TITLE: Top-Down Cracking: Myth or Reality? by:

In the late 1970s and early 1980's most highway engineers did not believe that top-down cracking occurred at all or, if they did, they thought that it was a relatively rare event that did not warrant serious attention. This paper describes research that led TRL to believe that the problem was much more serious than engineers had perceived. The paper begins with a summary of a large fullscale pavement design experiment in the tropics that in which the roads did not display the kind of structural behaviour that was expected. It explains how top-down cracking was recognised and describes the associated factors that seemed to determine its characteristics. At the same time, TRL engineers began to look for evidence of top-down cracking throughout the world and discovered that not only did it seem to be widespread in tropical environments, but there was also considerable evidence of its common occurrence.in more temperate climates such as France and Holland. The study identified the most probable cause of the problem and was followed by another experimental study of the behaviour of a range of asphalt mixes with the objective of finding a practical way of either preventing the problem from occurring or mitigating its adverse effects. The paper concludes with ideas for solving the cracking problem. Some of the possible solutions give rise to a high risk of deformation failures in asphalt materials and therefore bot