Determination of pavement number for flexible pavements using FWD deflection bowl information

Abstract

Structural Number (SN) is a well-known pavement index methodology derived from the product of structural layer coefficients, layer thicknesses and environmental (and drainage) factors. Subsequently, the Adjusted Structural Number (SNP) included the influence of the subgrade on pavement strength. The Pavement Number (PN) was recently developed in South Africa as an index similar to the SNP. However, in the PN calculation Equivalent Long Term Stiffness (ELTS) values are derived from material class inputs in a knowledge-based system. An approach to calculate the Effective Pavement Number (PNeff), is proposed which utilises the full deflection bowl more effectively in the calculation. It uses the Shape Factor (F1) to determine equivalent layer thickness (He) and FWD deflections at offset of 300mm from the centre of loading to derive Surface Modulus (SM) inputs for calculating an ELTS value representing the total pavement structure, SMpav. The product of the He and SMpav thus provides a derived PNeff value. A large database of flexible pavements was used to successfully validate this approach. It is demonstrated that PNeff, thus derived from the utilisation of the full deflection bowl and without detailed information of pavement layer thicknesses can be used to complement initial or preliminary structural evaluation. It is illustrated how PNeff is used in a benchmark methodology with FWD surveys. The well established FWD deflection bowl structural benchmark analysis method can then further enhance this preliminary structural analysis with PNeff by assisting in a preliminary analysis and helping to determine origin of distress. Hereafter detailed structural analyses can follow with detailed material type and pavement layer information in a much more focussed fashion.