Deformation Properties of Unbound Granular Aggregates

Abstract

This thesis discusses the resilient and permanent deformation properties of unbound granular aggregates for use in road structures. One of the objectives of the thesis is to identify the influence of the physical properties of the aggregate grains, such as grain size, grain shape, surface texture, mineralogy and mechanical strength through cyclic load triaxial testing. A second objective is to study the effect of water on the deformation properties of materials as well as their frost susceptibility. The third objective is to study the effect of micromechanical properties using a discrete element model (DEM).

Deformation in unbound granular materials under cyclic loading is divided into a resilient (recoverable) part and a plastic part that does not recover. The elastic strain represents the denominator in the resilient modulus and the non-recoverable strain results in permanent deformations over time. As the resilient response is non-linear, the resilient deformations may be interpreted using several models for curve fitting. Two of the simplest models are the k-θ model and Uzans model. The interpretation of the permanent deformation behaviour of unbound aggregates is complicated, as there is a need for a failure criterion to define when the material is at a failure stage. Two methods used for interpretation of the permanent deformations are mentioned in Chapter 3 of this thesis; the Shakedown approach and the “Coulomb approach”.

Many factors are known to affect the deformation properties in unbound materials. In this thesis the effect of most of these factors is investigated in the six papers. In Chapter 4 the influence of the different factors is discussed on the basis of the results from the papers and findings in the literature. Cyclic load triaxial testing has been the main method to test the deformation properties of the selected unbound materials. This is so far one of the best methods for laboratory simulation of traffic loading.

Discrete element modelling is performed to gain a better understanding of the deformation properties of unbound aggregates tested in a triaxial apparatus under cyclic loading. This method provides useful information about the contact mechanics between neighbouring particles and the interaction of the grains. In addition, unbound spherical aggregates have been tested in the laboratory using a triaxial apparatus.

The main factors studied in this thesis are the influence of grain shape, grain size distribution, fines content, mineralogy, dry density and water content. Useful information about these key factors has been obtained. However, there is still work to do in order to utilize the conclusions directly in a pavement design system.

The dry density, degree of saturation and stress level seem to be key parameters for determining the deformation behaviour, but mineralogy, fines content and grain size distribution are also of importance. Regarding the practical consequences, the results show that mineralogy, fines content and grain size distribution must be given more attention in the pavement design manuals. More effort should also be placed on the compaction control phase in situ, in order to avoid initial rutting in the road structure.