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Topic Comment from Prof. Ian D Walsh MBE, Consultant at Road Consultants Ltd   published
 

Having just come back from High Wycombe their pothole problem is extreme. It seems to be a combination of centre joints, removed white lines, stats causing fatigue and delamination wheer a 25,mm overlay was too thin. Out in the leafy( less) lanes the wet areas had obviously sufferred from frost damage particularly at the edges. This is what I have said in a recent ADEPT Guidance note ( not yet published):

The need for the correct selection of an appropriate surfacing for new and maintenance works has again been highlighted by the defects occurring in roads surfaced with Thin Surface Course Systems [TSCS] over the last 18 years. Many of these are coming to the end of their effective life and need replacement but money is scarce and predictive systems are still in their infancy to enable a scheme-specific forward programme to be developed.
The reasons for the explosion of potholes in winter are as a consequence of the nature of TSCS and maintenance practices.
Thin Surfacing and SMA
TSCS, as used to date, is an open textured aggregate skeleton mix which gains its strength from the thick film of bitumen holding the stones together in combination with the initial compaction and to an extent on straight roads, further compaction by traffic. Bitumen becomes brittle with exposure to air so that adhesion to the aggregate can start to be lost. Water is constantly being pumped down through the surface by tyres and the material is broken apart in wet/ frosty weather. Potholes will form first where initial compaction was less than ideal e.g. at joints, water is present on the surface e.g. under trees or at the carriageway edge, and turning traffic induces horizontal forces by scuffing. Delamination of the thin surface layer occurs where there was inadequate bond coat, the existing surface was dirty or the material had a cold spot on installation either because the layer was too thin or poor asphalt paving. The material therefore is not tolerant of abuse during manufacture, laying or through poor pavement design. Potholes can form very quickly with little warning to the maintenance engineer. TSCS has performed well for over 15years, conversely it has failed in 5 years. This makes maintenance planning very difficult.
Engineers also need to be aware that if the existing road is weak, it will deflect under heavy truck tyres and lead to fatigue failure in the surfacing layer. Although some TSCS can match Hot Rolled Asphalt for fatigue resistance, many are designed primarily to retain a high texture and be rut resistant on the Highways Agency heavy trafficked network.
Given the area of TSCS in service Local Authority maintenance teams must develop systems to inspect roads at much greater frequency by Inspectors, knowledgeable in pavement materials’ defects; they should not wait for public complaint and claims, and instigate a rapid effective reactive maintenance process.
This could involve velocity patching, hot box patching, inlay banding of joints, pre-emptive surface dressing or hot mix asphalt inlay/overlay based upon monitored deterioration. Cold lay materials have a role

  Catherine Birkner
28.03.2013 10:20:37

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