Behaviour of FRP-confined coal rejects based backfill material under compression Hongchao Zhao a,b,⇑ , Ting Ren a , Alex Remennikov a a School of Civil, Mining and Environmental Engineering, Faculty of Engineering and Information Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia b School of Geology and Mining Engineering, Xinjiang University, Urumqi, Xinjiang 830000, China highlights  Coal rejects and cementitious grout are combined to develop a novel backfill material.  FRP tubular standing support (FTSS) is developed for underground mines.  Ultra-high molecular weight polyethylene (UHMWPE) FRP jacket is wrapped on GFRP tube.  Both the strength and ductility of infill material in FTSS have been enhanced.  FRP-confined concrete model can predict the behaviour of confined infill in FTSS. article info Article history: Received 28 June 2020 Received in revised form 18 September 2020 Accepted 29 September 2020 Available online xxxx Keywords: UHMWPE FRP Coal rejects Confinement Underground mines Standing support abstract This paper introduces a novel standing support incorporating fibre-reinforced polymer (FRP), termed FRP tubular standing support (FTSS). The exterior container of FTSS is the small rupture strain (SRS) GFRP tube wrapped with ultra-high molecular weight polyethylene (UHMWPE) FRP jacket with large rupture strain (LRS), while the infill is the coal rejects based backfill material (CBM). The CBM is made of industrial by- product (i.e. coal rejects) and high flowable cementitious grout material, leading to some environmental and economic benefits when it is used in underground mines. Test variables included the type of coal rejects (i.e. screened and unscreened), the thickness of SRS FRP tube (i.e. 1.5 mm and 3.0 mm) as well as the thickness of LRS FRP jacket (i.e. 2-ply, 3-ply and 4-ply). Test results showed that both the load car- rying capacity and deformation ability of CBM in FTSS have been significantly enhanced attributed to the confinement provided by exterior FRP container. Existing stress–strain model for FRP-confined normal concrete can be used to reasonably predict the compressive behaviour of confined CBM in FTSS with large deformation ability. The ductile FTSS is suitable for underground spaces and infrastructures where the integrity of structure is the main concern. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Fibre-reinforced polymer (FRP), being an emerging material , has drawn much attention during the past decades in structural and civil engineering [1,2]. Due to its high strength-to-weight ratio and excellent corrosion resistance, FRP composites have been introduced to different areas with the combined utilization of steel [3,4], concrete [5,6], timber [7], aluminium [8], ice block [9], stone or masonry [10], and steel–concrete structures [11–13]. Among them, FRP-confined concrete columns are very popular either in the research or practical applications, in which the FRP tube will not only provide confinement to concrete core but also act as an on-site mould [14]. It has been well noted that both the strength and ductility of concrete will be enhanced with the confinement provided by FRP tube/jacket, if the type [15], thickness [16], wrap- ping method of FRP composites [17], the compressive strength of concrete [18] as well as the cross section [19–21] of the composite structures are well designed. Different from conventional constructions and infrastructures where both the stiffness and ductility are the main concerns, in some specific areas, the large deformation ability of the structure is critically important [22]. A good example is the standing support used in underground mines. When the rock mass is manually caved (Fig. 1a), it is impossible and not necessary to prevent the deformation of surrounding rock. By the contrast, the controllable https://doi.org/10.1016/j.conbuildmat.2020.121171 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: hz449@uowmail.edu.au (H. Zhao). Construction and Building Materials xxx (xxxx) xxx Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Please cite this article as: H. Zhao, T. Ren and A. Remennikov, Behaviour of FRP-confined coal rejects based backfill material under compression, Construc- tion and Building Materials, https://doi.org/10.1016/j.conbuildmat.2020.121171