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The combined use of environmentally-friendly plant-based natural fibre reinforced polymer (FRP) composites and recycled aggregate concrete (RAC) will be beneficial for the development of sustainable concrete. In literature, a large amount of research has been conducted on unconfined RAC where the recycled aggregates (RAs) mainly came from concrete rubble, but the use of RAs from masonry rubble for RAC is very rare. This study reported the compressive behavior of flax FRP tube encased RAC containing partially clay brick aggregate (CBA), which was termed as FFRP-rAC-CBA. For the RAC, up to 70% of natural coarse aggregates was replaced by recycled aggregates (RAs). The RAs consisted mainly of 60% of recycled clay brick aggregates and 40% of recycled concrete and mortar aggregates. A total of 36 cylindrical specimens including 24 FFRP-rAC-CBA and 12 unconfined rAC-CBA were tested under uni-axial compression. The testing variables included: (i) strength of rAC-CBA (i.e. 27.5 MPa and 32.8 MPa); (ii) thickness of FFRP tube (i.e. 3, 6, 9 and 12 FRP layers); and (iii) size of cylinder (i.e. 75 × 150, 150 × 300 and 300 × 600, unit of mm). Tests results indicated that the natural FFRP tube enhanced the ultimate strength and ductility of the confined rAC-CBA remarkably and the enhancement was more pronounced in specimens with higher concrete strength. Increasing FFRP thickness led to higher compressive stress and strain of the FFRP-rAC-CBA. In addition, stress and strain models were proposed to predict and compare with experimental results of FFRP-rAC-CBA. A size-dependent model was also used to predict the transitional strength and ultimate strength of FFRP-rAC-CBA. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]