## 4 Potential Improvements to ACR-PCR  
### Representative Subgrade Characteristic  
As shown in Figure 2, there are new limits and representative values for the subgrade bearing capacity associated with the transition from ACN-PCN to ACR-PCR. Under ACR-PCR, there is also consistency between characteristic subgrade bearing capacity values for flexible and rigid pavements, which is an improvement. Furthermore, the transition to layered elastic methods for ACR calculation, for consistency with contemporary pavement thickness determination software, means that an elastic modulus is a new characteristic used to represent different subgrades [16]. The transition to elastic modulus as the subgrade bearing capacity characteristic is supported, and the simple 10 times CBR provides for a convenient, albeit approximate, conversion for those that prefer to continue to think about subgrade support in CBR, which is how it is commonly measured in the laboratory [7; 30]. The alignment of flexible and rigid subgrade bearing limits and representative values is also supported, as this simplifies the strength rating system, particularly for airports with both flexible and rigid runway pavements in close proximity, because it is reasonable to expect both to have the same subgrade category for pavement strength rating. Despite these improvements, the change in subgrade category limits and representative values is a different matter. First, there is the potential to confuse airports, particularly those that will change the subgrade category when they transition from ACN-PCN to ACR-PCR. That includes those with an actual subgrade value equivalent to CBR 5, CBR 9, CBR 13, and CBR 14. The technical or operational benefit associated with aligning the representative subgrade support values to those used in French road pavement design methods [15] is questionable. Furthermore, as stated above, airport pavement thickness determination is generally performed over the range of CBR 3 to CBR 15 in Australia [7] and of CBR 3 to CBR 20 in the USA [30]. For subgrades with lower bearing capacity, improvement by stabilization or deep filling is common, allowing a CBR 3 or better value to be used in the design and to provide a platform able to support construction vehicles [7]. For subgrades that exceed CBR 15 (Australia) and CBR 20 (USA), a cap is usually placed on the value used for pavement thickness determination [30]. This upper-value capping reflects the minimum practical pavement thickness that should be provided for practicality and constructability. It also acknowledges that the effect of subgrade bearing capacity on flexible pavement thickness is greater at lower CBR values than at higher CBR values, as shown in Figure 4. Figure 4 also shows a similar trend for rigid pavements, with an actual increase in required slab thickness at subgrade support values above CBR 10, which is an illogical anomaly of FAARFIELD for some dual and dual-tandem wheeled aircraft. The greater influence of subgrade support on flexible pavements, compared to rigid pavements, is well established [7] and is also clear in Figure 4.  
The consequence of the change in representative subgrade support values associated with the transition to ACR-PCR, towards higher equivalent CBR values than those used for ACN-PCN, is to reduce the ability of the strength rating system to represent the significantly greater level of aircraft wheel interaction effects for pavements with low subgrade bearing capacity. This is because the difference between the actual p  
Figure 4: Flexible and rigid pavement thickness for A350-900 on different subgrade supports. representative or standard pavement thickness will be greater than it would have been under ACN-PCN. For example, under ACN-PCN, subgrade category D was represented by CBR 3 and represented all subgrades up to and including CBR 4. Given the practically viable minimum design subgrade support is CBR 3, the maximum difference was when CBR 3 was used to represent an actual CBR 4 subgrade.