Introduction

In the building structure, concrete is the most widely used because of its excellent performance, wide source of raw materials, low price and other advantages. With the continuous development of building science and technology, people have higher and higher requirements for buildings and structures, and their structures are more complex and changeable. In addition to meeting the basic bearing capacity and durability requirements, the building also has a beautiful appearance and meets the visual requirements. Concrete as a part of the building structure, the higher its requirements. Ordinary concrete has been unable to meet these requirements, and new materials and new combined structures are urgently needed to solve such problems. At this time, FRP (fiber reinforced composite material) came into being. The combination of new FRP materials and traditional materials such as concrete or steel pipes can form a variety of new combined structures. For example, FRP concrete composite structure1,2,3,4,5, FRP-CFST composite structure6,7,8,9,10, Frp-concrete-steel pipe composite structure11,12,13,14,15, FRP steel-bone concrete composite structure16,17,18, double FRP pipe composite structure19,20, etc. Other ways to improve concrete properties include new structures that combine traditional materials with another traditional material. For example, concrete filled steel tube structure21,22,23,24, double steel tube constrained concrete composite structure25,26,27,28 and so on. Glass fiber reinforced composite (GFRP) has the advantages of light weight, high strength, good energy absorption, etc. It is also widely used in the construction industry. In the previous research results on GFRP concrete materials, He et al.29 explored the axial compression performance of biochar concrete columns confined by glass fiber reinforced composite tubes. The results show that the ultimate compressive strength of the concrete specimens is increased by 490.4%~563.3% under the restraint effect of GFRP tube, and the yield stress and strain of the restrained specimens are much greater than that of the unrestrained specimens. The restraint effect of GFRP tube can effectively improve the strength and deformation capacity of biochar concrete. Zhang et al.30 explored the horizontal bearing performance of reinforced concrete piles reinforced by FRP sheets, and found that GFRP and CFRP sheets both improved the horizontal bearing capacity of concrete piles. The improvement effect increased with the increase of the number of layers. Jin et al.31 conducted an experimental study on the evolution law of the axial compression properties of concrete columns with GFRP tubes under sulphate environment, and found that GFRP tubes could effectively resist sulphate erosion under acidic environment, thus enhancing the corrosion resistance of the material. Li et al.32 explored the axial compression performance of sea sand concrete columns strengthened by GFRP, and found that the presence of GFRP tubes and GFRP bars could improve the ultimate bearing capacity. The addition of GFRP longitudinal reinforcement increased the bearing capacity of the specimen by 4.3%~49.8%. The bearing capacity and ductility of the specimen were further improved after the joint reinforcement of GFRP tube and GFRP bars. The overall shape of the specimen was intact and the failure form was improved. Zhang et al.33 studied the axial compressive properties of concrete short columns with elliptical GFRP tubes, and found that the confinement of elliptical glass fiber reinforced composite tubes could significantly improve the compressive strength of core concrete. With the increase of the number of glass fiber layers, the compressive strength of the core concrete are increased. The application of GFRP bars and GFRP pipes can effectively improve the bearing capacity and corrosion resistance of concrete materials, which is of great significance to improve the safe use of concrete buildings.

Concrete buildings are often subjected to dynamic loads such as seismic loads, blasting loads and impact loads of vehicles and ships during use, which have a greater impact on the safe use of buildings. The application of FRP materials can also enhance the dynamic load resistance of concrete materials. Ma et al.34 carried out multiple impact splitting tensile tests on GFRP tubebound concrete, and found that the binding effect of GFRP tubes can greatly enhance the dynamic tensile properties of specimens, and the greater the thickness of tube walls, the lower the damage degree of specimens. Duan et al.35 carried out a numerical simulation study on the impact resistance of damaged reinforced concrete beams reinforced by CFRP, and found that CFRP reinforced beams can significantly inhibit the development of shear oblique cracks, greatly reduce the range of cracks, and CFRP reinforced beams still show good impact resistance. The research results can provide reference for the optimization design of damaged RC beams reinforced by CFRP. Li et al.36 studied the lateral impact resistance of concrete-filled square steel tube members with I-section CFRP profiles, and compared with ordinary concrete-filled steel tube members, the impact resistance of concrete-filled square steel tube members with I-section CFRP profiles was significantly improved under lateral impact load. Yu et al.37 explored the impact resistance of damaged reinforced concrete beams reinforced by CFRP by using drop hammer test, and found that with the increase of the number of layers pasted by CFRP sheets, the specimen as a whole produced flexural shear failure. The impact resistance and stiffness of the damaged specimens after reinforcement and repair are significantly improved compared with that of the intact specimens, with an increase range of 70–72%. The reinforcement and repair of CFRP sheet can not only strengthen and reinforce the specimen, but also improve the overall impact resistance of the specimen to a certain extent. Therefore, the use of FRP materials can effectively enhance the impact resistance of concrete materials, and there are certain differences in the strengthening effect under different strengthening methods, among which FRP tube confined concrete is the most widely used in engineering.

