The Pavement Classification Number (PCN) is an International Civil Aviation Organization

International Civil Aviation Organization

The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...

 standard used in combination with the Aircraft Classification Number

Aircraft Classification Number

The Aircraft Classification Number is a number expressing the relative effect of an aircraft on the runway pavement for a specified standard subgrade category ....

 (ACN) to indicate the strength of a runway

Runway

According to ICAO a runway is a "defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft." Runways may be a man-made surface or a natural surface .- Orientation and dimensions :Runways are named by a number between 01 and 36, which is generally one tenth...

, taxiway

Taxiway

A taxiway is a path on an airport connecting runways with ramps, hangars, terminals and other facilities. They mostly have hard surface such as asphalt or concrete, although smaller airports sometimes use gravel or grass....

 or airport ramp

Airport ramp

The airport ramp or apron is part of an airport. It is usually the area where aircraft are parked, unloaded or loaded, refueled or boarded. Although the use of the apron is covered by regulations, such as lighting on vehicles, it is typically more accessible to users than the runway or taxiway...

 (or apron). This helps to ensure that the airport ramp is not subjected to excessive wear and tear, thus prolonging its life.

Usage

Although important for the runway the major use of this number is for the apron. On landing

Landing

thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...

 the aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 is light on fuel and usually less than 5% of the weight of the aircraft touches the runway in one go. On takeoff

Takeoff

Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...

 the aircraft is heavy but as the aircraft accelerates the weight gradually moves from the wheels to the wings. It is while the aircraft is being loaded and taxiing

Taxiing

Taxiing refers to the movement of an aircraft on the ground, under its own power, in contrast to towing or push-back where the aircraft is moved by a tug...

 prior to departure, that the apron experiences significant loads from aircraft weight.

Typically this is only used for asphalt

Asphalt

Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

 or concrete

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 runways and would not be used for grass

Grass

Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

 or gravel

Gravel

Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

.

How it works

The PCN is actually expressed as a five part code, separated by forward-slashes, describing the piece of pavement concerned.

The first part is a numeric value expressing the actual assessed strength of the pavement. The second part is a letter: either an R or an F, depending on whether the pavement itself is of a rigid (concrete) or a flexible (asphalt) design.

The third part is another letter from A to D expressing the strength of what is underneath the pavement, known as the subgrade

Subgrade

In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track. It is also called formation level.The term can also refer to imported material that has been used to build an embankment....

. So a subgrade of A would be very strong, most likely a reinforced concrete

Reinforced concrete

Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...

 subbase

Subbase (pavement)

In highway engineering, subbase is the layer of aggregate material laid on the subgrade, on which the base course layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles....

. A subgrade of D would be very weak, most likely uncompacted soil.

The fourth part is either a letter, or a number with units expressing the maximum tire pressure that the pavement can support. In terms of letters, W is the highest, indicating that the pavement can support tires of any pressure, the others are as follows:

Pavement Class	Maximum Tire Pressure
X	1.5 MPa Pascal (unit) The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...  (217 psi Pounds per square inch The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units... )
Y	1.0 MPa (145 psi)
Z	0.5 MPa (72 psi)

The fifth and final part just describes how the first value was worked out, a T indicates technical evaluation, and a U indicates usage -- a physical testing regime.

So a PCN of 80/R/B/W/T means that the underlying (probably concrete) has a bearing strength of 80, is rigid, it's on a medium subgrade, has no limit on tire pressure, and this has been calculated through technical evaluation.