Characterization of Unbound Granular Materials for Pavements

Abstract

This research is focused on the characterization of the mechanical behavior of unbound granular road base materials (UGMs). An extensive laboratory investigation is described, in which various methods for determination of the mechanical properties of granular materials are examined for their applicability, particularly in developing countries. Further, the mechanical behavior of unbound granular materials as a function of the moisture content and the degree of compaction is investigated. A study into the modeling of the stress dependent mechanical behavior of granular materials is presented. Finally, verification and validation of a relatively simple characterization technique, the repeated load CBR test (RL-CBR), by the results of cyclic load triaxial testing are provided and overall practical implications of the research are presented. The laboratory investigation involves a large range of granular materials, mainly (sub-)tropical road base and subbase materials. The performed tests yield fundamental parameters that describe the strength, stiffness and resistance to permanent deformation of the materials tested. In addition to the (sub-)tropical road base and subbase materials, a recycled mix-granulate widely used in pavement construction in the Netherlands and a base course and frost protection material from Austria are incorporated to a limited extent in the laboratory testing program. Most roads in developing countries are either unpaved or have a thin asphalt surfacing, and as a consequence the granular base and subbase layers provide the bulk of the bearing capacity. Although the important structural contribution of these unbound granular layers is understood, engineering practice still greatly relies on tests which mainly give index properties of these materials. Pavement structures are designed based on empirical design methods related to a single design chart, restricting the incorporation of marginal materials or new materials for which the empirical data sets are not available. The reasons that pavement design and construction in developing countries rely on empirical design procedures that are basically developed for completely different conditions are: - the affordability and complexity of the cyclic triaxial tests required to determine the stress dependent mechanical behavior of granular materials; - the perceived complexity and unfamiliarity with the computational tools (non-linear multilayer or finite element analysis) required to model the performance of pavements using this mechanical behavior despite the availability of powerful digital computers and their penetration even to remote places. In order to promote the introduction of Mechanistic-Empirical design methods in developing countries, this research was set up with two goals: i. to make the characterization technique for the mechanical behavior of granular road base materials more accessible to practice through the development of a simple and effective characterization technique; ii. to further develop the understanding of the stress dependent mechanical behavior of unbound (sub-)tropical base and subbase materials. To achieve the first aim an innovative and relatively simple testing procedure, the RL-CBR test, is developed to characterize the mechanical behavior of the UGMs. RL-CBR testing is performed on the various granular materials in steel moulds without and with strain gauges. With the strain gauges the confining condition and hence the stress state of the specimen is estimated through mould deformation measurements. The finite element method (FEM) is used to model the RL-CBR testing and interpret the test results into mechanical behavior. Due to the non-uniform complex stress distribution in the RL-CBR compared to the triaxial test, fundamental material properties such as the stiffness modulus are less easy to determine. Extensive triaxial testing on the various granular materials is performed to realize the second goal. The result of this investigation is also used to validate and verify the results of the RL-CBR tests. Moreover, the effect of influence factors such as moisture content, degree of compaction, material type etc. on the mechanical behavior is investigated. For the unbound granular road base materials, particularly the natural gravels, the effect of the moisture content on the mechanical behavior was found to be more significant than the effect of the degree of compaction. Relative to the failure and permanent deformation behavior the resilient deformation behavior is less affected by the moisture content and the degree of compaction. The RL-CBR testing serves well its purpose to get a good estimate of the fundamental mechanical properties of granular road base materials from a rather simple characterization technique. The practical accessibility of characterizing the mechanical behavior of UGMs can therefore be enhanced through RL-CBR testing. This is proven by the fact that good correlations have been found between the stiffness results of the two characterization techniques, i.e. the complex triaxial test and the newly developed repeated load CBR test.