GFRP materials are gradually being widely used in the construction industry, but there are few research results on the dynamic mechanical properties of GFRP concrete materials. In order to further explore the effect of GFRP tube wall thickness and concrete structure on the performance of restrained concrete under dynamic load, this paper uses SHPB device to carry out dynamic splitting tensile tests on several GFRP tube wall thickness (0, 2, 3, 4 mm) and several GFRP tube-mortar specimens with hollow ratio (0, 0.187, 0.292). The influences of tube wall thickness and concrete hollow rate on tensile mechanical properties, energy dissipation and fracture morphology of GFRP tubule-confined concrete were analyzed. The research results can provide experimental parameters for the safe application of GFRP pipe concrete materials in engineering sites.

Experiment

Materials

The GFRP pipe-mortar structure is composed of GFRP pipe and cement mortar. Among them, the cement is Conch 42.5 ordinary Portland cement, the sand is river sand with particle size ranging from 0.315 to 1.25 mm, and the water is laboratory tap water. The raw materials for GFRP pipes include high-quality glass fiber and high-temperature resistant epoxy resin, which are cross wound by uninterrupted fibers under microcomputer control. It has the advantages of high bending strength, corrosion resistance, excellent mechanical properties, etc. As shown in Table 1.

Table 1 GFRP tube properties.
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Specimen making

The experiment selected GFRP pipes with a diameter of 74 mm, a height of 37 mm, and wall thicknesses of 0, 2, 3, and 4 mm, respectively. GFRP pipe mortar specimens with different void ratios were prepared by pouring mortar inside the GFRP pipe, with void ratios of 0, 0.187, and 0.292, respectively. Use steel molds to pour specimens with different hollow ratios, as shown in Fig. 1. The mold base retains holes with diameters of 0 mm, 32 mm, and 40 mm respectively for casting specimens with hollow ratios of 0, 0.187, and 0.292.

Fig. 1
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Ring specimen mold.

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The composition ratio of each material in cement mortar is water: sand: cement = 0.4: 1: 1. The specimens are divided into two types: cement mortar specimens and GFRP pipe mortar specimens. When making cement mortar specimens, before pouring, place 40 mm and 32 mm PVC pipes into the holes reserved in the mold, and place the assembled mold on the vibration table. Pour the mixed cement mortar between the mold and PVC pipe, vibrate for 15 s after pouring, then firmly cover the top cover through the PVC pipe, and then place it under standard curing conditions for 28 days. When the cement mortar is not completely solidified, pull out the PVC pipe. When pouring GFRP pipe mortar specimens, first place GFRP pipes of different wall thicknesses into the mold, and insert PVC pipes into the reserved holes on the base. Pour the mixed cement mortar between the PVC pipe and GFRP pipe, vibrate for 15 s after pouring, and then firmly cover the top cover through the PVC pipe. After that, place it under standard curing conditions for 28 days. When the cement mortar is not completely solidified, pull out the PVC pipe and prepare the test piece as shown in Fig. 2.

Fig. 2
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Partial test piece.

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Test apparatus

Using SHPB testing equipment, non dynamic splitting tensile tests were conducted on the specimens, and the final impact air pressure was selected as 0.8 MPa. The main components of the device include a cylinder for storing nitrogen gas, an impact rod, a speed measurement system, a variable cross-section incident rod, a transmission rod, a damper, an ultra dynamic strain gauge, a waveform memory, a data processing system, and strain gauges attached to each rod. Among them, by adjusting the impact pressure generated by compressing nitrogen gas in the cylinder, the study of specimens under different strain rate loads can be achieved.

