Definition of PCR
PCR is a number that expresses the load-carrying capacity of a pavement for unrestricted operations. The PCR is assessed and assigned by the airport using the ICAO States computational method. An PCR can be based on LEA for flexible pavement or advanced Westergaard equations for rigid pavement.
The ACR/PCR method is strict towards the ACR procedure, but allows the airport to choose for any method suit-able for the determination of the pavement load bearing capacity or PCR. However, ICAO’s Aerodrome Design Manual, Doc. 9157-AN/901, part 3 Pavements and Aerodromes, Annex 14 to the Convention on International Civil Aviation presents some guidance for the PCR determination.
Determination of a rating in terms of PCR
In the most fundamental terms, the determination of a rating in terms of PCR is a process of deciding on the maximum allowable gross weight of a selected critical airplane for a pavement knowing its ACR at that weight, reporting it as PCR. This process can be as simple as knowing the operational gross weight of each aircraft that is currently using the pavement and looking up its ACR (referred to as the Using aircraft method). This method can be applied with limited knowledge of the existing aircraft and pavement characteristics. The second method is more complex and referred to as Technical evaluation. In order to be successfully implemented, the technical evaluation requires an intimate knowledge of the pavement and its traffic, as well as basic understanding of engineering methods that are utilized in pavement design.
The ICAO PCR pavement strength reporting system involves publishing a five (5) part strength code in the form of 510 F/D/W/T for flexible pavements or 620 R/B/W/T for rigid concrete pavements.
Briefly, the first number is the reported PCR value on a scale of 1 to about 1300, with 1 representing a weak pavement and 1300 a very strong pavement. The second part of the code is either an "F" for flexible pavement systems or "R" for rigid pavement systems. The third part is a letter code A, B, C, or D indicating the subgrade/bearing strength, with A representing a high supporting strength and D a very low strength. The fourth part indicates the tire pressure limitation in MPa if applicable (X, Y, Z otherwise W). The fifth and final part of the PCR code indicates the evaluation method used to determine the pavement strength - "T" if derived from an engineering study or "U" if based on satisfactory aircraft usage.
Subgrade Strenght Category
PCR reporting format
Aircraft overloads beyond published limits and ICAOs overload guidance
In the life of a pavement, it is possible that either the current or future traffic will load the pavement in such a manner that the assigned PCR rating is exceeded. Loads larger than the original design or as-built evaluated load may shorten the pavement service life, while smaller loads will usually have a minimal effect on pavement deterioration. Aircraft with load ratings greater than the reported pavement load ratings may still be allowed to use the pavement subject to the approval of the airport operating authority.
Pavements if overloaded beyond their original design strength can deteriorate very quickly leading to both FOD and roughness safety problems for aircraft. According to clause 19.1 ’Overload operations‘ of ICAO Annex 14, occasional movements (5 percent) by aircraft on flexible pavements with ACR values no more than 10 percent above the reported PCR should not adversely affect the pavement. For rigid pavement types, the ACR should not exceed the reported PCR by 5 percent. Overloads beyond these limits should be based on the results of a detailed engineering study that compares the individual aircraft load to the structural capability of the pavement.
The implications of allowing overload operations should be fully understood by the Airport Authority in terms of the accelerated structural deterioration and the reduction in pavement service life which may occur. When overloads are allowed, the pavement should be inspected regularly by the airport authority to ensure that unacceptable structural damage is not taking place.