Bending Properties of Short-Cut Basalt Fiber Shotcrete in Deep Soft Rock Roadway

To study the effect of short-cut basalt fiber (BF) on the bending and toughening properties of shotcrete, bending toughness tests of short-cut BF shotcrete slabs with different volume fractions were carried out. A transparent soil model and Scanning Electron Microscopy (SEM) were used to observe the distribution of BF with different volume fractions and analyze the toughness enhancement mechanism of basalt fiber shotcrete. ,e supporting effect of basalt fiber shotcrete with different volume fractions was verified by an underground engineering test. ,e test results show the following: (1) when the fiber content is within 3∼4.5kg/m, the distribution of BF in transparent soil is uniform, and it does not easily agglomerate, which is beneficial to improving the bending toughness of shotcrete. (2),e shotcrete slab with 4.5 kg/m fiber content had the best reinforcing effect. Compared with the control group, the peak load and absorption capacity increased by 56.67% and 636.96%, respectively, and the maximum crack width decreased by 32.10%. (3),e SEM analysis indicated that the basalt fibers distributed randomly and evenly in the concrete, which can form a three-dimensional spatial skeleton with a stable structure. Excessive fiber incorporation can increase fiber agglomeration during stirring and spraying. (4) ,e support results of an underground engineering test show that, in 35 days, short-cut basalt fiber shotcrete with 4.5 kg/m fiber content is better at restraining the surrounding rock than shotcrete with other fiber contents. BF has the properties of crack resistance, bridging, and toughening for shotcrete and can significantly improve the ability of shotcrete to restrain surrounding rock deformation. As a new type of fiber, BF has great significance in deep soft rock underground engineering.


Introduction
Surrounding rock stability control is one of the important research areas in the field of deep soft rock underground engineering, especially for practical problems such as difficult control of the surrounding rock stability and large deformation in the deep 800∼2000 m high-stress condition [1,2]. Traditional shotcrete has some shortcomings, such as small elongation, poor crack resistance, and high brittleness, which reduce the support effect and seriously endanger the stability of the surrounding rock structure and personal safety [3,4]. Basalt fiber (BF) is a new fiber material with a high inorganic environmentally friendly green performance. Compared with other fibers (glass fiber and steel fiber), basalt fiber has better corrosion resistance, a higher elastic modulus, better chemical stability, impact resistance, and fire resistance [5,6]. Basalt fiber was originally used in military and aerospace research programs [7,8] and is now widely used as a structural reinforcement material in buildings [9][10][11], bridges [12,13], highways [14,15], tunnels [16,17], and other fields. Basalt fibers have the functions of crack resistance, bridging, and toughening in concrete, which can significantly improve the fatigue resistance, postcracking toughness, and durability of the concrete [18][19][20].
In recent years, domestic and foreign scholars have carried out a large number of experimental studies on fiber reinforced concrete. Sim et al. [21] found that the toughness of basalt fiber reinforced concrete is 3∼5 times higher than that of ordinary concrete. Zhang et al. [22] suggested that basalt fibers could improve the toughness and crack resistance of concrete through experiments. Pickel et al. [23] tested the mechanical properties of basalt fiber reinforced concrete with different content. e basalt fibers were found to generally increase the tensile and flexural strength (modulus of rupture), and it was found on inspection that the fibers had ruptured upon macrocracking. Ayub et al. [24] suggested that the addition of basalt fibers significantly increased the tensile splitting strength and the flexural strength of HPFRC and slightly improved the compressive strength. Attia K et al. [25] found that basalt fiber can improve the bending resistance of the concrete sheet strips. Abed and Alhafiz [26] researched the effects of adding different types of fibers to the concrete mixtures on the flexural behaviour of concrete beams, which were reinforced longitudinally with BFRP bars. eir results showed that basalt fibers can improve the bending resistance of these beams. Wang et al. [27] found that the bonding between the fibers and concrete matrix is the main factor that improves the effect of the strengthening and toughening of fiber reinforced concrete. Iyer [28] conducted mechanical property tests on concrete with different lengths and quantities of basalt fiber and found that clumping of fibers at a high fiber content will cause mixing and casting problems. Li et al. [29] studied the effect of hybrid fibers on the flexural toughness of concrete square slabs and beams and used European and American codes to evaluate the flexural toughness characteristics. Li et al. [30] studied the early strength of steel fiber shotcrete slabs with different content according to the European EFNARC standard and found that steel fiber can significantly improve the impact resistance of early concrete. However, the research on the bending characteristics and strengthening and toughening of BF shotcrete is not perfect, so it is necessary to study the application of BF shotcrete in deep soft rock underground engineering.
At present, the research on the bending characteristics and strengthening and toughening effect of BF shotcrete is limited to the laboratory only, and the research results cannot be used in underground engineering. erefore, this paper studies the influence of short-cut BF shotcrete slabs with different volume fractions on the bending characteristics and toughness by using a test of underground shotcrete slabs. e distribution of fiber in the shotcrete and the strengthening and toughening mechanism are investigated and analyzed by means of a transparent soil model and SEM technology. Finally, a field test is used to systematically monitor the convergence deformation of different volumes of BF sprayed concrete, to verify the support effect.

