Dynamic compressive behavior of concrete confined with unidirectional natural flax FRP based on SHPB tests

This paper presents an experimental study on the dynamic compressive behaviors of concrete confined with unidirectional natural flax fiber reinforced polymers (FFRPs) under an axial impact load. A total of 80 specimens were prepared and tested using a 75 mm diameter split Hopkinson pressure bar (SHPB) at different strain rates varying from 50 to 200 s(-1). The experimental results showed that the failure modes and dynamic compressive mechanical properties (i.e., dynamic compressive strength, critical compressive strain, and energy absorption capacity) of FFRP-confined concrete were sensitive to the strain rate. The unconfined concrete specimens were crushed into small pieces at relatively low strain rates, whereas the FFRP-confined concrete specimens failed with FRP rupture and partial damage to the core concrete at relatively high strain rates. This indicates that the confinement of FFRP jackets can alleviate concrete damage and improve impact resistance. Compared with unconfined concrete, the application of the FFRP jacket remarkably improves the compressive strength, critical strain, and toughness, which indicates outstanding impact resistance. Increasing the confinement stiffness of the FFRP contributed to increasing the compressive strength, critical strain, and toughness. Based on the experimental results, the confinement mechanism of external flax FRP jackets on concrete was discussed, and a new dynamic strength model was proposed to predict the dynamic compressive strength of FFRP-confined concrete within the investigated strain rate range.