Hybrid effect of fibre mesh and short fibres on the biaxial bending behaviour of TRC

1 Hybrid effect of fiber mesh and short fibers on the biaxial bending behavior of TRC Dong LIa, Yining DINGa,, Qing WANGa, Yongchao ZHANGa, Cecilia AZEVEDOb, Yulin ZHANGb a State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China, bCentre of Mathematics, University of Minho, Braga 4700-052, Portugal Abstract: The textile mesh reinforced concrete/mortar (TRC/M) has been studied in recent years. However, the current testing methods are focused on simply supported member under uniaxial bending, which are inadequate for analyzing of the biaxial tensioned fiber mesh and also incapable to reveal the biaxial behavior of the TRC panel. Besides, the fibers can be damaged by the alkali ambient of concrete. In order to overcome the inadequacy, a series experiment of two-way panels is carried out. The methodology used here consists of the experiment and the analysis of the experimental data, including the evaluation of alkali resistance, the biaxial bending capacity and toughness of two-way slabs. The suitable fibers are selected based on alkali resistance, and the effect of fiber meshes on the biaxial bending capacity of the two-way TRC slabs is studied. Through addressing the disadvantages of brittle fiber mesh reinforced TRC compared to conventional reinforced concrete panel, a significant increment of the ductility of TRC panel with steel fibers is achieved. Following the analysis of the experimental data, it can be concluded that the conventional steel mesh with reinforcement ratio of 0.2% can be replaced by the combined use of glass/basalt fiber mesh and steel fibers. Keywords: composite materials; reinforcement; durability-related properties; slabs & plates; load & loading; toughness  Corresponding author. Tel.: +86 411 84709756. E-mail address: [email protected] (Y. Ding). 2 Notation TRC textile reinforced concrete TRM textile reinforced mortar PC plain concrete RC conventional steel mesh reinforced concrete BTRC textile reinforced concrete with basalt fiber HSBTRC hybrid use of 50kg/m3 macro steel fiber and basalt fiber mesh reinforced concrete GTRC textile reinforced concrete with AR glass fiber F1 first-peak load HSGTRC hybrid use of 50kg/m3 macro steel fiber and AR glass fiber mesh reinforced concrete F2 second-peak load Fu ultimate load l span length F500 load at deflection of l/500 Q500 energy absorption in deflection from 0 to l/500 F250 load at deflection of l/250 Q250 energy absorption in deflection from 0 to l/250 F100 load at deflection of l/100 Q100 energy absorption in deflection from 0 to l/100 F50 load at deflection of l/50 Q50 energy absorption in deflection from 0 to l/50 1. Introduction Series of previous investigations have reported that the mechanical properties of the concrete can be increased noticeably by the addition of discrete short fibers such as steel fiber, basalt fiber and plastic fiber etc (Alhozaimy et al., 1996; Ding et al., 2014; Enfedaque et al., 2010; Jain and Singh, 2013; Lee et al., 2016; Li and Xu, 2009; Lim et al., 2011; Ros et al., 2016; Sim et al., 2005; Singh, 2016; Song et al., 2004; Wang et al., 2009; You et al., 2017; Zollo, 1997). Due to the high loadbearing capacities, the performance of the textile mesh reinforced concrete or mortar (TRC/M) has been much concerned by the investigators in recent years (Bernat et al., 2013; Chira et al., 2016; Gopinath et al., 2016; Larrinaga et al., 2014; Papanicolaou et al., 2007; Shams et al., 2014a; Shams et al., 2014b; Triantafillou et al., 2006; Zargaran et al., 2017). Shams et al. (2014a; 2014b) evaluated the flexural behavior of two sides simply supported TRC panels, and presented an analytical model to calculate the bending behavior of the one-way TRC slab. Chira et al. (2016) studied the flexural properties of a newly developed TRC façade panel, and Zargaran et al. (2017) verified the effect of the parameters of the fiber mesh, the reinforcement ratio and the thickness of the specimen on the 3 bending performance of TRC composites. But, the above mentioned investigations (Chira et al., 