## 3 Review of the ACN-PCN and ACR-PCR Systems
### New Strength Rating System  
As stated above, to reduce the occurrence of minor contradictions between pavement thickness determination and pavement strength rating, ICAO developed a new pavement strength rating system, known as ACR-PCR. The new system is due to be implemented by member states in November 2024 [18] although some member States have already declared a delay to the transition. The ACR-PCR system is designed to operate and function in the same way that the ACN-PCN system has operated since 1981. That is, every aircraft will have an ACR value at a given aircraft weight and subgrade support, and this ACR will be compared to the PCR assigned to a given pavement. Consistent with ACN-PCN, when the ACR exceeds the PCR or the assigned tire pressure limit is exceeded by the aircraft tire pressure, a pavement concession is required. Despite the apparent similarities in the two systems, the ACR-PCR system includes significant changes [38]. * Strain is used as the relative damage indicator, rather than deflection;
* All wheels are considered explicitly, rather than being converted to an equivalent single wheel;
* Actual pavement materials and composition are considered explicitly, rather than being converted to a standard composition;
* Load repetitions, tire pressures, and pavement structures are more comparable to typical modern airport pavement structures;
* Elastic modulus is used as the subgrade bearing capacity characteristic, replacing CBR for flexible pavements and the modulus of subgrade reaction (k-value) for rigid pavements;
* Rigid and flexible pavement subgrade categories use the same elastic modulus ranges and characteristics values for subgrade characterization. The term ACR-PCR was adopted to avoid confusion with ACN and PCN values [17]. Furthermore, the ACR is defined as twice the equivalent wheel load in hundreds of kilograms, rather than in tones. This means that ACR values generally range from 50 to 1000, compared to ACN values, which generally range from 5 to 100. This change was also designed to avoid confusion between the two systems [15]. For rigid pavements, a stress-to-strength ratio is used to determine ACR values for aircraft, so the number of traffic repetitions is not relevant. However, for flexible pavements, 36,500 passes of the aircraft are used to calculate the ACR [16]. That is an increase from the 10,000 passes used in ACN-PCN and is intended to reflect the increase in aircraft traffic at airports. However, this remains an arbitrary value because many runways are exposed to much higher traffic loadings. For example, the busiest airport in the world by aircraft movements in 2023, was Atlanta in the USA. This is based on the 775,000 annual movements at Atlanta airport [39], assuming equal distribution across the five runways. Assuming a pavement life of 20 years [30] that is over 3,000,000 passes of a runway cross-section, which is 82 times greater than the 36,500 adopted by ACR-PCR. Despite this, standardization is necessary, but a high number of passes would have been more representative of most major international airports. The standard wheel load, to which other landing gear loads are converted, now has a 1.50 MPa tire pressure to better reflect large modern aircraft, compared to the 1.25 MPa tire pressure used to calculate ACN values [16]. The flexible standard pavement structure also has greater asphalt thickness and for flexible pavements that now depends on the number of wheels in the landing gear being considered. Table 2 shows the standard flexible pavement structures used in ACN-PCN and in ACR-PCR. For rigid pavements, the subbase is now explicitly considered under ACR-PCR, whereas the effect of the sub-base was combined with the subgrade support condition, expressed as a k-value, under ACN-PCN.