## 4 Potential Improvements to ACR-PCR
### Tire Pressure Limits for Surface Protection  
The tire pressure limit element of ACN-PCN, intended to protect flexible pavement surfaces from over-stress, does not change with the transition from ACN-PCN to ACR-PCR. That is, the tire pressure limit remains a category of acceptable tire pressure, which is compared to the tire pressure of the main landing gear of the aircraft, as detailed in Table 1.  
Limiting tire pressure is a simple approach, although it does not capture the interaction between tire pressure and wheel load when determining the relative damaging stressesand strains in flexible airport pavement surfaces. For example, a military jet with 2300 kPa tire pressure may only have a relatively modest wheel load. However, under ACR-PCR (and ACN-PCN), these are considered to be more damaging to an asphalt surface than a B777-300 ER (29.3 tones per wheel at 1524 kPa) or an A350-900 (34.1 tones per wheel at 1662 kPa) because of the higher tire pressure. This is not actually the case.  
[PERSON] [29] determined the relative damage caused to asphalt surfaces by different aircraft, using octahedral shear stresses induced by braking and turning aircraft as the indicator of relative damage [49]. It was concluded that an index, called the Wheel Classification Number (WCN) based on equal octahedral shear stress, should be used to replace the tire pressure limit. The importance of wheel load, in combination with tire pressure, is shown in Figure 6, and the recommended WCN index values for different categories of wheel load and tire pressure are in Table 5.  
The retention of the simple tire pressure categories and limits, rather than the adoption of a WCN or a similar index, based on both tire pressure and wheel load, is a missed  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Wheel Load (t)**} & \multicolumn{8}{c}{**Tire Pressure (kPa)**} \\ \cline{2-10}  & **\textless{}500** & **500–750** & **750–1000** & **1000–1250** & **1250–1500** & **1500–1750** & **1750–2000** & **2000–2250** & **\textgreater{}2250** \\ \hline \textless{}5 & 11 & 14 & 16 & 17 & 18 & 19 & 19 & 19 & 20 \\ \hline \textless{}5-10 & 13 & 17 & 19 & 21 & 22 & 23 & 24 & 24 & 24 \\ \hline \textless{}10-15 & 14 & 18 & 21 & 23 & 24 & 26 & 27 & 27 & 27 \\ \hline \textless{}20 & 15 & 19 & 22 & 24 & 26 & 27 & 28 & 29 & 31 \\ \hline \textless{}20-25 & 15 & 19 & 23 & 25 & 27 & 28 & 29 & 30 & 31 \\ \hline \textless{}25-30 & 15 & 20 & 23 & 26 & 28 & 29 & 30 & 32 & 33 \\ \hline \textgreater{}30-35 & 15 & 20 & 24 & 26 & 28 & 30 & 32 & 32 & 34 \\ \hline \textgreater{}35 & 15 & 20 & 24 & 26 & 29 & 31 & 32 & 33 & 35 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Wheel Classification Numbers (adapted from [49]).  
Figure 6: Iso-octahedral shear stress and WCN index for different wheel loads and tire pressure (adapted from [29]).  
opportunity for improving the airport pavement strength rating system. In light of the irrelevance of tire pressure on its own, combined with the general absence of surface distress from contemporary pavement design methods [7], it is recommended that either a WCN (or similar) be adopted, or the otherwise meaningless tire pressure limited be removed from ACR-PCR.