The impact rod is emitted by the impact air pressure generated by the cylinder and collides with the incident rod. At the same time, a one-dimensional stress wave is generated in the incident rod, and then the stress wave is transmitted to the specimen, causing deformation of the specimen. Due to the difference in wave impedance between the specimen and the incident rod, there will be wave reflection and transmission at the contact surface between the specimen and the rod. When the stress wave propagates through the contact surface between the two, a portion of the stress wave will return to the incident rod to form a reflected wave, and another portion of the stress wave will pass through the specimen to form a transmitted wave. The smaller the difference in wave impedance between the specimen and the incident rod, the larger the transmitted wave. Measure the incident strain signal, reflected strain signal, and transmitted strain signal generated in each rod by using resistance strain gauges or semiconductor strain gauges attached to the incident rod and transmission rod. After collecting strain signals from strain gauges using a super dynamic strain gauge, the stress waveform is stored in a waveform memory. Then, the transmission rod collides with the damper, dissipating energy and coming to a standstill38. The structure of the experimental device is shown in Fig. 3.

Fig. 3
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SHPB structure diagram.

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By processing the data obtained during the test, mechanical parameters can be obtained, and the calculation method is as follows:

$$\sigma \left( t \right)=\frac{{2P}}{{\pi (D - d)L}}=\frac{{2A0E\varepsilon t(t)}}{{\pi (D - d)L}}$$
(1)

. where D is the outer diameter of the specimen, m; d is the diameter of the hole in the specimen, m; L is the height of the specimen, m; P is the loading load, N.

Each energy can be calculated according to the one-dimensional stress wave theory, and its calculation formula is as follows.

$$\left\{ \begin{gathered} {W_{\text{i}}}(t)=AE{C_0}\int_{0}^{t} {{\varepsilon _{\text{i}}}^{2}(t)dt} \hfill \\ {W_{\text{r}}}(t)=AE{C_0}\int_{0}^{t} {{\varepsilon _{\text{r}}}^{2}(t)dt} \hfill \\ {W_{\text{t}}}(t)=AE{C_0}\int_{0}^{t} {{\varepsilon _{\text{t}}}^{2}(t)dt} \hfill \\ \end{gathered} \right.$$
(2)
$${W_{\text{s}}}(t)={W_{\text{i}}}(t) - {W_{\text{r}}}(t) - {W_{\text{t}}}(t)$$
(3)
$$\xi =\frac{{{W_{\text{s}}}(t)}}{V}$$
(4)

where \({W_{\text{i}}}\), \({W_{\text{r}}}\), \({W_{\text{t}}}\), and \({W_{\text{s}}}\) are the incident, reflected, transmitted, and dissipated energy. \(\xi\) is the crushing energy dissipation density, and V is the volume.

Discussion

Stress time history curve

The results of dynamic tensile properties are shown in Table 2. When the wall thickness of GFRP tube is 0 mm, the specimen stress time history curve is shown in Fig. 4a, and when the wall thickness is 4 mm, the specimen stress time history curve is shown in Fig. 4b.

Table 2 Test results.
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Fig. 4
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Stress time history curve.

