Development of a New Standardized Method
It must be noted that the ACR/PCR method is not a design or evaluation method, but purely a classification system. Unfortunately the fact that the method of calculating ACR utilizes a common design and analysis method (layered elastic) has led a surprisingly large number of people to assume that it is a design and evaluation method. It is not uncommon for reference to be made to PCR’s calculated by the ACR/PCR method. In fact the ICAO documentation makes it very clear that it is not a design/evaluation method and that the PCR is simply the ACR of the most damaging aircraft that can use the pavement on a regular basis (regular being defined by the operator).
The ACR-PCR system is easy to use. Each aircraft is assigned a number that expresses the structural effect on a pavement for a specified pavement type and a standard subgrade category. Each airport operating authority reports site pavement strengths using the same numbering system. The pavement is capable of accommodating unrestricted operations provided the aircraft load number is less than or equal to the pavement strength number. Maximum tire pressure limitations may also be applied to some pavements which may further restrict certain aircraft operations. The ACR is based on the static application on aircraft loads to the pavement surface making them somewhat conservative in nature. Member States to ICAO are required to evaluate and publish the strength of airport systems using the ACR-PCR system. The national CAA publishes weight bearing limits in terms of ACR/PCR in an Aeronautical Information Publication for civil and international use. The intent is to provide planning information for individual flights or multi-flight missions which avoid either overloading of pavement facilities or refused landing permission.
The ACR and PCR are defined as follows:
- ACR is a number that expresses the relative structural damage effect of an aircraft on different pavement types for specified standard subgrade strengths in terms of a standard single-wheel load.
- PCR is a number that expresses the relative load-carrying capacity of a pavement in terms of a standard single-wheel load.
- The system is structured so that a pavement with a particular PCR value can support, without weight restrictions, an aircraft that has an ACR value equal to or less than the pavement's PCR value.
- ACR values will normally be provided by the aircraft manufacturers. The ACR has been developed for two types of pavements, flexible and rigid, and for four levels of subgrade strength.
- The PCR value is for reporting pavement strength only. The PCR value expresses the results of pavement evaluation in relative terms and cannot be used for pavement design or as a substitute for evaluation. It must be noted that the ACR/PCR method is not a design or evaluation method, but purely a classification system.
The method of calculating the ACR to calculate the damaging effect of an aircraft is based on the linear elastic analysis (LEA) theory. The method for calculating an PCR is left up to the ICAO Member State. A State may either use Burmister linear elastic theories for flexible pavement and/or Westergaard slab theory for rigid pavement taking into account the slab dimensions and load transfer across joints as intended in a sound mechanistic-empirical design method. The latter is more advanced and theoretical sound when using the closed form formula for concrete pavement developed by Dr. Frans Van Cauwelaert.