Implementation of Mechanistic Pavement Design: Field and Laboratory Implementation

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CAIT project no.: ELF RU7072

Fiscal Year: 2001/2002

Status: Final

Rutgers-CAIT Author(s): Ali Maher, Nenad Gucunski, Thomas Bennert

Sponsor(s): RITA


One of the most important parameters needed for 2002 Mechanistic Pavement Design Guide is the dynamic modulus (E*). The dynamic modulus (E*) describes the relationship between stress and strain for a linear viscoelastic material. The E* is the prime material parameter used for calculating both rutting and fatigue cracking in hot mix asphalt. The parameter is traditionally measured in the laboratory under an axial compressive type testing condition. Under the recommendations of the 2002 Mechanistic Design Guide, this is the preferred method for reconstruction or new construction. However, if a rehabilitation is to be conducted, the 2002 Mechanistic Design Guide prefers the use of the Falling Weight Deflectometer (FWD) because of its capability of determining the E* parameter in-situ and in a non-destructive way. Unfortunately, this is not 100% true since most PMS procedures require that cores of the pavement be taken so accurate layer thickness’ can be determined for back-calculation purposes. If FWD testing is not available, then the 2002 Mechanistic Design Guide recommends using the laboratory testing of cores from the pavement.

Research showed that shear modulus testing (G*) from the Superpave Shear Tester provides modulus values from readily attained cores. The 50 mm samples required by the SST can easily be cut from 6 inch diameter cores. However, as indicated,
the G* values obtained can not be directly used in the 2002 Mechanistic Pavement Design Guide without using an assumed Poisson’s Ratio and elastic theory concepts that relate G* to E* (Dynamic Modulus). Intact cores can be taken and trimmed to provide dynamic modulus test specimens, as long as the asphalt pavement thickness is greater than 6 inches thick. A Master Stiffness Curve can then be developed using the E* data determined at various test temperatures and loading frequencies and shifted to the in-situ asphalt pavement temperature. In all test sections evaluated in the study, the Falling Weight Deflectometer (FWD) back-calculated asphalt modulus showed excellent correlation the corresponding Master Stiffness Curve, when it was assumed that the loading frequency of the FWD is 16.7 hertz. This illustrates that if field testing is not available, cores can be taken and tested using the dynamic modulus testing protocol to provide reasonable estimates of the asphalt modulus. And, although further validation is required, the generated Master Stiffness Curve should provide a reasonable estimate of the seasonal variation in asphalt modulus, which would required the FWD to test the identical location at least once every month of a full year.