maintenance, with congestion adding considerable hidden costs.

'Our road network is congested and is wearing out, ' says Graham Bowskill, head of road layout and pavements at the Highways Agency. 'We must determine our future needs, and HVS may well be part of it.'

A group of interested parties, led by Nottingham University, is being formed to investigate the potential of bringing an HVS to the UK, so it may soon become a familiar sight on UK roads.

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