Design Inputs

The backcalculation results exported from CalBack can be found here.

Structure Information

Layer Materials and Thicknesses

As-built plans for this section of highway are illustrated below.

A site investigation was initiated because this is a rehabilitation project. The investigation consisted of FWD testing, coring, and soil sampling. The cores showed that the combined HMA layers (RHMA and HMA) thickness varied between 0.63 ft and 0.89 ft, with a median thickness of 0.7 ft. This provided the information needed to update the pavement structure as below:

Structure Conditions

The updated pavement structure together with the FWD deflection data was used in CalBack to backcalculate the layer stiffnesses and to divide the section of highway into uniform sections with similar stiffness values. CalBack determined there to be 6 different sections. The average layer stiffnesses for the different sections are provided below:

The cores also showed that the HMA layer was debonded between 0.25 ft and 0.30 ft below the surface for the entire project.

Maintenance history has also shown that this area was prone to alligator cracking in the wheelpaths, requiring excessive digouts along wheelpaths. The existing surface cracking exceeded 35% of the project area.

The site investigation further showed the following:

•Adequate drainage is present around the highway

•The distresses are bottom up in the AC layers in the wheelpaths.

•There are no fabric or geogrid present in the structure.

•The AB layer has permanent deformation in the wheelpaths.

Material Properties

This area has a history of low R values for the subgrade. The subgrade soil samples were tested for R-value and to classify the soil following the unified soil classification system (USCS). The R-value results and USCS classifications for each section are provided in Table 1. Note that there were no data for Sections 1 and 2.

To determine the viability of full depth recycling (FDR), the "Guide for Partial- and Full-Depth Pavement Recycling in California" was followed for guidance.

The available material for FDR consists of approximately 0.65 ft of HMA and 0.4 ft of AB, for a 1.05 ft in total thickness. The cores were crushed and mixed with the AB material sampled from the site.

Since no blending tests were conducted for this project, it was assumed that the RAP and AB were blended at a ratio of 60 % RAP and 40% AB for each section. The blended material for sections 1 through 6 is assumed to meet the Class-2 AB specifications, with a USCS classification of GP, less than 15% passing #200, and a PI of less than 6. These numbers should be obtained from proper sampling and testing rather than being assumed in actual designs.