Assessment of the Punching Shear Behavior of Interior Slab-Column Connections Strengthened by Hemp Fibers Sheet Fabrics

Abstract

The replacement of synthetic fibers by natural fibers such as hemp fibers in concrete has been investigated to achieve a sustainable construction material. Hemp fibers are extracted from the plant's stem, which makes them strong and stiff. These fibers are environmentally friendly and are used in the manufacturing of composite materials. In this research, the assessment of hemp fiber reinforced polymer (HFRP) fabric sheets' performance as punching shear strengthening material for interior slab-column connections is investigated experimentally. The experimental results were validated through numerical and analytical approaches. Twenty-four lab-scale interior slab-column connections (670x670 mm) were designed to experience punching shear failure. The main test variables are the slab thickness (55 or 75 mm), the ratio of steel reinforcement (1% or 1.5%), the width of hemp fiber fabric sheet (100, 150, or 200 mm), the number of layers of the HFRP sheets (one or two layers), the type of strengthening material (HFRP sheets or carbon fiber reinforced polymer (CFRP) sheets), and the location of hemp sheets (adjacent or offset by 1.5d from the face of the column). All tested specimens were loaded centrally through the column stub using the MTS universal testing machine up to failure. The assessment was based on a comparison of load capacity, mode of failure, load-displacement history, and cracking patterns of different specimens with different test parameters. Three finite element models were developed using ABAQUS software to predict the behavior of simulated specimens: un-strengthened specimen, specimen strengthened by HFRP, and specimen strengthened by CFRP. The experimental results demonstrate that HFRP strengthening sheets led to a significant improvement in the slab-column connections' structural behavior, depending on the slab thickness, steel content, width and configuration for HFRP sheets. The improvement in the ultimate shear capacity ranged between 5.5% and 41.14%, while the increase in stiffness reached up to 56.8% relative to the control specimen. It was found that although the natural confining HFRP sheet led to lower improvement in the performance of the strengthened specimen as compared to the synthetic CFRP sheet, the same improvement could be reached by the HFRP sheets when applied in larger width or in different configurations. The numerical findings show that the models predicted the connections' behavior in good agreement with the test results. The comparison between test results, available analytical model, and ACI Code shows that the analytical model provides good predictions for the test specimens' punching shear capacity. However, the ACI equations provide conservative predictions for the punching shear capacity of the test specimens.