INADEQUATE COMPACTION is being investigated as the possible cause of extensive break-up of the porous asphalt wearing course on the recently opened Newbury bypass.
Engineers admitted they are baffled by the first problem encountered with this material in eight years of use.
The damage has been identified at 31 isolated patches on the noise and spray reducing surfacing on the 13km bypass.
Surface stone has been lost from the top 20mm layer of the 50mm thick wearing course. The mix consists of average 15mm aggregate held by a modified asphalt binder to give an open textured surfacing.
'The patches are totally random with no obvious pattern and the cause remains a complete mystery,' a Highways Agency spokeswoman admitted. 'At present no-one is blaming anyone for the problem.'
The £100M dual carriageway opened four months ago. Contractor Costain, consultant Mott MacDonald and the Transport Research Laboratory are conducting core tests to compare failed areas with sound asphalt.
Lack of compaction is a possibility as it is understood the specification for laying the asphalt does not define a required number of roller passes. Instead it calls for rolling to continue until no marks from the roller remain on the surface.
There is also no specific density check and it is thought future specifications may need tightening.