2016; Shams et al., 2014a; Shams et al., 2014b; Zargaran et al., 2017) was focused on the uni-axial flexural properties of TRC member using static determinate testing system (such as two sides simply supported beam or panel). Bernat et al. (2013) investigated the performance of the masonry wall strengthened by the TRM under eccentric compressive loading. Larrinaga et al. (2014) verified the uni-axial tensile behavior of the TRM. However, the strong biaxial load-bearing capacity of fiber mesh is still experimentally poor explored. In addition, it is well known that glass fiber, basalt fiber and concrete are all brittle materials. Hence, the incorporation of brittle fiber mesh into brittle concrete/mortar may enhance the strength of TRC/M, but it is incapable to reduce brittleness of the TRC/M member and to prevent catastrophic brittle failure by several mechanisms. Compared to the conventional steel mesh reinforcement, the basalt/glass fiber mesh may show strong corrosion resistance and excellent biaxial tensile strength. The TRC structural elements can be used as facades (see Fig.1), roofs, panels, permanent formwork elements, tanks and precast elements, like manhole covers for the drainage and cover plate for the urban underground pipelines (Brameshuber, 2006). The concrete matrix shows a strong alkaline ambient, the pH value of the concrete can be over 12 (Mehta, 2006). The prerequisite for the constructive application of various fiber mesh in the infrastructure consists mainly of the following three points: 1) alkaline resistance of fiber mesh in the concrete matrix; 2) the positive effect of fiber mesh on the mechanical properties of concrete, especially on the biaxial tension behavior, and 3) the similar toughness and ductile failure pattern of TRC member compared to conventional steel mesh reinforced RC panel under bending. Fig.1 TRC facades 4 In this study, the accelerated alkali resistance test was adopted to verify the chemical corrosion of the medium-alkali glass fiber (C-glass fiber), alkali resistance glass fiber (AR glass fiber) and the basalt fiber. Although some similar investigations have been made in the previous studies (Liu et al., 2013; Purnell et al., 2000; Rybin et al., 2013; Wei et al., 2010), but our focus was given on selecting the suitable fiber mesh to be used as the reinforcement of the cementitious materials. The two-way slab test was used to evaluate the bi-axial flexural property of the TRC, the test involves a 600×600×100 mm plate, simply supported on four sides by a rigid metallic frame, and the centre point load applied through a contact surface of 100 x 100 mm (European Federation of Producers and Applications of Specialist Products for Structures, 1996). Additionally, in order to enhance the toughness of the TRC and to replace the conventional steel mesh, the macro steel fiber was adopted, the hybrid effect of the combined use of fiber mesh and macro steel fiber was investigated. 2. Experiments 2.1 Materials The basic mix proportion was illustrated in Table 1. The cement was ordinary Portland cement (P·O 42.5R); the fine aggregates were quartz sand with the particle size 0-2 mm. The 28 d compression strength of the fine grained concrete was 51.2MPa. Fig.2 demonstrated the basalt fiber used in the test, the fiber mesh (Fig.2 (b)) was manufactured by the corresponding fiber filaments (Fig.2 (a)). The fiber filaments were used for the alkali resistance, and the fiber meshes were used in the concrete slab as reinforcement. There were two types of glass fibers: C-glass fiber (ZrO2 content 5 0%) and AR glass fiber (ZrO2 content 16.7%) used for the alkali resistance test. The 65/35BG macro steel fiber (fiber length 35 mm, aspect ratio 65, tensile strength 1345 N/mm2) was used as shown in Fig.3. The steel mesh was made by the hot-rolled plain bars with the diameter of 6.5 mm, the mesh size was 150 × 150 mm and the reinforcement ratio was 0.2%. This ratio is often used for members like roofs, facades, floors and manhole cover, etc. Table 1 Mix proportion of the fine grained concrete Cement (kg/m3) Water (kg/m3) Sand(kg/m3) Super plasticizer (kg/m3) 