An Innovative Tubular Standing Support Incorporating PVC and FRP Composites: Laboratory Tests

The importance of developing innovative support structure for underground mines has drawn much attention with the depletion of shallow resources. This paper presents a hybrid tubular standing support consisting of an exterior container made of the polyvinyl chloride (PVC) and fibre-reinforced polymer (FRP) while the infill is the coal rejects based backfill material. The FRP jacket featured with the high strength-to-weight ratio is attached on the exterior surface of the PVC tube with the large rupture strain to provide confinement to infilled backfill material. A series of laboratory tests were conducted on this FRP-PVC tubular standing support (FPTSS) with different FRP thickness. Meanwhile, the FRP tubular standing support and the PVC tubular standing support with the sole confining material were also prepared and tested. Tests results showed that the FPTSS specimen exhibited the typical strain hardening behaviour attributed to the effective confinement provided by the combined container. It is also indicated that the enhancement of either the compressive strength or the axial deformation ability is closely related to FRP thickness. The other comparative advantage of FPTSS obtained from this research is its ductile post-peak behaviour after the rupture of the exterior FRP jacket, indicating the importance of using PVC tube in FPTSS. Except for its superior compressive behaviour, the cost effectiveness and ease of construction of FPTSS are also attractive from the design aspect.


Introduction
The integrity of rock surface for underground roadway is always the main concern for strata controlling engineers, the difficulty of which generally increases with the depletion of shallow coal resources (Ghorbani et al. 2020;Peng 2015;Qiao et al. 2015). With the increase of the mining depth, the surrounding rock of the roadways tends to plastic from elastic, resulting in the side effect on the primary support system (Kang 2014). Taking the bolting system for example, it is difficult to find out the stable area to be anchored in the plastic zone of surrounding rock. In addition, the underground water will also weaken the interface connection between the bolt and surrounding rock. As indicated by numerous practical applications, it is impossible to use the primary support alone to control the large deformation of the surrounding rock in deep mining (Kang 2014;Yu et al. 2020).
As a critical component of underground support systems, the significant importance of secondary standing support has therefore drawn much attention from coal operators (Yu et al. 2020;Tai et al. 2020). Different from the bolting system, the additional resistance provided by the secondary standing support makes a contribution to the integrity of surrounding rock. These secondary standing support systems include the timber chock, concrete crib, I-sectional steel beam, CanÒ support, pumpable standing support and other structural form (Kang et al. 2016;Shook et al. 2017;Huang et al. 2018;Li et al. 2018). Compared with their counterparts, these composite structures such as the CanÒ support and the pumpable standing support always obtain some superior performance attributed to the combination effect of different materials. Taking the CanÒ support for example, with the confinement provided by the exterior steel tube, the inner concrete core is under the tri-axial state. As a result, both the axial deformation and the compressive strength of the concrete core are significantly enhanced. Correspondingly, the inner buckling of the hollow steel tube can be prevented when it is used as an exterior container in the CanÒ support.
Although the CanÒ support is believed to be one of the most stable secondary standing supports in the market, there are still some drawbacks to be accounted for (Zhao (2019)). Among them, the heavy duty of the steel container is the main concern which may significantly affect the normal transportation and installation process. Due to the specific tensile behaviour of steel, the confining pressure provided by the exterior steel tube will be constant, once the buckling of the steel tube is occurred (see Fig. 1). As a result, the load carrying capacity of the CanÒ support will experience the unexpected decline, which is not allowed from the design aspect for underground mines (Yu et al. 2019).
Recently, the conceptual standing support incorporating fibre-reinforced polymer (FRP) composite and waste lump filled backfill (LFB) material has been proposed at the University of Wollongong (Yu et al. 2019;Zhao et al. 2021a). The main feature of this FRP tubular standing support (FTSS) is that the exterior container is made of FRP composite with high strength-to-weight ratio (Wang et al. 2020;Zeng et al. 2019), whereas the lump-filled backfill (LFB) material is made of coarse lumps (e.g., coal rejects) and high flowable cementitious grout material. The infilled LFB material can be prepared on-site and thus only the light weight FRP tube should be transported. As demonstrated by the preliminary tests, the FTSS exhibits the superior compressive behaviour featured with the typical strain hardening response, compared with its market available counterparts including the CanÒ support. However, the load carrying capacity will be totally lost once the limited tensile rupture strain of FRP container is reached, which will result in the potential roof fall in this case.
To further improve the post-peak behaviour of FTSS, this paper presents an innovative standing support incorporating the polyvinyl chloride (PVC) and FRP composite. For ease of reference, this modified standing support is termed as FRP-PVC tubular standing support (FPTSS) hereafter. As depicted in Fig. 2, the FRP jacket was fully wrapped on the surface of the PVC tube to develop the exterior container. The main aim of using additional PVC tube is to maintain the structural integrity after the rupture of the exterior FRP jacket (Zhao et al. 2021b). It is also expected that the additional application of the inner PVC tube will somehow increase the axial load resistance. It is apparent in Fig. 2 that the FPTSS is also the modification of the PVC tubular standing support (PTSS) with the additional application of the FRP jacket. The simple change of the exterior container will not significantly increase the weight of PTSS specimen with the consideration of the high strength-to-weight ratio of the FRP composite.
When the combined FRP jacket and PVC tube in FPTSS is regarded as the sole material, as the updated CanÒ support, its light weight is believed to be most attractive. Except for the simple change of the exterior container, the infill material of this hybrid support will also bring some attractive advantages. As stated earlier, the direct use of coal rejects as the coarse lump in developing the backfill material with the simplified manufacture process earn its reputation. With the development of the coal rejects separation technique, the coal rejects can be easily obtained from underground space and thus only the cementitious grout material is requested (Zhang et al. 2019). Note that the coal reject is the industry by-product, the FPTSS is thus cost-effective and environment friendly.
In order to obtain an in-depth understanding on the mechanical behaviour of the FPTSS, a series of compression tests were conducted on FPTSS with different FRP thickness. In parallel, both the FTSS and PTSS were prepared and tested in this research. Both the failure mode and the confining action of the FRP-PVC container on infilled backfill material are discussed in this research. Some other advantages obtained by FPTSS will be highlighted with the further comparison between FTSS and PTSS.

