Case Histories of Premature Distress on Unbound Basecourses


Most of the following are recent examples of premature distress, with further details of most obtained from internal files (Tonkin & Taylor, Beca and Opus, with acknowledgements to others including John Hallett and William Gray). The focus is where subgrade deformation was not a significant factor, i.e. deformation within the pavement layers was involved, especially shear instability in the basecourse. Where deflection bowls have been measured, the expected total pavement life is reported, assuming the tests reflect the condition of the pavement immediately after bedding-in is complete (ie after initial in-service trafficking). The model used is based on Austroads principles supplemented by performance data from CAPTIF and the state highway LTPP sites. The pavement total life is predicted in terms of ESA to a terminal condition by addressing 4 discrete structural distress modes (rutting, roughness, flexure/cracking and shoving). Ref. For pavements that are not new it is important to distinguish between total life and remaining life. The remaining life takes into account the condition of the pavement at time of FWD testing and is the additional number of ESA expected until the treatment length reaches a terminal condition for each structural distress mode if there is no addition or replacement of bitumen bound layers. (Additional non-structural sprayed seal layers may be applied if markedly reducing seal lives are not being experienced.) On the other hand the total life or “potential life” ignores the current condition of the pavement and is the number of ESA expected from the time of FWD testing until the treatment length reaches a terminal condition for each distress mode assuming that the pavement is either new or if not, any departure from a new condition is first rectified by smoothing of the surfacing, but without any form of structural strengthening. Total life is the principal parameter of interest when deciding whether or not structural strengthening is required for a pavement surface that is in a terminal condition. For this reason total life expectations for each distress mode are reported below, plotted versus chainage along the road and also as a cumulative distribution so that the characteristic design value (usually the ten percentile) can be readily identified. For each case history possible corrective actions for tightened quality assurance are summarised. 

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