Retrofitting of axially loaded damaged concrete square columns by combination of UHPC cover concrete and CFRP wrapping

The members of concrete structures may need to be retrofitted for various reasons, including poor quality of materials, design errors, structural changes, non-compliance with the requirements of design codes and also losing cover concrete due to rebar corrosion. Steel jackets, concrete jackets and fiber reinforced polymers can be mentioned as commonly used retrofit methods for concrete members. There are also several methods for retrofitting concrete columns that have lost their concrete cover. Using concrete jacket has some disadvantages, such as a significant increase in the weight of structure, increasing the element dimensions and the required time for the implementation of the rehabilitation. On the other hand, steel jackets have various difficulties in implementation stage. In this study, specimens of square concrete columns that have lost their cover concrete due to rebar corrosion have been investigated and have been retrofitted by combining a number of methods. These methods include the use of a new concrete layer and the wrapping of columns with carbon fiber. Combining these methods will result in the enhanced performance of the rehabilitation technique since these methods will cover the deficiencies of each other. It is expected that these combined methods will result in increased load capacity, energy absorption and ease of forming. Therefore, in this study, the combined effect of carbon fibers and high performance concrete layer is investigated. The combination of high compressive strength of this type of concrete and high tensile strength of carbon fibers can be used to increase the axial load capacity and energy absorption of square concrete columns. The variables of this study include the type of cover concrete (UHPC, UHPFRC and SCM) and the number of layers of carbon fiber (one or two layers). The total number of specimens in this study was 42, of which 6 were control specimens, 6 were damaged control specimens and 30 were damaged specimens, which were retrofitted with cover layer and carbon fiber. All of the specimens are placed under uniform axial load. The results of the experimental study show that in the retrofitting of the square column, it is better to use the UHPC coating layer. While it is better to use a self-compacting mortar as a coating layer in retrofitting the circular column. Retrofitted columns have significant increase in strength and energy absorption capacity compared to the control columns. The least effect was seen for the columns retrofitted with the coating layer of ultra-high performance fiber reinforced concrete (UHPFRC), which showed an increase by 33% and 85% in terms of strength and energy absorption with respect to the control columns. The greatest effect was seen in the columns retrofitted with self-compacting mortar coating layer with two layers of carbon fiber, which increased the strength and energy absorption by 210 and 480%, respectively. Also, the results show that because the confinement effect in the circular sections is uniform and the entire concrete is effectively confined, the effect of the type of concrete on the coating layer is reduced. A numerical study on a real-dimension column was carried out to verify the results of the laboratory tests and also in order to allow the experimental results from small-sized samples be extended to large-scale columns. The results showed that by increasing the column dimensions, the carbon fiber confinement effect is significantly reduced.