ULTRA STIFF European-style high modulus asphalts used on up to 80km of trunk road in the UK are being investigated after one test stretch of the M65 was found to be weakening after only three years' service.
Highways Agency engineers are baffled by the degeneration. Agency pavement engineering group manager Graham Bowskill said: 'Received wisdom has it that bituminous compounds get stiffer with age.
'But on our three long-term trials using the very stiff mixes, two have not gained any significant strength and one is definitely getting less stiff.'
Among the roads with high modulus base sections that will be monitored by the Transport Research Laboratory are the M6, the A1(M) and the M32. Bowskill said none of the HMB stretches were showing signs of distress, and the Agency still had faith in the concept.
The new generation 'high modulus bases' were only approved for use in 1998. Since then 'less than 80km' of trunk road has been constructed using the material. Designed to last twice as long as conventional construction, HMBs are popular with contractors because they save up to 3% on costs.
The stiffest mix, HMB15, can be used 60mm thinner than conventional heavy duty macadam and up to 120mm thinner than hot rolled asphalt.
Discovery of the falling stiffness immediately prompted the Agency to suspend use of the two stiffer grades, HMB 15 and HMB 25, and order an investigation by TRL, which is expected to be complete by the autumn. HMB 35 can still be used.
The code numbers refer to the pen number of the bituminous binder used. The UK blacktop industry has more experience with 35 pen binders than the stiffer alternatives, Bowskill said.
He added: 'We have consulted with the Quarry Products Association and the Refined Bitumen Association. They support our decision to suspend the stiffer mixes.'