The conclusions from the backcalculation analysis are as follows:

The overall stiffness values of the two pavement structures indicated by the surface moduli are similar. The variation along the section is typical of that of other projects in which the pavement structure is relatively homogenous.

The average backcalculated stiffness of the cold foam material combined with the existing asphalt concrete layer below it is 1,778 MPa, with an 80 percent confidence range of 1,210 MPa to 2,660 MPa along the section tested.

The backcalculated stiffness of the cold foam mix is roughly 25 percent lower than that of the 37.5-mm asphalt concrete layer in the other structure. In both structures the layer in question was combined with the thin existing asphalt concrete layer below it for backcalculation.

The backcalculated CTB stiffness values of the two sections are very similar. The subgrade stiffness of the section using cold foam is slightly higher than the other section.

The coefficient of variation of the resilient moduli of Layer 2 (cold foam combined with existing asphalt concrete or 37.5-mm maximum size aggregate asphalt concrete) and Layer 3 (CTB) along the sections is relatively high (>30 percent) for both structures. This is likely due in part to variations in actual thickness compared to the assumed thickness, and in the case of the CTB, may indicate more damaged areas as well as variations in thickness. However, in both cases the variability is typical of structures with relatively homogenous structures. The coefficient

Caltrans is in the process of implementing Mechanistic-Empirical design procedures. All mechanisticempirical methods must be validated/calibrated against the behavior of real pavements. This should be done before implementing models in design methods to ensure that designs will be reasonable. The Heavy Vehicle Simulator (HVS) provides a first step in this validation/calibration process. The short test section can be carefully constructed with well characterized materials and instrumented to measure the pavement response. The climatic conditions may be controlled or monitored closely and all load applications are known exactly. The pavement may also be tested until it fails. The HVS may be seen as “large scale” laboratory equipment, between the “small scale” laboratory equipment (triaxial tests, bending tests etc.) and the reality of real pavements, which have uncertainties regarding materials, loads and climatic conditions. The two HVSs owned by Caltrans have been used on 27 flexible pavement test sections, with varying combinations of asphalt and granular layers. Temperature control was used during the tests. Most sections have been instrumented with Multi-depth Deflectometers (MDDs) to compare the measured pavement deflections (at several depths) to the deflections predicted by mechanistic methods, during the full duration of tests carried to “failure” (in terms of rutting or cracking). Results from mechanistic models have been compared with the deflection measurements and performance as a first step prior to empirical calibrations with field results. The complete time history of each test has been compared rather than just the beginning and end measurements. This report presents the validation of the mechanistic models for asphalt fatigue and for permanent deformation with the HVS test results.

This report follows a series of seven first-level Heavy Vehicle Simulator (HVS) testing reports, two laboratory reports on shear and fatigue testing, a forensic investigation report, and a report on the backcalculation of deflection measurements, all of which document an investigation undertaken to validate Caltrans overlay strategies for the rehabilitation of cracked asphalt concrete. It presents the findings from a detailed analysis of the laboratory fatigue and shear results, and a series of simulations using CalME mechanistic-empirical design software and continuum damage mechanics implemented using a finite element method. The work was conducted by the University of California Pavement Research Center (UCPRC) as part of Partnered Pavement Research Center Strategic Plan Item 4.10: “Development of Improved Rehabilitation Designs for Reflective Cracking.” This work was originally requested by the Caltrans/Industry Rubber Asphalt Concrete Task Group (RACTG) to compare the performance of one set of examples of thin overlays of cracked asphalt pavement containing different types of binders modified with recycled tire rubber. This work, included as Appendix H of the Rubber Modified Binder Pilot Projects Review prepared by the RACTG is part of a more comprehensive work plan prepared by the Task Group that included evaluation of pilot projects and construction and monitoring of field test sections (undertaken by Caltrans). The objective of this UCPRC project will be met after completion of the following four tasks: 1. Develop improved mechanistic models of reflective cracking in California, 2. Calibrate and verify these models using laboratory and HVS testing, 3. Evaluate the most effective strategies for reflective cracking, and 4. Provide recommendations for reflective cracking strategies. This report addresses all the tasks and consists of six main chapters. Chapter 2 provides an overview of the HVS testing program for the study. Chapter 3 summarizes a second-level analysis of the laboratory fatigue and shear test results. Chapter 4 discusses mechanistic-empirical modeling and simulations of the HVS results using CalME software, which is currently being developed for Caltrans by the UCPRC. Chapter 5 discusses modeling and simulations using continuum damage mechanics implemented using finite-element models. Chapter 6 lists key findings of the study, and provides recommendations for implementation.

Caltrans is in the process of implementing Mechanistic-Empirical design procedures. All mechanisticempirical methods must be validated/calibrated against the behavior of real pavements. This should be done before implementing models in design methods to ensure that designs will be reasonable. The Heavy Vehicle Simulator (HVS) provides a first step in this validation/calibration process. The short test section can be carefully constructed with well characterized materials and instrumented to measure the pavement response. The climatic conditions may be controlled or monitored closely and all load applications are known exactly. The pavement may also be tested until it fails. The HVS may be seen as “large scale” laboratory equipment, between the “small scale” laboratory equipment (triaxial tests, bending tests etc.) and the reality of real pavements, which have uncertainties regarding materials, loads and climatic conditions. The two HVSs owned by Caltrans have been used on 27 flexible pavement test sections, with varying combinations of asphalt and granular layers. Temperature control was used during the tests. Most sections have been instrumented with Multi-depth Deflectometers (MDDs) to compare the measured pavement deflections (at several depths) to the deflections predicted by mechanistic methods, during the full duration of tests carried to “failure” (in terms of rutting or cracking). Results from mechanistic models have been compared with the deflection measurements and performance as a first step prior to empirical calibrations with field results. The complete time history of each test has been compared rather than just the beginning and end measurements. This report presents the validation of the mechanistic models for asphalt fatigue and for permanent deformation with the HVS test results.