ULTRA STIFF high modulus base asphalts (HMBs) will stay banned for at least another 18 months, the Highways Agency confirmed this week.
The thinner road bases, championed by the asphalt industry as a cheaper alternative to concrete roadbases, were hit by a ban on their two stiffest and most economic grades more than two years ago (NCE 9 March 2000).
Grades HMB15 and HMB25 were banned after cores taken from one test stretch of the M65 indicated that HMB was losing strength after only three years.
Subsequent research by TRL proved inconclusive. 'Our concerns about strength loss turned out to be unfounded, ' said Agency senior pavement engineer John Williams. 'However, there are continuing concerns over compaction with the original mix design, and we have been assessing French experience in this area.'
He added that the Agency was working closely with trade bodies such as the Quarry Products Association but it would take at least 18 months to complete the process.
French HMBs have smaller maximum aggregate size and higher binder contents than the first generation UK mixes. And UK contractors have less experience dealing with the stiffer bitumens needed, particularly 15 pen, which have to be laid at much higher temperatures.
'We did hope to see HMB15 approved for use again by the end of this year, ' said Refined Bitumen Association technical director Tony Harrison. 'The problem is how to demonstrate a roadbase has a potential life of 40 years. Laboratory tests and assessments of French experience can only go so far: there have to be full scale trials.'