Promotion Jiafeng Zhou

On 17.10.2025, Mr. Jiafeng Zhou, M.Eng., successfully defended his thesis as part of the doctoral procedure 👨‍🎓 with the topic "Investigations on the bond behavior of a hybrid lap-spliced connection of GFRP and stainless steel bars". In addition to the chairman of the doctoral committee, Prof. Dr. Jens Otto (TU Dresden), Prof. Dr. Steffen Marx (TU Dresden) was present as an assessor. Prof. Dr. Robert Jockwer (TU Dresden) was also present as a member of the doctoral committee.

Abstract:

As the global population continues to grow, the demand for energy has also increased, leading to a significant rise in the resource consumption, notably the electricity and fuel oil. Consequently, enhancing energy efficiency has become an urgent necessity, not only to protect the environment but also to mitigate financial burdens. In the insulation joint of a traditional balcony structure, normal steel bars with high thermal conductivity are used as tension bars to connect the interior and exterior structures. This has caused significant energy losses due to the formation of serious thermal bridges. In comparison, the thermal conductivity of glass fiber reinforced polymer (GFRP) material is significantly lower, at only 1% of that of steel. It can be an ideal substitution for the tension steel bars in the joints. Thus, this introduces a novel hybrid lap-spliced connection between GFRP bars and steel bars in the adjacent structures, which has been rarely investigated.

In the first publication of this cumulative dissertation, comprehensive knowledge regarding the tensile lap-spliced steel and FRP bars in flexural concrete specimens is collected and systematically analyzed. Building on the results, a three-level based investigation method is outlined to study the mechanical behavior of the hybrid lap-spliced connection between GFRP bars and steel bars. In the second publication, the first two levels are presented. First, the tension tests on two types of GFRP bars and one type of stainless steel bar are conducted to obtain their mechanical properties, respectively. In the subsequent bond tests, the bond performance between concrete and one single bar is investigated. Four combinations of distributed fiber optical sensors (DFOSs) and adhesives are applied in the grooves of one type of GFRP bars to investigate their influence on the measured bar strains. Next, the determined best variant is further implemented in other two types of bars to study their different bond behaviors in concrete. The third publication primarily focuses on investigating the third level, specifically the actual lap-spliced connections. In the first step, the microstructural characterization of GFRP bars is analyzed to comprehend their specific material properties. In the second step, the optimal direction of grooves for placing DFOSs in the bars is determined through cantilever tests. In the third step, the influence of lap-spliced length and lap-spliced type on the bond performances of hybrid lap-spliced bars is investigated with four-point flexural tests. The strain distributions of lap-spliced bars, in particular in the lap-spliced area are investigated in-depth. Besides, the strains and stresses at the lap-spliced ends at different loads are analyzed. Furthermore, the maximum experimental loads are compared with calculated results of the ultimate strength method.

In the end of this work, several further studies are carried out. Firstly, the bar strain distributions in bond tests and four-point flexural tests are compared. Besides, the cracks focusing on the lap-spliced area in the four-point flexural tests are analyzed. Moreover, numerical simulations are conducted to validate the experimental results of the bond tests and the four-point flexural tests. Subsequently, parameter studies are carried out base on the validated models to determine the critical development length of the GFRP bars in bond test as well as the optimal lap-spliced length between GFRP bars and stainless steel bars in four-point flexural test. The results are then compared with the results calculated with different national and international standards.

Based on the abovementioned investigations, it can be concluded that the substitution of steel bars with GFRP bars not only enhances the thermal behavior of the lap-spliced connection, but also increases the mechanical performance such as load capacity of the reinforced concrete structures. In addition, the provisions in current standards to calculate the development and lap-spliced lengths require further improvements, including experimental investigations and numerical validations, to achieve more accurate results.

Dear Jiafeng Zhou, we wish you every success in your future scientific career and, of course, all the best for the future! 👍🥳