The thesis investigates the mechanisms and restraints which influence transverse crack propagation through the bituminous surfacings of semi-flexible pavements. These pavements incorporate continuously laid cement bound roadbases which, during curing, crack into slabs of varying length, ranging from 4-25m. Reciprocal crack growth can occur in the surfacing, known as 'reflection cracks', located through stresses concentrated at the discontinuities within the roadbase . Three mechanisms have been identified and are described as contributing to reflection crack propagation. They have been analysed independently although the majority of conclusions drawn are applicable to their combined action. Their relative importance will vary with respect to pavement geometry, material properties, environmental conditions and traffic intensity. The first mechanism, 'tensile fatigue', induces crack propagation vertically upward through the surfacing. Tensile strains are developed during daily and ru1nual fluctuations of temperature, which cause expansion and contraction of the cement bound roadbase. This mechanism is most prominent on pavements with thin surfacings and long slab lengths. The rate of crack growth is dependent on the range of temperature within the roadbase , slab length, thermal characteristics of the roadbase material and resistance of the surfacing to this form of fatigue . A model has been developed based on a combination of results from an extensive testing programme, the use of fracture mechanics theory and computer simulation of the condition. The results quantify the resistance shown by conventional bituminous mixes to reflection cracking in terms of their mix parameters. Also considered are the use of stress relieving membranes, reinforcement material and modified binders to inhibit crack growth. The second mechanism, 'tensile yield' is also thermally induced but associated with cold weather conditions. Temperature gradients through the pavement structure induce warping and contraction within the uppermost layers. Tensile strains developed at the surface can, under U.K. winter temperatures, exceed the ultimate yield strain of the wearing course material. Preliminary. investigations of four pavements constructed in the early 1970's to motorway specifications indicate that reflection cracking will initiate at the surface if the yield strain, as defined through tensile creep tests, is reduced through binder oxidization to a value of 0.5%. This mechanism will operate on pavements with greater structural layer thicknesses and is only partially dependent on slab length. The influence of a further mechanism, 'shear fatigue' induced through trafficking of the pavement, has been shown to be confined to the acceleration of crack growth in the final stages of propagation unless a breakdown of interlock occurs between adjoining roadbase slabs.

Most engineering as well as pavement materials are composites composed of two or more components to obtain a variety of solid properties to support internal and external loading. The composite materials rely on physical or chemical properties and volume fraction of each component. While the properties can be identified easily, the volume fraction is hard to be estimated due to the volumetric variation during the performance in the field. Various test procedures have been developed to measure the volume fractions; however, they depend on subjective determination and judgment. As an alternative, electromagnetic technique using dielectric constant was developed to estimate the volume fraction. Empirical and mechanistic approaches were used to relate the dielectric constant and volume fraction. While the empirical models are not very accurate in all cases, the mechanistic models require assumptions of constituent dielectric constants. For those reasons, the existing approaches might produce less accurate estimate of volume fraction. In this study, a mechanistic-based approach using the self consistent scheme was developed to be applied to multiphase materials. The new approach was based on calibrated dielectric constant of components to improve results without any assumptions. Also, the system identification was used iteratively to solve for dielectric parameters and volume fraction at each step. As the validation performed to verify the viability of the new approach using soil mixture and portland cement concrete, it was found that the approach has produced a significant improvement in the accuracy of the estimated volume fraction.