Experimental Materials and Mix Proportion.
e experimental materials are as follows: PO 42.5R Portland cement produced by Qianye Cement Plant; the coarse aggregate is calcareous macadam with a mud content of 0.2 and maximum particle size of 10 mm; fine aggregate is manufactured sand with fineness modulus of 3 and particle size of 0∼3 mm; short-cut BF with 18 mm fiber, with parameters as shown in Table 1.
e accelerator is Xingde Brand accelerator, Wuan City, Hebei Province, and the water is tap water from Jiaozuo City. e specimen mix is shown in Table 2.

Fiber-Transparent Soil Model Experiment.
To prepare the fiber-transparent soil model, different sizes of quartz sand were mixed in the mixer for 30 seconds, adding different amounts of short-cut basalt fiber, stirring for 40 seconds, pouring the stirred mixture into a plexiglass box (100 × 100 × 100 mm), and then adding the refractive index 1.4585 calcium bromide solution prepared by using an abbe refractometer. e distribution of different fiber content in transparent soil was observed.
e fiber-transparent soil model was binarized by Image J software, and the fiber distribution was counted.

Bending Toughness Experiment of Shotcrete Slab.
According to the European EFNARC standard for making 600 × 600 × 100 mm shotcrete slab [31], specimens with fiber volume fractions of 0 kg/m 3 , 3 kg/m 3 , 4.5 kg/m 3 , and 6 kg/m 3 were made in the bottom drainage roadway of Zhaogu No.2 coal mine, respectively, while square slabs with fiber volume fractions of 0 kg/m 3 were used as the control group. ree parallel specimens were made for each group. e specimens were demoulded 24 hours later and kept underground for 28 days before being transported to the laboratory for testing. e flexural toughness test of the shotcrete slabs is divided into three steps.
Step one: place the square slab on the rigid frame, and place a 100 × 100 × 100 mm loading rigid block at the centre of the specimen.
Step two: apply load with a TAW-3000G electrohydraulic servo high rigid pressure testing machine, with a loading speed of 1.5 mm/min.
Step three: the CY050-2A deform meter (range 50 mm) and crack width gauge were used to monitor and record the load and deflection of the centre slab, the deformation of the centre bottom slab, and the crack width and extension crack width of the loading specimen. e flow of the square slab test is shown in Figure 1.

SEM Experiment.
For choosing failure specimens to conduct SEM, in order to ensure focused scanning, the test pieces were sliced and selected to be placed in a sample bag for keeping. e enclosed sample was taken out during the test and fixed on the sample stage with a conductive paste. e sample stage to which the sample is fixed is placed in an ion sputtering equipment for gold coating. en, the SEM instrument switch is turned on, the appropriate beam spot value is selected, and a SEM test is performed.

