Investigations on the stress-strain-relationship of artificially damaged reinforcing bars

(original title: Untersuchung des Spannungs-Dehnungsverhalten künstlich geschädigter Bewehrungsstäbe)

Author: Sara Fomasi
Language: German

Abstract

The Federal Roads Office (FEDRO) has carried out a pilot project as part of the examination and maintenance of the infrastructure of the national road network. In this study, it was discovered that in many cantilever retaining walls, the main reinforcement is damaged due to strongly localized corrosion. This type of corrosion significantly influences the load-deformation behaviour of the reinforcing bars and thus the behaviour of the entire walls. For this reason, the reduced load-bearing capacity and the remaining deformation capacity should be determined in order to be able to plan a monitoring concept and to assess necessary safety measures. The present Master’s thesis experimentally investigates the quantitative effects of individual phenomena on the stress-strain behaviour of reinforcing bars. In addition, a model for predicting the stress-strain relationship of damaged Tempcore reinforcing bars is developed.
In order to separately analyse different influences on material behaviour, four test series were conducted. Each test series differs from the others in the type of steel used, the damage type taken into account, the necessary specimen preparation, the testing procedure and the selected measuring systems. The results of approximately 80 tensile tests confirm statements already documented in literature and provide new contributions to the next research steps.
The obtained results show a high sensitivity of the material properties and the relaxation behaviour of reinforcing bars concerning the applied strain rate , independently of the steel type. Other important findings concern the mechanical properties of individual material zones, which compound the cross-section of Tempcore reinforcing bars. Ultimate tensile stresses are not uniformly distributed over the cross-section, which has to be taken into account especially for corroded reinforcing bars. Furthermore, the damage geometry was identified to be an important parameter. affecting the yielding behaviour of steel. This is related to the presence of a 3D-stress state in the damaged area.
In a further step, a semi-empirical model was derived from the results of the conducted experiments. This model is capable to predict the stress-strain behaviour of the various material zones building the cross-section of Tempcore reinforcing bars. Although the overall behaviour can be described accurately, the achieved strength is slightly over-estimated and the ductility values underestimated. An optimization of the model is therefore to be considered.
The final results of this Master’s thesis provide a solid basis for the next steps of the entire research project: the additional investigation of bending effects and the quantitative description of the various analysed effects on the stress-strain behaviour of reinforcement bars, as well as the final analysis of the deformation capacity of cantilever retaining walls affected by local corrosion.