The objective of this study is to develop a mechanistic-empirical method for assessing pavement layer conditions and estimating the remaining life of flexible pavements using multi-load level Falling Weight Deflectometer (FWD) deflections. A dynamic finite element program, incorporating a stress-dependent soil model, was developed to generate the synthetic deflection database. Based on this synthetic database, the relationships between surface deflections and critical pavement responses, such as stresses and strains in each individual layer, have been established. A condition assessment procedure for asphalt pavements that uses multi-load level FWD deflections has been developed using these relationships. The verification study was conducted using field data. The results indicate that the proposed procedure can estimate the base and subgrade layer conditions. It was found from the study for the nonlinear behavior of a pavement structure that an FWD test with a load of 12 kip or less does not result in any apparent nonlinear behavior of the subgrade in aggregate base pavements. The study also indicated that the deflection ratio obtained from multi-load level deflections may predict the type and quality of the base/subgrade materials. With regard to the condition assessment of the asphalt concrete (AC) layer, the AC layer modulus and the tensile strain at the bottom of the AC layer were found to be better indicators than deflection basin parameters.
The procedures for performance prediction of fatigue cracking and rutting were developed for flexible pavements. The drastically increasing trend in fatigue cracking with time may not be predicted accurately using the proposed procedure. Such trends may be due to the environmental effects and the inconsistent distress measurements. Predicted rut depths using multi-load level deflections show good agreement with measured rut depths over a wide range of rutting. However, the procedure using single load level deflections consistently under predicts the rut depths. It was concluded that the rutting prediction procedure using multi-load level deflections can estimate an excessive level of rutting quite well and, thus, improve the quality of prediction for rutting potential in flexible pavements.
The layer condition assessment procedure and the remaining life prediction algorithms developed in this project were incorporated into APLCAP (Asphalt Pavement Layer Condition Assessment Program) version 2.0, the VisualBasic program developed under the NCHRP 10-48 project.
The major products from this research project are the asphalt pavement layer condition assessment and remaining life prediction procedures displayed in Figures 4.55, 5.33, and 5.34. These procedures are programmed into APLCAP (Asphalt Pavement Layer Condition Assessment Program), a VisualBasic software developed at North Carolina State University for the NCHRP 10-48 project. The resulting product is an APLCAP version 2.0, for which the operational guideline is given in Appendix C.
In order for this program to be successfully implemented by the NCDOT, the program needs to be tested by PMU engineers for various field cases. Their feedback on the performance of the program is necessary to customize it to meet the needs for routine operation. Some parts of the program require further research and refinement, including: (1) traffic data analysis; (2) the effect of damage on the modulus of asphalt concrete; and (3) lab-to-field shift factors or transfer functions. Also, rigorous beta testing is necessary for the program to be used more widely.
APLCAP version 2.0 provides an excellent modular framework for layer condition assessment and remaining life prediction of asphalt pavements using single- and multi-load FWD deflections. It is recommended that other pavement design and rehabilitation procedures used by the PMU engineers be added to this program. The consolidation of deflection analysis algorithms into a single program has many advantages, including easy implementation of new research findings in this subject area, consistency within the NCDOT in deflection analysis, and efficient evaluation of the effects of changing design parameters on pavement design and rehabilitation decisions.
When APLCAP version 2.0 is ready to be disseminated more widely within the NCDOT, one-day workshop on the background concepts and the use of the software may be beneficial to ensure a smooth start in the implementation effort.
