Promotion Marc Koschemann

On 30.04.2024, Dipl.-Ing. Marc Koschemann successfully defended his scientific thesis as part of the doctoral procedure with the topic "Local bond behavior of reinforced concrete under short-term and long-term loading". In addition to the chairman of the doctoral committee, Prof. Dr. Peer Haller (TU Dresden), Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach (TU Dresden) and Prof. Dr. Nguyen Viet Tue (TUD Dresden University of Technology Graz) were present as reviewers. Prof. Dr. Jan Hofmann (University of Stuttgart) was connected online.

Abstract:

The bond effect between steel and concrete is the essential basis for the functioning of reinforced concrete components and influences, among other things, crack spacing and crack widths as well as the concrete's contribution to tension between the cracks. To ensure the serviceability and durability of buildings, it is necessary to limit the crack widths. Due to the time-dependent behavior of concrete, the bonding effect changes during the service life of structures, which leads to an increase in crack widths. A realistic condition assessment therefore requires precise knowledge of the bond behavior and crack width development under short-term and long-term loading.

The influence of the concrete strength, the bond length and the specimen type on the local bond behavior under short-term and long-term loading was investigated as part of the work. For this purpose, a total of 132 bond tests were carried out on pull-out and beam end specimens. In order to obtain information about the local and temporal course of the bond force transmission along the embedment length, fiber optic sensors were applied to the reinforcing bars and the steel strain distribution was recorded.

Based on the results of tests with a very short bond length, analytical descriptions of the local bond stress-slip behavior under short-term loading were formulated as a function of the type of failure. The bond resistance increases almost linearly with the concrete compressive strength in the case of pull-out failure and as a function of the splitting tensile strength when splitting cracks occur. Furthermore, the fracture displacement decreases with increasing concrete strength. Increasing the bond length in the test resulted in a decrease in the average bond strength.

Based on the measured strains at different application locations on the bar, it was possible to derive statements about the stress distribution within the bar and about the local influence of the ribs. The experimental findings were qualitatively confirmed by numerical simulations with discrete modeling of the ribs. Using fiber optic sensors, it was demonstrated that the force transmission is uneven even with very short bond lengths and that the bond behavior is locally dependent.

In the bond tests under long-term loading, significant increases in deformation were recorded during the loading period, which showed no dependence on the bond length and only a slight influence of the concrete strength. Existing models underestimate the effect of bond creep. The proposed calculation approach can be linked to the local bond stress-slip relationship and the slip increase can be determined for any load duration.

To investigate the development of crack widths under permanent load, expansion body tests were carried out with a centrally embedded bar and different cross-sectional dimensions. Using fiber optic sensors, an increase in crack width of up to 20 % was determined during a long-term load of 1,000 hours. Based on the strain body tests, it could be deduced that the effects of bond creep on the crack width decrease with increasing reinforcement ratio and increasing load level.

Using the stepwise integration of the differential equation of the displaceable bond, comparative calculations were carried out on the basis of existing models for the bond and bond creep as well as the derived local relationships. The calculated slip and bond stress distributions show a high degree of agreement in comparison with the experimentally determined curves. Larger deviations were achieved in the case of a splitting failure. The crack widths determined with the expansion body tests could be predicted correctly using the derived relationships for small cross-sectional dimensions. The calculation results for the increase in crack width due to bond creep qualitatively confirm the findings obtained in the tests.

We would like to congratulate Mr. Marc Koschemann on completing his doctorate and wish him all the best and every success for the future.