Tested Specimens
As listed in Table 1, a total of 13 tubular specimens have been prepared and tested, including 3 LFB specimens without confinement, 2 PTSS specimens, 2 FTSS specimens and 6 FPTSS specimens. For these specimens with the exterior containers, there are two identical specimens prepared in each series. All specimens had an inner diameter of 100 mm and a height of 200 mm. The same batch LFB material was used to prepared all specimens listed in Table 1. The only difference between specimens in series 2 and series 3 is the type of the exterior container, the former of which is made of the 3-mm PVC tube while the latter of which is the 3-ply FRP jacket. These FPTSS specimens differentiate from each other in terms of the FRP thickness, in which 2-ply, 3-ply and 4-ply FRP jackets were wrapped on the surface of the PVC tube. For these specimens with the FRP jacket, the 100-mmlength overlapping zone was applied to prevent the unexpected debonding. For ease of reference, each specimen was given a name, starting with the short name of the columns. The followed number is used to represent the ply of the FRP jacket. The last Roman number is used to differentiate two or three nominally identical specimens from each other. Taking FPTSS-2-II for example, it is the second FPTSS specimen covered with a 2-ply FRP jacket.

Material Properties
Coal rejects used in this research was provided by the local coal mine located at the New South Wales in Australia, more detailed information about the mechanical and physical properties of which can be seen from previous research (Zhao et al. 2021c). Herein, the coal rejects with the nominal particle size of 10 mm were adopted to generate LFB material. It is apparent in Fig. 3 that the coal rejects share the relatively constant particle size distribution when the sieving curves of three bathes tests were compared in accordance with the standard (ASTM-C136/C136M 2014).
The cementitious grout material (CMT) was provided by Minova Australia. According to the technical data sheet provided by the supplier (Minova 2020), the main content of CMT grout material is the CSA cement and some additional components such as the gypsum and quick lime. Table 2 presents the mechanical properties of CMT grout material, which was also provided by the material supplier. As can be seen from Table 2, the largest water-to-powder ratio of CMT grout is 2.0, which can significantly reduce the total cost of infill material because the water is ease to be obtained from underground.

