The prediction of permanent deformation in flexible pavements

At the present time (1978) new roads in the United Kingdom are designed empirically. However, over approximately the last twenty years pavement technologists throughout the world have been advocating fundamentally based procedures. Such procedures, when applied to flexible pavements, require a detailed knowledge of the two main modes of failure, fatigue of the bituminous bound layers and excessive permanent deformation of the pavement as a whole. The research described herein was aimed at improving knowledge of the permanent deformation behaviour of flexible pavements, and at developing a technique for predicting this deformation which could be incorporated in a fundamental design procedure. Three laboratory experimental pavements were trafficked with a rolling wheel facility under conditions of constant temperature, load and speed, and the resulting deformations were monitored. These deformations were compared with predictions using models developed from the results of repeated load characterisation tests on the pavement materials and utilising a digital computer. A review of previous work is presented ill two parts, the first considering the resilient and permanent deformation response of pavement materials, the second considering proposals for the prediction of permanent deformation in pavements. The development of the wheel loading facility, and the construction, instrumentation and measurements from the pavements are described briefly. More attention is given to the theoretical approach, materials characterisation testing and the prediction of resilient and permanent deformation response of the pavements. The prediction techniques and their accuracy are assessed, the problems involved in the development of such procedures are discussed, and the conclusions which can be drawn from the work are presented. A number of suggestions regarding further work in this field are made. Finally, an Appendix considers an alternative to the method of modelling permanent deformation presented in the main part of the text