Sabine Werkmeister

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.

Shakedown Analysis of Unbound Granular Materials using Accelerated Pavement Test Results from New Zealand's CAPTIF Facility

This paper reports on one aspect of an ongoing research project that is aimed at investigating the nonlinear resilient and permanent deformation behaviour of Unbound Granular Materials (UGM) using laboratory and field test results. Insufficient evidence is currently available to confirm the reliability of the proposed linkage between the Shakedown ranges defined via the Repeated Load Triaxial (RLT) test and in-situ performance of UGM. Further investigations on this specific topic were necessary to confirm predictions made by the laboratory test results by examining the results of a series of accelerated pavement tests. The examination of the relationship between laboratory shakedown parameters and field test results is described in detail. The paper shows that the shakedown behaviour of UGM observed in RLT tests can also be observed in field tests. In particular an empirical approach was formulated and calibrated to determine basecourse permanent strain rate using data from RLT and field tests for two different UGM. From the results of the tests a correlation was found to exist between the resilient strain rate and the permanent strain rate as long as the shear stresses within the basecourse material were sufficiently small.

Analytical Design Methodology for Thin Surfaced Asphalt Pavements in Germany

The paper presents the fundamental basis of an analytical design methodology for roads with thin asphalt surfaces in Germany. A new mechanistic-empirical design approach was developed which is focused on limiting the main sources of rutting of thinly sealed pavement structures: 1) excessive plastic deformation in the subgrade and 2) excessive plastic deformation of the unbound granular materials in the basecourse in the vicinity of the wheel load. The procedure is based on two steps: first, a response step is carried out where the response of the structure is calculated due to traffic loading, taking into account the effect of climatic conditions on material properties. Thereafter, a distress prediction is undertaken to evaluate the accumulated damage in terms of rutting originated in the subgrade and granular basecourse.