FRP and PVC Composite
Tensile tests on six FRP coupons and PVC coupons were conducted in accordance with ASTM-D3039/ D3039M (2017). The test results showed that the average tensile strength of FRP composite based on a nominal thickness of 0.17 mm per ply is 1129.8 MPa, PVC tube, FRP tube as well as the FRP-PVC tube were tested by the compression machine with the displacement control mode. The constant loading speed of 0.6 mm/min was adopted to drive the compression machine. For each type of hollow tube, two identical specimens were prepared. Note that the same length of 100 mm was adopted for all tested specimens. The peak loads of these hollow tubes are listed in Table 1 for comparison. As expected, the axial load carrying capacities of these combined tube made of PVC and FRP increase with the layer of the FRP jacket.

Preparation of Specimens
The preparation of FPTSS specimens consists of the flowing steps: (1) fixing the hollow PVC tube on the water-proof wooden base (Fig. 4a); (2) filling the fixed hollow PVC tube with coal rejects (Fig. 4b); (3) mixing the CMT grout material with requested water and then dumping the mixed slurry into the lumped coal rejects (Fig. 4c); (4) cutting E-glass fibre sheet with requested length (Fig. 4d); and (5) embedding the fibre sheet into the epoxy resin to generate the FRP jacket and then wrapping the FRP jacket on the surface of the PVC tube (Fig. 4e). Note that only steps (1)-(3) are requested for preparation of LFB specimens. After three days curing in the laboratory condition, the exterior PVC container was removed and the LFB specimen was obtained. For FTSS specimen, the FRP jacket was wrapped on the surface of the LFB specimen by the wet-layup procedure as shown in steps (4) and (5). Note that the 100-mm-length overlapping zone of FRP jacket was left to prevent the debonding of FPTSS specimens and FTSS specimens.

Test set-up and Instruments
All specimens were tested on the 500-tones compression machines with the displacement control. Except for LFB specimens which was tested with the speed of 0.6 mm/min, the same displacement rate of 1.5 mm/ min was adopted for other specimens with the confining container in accordance with AS 1012.9 (2014). As shown in Fig. 5, two linear variable displacement transducers (LVDTs) were placed at the opposite corners on the bottom loading plate to measure the overall axial shortening of all specimens. The Cannon camera was set up in front of the specimens to record the progressive failure process of tested specimens. All data including the axial load and the axial displacements were recorded by a data logger simultaneously.

Failure Modes
It is apparent that in Fig. 6 that these three LFB specimens are all with the typical shear failure modes. In detail, only limited fresh failure surface observed from the coal rejects. A large amount of obvious separation was found from the interface between the coal rejects and the cementitious grout material, indicating that the interface is the weakest zone for LFB material. This observation agrees well with the findings presented in the previous research (Zhao et al. 2021c).
Different from LFB specimens without any confinement, all FPTSS specimens failed by the rupture of exterior FRP composite under hoop tension (see  123 confinement provided by the FRP jackets, the axial load experiences the sharply decline, which can also be found from the axial load-axial shortening curves shown in Fig. 7. During this period, the inner PVC tube experience the outward buckling. For FTSS specimen presented in Fig. 8, the exterior FRP tube in FTSS specimen experienced a rupture under tension associated with a sudden load shedding event after the bilinear increasing. Whereas, the serious buckling was observed from the exterior PVC tube of PTSS specimens shown in Fig. 9.