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Figure 4 shows that the stress time-history curves of cement mortar specimen are different from those of GFRP tube-mortar specimen. The stress time history curve of the cement specimen is unimodal. The stress rises linearly and rapidly at the initial stage of load action, and then decreases after the stress reaches its peak rapidly. With the increase of hollow ratio, the peak stress of the specimen decreases and the peak strain increases. When the hollow ratio is 0, 0.187 and 0.292, the peak stress of the cement mortar specimen is 10.56 MPa, 8.78 MPa and 4.19 MPa, respectively. Compared with the peak stress with hollow ratio 0, the peak stress of the cement mortar specimen with hollow ratio 0.187 and 0.292 decreased by 16.87% and 60.32%, and the peak strain increased by 2.71% and 5.43%, respectively. The presence of empty holes reduces the bearing capacity and increases its deformation degree under load, and the material properties decrease accordingly. The larger the hollow ratio, the more significant the decline in the specimen properties. The stress time history curve of GFRP tube mortar specimens is bimodal, and the first peak is higher than the second peak. At the initial stage of loading, the stress continues to rise, and then decreases after reaching the first peak, but the decreasing speed decreases. After a period of decline, the stress increases to the second peak, and then the stress decreases continuously until it reaches 0. The reason for the double peak of GFRP tube-mortar specimen: When the stress–strain curve of GFRP tube-mortar specimen reaches the first peak stress point, due to the constraint effect of GFRP tube outside the specimen, only the internal mortar cracks, and the whole specimen does not completely fail, and still has a high residual bearing capacity. Due to the high tensile strength of GFRP tubes, the energy required for crack generation and propagation is high. Under the action of higher impact pressure, the stress value required for crack generation is higher. Therefore, under the action of higher impact pressure, a second peak stress point will appear, and then the stress unloading will show a “double peak curve”, which is similar to the research results of Li et al. on the dynamic mechanical behavior of pure water ice and impurity ice39. When the hollow ratio is 0, 0.187 and 0.292, the peak stress of the 4 mm GFRP pipe-mortar specimen with wall thickness is 18.52 MPa, 15.29 MPa and 9.81 MPa, respectively. When the hollow ratio is 0.187 and 0.292, the peak stress of GFRP tube-to-mortar specimen decreases by 17.44% and 47.03%, and the peak strain increases by 14.98% and 53.30%, respectively. The increase of hollow rate also reduces the bearing capacity and deformation resistance of GFRP tube-mortar specimens under external loads, indicating that the existence of hollow rate will cause great deterioration of the specimens, and the greater the hollow rate, the more significant the deterioration degree of the specimens.

By comparing the stress time history curves of the two specimens, the splitting tensile strength of GFRP tube-mortar specimen is significantly higher than that of mortar specimen, and the peak stress of GFRP tube-mortar specimen with 4 mm wall thickness is 1.75, 1.74 and 2.34 times that of mortar specimen with hollow ratio 0, 0.187 and 0.292. In combination with Table 1, GFRP pipe has extremely high compressive and tensile strength, which plays a good role in protecting mortar materials, strengthening the protection of traditional concrete structures, and enhancing the impact resistance, deformation and damage resistance of structures. The larger the hollow ratio is, the more obvious the effect of GFRP pipe is.

Peak stress

The relationship between the peak stress and the thickness of the tube wall is shown in Fig. 5.

Fig. 5
figure 5

Peak stress.

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Figure 5 shows that there is a linear positive correlation between peak stress with different hollow ratios and tube wall thickness. Compared with the specimen with 0 mm wall thickness, the increase of peak stress with 0 hollow ratio and the specimen with 2 mm wall thickness, 3 mm and 4 mm wall thickness is 30.40%, 35.23% and 75.38%, respectively. For the specimen with hollow ratio 0.187 and tube wall thickness of 2 mm, 3 mm and 4 mm, the increase of peak stress is 17.65%, 34.17% and 74.15%, respectively. For the specimen with hollow ratio 0.292 and tube wall thickness of 2 mm, 3 mm and 4 mm, the increase of peak stress is 95.94%, 124.82% and 134.13%, respectively. The increase of tube wall thickness has a significant effect on the tensile strength of GFRP-mortar specimens, and the greater the thickness, the more significant the effect. The reasons for the increase of dynamic tensile strength mainly include the following aspects: (1) The high strength of GFRP material itself has a significant improvement effect on GFRP-mortar specimen. Its axial tensile elastic modulus is 14,000 MPa, axial tensile strength is 280 MPa, circumferential tensile strength is 600 MPa, and axial compressive strength is 240 MPa, and its performance is much higher than that of mortar material itself. Therefore, it has a good protection effect on the mortar material, and the greater the thickness of the pipe wall, the better the performance of the GFRP tube and the higher the strength of the GFRP-mortar specimen. (2) The restraint protection effect of GFRP tube improves the overall performance. Under the action of load, the specimen deforms to a certain extent, but the GFRP tube exerts a restraining effect on the internal mortar, and the internal mortar is subjected to “confining pressure”. The existence of confining pressure significantly improves the mechanical properties of mortar materials, and the performance of GFRP-mortar specimens increases accordingly. (3) Synergistic effect of two materials. The tensile strength of GFRP tube is higher than the compressive strength, and the compressive strength of mortar material is higher than the tensile strength. The performance of the two materials is “complementary”. Under the external load, the synergistic effect can give full play to the advantages of the material, and the strength increases accordingly. By comparing the effect of hollow ratio on the performance of the specimen, it can be seen that the higher the hollow ratio, the smaller the peak stress with the same GFRP tube thickness, and the existence of hollow ratio reduces the mechanical properties of the material.

