New insights in bond and tension stiffening of reinforced concrete under monotonic and high-cyclic loading
Author: Yasmin Lemcherreq
Language: English
external pageDOI: 10.3929/ethz-b-000610913call_made
Abstract
The interaction of reinforcement and concrete through bond is fundamental to the structural behaviour of reinforced concrete structures and must be preserved throughout their service lives. Bond behaviour has, thus, been the subject of intensive investigations since the advent of reinforced concrete. However, many aspects still lack a clear, mechanically consistent characterisation due to limitations of the conventional measurement techniques and the difficulty to obtain measurements at the reinforcement-concrete interface without modifying its structural behaviour. Consequently, the complex bond mechanisms are predominantly idealised as nominal bond shear stresses with a uniform distribution along the reinforcing bar perimeter, of which the magnitude is expressed as a function of the relative displacement between concrete and reinforcement. This thesis provides new insights into the bond behaviour through the application of fibre optical sensors mounted directly on the reinforcing steel bars embedded in concrete. The focus of the investigation is to characterise the effects of repeated tensile loading and unloading on bond and their implications for the tension stiffening.
In the first part of the thesis, the potential of distributed fibre optical sensing in the field of reinforced concrete research is explored. An optimal instrumentation of reinforcing bars as well as the data acquisition and post-processing methods, as identified in a series of pilot tests, are described. Subsequently, the performance of fibre optical sensors applied to measure elastic strains in high-cyclic tests and strains exceeding the yield limit of the reinforcement is assessed.
The second part of the thesis presents the results of a comprehensive experimental programme comprising a total of 31 pull-out tests and 28 reinforced concrete ties subjected to monotonic, cyclic and high-cyclic loading. Detailed measurements obtained in the former test configuration allow reconsidering the suitability of this test set-up for the description of the bond behaviour. The assessment of local strains observed in the reinforced concrete ties allows a qualitative description of the bond deterioration mechanism under repeated loading. From these observations, two approaches are proposed to account for the effects of repeated loading in the load-deformation analysis. The first approach is centred around solving the differential equation of slip using various bond stress-slip relationships accounting for bond degradation near the cracks and through repeated loading. The second approach is based on the Tension Chord Model, which considers the interaction in a simplified, yet mechanically consistent way. This model is amended to account for repeated loading and calibrated with the experimental results presented in this thesis.
The final part describes the yield phenomenon in bare and embedded reinforcing steel bars with a defined yield plateau. Discrepancies between the presented experimental results and the currently accepted bond theory are addressed and their implications are discussed.