Ultra Low Strength – CBR 3 (For CBR below 4%). Medium Strength – CBR 10 (For CBR between 8% to 13%). High Strength – CBR 15 (All CBR above 13%). The flexible pavements have four subgrade categories: So a subgrade of A would be very strong (e.g. The third part is another letter from A to D expressing the strength of what is underneath the pavement section, known as the subgrade. The second part is a letter: either an R or an F, depending on whether the pavement itself is of a rigid (most typically concrete) or a flexible (most typically asphalt) design. The value is calculated based on a number of factors, such as aircraft geometry and a pavement's traffic patterns, and is not necessarily the direct bearing strength of the pavement. This is always reported as a whole number, rounded from the determined capacity. The first part is the PCN numerical value, indicating the load-carrying capacity of the pavement. The PCN is actually expressed as a five-part code, separated by forward-slashes, describing the piece of pavement concerned.
The PCN values are published in the Aeronautical Information Publications (AIPs), part AD (aerodromes). The PCN is the ACN of the most damaging aircraft that can use the pavement on a regular basis (regular being defined by the operator). This helps to ensure that they are not subjected to excessive wear and tear, thus prolonging their usable life. PCN is an International Civil Aviation Organisation (ICAO) standard used in combination with the Aircraft Classification Number (ACN) to indicate the strength of a runway, taxiway or apron. Pavement Classification Number (PCN) is a number that expresses the load-carrying capacity of a pavement for unrestricted operations.
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