## 3 Review of the ACN-PCN and ACR-PCR Systems
### New Strength Rating System
The rigid pavement structure is not affected by the number of wheels in the landing gear, as shown in Table 3.  
The subgrade categories have also been adjusted to be the same for rigid and flexible pavements [17] and now correspond to subgrade categories used in France for road and highway pavement design [15]. The current and new subgrade categories are summarized in Table 4. For simplicity, equivalent CBR values are shown, even though ACR-PCR uses elastic modulus as the subgrade characteristic for ACR calculation. Figure 2 compares the subgrade categories, also as equivalent CBR values, and highlights the representative values used for each category in the respective systems. Figure 2 also highlights the equivalent CBR values that will necessitate airports changing from one subgrade category to another, such as CBR 5, 9, 13, and 14. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Subgrade Category**} & \multicolumn{2}{c}{**ACN–PCN System**} & \multicolumn{2}{c}{**ACR–PCR System**} \\ \cline{2-5}  & **Nominal CBR** & **CBR Range** & **Nominal CBR** & **CBR Range** \\ \hline A & 15 & 13 and above & 20 & 15 and above \\ \hline B & 10 & 8–12 & 12 & 10–14 \\ \hline C & 6 & 4–8 & 8 & 6–9 \\ \hline D & 3 & 4 and below & 5 & 5 and below \\ \hline \end{tabular} Note: elastic modulus (MPa) is commonly calculated as 10 \(\times\) CBR.  
\end{table} 
Table 4: ACN–PCN and ACR-PCR flexible subgrade categories. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Layer**} & \multirow{2}{*}{**Thickness under ACR–PCN**} & \multicolumn{2}{c}{**Thickness under ACR–PCR System**} \\ \cline{3-4}  & & **For Aircraft with 1–2 Wheels** & **For Aircraft with 3 or More Wheels** \\ \hline Asphal surface (P-401) & 75 mm & 76 mm & 127 mm \\ \hline Crushed rock (P-209) & 150 mm & As required & As required \\ \hline Uncrushed gravel (P-154) & As required & Not used & Not used \\ \hline Subgrade & Infinite & Infinite & Infinite \\ \hline \end{tabular}
\end{table} 
Table 2: ACN–PCN and ACR–PCR standard flexible pavement structures. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Layer**} & \multirow{2}{*}{**Thickness under ACR–PCR System**} & \multicolumn{2}{c}{**Thickness under ACR–PCR System**} \\ \cline{2-2}  & **Nominal CBR** & **CBR Range** & **Nominal CBR** & **CBR Range** \\ \hline A & 15 & 13 and above & 20 & 15 and above \\ \hline B & 10 & 8–12 & 12 & 10–14 \\ \hline C & 6 & 4–8 & 8 & 6–9 \\ \hline D & 3 & 4 and below & 5 & 5 and below \\ \hline \end{tabular}
\end{table} 
Table 4: ACN–PCN and ACR–PCR flexible subgrade categories. Figure 2: Flexible subgrade category ranges for ACN–PCN and for ACR–PCR. Furthermore, as stated above, the ACR-PCR system now uses the elastic modulus of the subgrade (expressed in MPa) to characterize subgrade bearing capacity. This better reflects the input into modern pavement thickness design software. Furthermore, the use of elastic modulus for both rigid and flexible pavement types avoids the need to estimate k-values for rigid pavements, which simplifies the ACR-PCR system for rigid pavements. The ACR-PCR indicator of relative damage caused by different aircraft is vertical strain at the top of the subgrade, instead of maximum deflection at the top of the subgrade. Furthermore, the layered elastic models in FAAFRIELD [8] are used to calculate the magnitudes of strain, rather than the simpler (Boussinesq) models used in COMFAA. This change reflects the more sophisticated computer power that is now readily accessible and greatly reduces the potential for anomalies between pavement thickness determination and strength rating. However, it means that the relationship between ACN and ACR is not fixed. In fact, the ratio of ACR to ACN for any given aircraft, pavement type, and subgrade category varies from approximately 7.5 to 12.5, as shown in Figure 3.  
The change from ACN-PCN to ACR-PCR includes many improvements in the strength rating of aircraft pavements.