0-1 mm 1-2 mm 600 210 360 840 7.2 Fig.2 Basalt fibers: (a) basalt fiber filament; (b) basalt fiber mesh Fig.3 Macro steel fiber 2.2 Alkali resistance test 6 The concrete matrix shows a strong alkaline ambient, the pH value of the concrete ranges from 12.5 to 13.5 (Mehta, 2006). The accelerated alkali resistance test was carried out based on Ref. (Hou et al., 2007). The fiber filaments were treated in 5% NaOH solution with the constant temperature of 80℃, and the sustained time varied from 0 to 24 h. After the alkaline treatment, the samples were washed by distilled water and dried. The scanning electron microscope (SEM) was adopted to characterize the surface morphologies of the original fibers and those after the alkaline treatment of 24 h in NaOH solution. The energy dispersive spectroscopy (EDS) was introduced to analyze the significant chemical modification, especially the change of Si element of the fiber filaments. 2.3 Two-way slab test In order to evaluate the bi-axial bending behavior and the energy absorption of fiber mesh reinforced concrete, the two-way slab test recommended by the EFNARC was adopted (European Federation of Producers and Applications of Specialist Products for Structures, 1996). Similar to the steel mesh, the fiber mesh also showed strong bi-axial tension behavior. The two-way slab was considered to sufficiently reflect the influence of fiber mesh on the mechanical property of cementitious material, especially the fiber mesh effect on the bi-axial bending behavior of mortar/concrete slab. In this experiment, the dimension of the slabs was 600×600×100 mm, the fiber meshes were arranged at 30 mm from the bottom of the slab. After demoulding, the slabs were placed in curing room immediately before testing. A rigid metallic frame was introduced to simply support the slab on its four edges. A central load was applied through a 100×100 mm precast stiffness bearer (see Fig.4). The deformation rate of the central point was about 1.5 mm/min. Fig.4 Set-up for bending slab test 7 Compared to the conventional two sides simply supported beam test, there are some advantages of four edges simply supported slab as follows: 1) the effect of fiber mesh on the bi-axial bending capacity of the two-way TRC slabs can be better analyzed; 2) the four edges simply supported TRC slab is a three times statically indeterminate structure and allows both inner force redistribution and stress redistribution, hence the loading behavior and failure pattern of TRC related to the inner force redistribution could be evaluated more exactly, whereas the two sides simply supported conventional beam/panel is a static determinate structure, only stress redistribution occurs and no inner force redistribution can take place during the loading process. 3 Test results and discussions 3.1 Alkaline resistance The prerequisite for the investigation of the mechanical behavior and practical application is the alkaline resistance of the fiber mesh. In order to choose the suitable fiber mesh for the further study on the load bearing capacity of TRC slab, the different filaments are treated in the alkali solution for 24 hours. After the accelerated testing of the alkaline resistance, the corrosion-damaged surfaces of C-glass fiber, AR glass fiber and basalt fiber were compared with the corresponding original fibers without alkali treatment and analyzed using SEM. Fig. 5, Fig. 6 and Fig. 7 show the comparison of surface analysis of different fibers before and after alkali treatment in the 2 mol/L of NaOH solution. Fig.5 SEM analysis of the C-glass fiber surface: (a) original fiber, (b) after 24 h alkaline treatment 8 The C-glass fiber has relatively smooth surface relief before alkali attack. Small single surface defects can be found under large magnification (10000 times) as shown in Fig. 5(a). The strong corrosive damage on the surface of the fibers can be observed (see Fig. 5(b)), and for samples immersed into alkali solution for 24 hours, the corrosion shell is peeled off, and the effective diameter of the fiber is reduced clearly. So, such kind of C-glass fiber is not suitable as reinforcement for cement/concrete member, hence it is not selected to the further investigation on the mechanical property of TRM member. Fig.6 SEM analysis of the AR glass fiber surface: (a) original fiber, (b) after 24 h alkaline treatment The AR glass fiber has very smooth surface relief even under large magnification (10000 times) as shown in Fig.6 (a). The obviously corrosive damage is not observed on the surface of the fiber after 24h alkaline treatment, as shown in Fig.6 (b). 