Axial Load-Axial Shortening Behaviour
The key results of all tested specimens are listed in Table 1, in which the axial loads of LFB specimens and exterior container averaged from two or three identical specimens were termed as P infill and P exterior , respectively. The peak axial load of FPTSS specimen, PTSS specimen and FTSS specimens was denoted as P total . The axial shortening corresponding to the peak axial load was denoted as S total . P infill ? P exterior represents the ultimate load of constitute material regardless of the possible difference attributed to the combined effect of different materials. Then, the value of (P total -P exterior )/(P infill ) was adopted to represent the enhancement of load carrying capacity of LFB material with confinement. The axial shortening of LFB specimens corresponding to the peak load P infill is denoted as S infill, and S total /S infill presented in Table 1 Fig. 7 Axial load-axial shortening curves of FPTSS specimens The axial load-axial shortening curves of unconfined LFB specimens are plotted in Fig. 6, in which the axial shortening was averaged from two LVDT installed on the loading platens of the compression machine. The constant axial load-axial shortening curves of LFB specimens obtained from three identical specimens indicate that the preparation of LFB specimens with inhomogeneous coal rejects is reliable and reasonable. As mentioned earlier, the axial load carrying capacity of LFB specimen experiences the sharply decline due to the weak interface between the coal reject and cementitious grout material. Once the peak load was reached, as depicted in Fig. 6, the integrity of the LFB specimen will be significantly affected. It is thus believed that the LFB cannot be directly used to develop a secondary standing support.
Figures 7 present the axial load-axial shortening curves of FPTSS specimens covered with the FRP jacket with different thickness. Compared with these unconfined LFB specimens, all FPTSS specimens show the large deformation ability, not only in terms of the axial shortening at the peak axial load, but also the ultimate axial shortening ([ 60 mm, equivalent to 30% of the overall height of the specimen).
As shown in Fig. 7, the axial load-axial shortening curves of FPTSS specimens can be divided into three portions: (1) ascending part with a bilinear shape; (2) sudden load shedding part with a linear shape; and (3) slight ascending part with minor fluctuations. The transition point between the first portion and the second portion corresponds to the rupture of FRP composite which were observed from the testing procedure. It is not difficult to explain the specific behaviour of FPTSS specimens under compression. As mentioned above, the FRP-PVC container will not only act as the container for ease of construction but also provide confining pressure to infill material. At the beginning of the test, the axial load of FPTSS was initially resisted by the infill material and the exterior FRP-PVC tube. With the increasing lateral expansion of infill material, the confining pressure provided by the FRP-PVC container leads to the ascending prat of the axial load-axial shortening curves. This process will last for a long time until the rupture of the exterior FRP jacket associated with the sudden load shedding. Then, the confining pressure acting on the infill material is mainly from the PVC tube and the residual FRP jacket apart from the ruptured zone. It is apparent in Fig. 7 and Table 1 that the compressive behaviour of FPTSS is closely related to FRP thickness.

Comparison Between FPTSS and FTSS Specimens
As can be seen from the preparation procedure of the FPTSS specimen, the only difference between FPTSS specimen and FTSS specimen is the former of which contains the additional PVC tube. Therefore, it is believed that the difference between the compressive behaviour of these specimens is mainly attributed to the existence of the PVC tube. To verify the above conclusion, the axial load-axial shortening curves of these specimens are depicted in Fig. 8, in which all these specimens had a 3-ply FRP jacket. In addition, the plain LFB specimens without confinement was also plotted in Fig. 8 for comparison. It is apparent that the axial peak loads of FPTSS specimens are higher than that of FTSS specimens, mainly attributed to the additional use of PVC tube in resisting somewhat axial load at the initial stage. As listed in Table 1, the sum of the axial load resisted by the hollow PVC tube and the FTSS is approximately equal to the axial load of FPTSS, which is the other evidence to support above Assumption.
The other attractive observation obtained from the above comparison is the axial deformation ability of the FPTSS specimen is much superior than its counterparts. As shown in Fig. 8, the average axial Fig. 9 Comparison between FPTSS and PTSS specimens: axial load-axial shortening curves load of FPTSS specimens is about 125 kN (equal to 50% of the peak load) corresponding to the large axial deformation (i.e. 80 mm). However, the axial deformation of FTSS is only 40 mm when the axial load reduced to 50% of the peak load. This comparative advantage indicates that the effect of the PVC tube is not only increase the axial load carrying capacity but also enhance the axial deformation ability.

