Influence of strength grades of core concrete of reinforced concrete column with self-compacting mortar stay-in-place formwork

The behaviour of composite columns with prefabricated self-compacting mortar, stay-in-place (SCM-SIP) formwork, and grouted with reinforced concrete having two grads; the first normal-strength (NSC) and the second with high-strength concrete (HSC) was investigated. Twenty-four RC composite columns with two types of cross-sections [square and circular], filled with two types of concrete grads using formwork thickness of 10-mm, and one layer of carbon fibre reinforced polymer (CFRP) confinement were tested under axial compressive load. The infilled concrete grades were NSC with 36.64 MPa and HSC with 61.46 MPa. The results show that the enhancement ratios in the strength of composite columns for specimens with HSC concrete cores as percentages from the reference specimens were lower than their counterparts with NSC ones. While using square cross-section-shaped columns showed lower axial strength and strain capacities than the circular one. Peeling failure between SCM-SIP form and core concrete might have been a plausible failure mode during the design and testing stages because of differential material properties or poor interface bonding, which could lead to delamination under mechanical loads. However, the results indicate that the bonding was strong enough to prevent such issues, demonstrating the integrity of the composite columns. The confinement systems with one layer of the CFRP showed an enhanced compression strength result as compared with column without warping. The lateral strain under the axial compression load for each specimen was measured, thus models were suggested to predict the ultimate axial strength and corresponding strain of the composite columns.