9 Fig.7 SEM analysis of the basalt fiber surface: (a) original basalt fiber, (b) basalt fiber after 24 h alkaline treatment Similar to the C-glass fiber, small single surface defects can also be detected on the basalt fiber (Fig.7 (a)). After 24 h alkaline treatment, only pit areas on the surface of the fiber can be observed (see Fig.7 (b)), and the effective diameter of the fiber will not be affected by the pit areas obviously. From Figs.5-7, it can be seen that:  Before the alkaline treatment, the surface of the C-glass fiber, the AR glass fiber and the basalt fiber are very smooth, although some points of the fiber display surface defect which may probably be caused by the abrasion during the manufacturing (Fig.5 (a) and Fig.7 (a)).  After 24 h alkaline treatment, the formation of a brittle shell was found on the C-glass fiber and basalt fiber filament surface (see Fig. 5 (b) and Fig. 7 (b)). This shell was formed with a certain thickness around the filament and was partially peeled off in some areas. Although the chemical composition of the fibers is different, both fibers react in the similar way. The underlying fiber surface appeared smooth and uncorroded. This was in agreement with observations made by other investigators (Liu et al., 2013; Rybin et al., 2013; Wei et al., 2010), while the surface of the AR glass fiber is still very smooth, and demonstrates good alkali resistance (Fig.6 (b)). The changes in the composition of the different fibers were investigated by EDS analysis. Integrity and representativeness of the morphological features of the corrosion shell for each sample were taken into consideration in order to choose the suitable filaments for further investigation on the 16 HSBTRC slab increases by about 366%.The ability of plastic redistribution of inner force is enhanced strongly by the hybrid use of basalt fiber mesh and steel fibers. iii) The F500 of the HSBTRC slab and the RC slab are 59.0kN and 52.8kN, respectively. Compared to the RC slab, the F500 of the HSBTRC slab increases by about 12%. iv) The F250 of the HSBTRC slab and the RC slab are 73.1kN and 54.8kN, respectively. Compared to the RC slab, F250 of the HSBTRC slab increases by about 33%. v) The F100 of the HSBTRC slab and the RC slab are 65.5kN and 59.5kN, respectively. Compared to the RC slab, F100 of the HSBTRC slab increases by about 10%. vi) The F50 of the HSBTRC slab and the RC slab are 38.5kN and 53.1kN, respectively. Compared to the RC slab, F50 of the HSBTRC slab decreases by about 27%. vii) The load-deflection curve of the HSBTRC slab is higher than that of the conventional RC slab over the deflection from 0 to 5 mm. From the discussion above, it can be summarized that the combined use of basalt fiber mesh and macro steel fiber can improve the flexural load bearing capacity, the toughness of the TRC slab and the ability of plastic redistribution of inner force significantly. Fig.13 shows the comparison of the load–deflection curves of hybrid use of 50 kg/m3 steel fiber and AR glass fiber mesh reinforced concrete (HSGTRC) slab, RC slab and GTRC slab. The comparison of the load bearing capacity of GTRC, RC and HSGTRC slabs are shown in Table 5. Fig.13 Comparison of load-deflection curves of GTRC, RC and HSGTRC slabs 17 Table 5 Comparison of the load bearing capacity Types Fu/kN δu/mm Load at different deflection /kN F500 F250 F100 F50 GTRC 45.8 0.4 24.6 23.0 3.3 2.9 RC 66.9 9.7 52.8 54.8 59.5 53.1 HSGTRC 88.0 2.1 69.6 86.6 61.3 37.0 From Fig.13 and Table 5, it can be seen that: i) The ultimate load Fu of HSGTRC slab is 88.0kN. Compared to the RC slab, Fu of the HSGTRC slab increases by about 32%. ii) The