The falling weight deflectometer (FWD) which measures pavement deflections was assessed for its ability to predict the life of a newly constructed or rehabilitated pavement. FWD measurements used in the study were from NZ Transport Agency’s test track CAPTIF, roads that have failed and from two Performance Specified Maintenance Contracts where the actual life from rutting and roughness measurements could be determined. Three different methods to calculate life from FWD measurements were trialled. The first, a simple Austroads method that uses the central deflection only and was found to either grossly over predict life by a factor of 1000 times more than the actual life or grossly under predict the life. The second two methods trialled were based on Austroads Mechanistic Pavement Design where the life is determined from the vertical compressive strain at the top of the subgrade. For the mechanistic approach the FWD measurements are analysed with specialised software that determines a linear elastic model of the pavement which computes the same surface deflections as those measured by the FWD. From the linear elastic model the subgrade strain is determined and life calculated using the Austroads equation.
It was found when using this approach that predictions of life from individual FWD measured points within a project length can range from nearly 0 to over 100 million ESAs (Equivalent Standard Axles). To cater for this large scatter in results the 10th percentile value was used as the predicted life of the pavement. In general the Austroads Mechanistic approach under-predicted the life, sometimes by a factor of 10 or more. The third approach trialled was adjusting the Austroads Mechanistic approach by applying a factor determined from past performance to calibrate the subgrade strain criterion to local conditions. This third approach greatly improved the predictions but it was found that the multiplying factor was not consistent for a geographical area and thus the factor found from one project may not be suitable for the another similar project.