In the past decades, there has been an increasing effort to describe the local bond behavior with a bond stress-slip model. Such bond models are often used in studies of long bond lengths, numerical calculations and crack width determination. Local bond behavior is usually investigated experimentally in pull-out tests with short bond lengths equal to or less than 5 times the bar diameter ds, recording the pull-out force and bar slip. Therefore, existing bond models, like the approach of fib Model Code 2010, are mainly based on data determined outside the bond zone. Distributed optical fiber sensing opens up the possibility to get an insight of the processes within the bond zone during testing.
Current investigations at Technische Universität Dresden focus on the bond behavior and the bond stress distribution for ribbed bars with bond lengths from 1·ds up to 60·ds under various loading conditions. The test program includes systematic investigations of the influence of the bond length on the ultimate bond stress testing pull-out, beam-end and tensile tie specimens as well as the targeted use of distributed optical fiber sensing. Therefore, the hair thin sensors are applied directly on the pull-out bar and enable quasi-continuously strain reading along the bond length. Based on the local resolution of the strain, conclusions about the local force transmission between reinforcement and concrete at different load levels or after a certain load duration can be drawn. Hence, already with a bond length of 2·ds, non-linear distributions of the bond stresses and local peaks in front of the ribs could be observed.
This article deals with the experimental and instrumental setup, test results and the procedure of evaluation. Furthermore, it will be discussed how distributed optical fiber sensing can help to derive a local bond stress-slip model.