Distribution of BF in Transparent Soil Model with Different Contents.
e fiber-transparent soil model was divided into 9 cells, binary analysis was conducted by Image J software, and the visible fiber distribution in different cells was counted.
e distribution morphology of fiber with each content in the transparent soil model is shown in Figure 2, and the binarization image of the fiber distribution in the transparent soil is shown in Figure 3.
In order to quantitatively evaluate the dispersion of fiber in the transparent soil, the dispersion coefficient of fiber is proposed, and its expression is shown in the following formula: In the formula, T is the number of cells in the field of view, X i (i � 1∼9) is the number of fibrous roots in different cells, X a is the average number of fibrous roots in all cells in the entire image, and β is the dispersion coefficient of the fiber.
Define the dispersion coefficient of the weighted average value of the content of BF contribution value α j (α 0 � 1) and the contribution growth coefficient c j (j � 1∼5), where j corresponds to (1.5, 3, 4.5, 6, and 7.5 kg/m 3 ), five kinds of contents, respectively. Its expression is shown in the following formulas: V f is the BF content and β j is the BF dispersion coefficient. e dispersion of different content of BF in the transparent soil is shown in Table 3.
According to Table 3, the BF content has a great influence on the fiber distribution. When the content is 1.5 kg/m 3 , the fiber dispersion coefficient is 0.7 and the fiber contribution rate is 0.05. In the transparent soil model, the fiber distribution is random, with large gaps, and a uniform network structure cannot be formed. When the content is 3 kg/m 3 , the fiber dispersion coefficient is 0.76 and the fiber contribution rate is 1.171. e gaps between fibers are small, and multiple fibers are connected to form a stable spatial network structure. A few bubbles appear in the transparent soil model. When the fiber content is 4.5 kg/m 3 , the fiber dispersion coefficient is 0.52, and the fiber contribution rate is 0.026, which indicates fiber agglomeration. When the fiber content is 6.0 kg/m 3 , the fiber   dispersion coefficient is 0.46 and the fiber contribution rate is 0.179. e fiber distribution is uneven and bubbles in the model increase greatly. It can be seen that the proper amount of fiber can exert a gain effect, and the excess fiber is entangled and overlapped. When the fiber dispersion amount is 3∼4.5 kg/m 3 , the BF distribution uniformly forms a dense network structure, which is less prone to clumping and has a small overlap ratio.
is is beneficial to improving the bending toughness of the sprayed concrete. e load-deflection curves of simply supported BF shotcrete square slabs under concentrated loads can be divided into the elastic stage, stress redistribution stage, and unloading stage. e load-deflection curve of a square slab with 4.5 kg/m 3 content is shown in Figure 4.

Effect of BF on Bidirectional Reinforcement
Elastic stage (OA segment) [32]: with the increase of load, the two-way bending moment of the slab increases continuously. When the bending moment is less than the ultimate bending moment (M 1 ) of the square slab, there is no cracking in the square slab and it is approximately elastic. When the unidirectional bending moment reaches the maximum bending moment (the load reaches point A), a unidirectional microcrack appears at the bottom of the square slab.
Stress redistribution stage (ABCDE segment) [33]: this stage can be divided into an internal microstructure effect (AB) stage and a BF effect (BE) stage. AB segment: under the condition of external force, the defects such as microcrack and pore expansion within the square slab leads to a decrease of the bearing capacity. BE segment: with the increase of load, the bonding effect between the fiber and matrix delays the development of cracks and increases the bearing capacity.
Unloading stage (EFGH segment) [30]: after reaching the peak load of 54.11 kN, the crack develops into a main crack penetrating the matrix of the shotcrete square slab. e bridging fibers under the effect of tension are pulled out of the matrix of the shotcrete square slab, and the bearing capacity of the slab decreases to 46.27 kN (EF segment). Before the fiber reaches its ultimate tensile strength, its own strength enhances the bearing capacity of the concrete matrix, and the bearing capacity increases to 49.06 kN (FG segment). e fracture of the bridging fibers leads to a sharp decline in the bearing capacity of the square slab until it is destroyed (GH segment).

