What is a Pavement Classification Number (PCN)?
The Pavement Classification Number (PCN) measures how much weight a pavement can handle during unrestricted use. This standard comes from the International Civil Aviation Organization (ICAO) and works alongside the Aircraft Classification Number (ACN). Together, they ensure that runways, taxiways, and aprons remain strong enough to avoid damage, ultimately extending their lifespan.
The PCN reflects the ACN of the most damaging aircraft that regularly use the pavement. The term “regular” is defined by the operator. You can find PCN values published in Aeronautical Information Publications (AIPs), specifically in part AD, which focuses on aerodromes.
The PCN is represented as a five-part code, separated by slashes. The first part is a number indicating the pavement’s load-carrying capacity. This number is always a whole number rounded from the actual capacity. It is calculated based on factors like aircraft size and traffic patterns, rather than being a direct measure of the pavement’s strength.
The second part of the code is a letter, either R or F. This shows if the pavement is rigid (usually concrete) or flexible (typically asphalt). Next comes another letter from A to D, which represents the strength of the material beneath the pavement, known as the subgrade. An A subgrade indicates high strength (like concrete-stabilized clay), while a D subgrade suggests low strength (like uncompacted soil).
For flexible pavements, the subgrade categories are:
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A: High Strength – CBR 15 (All CBR above 13%)
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B: Medium Strength – CBR 10 (CBR between 8% to 13%)
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C: Low Strength – CBR 6 (CBR between 4% to 8%)
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D: Ultra Low Strength – CBR 3 (CBR below 4%)
The California Bearing Ratio (CBR) is a common test used to evaluate the mechanical strength of subgrades. It measures the pressure needed to push a plunger into soil or aggregate, then compares this to a standard crushed rock material.
For rigid pavements, the subgrade categories are:
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A: High Strength – Subgrade k = 150 MN/m³ (k values above 120 MN/m³)
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B: Medium Strength – k = 80 MN/m³ (k values between 60 and 120 MN/m³)
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C: Low Strength – k = 40 MN/m³ (k values between 25 and 60 MN/m³)
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D: Ultra Low Strength – k = 20 MN/m³ (k values below 25 MN/m³)
The fourth part of the PCN code indicates the maximum tire pressure the pavement can support. Possible letters include:
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W: Unlimited – no tire pressure limit
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X: High – max tire pressure of 1.75 MPa
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Y: Medium – max tire pressure of 1.25 MPa
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Z: Low – max tire pressure of 0.5 MPa
Finally, the fifth part of the code shows how the PCN value was determined. A T means it was obtained through technical evaluation, while a U indicates it was based on physical testing.
Example: A PCN of 80/R/B/W/T means:
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Load-carrying capacity is 80 (no units)
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The pavement is rigid (R)
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It is on a medium-strength subgrade (B)
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There is no limit on tire pressure (W)
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The value of 80 was calculated through technical evaluation (T)
