Introduction. The introduction to the paper highlights the role of the modeling of deformation behavior of unbound granular mixtures (UGMs) under repeated mechanical loading as the base aspect of designing flexible road pavement with an unbound base layers due to the fact that UGMs have been recognized as the most vulnerable materials of the flexible road pavement under action of negative climatic (seasonal) or technogenic factors.
Problem statement. The issues of the paper concern the problems related to unstable deformation behavior of UGMs in unbound base/subbase layers during flexible road pavement designing and maintenance.
Purpose. The main goal of the paper is to perform an analytical review of the theoretical background to the construction and calculation of an unbound base layer of the flexible road pavement according to the results of the study of UGMs’ deformation behavior with their following categorization. The results of the analytical review are intended to be used during implementation of a national standard concerning the method of testing of unbound granular mixtures by the triaxial cyclic loading identical to the related European standard.
Results. The “shakedown concept” was reviewed which allows the categorization and also the appropriate selection of granular mixture during road pavement design and construction. It was established, that the “shakedown concept” sets up three types of permanent behavior of UGMs which, only after being tested via the triaxial cyclic loading method, can be categorized considering the design input data as stable or unstable ones. Also, an analytical comparison of various possible approaches to the UGMs’ categorization within the “shakedown concept” was fulfilled. By the standardized approach, the limits of three categories shall be established at progressively increasing stresses taking into consideration the restrictions on accumulated axial permanent strains. The other approach may be potentially effective for UGM selection and is based on the value of the permanent axial strain rate per one loading cycle. Further, the aspects of an empirical designing of flexible road pavements were reviewed where the parameters of deformation behavior of UGM were adopted, via the Finite Elements Method, to calculate the distribution of stresses and to model rutting in an unbound base layer considering variable thickness of asphalt pavement and variable climatic factors as well.
Conclusions. The results of the study of deformation behavior of UGMs by triaxial cyclic loading allow the categorization of such mixtures regarding their resilient or permanent properties.
Categorization of UGMs on the base of the “shakedown concept” shall be performed through the application of a mathematical model based on the dependence of values of maximal axial stress of the specimen during testing on its ratio to confining stress values. Potentially perspective also will be other methods for the categorization of mixtures on the base of the “shakedown concept”, for example those based on the value of the permanent axial strain rate per one loading cycle.The assurance of stable deformation behavior of UGM within the input data allows the implementation of an effective simple designing method that does not suppose the calculation of permanent strains of an unbound base layer or any subsequent limitations on loading.
