The strengthening of existing reinforced concrete structures (RC) with carbon reinforced concrete (CRC) has a high potential to save resources and to increase the lifespan of the whole strengthened structure immensely. However, when strengthening structures with CRC, in some cases, failure due to concrete cover separation is detected, leading to the fact that the potential of the carbon reinforcement cannot be exploited. The prediction and prevention of this type of failure is the subject of current research. In this paper, a strut-and-tie-model is presented for calculating a critical tensile force leading to failure due to concrete cover separation. Additionally, possible methods to avoid that kind of failure are suggested. One of these is doweling the ends of the strengthening layer. This paper presents the first experiments to test this method, which show that doweling the strengthening layer leads to much higher failure loads compared to a structure without doweling. However, further investigations have to be examined to verify these first results.

In this paper, we address the relatively underexplored topic of the bond behavior between various carbon rebars and high-strength concrete. This research aims to bridge the knowledge gap in understanding how different manufacturing processes and surface profiles of carbon fiber rods influence their bond strength with concrete. Through experimental bond tests comparing different carbon fiber rebars with varied surface profiles and manufacturing methods, we observed that the achievable bond stresses are significantly influenced by these factors. One carbon rebar variant was selected based on preliminary investigations for detailed analysis. Extensive investigations were conducted on the preferred carbon rebar. Factors such as concrete strength, bond length, and testing speed were experimentally explored. The results not only corroborate many findings from traditional reinforced concrete construction but also reveal new phenomena unique to carbon rebars. These insights are crucial for advancing the application of carbon rebars in modern construction, offering a potential solution to challenges faced in conventional concrete construction.

Carbon reinforced concrete is perceived by industry as a promising alternative to the currently established construction products. Previous building authority approvals and approvals for this construction method largely exclude questions of preventive fire protection with regard to load-bearing behavior under fire because there are hardly any reliable research results available in this field. This article shows the results of experimental investigations including thermogravimetric analyses of carbon reinforcement and tensile tests on the composite material carbon reinforced concrete. The thermogravimetric analyses show the loss of mass of the carbon reinforcement under a temperature load. A decomposition of the coating system of the carbon fibers and, with increasing temperature load, also of the carbon was observed. By varying various boundary conditions, such as the heating rate and the oxygen content present, their influences can be assessed. Stationary and non-stationary tensile tests on strip-shaped carbon reinforced concrete specimens were used to determine the load-bearing and deformation behavior in the high-temperature range up to 700 °C. The investigations were carried out under constant heating rates of 2 K/min and 10 K/min. This made it possible to obtain stress-strain curves and information on the various temperature-dependent deformation components from mechanical strains and load-independent strains. The time- and temperature-dependent decomposition of the carbon resulted in a reduction in the tensile load-bearing capacity of the reinforcement in the high-temperature range. This effect can be taken into account by considering the cross-sectional loss of the carbon reinforcement in a hot design.