Comparison Between FPTSS and PTSS Specimens
The axial load-axial shortening curves of FPTSS specimens and PTSS specimens are plotted together in Fig. 9 to evaluate the effect of FRP jacket on the compressive behaviour of FPTSS specimens. As mentioned above, the only difference between the FPTSS specimens and PTSS specimens herein is the additional FRP jackets. It is thus feasible to obtain the understanding of the contribution of the FRP jacket. It is evident that the peak axial loads of FPTSS specimens are larger than that of corresponding PTSS specimens. As can be seen from Table 1, the peak axial loads of FPTSS specimens are still higher than that of PTSS specimens when the axial load resisted by FRP composite was excluded. This observation suggests that the enhancement of load carrying capacity of FPTSS specimens is mainly attributed to the confining action of exterior FRP jacket. Different from PVC, the linear tensile behaviour of FRP jacket will result in the increasing confining action on the infill material and the infill material will be under the tri-axial state. If the confining pressure acted on the infill material is large enough, the axial load-axial shortening curve of confined material will be thus in the manner of strain hardening. Because the tensile rupture strain of FRP composite is generally smaller than that of these PVC tube, the axial load of FPTSS specimen will not sustain a high value for a very long time. Meanwhile, the axial load carrying capacity of FPTSS specimen will not decline to an extremely low level after the occurrence of the FRP rupture due to the existence of the ductile PVC tube. Note that the residual FRP jacket apart from the rupture zone will also provide somewhat confining pressure, the contribution of which may be the reason for the slightly increasing axial load of FPTSS specimen after the rupture of the FRP jacket.
Different from the obvious enhancement of the load carrying capacity, the effect of the utilization of FRP jacket in enhancing the deformation ability is limited. As seen from Fig. 9, there is no significant enlargement of the axial shortening when these two types of structural forms are compared. Because that the rupture strain of FRP jacket is much smaller than that of PVC tube in tension, the large deformation ability of FPTSS specimens is mainly attributed to PVC tube rather than the exterior FRP jacket.

Conclusions
This paper presents a novel tubular standing support termed FPTSS for underground mines. The main feature of FPTSS is its exterior container which is the combination of polyvinyl chloride (PVC) with large tensile rupture strain and the fibre-reinforced polymer (FRP) with high strength-to-weight ratio, whereas the infill is made of coal rejects and high flowable cementitious grout material. Compared to its counterparts, FPTSS is believed to be cost effective and environment friendly. To obtained an in-depth understanding of this composite structure, a series compression tests were conducted. The following conclusions can be drawn based on the discussions of the experimental tests presented in this research: 1. Different from the unconfined LFB material, the combination of three components (i.e. PVC, FRP and LFB) in FPTSS leads to the significantly enhancement of the load carrying capacity and the deformation ability; 2. The FPTSS incorporating FRP-PVC container obtained a superior compressive performance: an approximate bilinear axial load-axial shortening curves characteristics with a strain hardening behaviour, followed by a slight ascending portion after the load shedding event; 3. The main effect of the PVC tube in FPTSS is to sustain the integrity of the structural form after the rupture of FRP composite, while the FRP jacket makes a significant contribution to the enhancement of the load carrying capacity.
Note that the main aim of this paper is to obtain a first insight on the behaviour of FPTSS column under uniaxial compression. Only limited number of smallscale specimens were prepared and tested. From the design aspect, the systematic research covering the large range of critical parameters such as the type of FRP composite, the thickness of PVC tube as well as the sources of coal rejects, etc. should be well investigated before the trial tests in practical application. Moreover, the full-scale tests on FPTSS columns with the large rupture strain FRP jacket under different loading states should also be conducted.