Comparative Analysis of Bending Properties of Square Slab with BF Content.
e results of the mechanical properties of the basalt fiber shotcrete square slabs are shown  in Table 4 and Figure 5 which indicate that the peak loads of shotcrete square slabs with fiber content of 0 kg/m 3 , 3 kg/m 3 , 4.5 kg/m 3 , and 6 kg/m 3 are 34.67 kN, 43.51 kN, 53.22 kN, and 45.19 kN, respectively. e variance of the peak loads is 0.5110, 0.8091, 0.7562, and 0.6970, respectively. It can be seen from the variance of the different fiber content that the fluctuation of the test data is small, indicating that the dispersion of the test data is good. Compared with the control group, the peak loads with fiber contents of 3 kg/m 3 , 4.5 kg/m 3 , and 6 kg/m 3 increased by 11.11%, 56.67%, and 32.21%, respectively. e control group shotcrete square slab has high brittleness and low toughness. With the addition of BF to the shotcrete concrete, the ultimate load and toughness of the concrete can be improved appropriately.
In order to study the effect of different contents of basalt fibers on the toughness of the square slabs, the energy absorption value in the EFNARC standard is used to characterize the fracture toughness of the slabs. e calculation method is as follows: In the formula, W is the square slab energy absorption value (unit: J); δ is the central deflection of the square slab; and F(x) is the load corresponding to the central deflection of the square slab X.
According to the load-displacement curves of shotcrete square slabs with different contents of basalt fiber in Figure 5, the energy absorption values corresponding to different deflections are obtained by the integral method, as shown in Table 5. e energy absorption curve is shown in Figure 6. Figure 6 shows that the energy absorption capacity of square slabs with fibers of 3 kg/m 3 , 4.5 kg/m 3 , and 6 kg/m 3 is 322.08 J, 565.10 J, and 373.23 J, respectively. Compared with the control group (76.68 J), the energy absorption capacity increased by 320.03%, 636.96%, and 386.74%, respectively. erefore, BF can significantly improve the toughness and energy absorption capacity of BF shotcrete.
rough the analysis of the load-deflection curve and energy absorption curve, it can be seen that the square slab with the 4.5 kg/m 3 volume fraction of fiber shows the highest toughness and energy absorption capacity.

Analysis of Failure Form of Shotcrete Square Slab.
e crack development law and failure pattern of basalt fiber shotcrete square slab are shown in Table 6 and Figure 7. e conclusions drawn from Table 6 and Figure 7 are as follows.
e BF shotcrete square slabs have four cracks for each amount, and the fiber contents are 0 kg/m 3 , 3 kg/m 3 , 4.5 kg/ m 3 , and 6 kg/m 3 , where the mean values of the corresponding crack width are 0.28 mm, 0.25 mm, 0.19 mm, and 0.23 mm, respectively, compared with the control group. e crack width reduced by 10.7%, 32.1%, and 17.9%, respectively. With the increase of fiber content, the crack width of the square slab first increases and then decreases. e bridging effect of the fiber creates a bond between the fiber and the concrete matrix, which causes the stress redistribution and limits the generation and expansion of the crack. erefore, increase of the fiber content is beneficial to limiting the development of the crack in the square slab and appropriately raises its bearing capacity and ductility. However, excessive fiber content increases the porosity  Advances in Civil Engineering inside the concrete matrix, resulting in a decrease in the toughness of the matrix. e failure mode of the control group was brittle failure with a flat fracture surface. When the flexural specimens of plain sprayed concrete were destroyed, the cracks developed rapidly. With the addition of BF, the toughness of the basalt fiber shotcrete square slab was improved, the crack interface of the square slab was not intersected at the centre point, and the fracture interface appeared as an irregular fracture shape. When the fiber content is 4.5 kg/m 3 , the average crack width is the smallest, and the irregularity of cracks is poor. Combined with the analysis of the load and energy absorption of the slabs, the peak load value and energy absorption value are maximum of the slabs. e bridging connection provided by the basalt fiber across the crack can effectively improve the toughness and maximum elongation of the shotcrete square slab, avoiding sudden brittle fracture after the peak load of the slab, and making the slab gradually transition from brittle failure to ductile failure.

SEM Analysis.
Electron microscopic scanning is an effective method to study the internal distribution and microstructure of cement-based hydration products [34,35], allowing the observation of the distribution of fibers and the bonding mode between fibers and matrix and analyzing the effect of hydration products such as ettringite and C-S-H on the microstructure of the fiber matrix. e bonding mode of the fiber matrix is shown in Figure 8. Figure 8 shows that the basalt fibers are covered with dense cement hydration matrix, so there is a good chemical cementation between the fibers and the matrix (Figure 8(a)). e basalt fibers are distributed randomly and evenly in the concrete, which can form a three-dimensional spatial skeleton with a stable structure (Figure 8(b)). Excessive fiber incorporation can increase fiber agglomeration during stirring and spraying, which may reduce the bonding property between BF and shotcrete matrix (Figure 8(c)). Once a new crack appears in the specimen under external loads, the fiber spanning the crack consumes energy to restrain the crack development through the debonding and slipping of the fiber, thus delaying the driving force of crack propagation in the concrete (Figure 8(d)).
When the existing crack driving force is restrained and energy accumulates in the concrete, the anisotropic distribution of the BF will seek new cracks in the weak points of the matrix to release energy again. e toughness and strength of concrete is enhanced by the fiber skeleton distributed inside the concrete. e effect of single BFs dispersed in basalt fiber shotcrete is similar to reinforcing bars. e secondary micro-reinforcement effect of BF combined with shotcrete matrix can effectively improve the mechanical properties of basalt fiber shotcrete.

