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Runways: perfecting pavements

Airports thinking - perfecting runways

In this article, Matt Glenn, Airports Leader - Airside, looks at the business decisions that airport operators must make around pavements and air traffic.

He also looks at the effects of repeated aircraft overload operations on pavements and the best approach to pavement evaluation and maintenance.

Pavements are normally designed for a mix of traffic and a defined life. For flexible pavements, the functional life is usually 12 to 15 years before major pavement maintenance is required, and for rigid pavements the timeframe is usually 40 years. The true life expectancy of a pavement is a direct function of:

  • Environmental factors
  • Quality of pavement material
  • Traffic distribution
  • Number of operations/ repetitions of aircraft loading
  • Aircraft characteristics - weight, tyre pressure, wheel configuration; and
  • Overload operations.

The strength rating of airport pavements is commonly thought of, in terms of conventional structural concepts in which limiting loads are determined, based on ultimate strength or failure criteria.

Pavements, however, do not generally experience a loss in serviceability from instantaneous structural failure, but rather from an increase in surface roughness or deterioration of the surface resulting from the accumulated effects of traffic.

Structural failure is most often recognised, in terms of common pavement distresses such as rutting, cracking, and noticeable surface roughness that both pilots and passengers experience.

Pavement management and business decisions

Analysis of the adequacy of a pavement requires that a rating be assigned which not only considers the significance of load magnitude but also the effects of traffic volume over the intended life of the pavement.

The rate of deterioration of a pavement will, of course, be increased by any overload, particularly where high frequencies are involved.

The Aircraft Classification Number (ACN) – Pavement Classification Number (PCN) system is the means by which airport owners and operators regulate the use of their pavements and balance pavement strength and maintenance strategies in relation to revenue from landing fees, ACNs must be calculated using a fixed technical method and are intended to indicate the relative pavement damaging effect of each aircraft.

An aircraft’s ACN is fixed by its weight, its wheel layout and the subgrade strength. The airport owner has no say in what the ACN of an aircraft is. It is a technical fact. By contrast, the PCN functions as a pavement management tool, and its selection is largely a business decision.

The ACN-PCN system functions in practice as a pavement management tool for airport owners, not to protect aircraft. An airport owner might allow a particular aircraft to use the airport, only, if it operates at a restricted weight, and/or at reduced tyre pressure and/or at a limited frequency. Permanent concessions are often granted to allow aircraft with ACNs higher than the PCN to use a runway.

Pavements can sustain some overload, that is, pavement ratings are not absolute. There may be good reason why overload operations should be approved. For instance, the design traffic is operating at less than design capacity and limited overload may not reduce the life of the pavement or, depending on the overload, may only marginally reduce the life of the pavement. This reduction in pavement life may be preferred to the alternative of refusing a desirable operation or having to strengthen the pavement for infrequent operations.

Pavement evaluations to support decision making

At some stage in the life cycle of the pavement, failure modes will start appearing. The pavement is a structure, and, like all structures which are exposed to repeated loadings, will eventually fail. Pavement distress can be arrested by following planned maintenance practices through an established pavement management system.

Repeated overloads may lead to the following failure conditions:

  • excessive roughness caused by general loss of shape after repeated operations by heavy wheel loads;
  • cracking of the seal surface where deflections caused are high or compaction of the pavement material is poor; and
  • surface rutting and cracking of the surface and stripping of aggregate from high tyre pressure.

It may also result in high pavement maintenance costs. Repeated overloads may lead to the following effects on aircraft operations:

  • reduced braking characteristics through the tyre/ pavement interaction;
  • an increase in the required operational length of runway;
  • a potential increase to structural fatigue to aircraft;
  • an increase in the likelihood of foreign object damage to aircraft structures from loose stones and material; and,
  • discomfort to passengers.

Airport pavement evaluations are necessary to assess the ability of an existing pavement to support different types, weights, or volumes of aircraft traffic. They are also critical in assisting airport owners with making a decision with regard to the most applicable PCN and acceptable pavement overloads.

The load carrying capacity of existing bridges, culverts, storm water drains, and other structures should also be considered in these evaluations. Evaluations may be also necessary to determine the condition of existing pavements for use in the planning or design of improvements to the airport.

Pavement Design versus Evaluation

In Pavement Design, the task is to determine the pavement thickness (and composition) required to support a specified aircraft traffic scenario. The thickness is dependent upon the strength of the in-situ soil material (the subgrade), the strength of the pavement layers and the applied loading.

The applied loading is normally defined, in terms of:

  • aircraft types (which determines undercarriage geometry, tyre spacings, tyre pressures, and percentage of the total mass on each gear);
  • frequency of operation of each aircraft type (operations per day, week, month or year); and
  • design period.

For a given subgrade strength and aircraft traffic scenario, there can be a range of pavement types and compositions that will support the traffic over the design period without significant deformation or loss of functionality and fitness for purpose. For each particular pavement type and composition, however, there is a unique solution for the total pavement thickness and the thickness of intermediate pavement layers.

In Pavement Evaluation, the pavement thickness and composition are known, and the task is to determine the traffic that can operate on the pavement. Again, the traffic is defined, in terms of the allowable operations (operations per day, week, month or year) of a combination of aircraft types at a range of loads. Clearly, for pavement evaluation, there is an extensive range of possible results because there can be a multitude of combinations of aircraft types, loads and frequency of operation.

The challenge, therefore, is to be able to assess the relative damaging effect of any single aircraft type at a specific load and frequency of operation, and then to be able to determine the cumulative damage caused by any such combinations.

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