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.

Carbon reinforced concrete (CRC) has been researched in Germany for around 20 years by now. To strengthen structures with this building material, a grid of carbon fibre in a fine-grained concrete layer is applied e.g. to the bending tension zone of an existing reinforced concrete structure (RC). This increases the load bearing capacity of the structure. The grids have changed significantly over the past few years. They have larger fiber cross-sections and different impregnations, which results in tensile forces that are approx. 3 times higher than those of textiles of the older generation. However, this increase in tensile strength can entail additional failure mechanisms: In some tests on RC specimens strengthened with a higher number of CRC layers the tensile forces in the strengthening layer could no longer be fully transferred into the steel reinforcement concrete member and damage occurred, either in the RC structure, in the bonding area between the RC structure and the strengthening layer or within the strengthening layer. To enable an optimized utilization of the carbon grids in the future, the various forms of failure are examined more closely in a research project. Different solutions are derived. The focus of the investigationsis on the anchoring of the ends of the strengthening layers. The aim is to find constructive solutions to make better use of the carbon grids and thereby further increase both the economic efficiency and the ecological balance of carbon reinforced concrete.

Carbon reinforced concrete (CRC) has been researched in Germany for around 20 years by now. To strengthen structures with this building material, a grid of carbon fibre in a fine-grained concrete layer is applied e.g. to the bending tension zone of an existing reinforced concrete structure (RC). This increases the load bearing capacity of the structure. The grids have changed significantly over the past few years. They have larger fiber cross-sections and different impregnations, which results in tensile forces that are approx. 3 times higher than those of textiles of the older generation. However, this increase in tensile strength can entail additional failure mechanisms: In some tests on RC specimens strengthened with a higher number of CRC layers the tensile forces in the strengthening layer could no longer be fully transferred into the steel reinforcement concrete member and damage occurred, either in the RC structure, in the bonding area between the RC structure and the strengthening layer or within the strengthening layer. To enable an optimized utilization of the carbon grids in the future, the various forms of failure are examined more closely in a research project. Different solutions are derived. The focus of the investigationsis on the anchoring of the ends of the strengthening layers. The aim is to find constructive solutions to make better use of the carbon grids and thereby further increase both the economic efficiency and the ecological balance of carbon reinforced concrete.