Engineering Background.
e short-cut basalt fiber shotcrete is applied to the bottom extraction roadway of Zhaogu No.2 coal mine (the roadway elevation is −848∼−850 m). e test section of roadway is located in the fractured shatter zone. Because of the action of the fault zone, the marl fracture zone in the test section of the roadway is well developed. e surrounding rock of the roadway has a weak layer and a strong water-rich layer. e lithology soft easy to produce large deformation (the maximum deformation of the support is 300 mm). Concrete cracking and detachment, deformation of steel support, and other phenomena occur very often, resulting in the failure of supports, a great threat to mine safety. erefore, the short-cut basalt fiber shotcrete was used for the field test, and 200 m of the bottom extraction roadway was selected for the underground support test. Every 40 m, a different fiber content was selected for the bolt-shotcrete support. After the shotcrete was completed, two sets of monitoring points were used to monitor the convergence deformation of the roadway. e layout of the convergence survey points in the bottom drainage roadway is shown in Figure 9.

Analysis of Convergence Deformation of Roadway.
After the basalt fiber shotcrete has set and hardened, convergence deformation monitoring is carried out for 35 days on the bottom drainage roadway. e monitoring results are shown in Figure 10. e addition of BF increases the deformation resistance of the supporting structure. e     Advances in Civil Engineering maximum deformation (7 days) of the plain sprayed concrete retaining structure is 1.44 mm. When the BF content is 3 kg/m 3 and 4.5 kg/m 3 , the deformation of the roadway is only 0.12 mm in the first 7 days. e total convergence displacement of the plain sprayed concrete at 35 days is 2.47 mm. When the BF content is 4.5 kg/m 3 , the convergence displacement of the bottom drainage roadway is 0.21 mm. In the stirring and spraying process, the uniformly distributed BF will form a stable bearing structure inside the basalt fiber shotcrete matrix, thereby improving the resistance of the roadway to deformation.

Conclusions
In this paper, the effect of BF on the flexural toughness of shotcrete square slab is studied. e fiber-reinforced mechanism is analyzed by means of a synthetic fibertransparent soil model and SEM technology, combined with a field test, and the following conclusions are drawn: (1) rough the fiber-transparent soil model, it is found that when the fiber content is within 3∼4.5 kg/m 3 , the distribution of BF is uniform and it forms a dense network structure, in which agglomeration appears and has a small ratio of pores, which is beneficial to improving the bending toughness of the shotcrete concrete. (2) According to the comparison test of the bending toughness of the square slab, it is found that when the fiber content is 4.5 kg/m 3 it had the best reinforcing effect. Compared with the control group, the peak load and absorption capacity increased by 56.67% and 636.96%, respectively, and the maximum crack width decreased by 32.10%. (3) e SEM analysis indicated that the three-dimensional random distribution of BF forms a stable loadbearing skeleton in the shotcrete, improves the stress distribution in the shotcrete matrix, reduces the crack driving force, and restrains the generation and development of microcracks. Excessive fiber incorporation can increase fiber agglomeration during stirring and spraying, which may make BF and shotcrete matrix not fully bonded, and affect the toughness of basalt fiber shotcrete. (4) BF shotcrete has achieved a good support effect in a deep soft rock underground engineering test: the basalt fiber shotcrete support effect is improved compared with plain shotcrete, and the ability of restraining the deformation of surrounding rock is enhanced. e convergence displacement of the plain shotcrete support section is 2.47 mm in 35 days, while the convergence displacement of the roadway in the basalt fiber shotcrete support section with a fiber dissolution of 4.5 kg/m 3 is only 0.21 mm.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.