## 3 Review of the ACN-PCN and ACR-PCR Systems
### New Strength Rating System
The adoption of consistent and elastic modulus-based values for subgrade characterization for both flexible and rigid pavements is a good simplification and aligns strength rating with modern airport pavement thickness design practices. Similarly, the changes to the representative rigid and flexible pavement structures, as well as the aircraft traffic loadings, are more representative of typical airport pavement design scenarios, and this can only reduce anomalies between pavement strength rating and pavement thickness design. However, the adoption of strain as the indicator of relative damage caused by different aircraft rather than deflection, the avoidance of the equivalent aircraft concept by considering all the aircraft in the traffic loading, and the use of layered elastic methods for the calculation of critical damage indicators at critical locations with  
Figure 3: Relationship between ACR and ACN values for common aircraft. the pavement are the changes with the greatest potential to reduce anomalies between pavement design and pavement strength rating. The result is a modernized strength rating system that is far more aligned with the methods used in contemporary pavement thickness determination, which is a scientific improvement and provides many practical advantages. Despite these improvements, some have criticized the change from ACN-PCN to ACR-PCR because it places a change-management burden on most airports around the world but provides no improvement in safety or operational efficiency. Consequently, some promoters of the change have made claims that ACR-PCR will provide benefits that are not realistic. For example, it has been claimed or implied that ACR-PCR will  
* Increase the knowledge of pavement thickness, material characteristics, and bearing capacity [40];
* Reduce greenhouse gas emissions [40];
* Prevent underestimation of pavement strength that is associated with ACN-PCN [40,41];
* Result in an extended pavement design life [42];
* Provide reduced pavement thicknesses for new pavement construction [40];
* Improve pavement life prediction [40,41,43];
* Provide more sustainable airport pavements [42,43]. In reality, it is not possible for a pavement strength rating system to increase the knowledge of the pavement thickness, material characteristics, or bearing capacity of the pavement owner. The knowledge of the pavement characteristics is a function of historical and contemporary design and construction records, as well as any investigation and testing results. A strength rating system cannot change the existence (or not) of those factual information sources. Furthermore, reduced greenhouse gas emissions, extended pavement design life, and reduced pavement thickness all relate to the avoidance of underestimated pavement strength ratings, by removing conservativism in historical pavement thickness design methods. However, ACR-PCR (just like ACN-PCN) is not (and was not) a pavement design method [16]. That is, a pavement that is designed for a specific aircraft will be equally strong enough to support that aircraft, no matter whether it is subsequently strength-rated by ACN-PCN or by ACR-PCR. The strength, structural life, predictability, and embodied greenhouse gas emissions associated with that pavement will be no different under ACR-PCR than it was under ACN-PCN. Similarly, the sustainability of that pavement will be unaffected by the strength rating system that is adopted for the management of its aircraft traffic loadings. Any claim that ACR-PCR will provide stronger, thinner, more sustainable, lower carbon content, or more predictable aircraft pavement structures is simply not true. In addition to these unreasonable claims regarding safer, more efficient, and more predictable airport pavements under the ACR-PCR system, the new system also includes some changes that are not optimal, and some elements fail to incorporate additional potential benefits.