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Runways can be made of asphalt, concrete, gravel, grass, dirt or a mixture of these materials. Maximum grades for commercial airfields are generally between 1 and 2%. Smaller aircraft can take off and land on steeper grades.
Pavement depths of runways vary depending upon the type of aircraft that is using the runway. For paved runways, the finished surface consists of concrete or asphalt pavement. Beneath the finished surface there are one or more layers called subbase. Subase material usually consist of gravel or a gravel/asphalt/concrete mixture. Gravel diameter varies depending upon the weight that it is designed to carry.
For asphalt or conrete runways, determining the depth of pavement involves complex engineering formulas. These formulas must take into consideration:
type of earth the pavement is placed on (load bearing capacity);
type and weight of the aircraft that will be using the runway (known as the design aircraft);
minimum and maximum operating temperatures of the airport;
type of landing gear utilized by the aircraft;
tire pressure of the design aircraft;
volume of traffic that will be using the runway.
In general, runway pavements are designed to last for 20 years.
There are two basic types of pavements, rigid and flexible. Rigid pavements are those pavements that are made of Portland cement concrete. They generally last longer than flexible paving and are more expensive to build. Flexible pavements are made of hot mix asphalt that are similiar to highway pavement. It may be necessary to patch and overlay the pavement during the expected life cycle as part of normal maintenance. Climatic conditions such as ice, snow, and temperature will often determine the actual lifespan of the pavement.
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