Is Flexible Pavement Design on the Right Road?

The principles underlying current mechanistic–empirical pavement design methods for fully flexible pavements have changed little in the past 30 years. They generally use linear elastic theory to determine the permissible strain at critical locations in the pavement structure to safeguard the road against excessive fatigue and structural deformation. These criteria are calibrated using performance data from structures that experience has shown to perform well. In recent years, the traditional design concepts have been brought into question. The concept of long-life or perpetual pavements and the recognition of top-down cracking has initiated research that suggests that fatigue damage is not necessarily a cumulative damage mechanism and that rutting in many asphalt roads is confined to the surfacing. This paper, sponsored by the UK Highways Agency, examines the reasonableness of current design methodology and identifies shortcomings. Basic information on pavement behaviour of which we are more certain is identified and it is proposed that this should form a platform for the development of a new approach to pavement design, which offers a truer reflection of performance. The review suggests that threshold levels exist and that above these levels, a risk analysis approach to pavement design should be developed. This change in thinking will have far reaching implications for pavement research, design, condition assessment, construction and maintenance.