This paper presents the approach of the Retained ThicknessIndex (RTI) for the assessment of the residual structural integrityof asphalt pavements on the network level. Lacroix Deflectographdeflections and distress identification served as major inputvariables. The procedure makes use of the whole deflection basinand is not restricted to application of the maximum deflectiononly. The approach provides additional data on cracking origin(top-down or bottom-up) and crack propagation, and can be usedin combination with the commonly used analysis methods.

The Retained Thickness Index is based on the flexural rigidity,inferred directly from the deflection bowl, and the occurrenceof cracking observed in the wheelpaths. Both input parameterscan be collected quite quickly.

Verification of the model was performed in 1996 on some fiftytest sections of the SHRP-The Netherlands programme. This paperdescribes an attempt to fine tune the methodology. To this endsome thirty test sections on the county network of Gelderlandwere made available, which is gratefully acknowledged by the authors.Analysis of Deflectograph testing, cracking surveys and asphaltcoring on cracks attributed to establishing the accuracy of thepredictive method.