# Comparison of Lab vs. Backcalculated Moduli of Virgin Aggregate and Recycled Aggregate Base Layers
## 3 Results and Discussion
### Comparison of Vertical Strains  
In this section, the vertical strain responses of the test sections at the mid-depth of the base layer (9.5-inch depth) and the top of the subgrade layer (20-inch depth) are calculated and compared. For this purpose, the coefficients of the NCHRP 1-37A model for the base + subbase and subgrade layers were inputted into the MatFEA program (FE approach) using the lab-measured \(M_{R}\) and multi-load FWD test data. In addition, the test sections were also modeled using the KENLAYER program (LEA approach) with the backcalculated \(E_{FWD}\) values from the field \(E_{FWD}\) test data. It must be noted that the FWD-based data represent the field conditions over three years, starting two years after construction, while the laboratory-based data represent the conditions of the pavement layers at the time of construction. Figure 6 shows a comparison of the calculated vertical strain at the mid-depth of the base layer. It was observed that the vertical strains calculated by the \(M_{R}\)-based coefficients for the coarse and fine RCA were much higher than those calculated from the \(E_{FWD}\) based coefficients (Figure 6a,b). Both materials exhibited stiffer behavior in the field, as indicated in the deflection and \(E_{FWD}\) data earlier, the extent of which could not be ascertained using lab testing during construction. Also, from the EFWD-based coefficients, vertical strains in the RCA + RAP blend were found near the projected vertical strains (lab \(M_{R}\) based coefficients) initially. Still, significant variations were observed during the latter years (Figure 6d). Obtaining a field representative stiffness value for any material in the lab can be challenging, but the same is becoming more complex for RABs. The presence of unhydrated  
Figure 6: Vertical strains at mid-depth of base layer (9.5-inch depth) found using MatFEA and KENLAYER: (**a**) Cell 185; (**b**) Cell 186; (**c**) Cell 188; (**d**) Cell 189. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{2}{c}{**Relative Difference (\(E_{FWD}\))**} \\ \cline{2-3}  & **Base + Subbase** & **Subgrade** \\ \hline Cell 188 & & \\ (VA base, clay loam subgrade) & 0.97 & 1.12 \\ \hline Cell 189 & & \\ (RCA + RAP base, clay loam subgrade) & 0.99 & 1.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._cement in RABs makes hydration a critical attribute of their performance. As it occurred gradually over time, the lab \(M_{R}\) was found to be insufficient to assess the degree of performance of RABs in the field. No workable relationship existed between the lab \(M_{R}\) and the \(E_{\text{FWD}}\) due to the drastically different behaviors of the recycled aggregate bases in the field and the lab. The vertical strains in limestone obtained from the FWD data using the FE approach tended to have values near the projected vertical strains from the lab \(M_{R}\) (Figure 6c). LEA also exhibited a similar trend in the middle of the limestone base. Therefore, a correlation of the lab \(M_{R}\) with the \(E_{\text{FWD}}\) was deemed possible for the unbound materials (base and subgrade) in the VA (limestone) section. Similarly, Figure 7 shows a comparison of the calculated vertical strain at the top of the subgrade layer for the different test sections. As this figure shows, different materials in the base layer significantly affected the calculated strain response at the top of the subgrade layer. The laboratory-based coefficients of the NCHRP 1-37A model for Cells 185 and 186, with the RCA base layers, presented higher vertical strains compared to those calibrated based on the field FWD test data. Similar trends with Figure 6 were also observed for Cells 188 and 189 with the VA and RCA + RAP base layers. Furthermore, while both the FE approach and LEA projected closer values of strains for the VA and RCA + RAP at a 9-inch depth (Figure 6c,d), the FE approach presented higher strains at a 20-inch depth than the LEA (Figure 7c,d).