deflection of the GTRC slab and the HSGTRC slab corresponding to Fu are 0.4 mm and 2.1 mm, respectively. Compared to the GTRC slab, the δu of the HSGTRC slab increases by about 425%. iii) The F500 of the HSGTRC slab and the RC slab is 69.6kN and 52.8kN, respectively. Compared to the RC slab, F500 of the HSGTRC slab increases by about 32%. iv) The F250 of the HSGTRC slab is 86.6kN. Compared to the RC slab, the F250 of the HSGTRC slab increases by about 58%. v) The F100 of the HSGTRC slab is 61.3kN. Compared to the RC slab, the F100 of the HSGTRC slab increases by about 3%. vi) The F50 of the HSGTRC slab and the RC slab are 37.0kN and 53.1kN respectively. Compared to the RC slab, the F50 of the HSGTRC slab decreases by about 30%. vii) The load-deflection curves of the HSGTRC slab is above the conventional RC slab over the deflection from 0 to 5 mm. From the discussion above, it can be concluded that the combined use of AR glass fiber mesh and 50kg/m3macro steel fiber can increase the post-peak load bearing capacity, the ductility and the ability of plastic redistribution of inner force of the TRC slab significantly. According to Chinese guideline (National Standard of the People’s Republic of China, 2010), the allowable deflection of the slab is l/200 (l is the span length) in the serviceability stage, the load-bearing capacity of the 18 HSGTRC slab is higher than the RC slab even at the deflection of l/100 (corresponding to F100), therefore, the hybrid use of the 50kg/m3 steel fiber and AR glass fiber mesh is suitable for replacing of the conventional steel mesh with steel ratio of 0.2%. One of the disadvantages of glass/basalt fiber mesh and TRC members without steel fibers is the brittleness and the poor energy absorption capacity after peak-load. The absorbed energy is an effective parameter to evaluate the effect of additional steel fibers as well as the hybrid effect of fiber mesh and steel fibers on the toughness of the two-way slab (Bernard, 2002;Cengiz and Turanli, 2004; Ding and Kusterle, 1999). The absorbed energy of different slabs is found by integrating the area under load-deflection curves, and can be calculated by Eqn.1 (European Federation of Producers and Applications of Specialist Products for Structures, 1996). The results of the energy absorption for different slabs can be seen in Table 6.   0 Q F x dx   (1) Where Q is the absorbed energy, J; δ is the central deflection of the slab, m; F(x) is the force corresponding to x, N, respectively. Table 6 Energy absorption of different slabs Types Energy absorption (J) Q500 Q250 Q100 Q50 PC 15.9 — — — BTRC 26.3 37.3 60.1 76.6 GTRC 27.8 53.3 71.7 84.3 RC 34.6 89.0 201.4 576.0 HSBTRC 41.3 108.6 324.8 578.4 HSGTRC 40.3 122.0 343.4 582.0 Note: Qa (a=500, 250, 100, 50) means the energy absorption in the deflection from 0 to l/a, l is the span length. Fig.14 shows the comparison of the energy absorption of BTRC slab, RC slab and HSBTRC slab. Fig.14 Comparison of energy absorption of BTRC, RC and HSBTRC slabs 19 From Table 6 and Fig.14, it can be seen that: i) When the deflection reaches to 2 mm, the Q250 of the RC slab, HSBTRC slab and HSGTRC slab are 89.0 J, 108.6 J and 122.0 J, respectively. Compared to the RC slab, the Q250 of HSBTRC slab and HSGTRC slab increase by about 22% and 37%, respectively. ii) When the deflection reaches to 5 mm, the Q100 of the RC slab, HSBTRC slab and HSGTRC slab are 201.4 J, 324.8 J and 343.4 J, respectively. Compared to the RC slab, the Q100 of HSBTRC slab and HSGTRC slab increase by about 61% and 71%, respectively. iii) When the deflection reaches to 10 mm, the Q50 of the RC slab, HSBTRC slab and HSGTRC slab are 576.0 J, 578.4 J and 582.0 J, respectively. Compared to the RC slab, the Q50 of HSBTRC slab and HSGTRC slab increase slightly (about 1%). iv) The energy absorption up to a deflection of 10 mm of the HSBTRC and HSGTRC slabs are higher than that of the RC slab, as shown in Fig.14 for the comparison of the energy absorption of the RC and HSBTRC slab. v) Based on the testing results and the ductile failure pattern, the conventional steel mesh can be replaced by the hybrid use of