The findings of an extensive literature survey on the structural response of unbound aggregates are discussed in two companion papers. In this paper, a state of the art is presented on resilient stress-strain characteristics of such materials, as well as different modeling techniques. The resilient response of aggregates is affected by several factors with varying degrees of importance. These are presented, and different views on the impact of each individual factor are discussed. Research efforts in the past have resulted in different mathematical models for predicting the resilient response of aggregates under repeated traffic-type loading. The models found in the literature are listed, and their advantages and shortcomings are reviewed. The permanent strain characteristics of unbound aggregates are considered in a separate companion paper.

The permanent strain behavior of unbound aggregates under repeated loading has been studied by many researchers over the years. A summarized review of the research into this area is given based on the literature available to date. Different views from various sources are outlined and discussed. A state of the art is presented regarding the factors affecting the permanent strain development in these materials and the modeling techniques available. This paper is the second of two companion papers discussing the current state of knowledge on the structural behavior of unbound aggregates. The resilient properties of these materials are reviewed in detail in the first paper.

The shakedown concept has been used to describe the behavior of conventional engineering structures under repeated cyclic loading. The possibility has been raised that a critical stress level exists between stable and unstable conditions in pavement. According to the “shakedown” concept, this level is termed the “shakedown limit.” Several repeated load triaxial tests were performed on crushed rock aggregates at different stress levels. The resulting permanent deformation, which accumulated with the repeated loading, was described and compared with the types of responses usually described by the shakedown approach. The existing shakedown approach can describe some, but not all, of the observed responses. Thus, a modified set of possible responses was defined in shakedown terms. The method of description could provide a powerful material assessment and pavement design tool for engineering unbound pavement bases. A design chart derived from the data illustrates a possible design approach.

The paper reports the results of several repeated load triaxial tests performed on a crushed rock aggregate at different stress levels. The development of the resulting permanent deformation that accumulates with the repeated loading is described and compared with the types of responses usually described by the shakedown approach. It is shown that the existing shakedown approach can describe some, but not all, of the observed responses. Thus a modified set of possible responses is defined in shakedown terms and some explanations of the differences from the conventional approach are given. The elastic shakedown limit can be used to predict whether or not stable behavior occurs in the unbound granular material layer of a pavement construction or whether rutting will occur. With this information a pavement’s granular base may be dimensioned. The elastic shakedown limit of a Granodiorite has been used in this manner and the resulting design compared with an empirical German design procedure. The paper conclud...