Comparison of wall thickness effect and hole effect

In order to better compare the influence of GFRP tube wall thickness and hollow rate on the specimen, φ1 was introduced to represent the wall thickness growth factor of the specimen peak stress, and φ2 was introduced to represent the hollow rate growth factor peak stress, as shown in Eq. (5). The changes of the two growth factors are shown in Fig. 6.

$$\varphi _{{i,1}} = \frac{{\sigma _{{i,4}} }}{{\sigma _{{i,0}} }},\varphi _{{j,2}} = \frac{{\sigma _{{j,0}} }}{{\sigma _{{j,0.292}} }}$$
(5)

where φi,1 is the wall thickness growth factor of the peak stress when the hollow ratio is i, and i is 0, 0.187, 0.292, respectively. φj,2 is the hollow rate growth factor of the peak stress when the tube wall thickness is j, and j is 0 mm,2 mm, 3 mm, 4 mm, respectively.

Fig. 6
figure 6

Growth factor.

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Figure 6 shows that both the wall thickness growth factor and the hollow-ratio growth factor of GFRP tube-mortar specimen are greater than 1, indicating that the increase of wall thickness and the decrease of hollow-ratio will increase the dynamic tensile strength. Figure 6a shows that when the hollow ratio is 0, 0.187, and 0.292, the wall thickness growth factor is 1.75, 1.74, and 2.34, respectively. The increase of the hollow ratio increases the wall thickness growth factor of the specimen. When the hollow-ratio of mortar material increases, its peak stress decreases significantly, but the constrained protection effect of GFRP tube will increase the peak stress of specimen, and the greater the wall thickness, the higher the protection effect on mortar material, the more significant the increase in stress of specimen, and the greater the wall thickness growth factor. Figure 6b shows that with the increase of tube wall thickness, the growth factor of hollow percentage first decreases and then increases. When the tube wall thickness is 0 mm, 2 mm, 3 mm and 4 mm, the growth factor of hollow percentage of specimens is 2.52, 1.68, 1.52 and 1.89, respectively. The presence of GFRP tube can reduce the influence of hollow ratio of mortar on its tensile properties. The tensile strength of mortar material itself is low, when there are holes (defects) in the interior, its performance is greatly reduced, and the ability to resist dynamic tensile load is significantly reduced. When the GFRP tube restraps the mortar material, its effect will reduce the influence of the hollow rate on the mortar material properties, which indicates that the presence of GFRP tube will not only enhance the mechanical properties of the mortar material, but also reduce the weakening of the internal holes of the material on the mortar property. When the wall thickness of GFRP pipe is 3 mm, the hollow rate growth factor is smaller, and the influence of the holes in the mortar on the overall material properties is minimal.

Energy dissipation

Under the action of impact load, external input energy is stored inside the concrete material. When the stored energy exceeds the capacity of the specimen itself, the specimen undergoes deformation and failure. Therefore, exploring the energy evolution process of GFRP pipe mortar specimens under impact loads is an important aspect of revealing their damage and failure. The energy changes under dynamic tensile load are shown in Table 3. The energy time history curve with a wall thickness of 0 mm and a hollow rate of 0 is shown in Fig. 7, the relationship between each energy and the wall thickness of GFRP tube is shown in Fig. 8, and the relationship between the crushing energy dissipation density and the hollow rate is shown in Fig. 9.

Table 3 Evolution law of each energy.
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Fig. 7
figure 7

Energy time history curve.

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Fig. 8
figure 8

The relationship between energy and wall thickness of GFRP tube.

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Fig. 9
figure 9

Relationship between crushing energy density and hollow ratio.

