Cracking Behaviour of Bitumen Stabilised Materials (BSMs): Is There Such a Thing?

Abstract

The behaviour of bitumen stabilised materials (BSMs) is uniquely different from all other materials used to construct road pavements. Unlike asphalt, where the bitumen as a continuum binds all the aggregate particles together, the bitumen in a BSM is dispersed selectively amongst only the finer particles, regardless of whether bitumen emulsion or foamed bitumen is used as the stabilising agent. When compacted, the isolated bitumen-rich fines are mechanically forced against their neighbouring aggregate particle, regardless of size, resulting in localised bonds which are non-continuous.

The purpose of this paper is to explore the question: Is the mode of failure of BSMs purely permanent deformation, similar to granular materials? Some engineers continue to argue that, similar to thick asphalt layers, BSMs fail in fatigue, supporting this stance by means of repeated-load tests carried out in a laboratory on beam specimens.

In this paper, the principles of fracture mechanics are employed to demonstrate that non-continuously bound materials experience different modes of failure to continuously bound materials. For a crack to propagate through a layer, the material must have sufficient internal cohesion to allow applied stresses to concentrate at the crack tip. In addition, the performance of several pavements, constructed at least five years ago, each with a base layer of high quality BSM material, is reviewed. Deflection measurements taken at regular intervals show none of the symptoms that would indicate deterioration due to fatigue initiation. Deflection measurements suggest that the pavement stiffness increases in the first year after construction.