Frohmut Wellner

Permanent Deformation Behavior of Granular Materials and the Shakedown Concept

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.

Permanent Deformation Behaviour of Granular Materials

ABSTRACT The paper describes the accepted understanding of the shakedown concept. The results of several repeated load triaxial tests performed on crushed rock aggregates at different stress levels for a large number of repetitions of loading are given. The development of the resulting permanent deformation which accumulates with the repeated loading is depicted and compared with the types of responses usually described by the shakedown approach. It is shown that the existing shakedown approach can represent some, but not all, of the observed responses. Thus a modified set of possible responses is defined in shakedown terms and some explanation of the differences from the conventional approach are given. It is concluded that the method of description could give a powerful material assessment and pavement design tool for the engineering of unbound pavement bases. A design chart is derived from the data described, so as to illustrate a possible design approach.

DESIGN OF GRANULAR PAVEMENT LAYERS CONSIDERING CLIMATIC CONDITIONS

A new simple design approach that uses test results from the repeated load triaxial apparatus to establish the risk level of permanent deformations in the unbound granular layers (UGL) in pavement constructions under consideration of the seasonal effects was developed. From these data, a serviceability limit line (plastic shakedown limit) stress boundary for the unbound granular materials (UGM) was defined for different moisture contents. Below this line, the material has stable behavior. The serviceability limit line was applied in a finite-element (FE) program, FENLAP, to predict whether stable behavior occurs in the UGM. To calculate the stress in the UGL, a nonlinear elastic model (Dresden Model) was implemented into the FE program. The effects of changing moisture content during spring thaw period and asphalt temperature on pavement structural response were investigated. Additionally, permanent deformation calculations for the UGL were performed taking the stress history into consideration. The results clearly demonstrate that, for pavement constructions with thick asphalt layers, there is no risk of rutting in the granular base, even at a high number of load repetitions. The proposed design approach is a very satisfactory simple method of assessing the risk of rutting in the UGL, even without the calculation of the exact permanent deformation of the pavement construction.

Numerical modelling of tyre–pavement-interaction phenomena: constitutive description of asphalt behaviour based on triaxial material tests

As basis for the subsequent numerical modelling of tyre–pavement-interaction phenomena, an adequate representation of asphalt material behaviour is required. Therefore, in the present publication, experimental investigations by the repeated load triaxial test and a new approach of the interpretation of the test results are presented. Based on these results, an appropriate asphalt material model that considers elastic, viscous as well as plastic properties and that allows for large strain theory is selected for the numerical modelling by finite element method. The related material parameters are identified based on the experimental data by an optimisation procedure. Exemplarily, the numerical simulation of data points of the Cole–Cole-plot and the computation of an asphalt test sample in triaxial stress state are conducted. The comparison of experimental and numerical results shows good agreement. Therefore, the material model and the parameters may be used subsequently for structural pavement computations.

Rheological characterisation and modelling of bitumen containing reclaimed components

ABSTRACT Currently, many efforts are undertaken to reach asphalt performance from binder performance in order to reduce costly and time consuming laboratory tests on asphalt materials. In this context, it is important to use and develop appropriate testing methods for a comprehensive binder characterisation, which requires suitable material models. Depending on the application, different models can be used to model rheological bitumen behaviour. This paper focuses on the applicability of the Christensen and Anderson model (CA model) and the Olard-Di Benedetto (2S2P1D) model for bitumen samples containing different amounts of recycled components using various objective functions in the optimisation process. The purpose is to analyse the influence of the reclaimed asphalt rate on the parameters of the investigated models. Dynamic Shear Rheometer tests are conducted on binder samples extracted from Stone Mastic Asphalt mixes with polymer modified bitumen and Asphalt Concrete mixes with paving bitumen, each containing 0–100% reclaimed material components. The resulting influences of the recycled materials are illustrated through master curves, black diagrams and Cole–Cole plots by fitting these experimental data by the application of 2S2P1D and CA model.

Numerically Supported Experimental Determination of the Behavior of the Interlayer Bond in Asphalt Pavement

A good and durable interlayer bond is of high importance for the service life of asphalt pavements. Accordingly, the interlayer bond has long been the subject of active investigation. This paper describes a new test device for cyclic testing of the interlayer bond in double-layered asphalt laboratory specimens. Experiments were carried out to determine the shear stiffness at five temperatures, five normal stresses, and six loading frequencies. All three parameters had a strong influence on the shear stiffness and, therefore, on the operation of the interlayer bond. At low temperatures, high tack coat stiffness as well as good adhesion and aggregate interlocking was present. In contrast, rapid decrease in bonding stiffness occurred at rising temperatures because of the loss of adhesion. In addition, a total loss of interlayer bond occurred at low loading frequencies when no normal force was applied. Furthermore, it was observed that the tack coat amount applied at the interface had a significant effect on the shear stiffness. A numerical model based on the finite element method was developed to simulate the test. With the aid of sigmoidal regressions built for the lowest and highest test temperatures, it was possible to predict the shear stiffness analytically at any shear force and shear displacement and to compare the stiffness with the experimentally determined one.