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Figure 7 shows that under impact load, each energy increases with the increase of load action time. When the load is about 250 µs, the action ends and each energy tends to a fixed value. It can be seen from Fig. 8 that the reflected energy and dissipated energy decrease with the increase of GFRP tube wall thickness, while the transmitted energy increases accordingly. Compared with the specimen with 0 mm wall thickness, the reflection energy with 2 mm, 3 mm and 4 mm wall thickness decreased by 2.49%, 1.31% and 3.89%. The decrease of dissipative energy was 23.49%, 23.59%, 30.10%, and the increase of transmitted energy was 26.57%, 23.48%, 28.61%. Compared with the specimen with 0 mm wall thickness, the reflection energy with 2 mm, 3 mm and 4 mm wall thickness decreased by 3.13%, 2.04% and 2.79%. The dissipation energy decreased by 22.88%, 27.10% and 30.53%, while the transmission energy increased by 34.00%, 35.32% and 45.89%. The increase of tube wall thickness decreases the difference of wave impedance between the specimen and the bar, and the reflection energy decreases and the transmission energy increases under load. Combined with Fig. 8, it can be seen that the increase of tube wall thickness greatly reduces the degree of fracture and breakage, and the energy used for plastic deformation, crack propagation and breakage decreases accordingly, and more energy is generated through or accumulated and released in the form of elastic energy. Figure 9 shows that the hollow ratio also has a great influence on the energy dissipation density. When the tube wall thickness is the same, the energy dissipation density increases with the increase of the hollow ratio. When the tube wall thickness is 0 mm, the energy dissipation density of 0.292 and 0.187 specimens is 1.87 and 1.45 times of that of 0 specimens. When the tube wall thickness is 2 mm, the energy dissipation density of specimens with hollow ratio 0.292 and 0.187 is 1.88 and 1.55 times that of specimens with hollow ratio 0. When the tube wall thickness is 3 mm, the energy dissipation density of specimens with hollow ratio 0.292 and 0.187 is 1.79 and 1.25 times that of specimens with hollow ratio 0. When the tube wall thickness is 4 mm, the energy dissipation density of specimens with hollow ratio 0.292 and 0.187 is 1.86 and 1.48 times that of specimens with hollow ratio 0. The increase of the hollow rate will reduce the bearing capacity and deformation resistance, and the specimen is more prone to rupture and failure under the impact load. The external input energy is more used for the fracture and breakage, and the larger the hollow rate is, the higher the degree of breakage, and the density of the dissipated energy will increase accordingly. Therefore, for mortar concrete buildings, the restraint protection effect of GFRP tubes can greatly improve the dynamic load resistance of the structure, reduce the damage degree of the structure, and improve the safety performance of the building structure. At the same time, reducing the hollow ratio of the structure itself can enhance the safety performance of the building structure.

Failure pattern analysis

Figure 10 shows the fracture morphology of specimens with different hollow ratios and tube wall thicknesses under dynamic tensile load.

Fig. 10
figure 10

Effect of hollow ratio and tube wall thickness on fracture morphology of specimens.

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As can be seen from Fig. 10, specimens with hollow ratio of 0 and different tube wall thicknesses all present typical tensile failure modes. The left side is in contact with the incident rod, and the right side is connected with the transmission rod. There is a main crack in the loading direction, and a triangular fracture zone appears at both ends of the loading, and the fracture zone at the incident end is obviously larger than that at the transmission end. The main reason for the triangular fracture zone is stress concentration at both ends. This is consistent with Qi et al. ‘s dynamic cracking failure pattern of concrete, and the test results are reliable40. The presence of GFRP tube significantly reduces the fracture degree of the specimen, and the mortar specimen breaks along the loading direction, and a triangular shear fracture zone is generated at the loading point, and the fracture degree is larger. The GFRP tube-mortar specimen only produces penetrating cracks along the loading direction, and a small amount of debris is spalling at the loading point, so the specimen can still maintain its integrity. Moreover, the greater the thickness of the tube wall, the lower the overall fracture degree. It shows that the presence of GFRP tube can enhance the deformation ability against external load, and the thicker the tube wall, the stronger the deformation resistance and the higher the stability.

