There is no fundamental difference between roads and airfields, and the general principles of design apply to both of them. However, some distinct differences exist between the two types of pavements, sortable among them are
(i) the magnitude of applied load,
(ii) tire pressure,
(iii) geometric section of the pavement, and
(iv) the number of repetitions of weight used to the pavement during the design life.
Also, the rapid development of jet aircraft in recent years has a profound effect on pavement design concepts.
The total weight of an aircraft is usually greater than that of a truck. The design load for a major highway is ordinarily in the vicinity of 8200 kg on dual tires, and for a conventional tires truck, the pressure is in the vicinity of 4.2 - 6.3 kg/cm2. On the other hand, a heavy commercial jet aircraft with gross wt of 310000 kg (Boeing 747) may have wheel loads over 100000 kg with tire pressure as high as 14 kg/cm2. Heavier bombers have higher gross wt and higher tire pressure. For eg, B-52 bomber has tyre pressure of 18.2 kg/cm2.
The number of repetitions of loads is much higher on highways than on airports. On the road, the expected repetition may be as much as 1000 - 2000 trucks/day on major roads, but on airports, only 20000 - 40000 coverages may be considered for the life of the pavement.
For a given wheel load and a given tire pressure, higher pavements should be thicker than airfield pavements, bcoz repetition of load on a highway is much higher, and also loads are applied closer to the pavement edge. However, bcoz gross weights on the airfield are much higher; in actual practice, airfield pavement is thicker.
Regarding the geometry of the pavement, lateral placement of traffic on roads is such that nearly all truck traffic travel within 1 - 1.2 m of the edge of the pavement. On the other hand, the pattern of wheel load application varies across the width of a runway and is concerned in the central portion.
Analysis of about 9m of the center of the pavement with bicycle-type and steerable type landing gears have further resulted in channelized traffic, and studies show that 75% of this type of traffic will occur on about 2.25m of pavement. This indicated the possibility of some application of the saving in cost that could be made by taking into account this variation in wheel load application in determining the thickness of the pavement. As a result, variable thickness section may be designed instead of the uniform section for a runway.
Similarly, traffic follows a designated line among the critical areas such as aprons, taxiways, and at runway ends and as such will need to be built of thicker cross-section than the central portion of a runway.