glass/basalt fiber mesh and 50 kg/m3steel fibers. The energy absorption capacity of the TRC slab with macro steel fibers is improved strongly. This behavior can be attributed to the reinforcement mechanism of macro steel fiber. After cracking, the fibers spanning across the cracks can transmit tensile loads, and a large amount of energy can be 20 absorbed in the process of de-bonding, slipping and pulling out of fibers (Cengiz and Turanli, 2004; Ding and Kusterle, 1999). Fig.15 shows the comparison of the failure pattern and crack propagation at the bottom of the plain concrete slab and the basalt /glass fiber mesh reinforced concrete slab. Fig.15 Failure patterns: (a) PC slab; (b) GTRC slab; (c) Broken down of the glass fiber From Fig. 15 (a) and (b), it can be seen that the crack patterns of the PC slab and the fiber mesh reinforced concrete slab are similar. Due to the brittle properties of the basalt fiber and the AR glass fiber, the BTRC and GTRC slab still show clearly brittle behavior with small deflections. The fibers of the mesh across the cracks are mostly broken down, as shown in Fig.15 (c). Fig.16 shows the comparison of the failure pattern and crack propagation of the RC slab and the TRC slab with hybrid reinforcement of fiber mesh and macro steel fiber (50kg/m3). Fig.16 Failure patterns: (a) RC slab; (b) HSBTRC slab From Fig.16, it can be seen that: i) Compared to the failure pattern of the TRC slabs without steel fiber in Fig.15(b), lots of cracks in the RC slab and TRC slab with steel fibers are formed, developed and radiate 21 outward to the edges from the centrally loaded zone. One of the important reasons is that the combined use of fiber mesh and macro steel fiber may show strong positive hybrid effect on the plastic redistribution ability of inner force and stress during the loading process. ii) The crack widths are less and smaller than the slabs without steel fibers due mainly to the well distributed tensile stress. iii) The failure mode of the BTRC/GTRC slab changes from brittle pattern into a ductile one. It means that the addition of the macro steel fiber aids in converting the brittle properties of the concrete into a ductile composite member. 4. Conclusions Based on the experimental and analytical investigation, the following conclusions can be drawn: 1. The basalt fiber and AR glass fiber meshes demonstrate very good alkaline resistance and are selected to the further investigation on the mechanical property of TRC member. 2. For concrete with compressive strength about 51.0 MPa after 28d, compared to the PC slab, both the AR glass fiber mesh and basalt fiber mesh could enhance the bi-axial ultimate load of the two-way concrete slab by 49% and 58%, respectively, but the most brittle properties of the fiber mesh reinforced concrete remain unchanged. 3. Compared to the often used statically determinate beam or one-way panel test, the statically indeterminate two-way slab test is more suitable for investigation on the effect of fiber mesh on the bi-axial flexural property and ability of inner force redistribution of TRC slab. 4. The hybrid use of basalt/AR glass fiber mesh and macro steel fiber increase the loading bearing capacity and post-peak behavior as well as toughness of the TRC slab significantly. Compared to the BTRC slab, the energy absorption Q500, Q250, Q100, Q50 of the HSBTRC slab increase by 57%, 191%, 440% and 655%, respectively. 5. The addition of macro steel fiber can change the brittle failure pattern of the BTRC and 22 GTRC slab into the ductile ones; enhance the residual load bearing capacity as well as the energy absorption capacity of the slab over the whole post-peak region strongly. 6. The load-deflection curves of HSBTRC slab and HSGTRC slab are higher than those of the RC slab over the deflection from 0-5 mm. The conventional steel mesh with constructive steel ration (0.2%) can be replaced by the hybrid use of basalt /AR glass fiber mesh and steel fibers with fiber content of 50 kg/m3. The present study provides a method to improve the ductility of the TRC member. 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