FURTHER TESTS to evaluate the performance of European style ultra stiff high modulus asphalt road bases have failed to persuade the Highways Agency to lift its ban on their use on UK roads.
But research at TRL to test the material has also cast doubt on the validity of the procedures and computer software used.
The Agency suspended use of the two stiffest grades of HMB after cores taken from a high modulus base trial on the M65 in March showed an unprecedented loss of stiffness after only three years. (NCE 9 March).
TRL was asked to investigate normal bituminous mixes with higher pen number binders which were assumed to gain stiffness as they age.
Agency pavement engineering group manager Graham Bowskill said: 'Although it is too early to judge the final outcome of the investigation, recent results from the road trials suggest it is unlikely the suspension will be lifted in the short term.'
But he added: 'Given the variability of these results, the precision of the test method used to monitor these very stiff materials is also being examined.'
TRL is monitoring HMB areas on the M6, the A1(M) and the M32. Results from these also show deterioration, Bowskill confirmed, although the latest tests on the M65 showed a slight improvement. To handle materials with 15 and 25 pen binders rather than the usual 50 or 100 pen, a standard Nottingham asphalt tester had to be modified with a heavier actuator. In the test a 150mm diameter, 60mm thick disc of HMB is loaded to determine its indirect tensile stiffness modulus.
But TRL investigators are said to have detected problems with the controlling software that could account for the earlier anomalous results.