Executive summary
As part of the Scottish Road Research Board research and development work, WSP was commissioned to investigate the occurrence of early life pavement defects. The study was split into two phases: assessing the scale of the problem; and site visits to determine causes and potential solutions. The first phase involved collecting Information on early life defects from a range of sources, including Transport Scotland’s pavement management system and discussions with Operating Company (OC) representatives. The study covered the Scottish trunk road network that is maintained by the OCs. The objective of Phase I was to establish the scale of the problem and to identify a list of schemes for further examination. Phase II involved reviewing and visiting schemes identified as experiencing early life defects to identify any common causes or trends.
Transport Scotland’s Integrated Road Information System (IRIS) was used to collect construction records on structural maintenance schemes completed between January 2015 to December 2019. The schemes were then cross referenced with post construction defect records held within the routine maintenance section of IRIS. In addition, information relating to early life pavement failures was directly requested from OC representatives.
Category 1 and Category 2 defect data recorded over the five year study period were examined to quantify the number, type and timing of defects over the four geographical areas covered by the OCs. Analysis of the data provided an insight into how structural maintenance schemes deteriorate over time, and how the type and quantity of defects varies across the country. Fretting, cracking and edge deterioration are dominant defects in the northern parts of Scotland, whereas the occurrence of potholes appears to be major defect recorded in the south of Scotland.
The information collected as part of Phase I was used to identify 19 schemes that were deemed to provide useful information on the cause of early life defects. All the sites were visited and inspected, and the findings were analysed along with design scheme data supplied by Transport Scotland. Based on this analysis, the sites were split into four broad groups, namely: external or isolated defect; inadequate inlay depth; edge deterioration and drainage; and urban/utilities. The schemes that fell into these groups were studied to determine the likely reasons that contributed to the early life defects.
Positive outcomes resulting from the site visits included the finding that 42% of the schemes identified in Phase 1 contained only external or isolated defects. Additional analysis on older schemes supported this finding by showing that 30% of schemes constructed in 2015 where a Cat 1 defect had been recorded, required no subsequent treatment. The latter indicates that apart from isolated surface course defects associated with workmanship the schemes continue to perform well, particularly when surfaced with TS2010. The most commonly occurring defects were associated with inadequate inlay depths, and this was evidenced by the fact that deeper inlays used on the same scheme showed no defects. The observed failures on other schemes were related to inadequate pavement width and drainage, and weak subbase and foundation conditions.
The report discusses the likely failure mechanisms that caused the early life failures on the schemes visited and recommends potential solutions to reduce the incidence of early life defects. General advice is provided on reducing the risk of early life defects occurring, including optimising the selection of inlay depths, considering deeper construction and avoiding a piecemeal approach.