When the hollow ratio increases, the fracture morphology of the GFRP tube-mortar specimen changes significantly, and the fracture degree increases accordingly. When the hollow ratio is 0.292 and 0.187, the fracture morphology of cement mortar specimens does not change significantly with the increase of impact air pressure, and all of them are approximately symmetrically broken into four pieces, which is the same as the dynamic fracture failure morphology of rock ring specimens studied by Li et al.41. The GFRP tube-mortar specimen only cracks and spalling at the loading point under dynamic tensile load, but it can still maintain its integrity. The fracture morphology of GFRP tube-mortar specimens with different thickness of pipe wall is different, and the fracture degree decreases significantly with the increase of pipe wall thickness. The specimen with a hollow ratio of 0.292 and a tube wall thickness of 2 mm has a large “wedge” failure in the contact part of the incident rod, and cracks spread and small debris spalling at the end of the transmission rod, which indicates that the failure at the impact position is more serious. When the tube wall thickness increases from 2 mm to 3 mm and 4 mm, the “wedge” failure area is significantly reduced, and the number and expansion degree of cracks are significantly reduced, which indicates that the increase of the tube wall thickness can significantly enhance the impact resistance and provide the safety stability of the GFRP tube-mortar structure. Combined with the energy dissipation law of the specimen, it can be known that the increase of the pipe wall thickness will reduce the dissipated energy of the specimen, and the reduction of dissipated energy also reduces the degree of specimen rupture and fragmentation.

Compared with specimens with different hollow ratio, it can be seen that the fracture degree of GFRP tube-mortar specimens increases significantly with the increase of hollow ratio. The specimen with a wall thickness of 2 mm and a hollow ratio of 0 can still maintain its integrity under impact load. The specimen has a penetrating crack along the stress point and a small regional wedge failure area at both ends of the stress point, accompanied by a small amount of debris spalling. The specimen with wall thickness of 2 mm and hollow ratio of 0.187 will produce stress concentration due to the existence of hollow ratio, and the mortar is full of cracks in the hollow area, and more debris is associated with spalling. The specimen with a tube wall thickness of 2 mm and a hollow ratio of 0.292 has a large range of “wedge-shaped” spalling area in the contact part of the incident rod, and a large number of penetrating cracks around the empty hole, and the fracture degree of this specimen is more significant. The increase of the hollow ratio leads to a significant increase in the energy dissipation density of the specimen, and the degree of fracture and fragmentation accordingly increases. The increase of hollow rate increases the degree of breakage, and the increase of tube wall thickness decreases the degree of breakage. Therefore, the specimen with hollow rate of 0 and tube wall thickness of 4 mm has the lowest degree of breakage.

The results of this study indicate that the presence of GFRP pipes can effectively enhance the impact strength and deformation resistance of mortar materials, and the thicker the pipe wall, the more significant the increase in the performance of mortar and concrete materials. Although the increase in void fraction may reduce the impact resistance of the specimen to a certain extent, it can effectively reduce the weight of the concrete material itself. Therefore, in practical engineering applications, the thickness and hollow ratio of GFRP pipes should be selected based on the complexity of the building’s environment and the requirements it needs to meet, in order to ensure that the building meets safety, economy, and durability requirements.

Conclusions

  1. (i)

    There is a linear positive correlation between the peak stress with different hollow ratio and the tube wall thickness. The dynamic tensile strength of GFRP tube can be improved significantly due to its self-height strength, binding and protective effect on mortar and their synergistic effect. The higher the hollow ratio, the lower the peak stress of GFRP tube with the same thickness, and the existence of hollow ratio reduces the mechanical properties of the material.

  2. (ii)

    The increase of tube wall thickness and the decrease of hollow ratio will increase the dynamic tensile strength. The wall thickness growth factor increases with the increase of hollow ratio, and the wall thickness increases with the increase of tube wall thickness. When the wall thickness of GFRP pipe is 3 mm, the hollow rate growth factor is smaller, and the influence of the holes in the mortar on the overall material properties is minimal.

  3. (iii)

    The presence of GFRP tube can enhance the deformation ability of the specimen against external load, and the thicker the tube wall, the stronger the deformation resistance and the higher the stability of the specimen. The degree of fracture of GFRP tube-mortar specimen increases with the increase of hollow ratio.

  4. (iv)

    The reflection energy and dissipation energy of the specimen decrease continuously with the increase of GFRP tube wall thickness, while the transmission energy increases accordingly. The increase in void fraction will reduce the load-bearing capacity and deformation resistance of the specimen, and the energy dissipation of fracture will increase accordingly. Therefore, for mortar concrete buildings, the restraining and protective effect of GFRP pipes can greatly enhance the structural resistance to dynamic loads, reduce the degree of structural damage, and improve the safety performance of building structures. At the same time, reducing the hollow ratio appropriately according to the environment in which the building is located and its own needs can better meet the safety, economy, and durability requirements of the building.