Keywords

1 Introduction

As a composite material, concrete is often affected by load, resulting in internal fracture, which affects the stability of the structure. Reinforced concrete is an upgraded component of concrete, and micro-cracks often occur in it. Under the action of environmental load, the whole concrete structure is gradually destroyed [1,2,3]. Reinforced concrete cracks are divided into macro and virtual parts. Macro cracks can be detected by instruments, which is a real hidden danger. Virtual cracks can’t be detected by instruments, and they are small in size and narrow in width. They are continuously destroyed by the load during construction, which affects the final construction quality [4,5,6]. Reinforced concrete will be damaged by temperature, water pressure, wind, waves and earthquake load, which will affect the mechanical properties of reinforced concrete. By analyzing the load-strain and load-displacement of reinforced concrete, we can fully grasp the changing law of concrete material properties, thus ensuring the stability of reinforced concrete in complex environment [7,8,9]. Reinforced concrete structures not only bear static load, but also bear complex dynamic load and repeated load, which leads to structural damage and fracture failure, and affects the bearing capacity and durability of components. In order to meet the bearing demand of reinforced concrete, it is necessary to optimize the bearing capacity of reinforced concrete under reciprocating load. CFRP is a fiber-reinforced polymer, which is composed of carbon fiber, glass fiber, aramid fiber, basalt fiber and metal fiber. It has the advantages of light weight, high tensile strength, good fatigue resistance and corrosion resistance, and is widely used in the field of civil engineering [10]. Therefore, the ultimate bearing capacity of CFRP-reinforced concrete under reciprocating load is studied in this paper.

2 Specimen Production and Preparation

In this experiment, CFRP-reinforced concrete specimens are made on the test platform, and the concrete mixing materials include tap water, cement, fine aggregate and coarse aggregate. Among them, the water contains no impurities, and the fine aggregate is sun-dried intermediate river sand to avoid concrete strength damage, and the fine aggregate [11,12,13] is stored in barrels. Coarse aggregate is filtered out of granite gravel below 20 mm, washed, dried and stored in barrels. At the same time of concrete mixing, the 5 m-long steel pipe is cold-bent, and after cooling, the redundant part is cut off. Install the steel pipe in the flange and fix it on the reaction frame. When concrete is poured into the steel pipe, the flange and concrete are on the same level during the loading process, and the concrete is uniformly stressed and sealed, and naturally cured. After 28 days of concrete curing, CFRP is wound around the steel pipe, and the curing agent and epoxy resin adhesive matched with CFRP are used to make CFRP-reinforced concrete more in line with the construction requirements. The test platform is shown in Fig. 1 below.

Fig. 1.
figure 1

Schematic diagram of test platform device.

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As shown in Fig. 1, this test used supports, pads, hydraulic jacks, lateral supports, displacement meters, force sensors, reaction frames, moving rollers, TDS collectors and other devices. In this paper, the bearing is taken as the boundary condition, and the two ends of the CFRP-reinforced concrete specimen are fixed. Four sections, L/6, L/3, 2L/6 and 2J/3, are loaded on the loading platform, and the changes of compression bearing capacity in different section states are analyzed. Other parameters of the specimen are shown in Table 1 below.

Table 1. Specimen Parameter Table.
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As shown in Table 1, according to the amount of concrete, this paper made 11 specimens, including CFRP_0, CFRP_15, CFRP_30, CFRP_45, CFRP_015, CFRP_030, CFRP_045, CFRP_090, CFRP_L8_090, CFRP _L10_ 090 and CFRP_L12_090, with a height of 500 mm. Using C30, C40 and C45 strength concrete respectively, CFRP-reinforced concrete specimens with different thicknesses were made, and they met the test requirements in the form of overall compression.

3 Test Methods

In this experiment, CFRP_0, CFRP_15, CFRP_30, CFRP_45, CFRP_015, CFRP_030, CFRP_045, CFRP_090, CFRP_L8_090, CFRP_L10_090 and CFRP_L12_090 11 specimens were selected and CFRP with different layers on them. Among them, CFRP_0 means that the number of layers of reinforced concrete wrapped with CFRP is 0, CFRP_15, CFRP_30 and CFRP_45 are circumferential wound to strengthen CFRP layers with 15, 30 and 45 respectively; CFRP_015, CFRP_030, CFRP_045 and CFRP_090 are circumferentially wound with CFRP1, 3, 4 and 9 layers, vertical winding 5,0,5 and 0. CFRP_L8_090, CFRP_L10_090, CFRP_L12_090 shall wrap the CFRP 9 layer around L 8, L 10 and L 12. Under the condition of reciprocating load, the load-displacement changes of different specimens are different. This paper takes CFRP_090 as an example to analyze the load-displacement change of this specimen, as shown in Fig. 2 below.

Fig. 2.
figure 2

Load-displacement curve of specimen CFRP_090.

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As shown in Fig. 2, in the L/6 section, the displacement is −25 mm, and the load is the largest; In the L/3 section, the displacement is −10 mm, and the load is the largest; In the section of 2L/6, the displacement is −18 mm, and the load is the largest; In the 2L/3 section, the displacement is +3 mm, and the load is the largest. Under the condition of consistent L/6 section, CFRP_30, CFRP_015, CFRP_045 and CFRP_L8_090 are selected for load-displacement analysis. As shown in Fig. 3 below.

Fig. 3.
figure 3

Load-displacement curves of different specimens at L/6.

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As shown in Fig. 3, when the displacement of CFRP_30 is −20 mm, the load is the largest. When the displacement of specimen CFRP_015 is −30 mm, the load is the largest; When the displacement of CFRP_045 is −32 mm, the load is the largest; When the displacement of CFRP_L8_090 is −28 mm, the load is the largest. Among them, the overall load of specimen CFRP_L8_090 is the largest, and that of specimen CFRP_30 is the smallest. After the load-displacement analysis is completed, the plastic deformation of CFRP-reinforced concrete specimens is analyzed. The formula is as follows:

$$ \delta_{c} = E_{c} \varepsilon_{c} - \frac{{(E_{c} - \varepsilon_{c} )^{2} }}{{4f_{c} }}\varepsilon_{c}^{2} $$
(1)

In the formula (1), \(\delta_{c}\) is the plastic deformation of CFRP-reinforced concrete specimen; \(E_{c}\) is the slope of a straight line; \(\varepsilon_{c}\) is the strain value; \(f_{c}\) is the number of CFRP layers. Considering the tensile, compressive and failure modes of CFRP- reinforced concrete specimens, the tensile failure of fibers is expressed as:

$$ F_{f}^{t} = \left( {\frac{{\delta_{c} }}{{X^{T} }}} \right)^{2} + \alpha \left( {\frac{{\varepsilon_{c} }}{{S^{L} }}} \right)^{2} $$
(2)

In the formula (2), \(F_{f}^{t}\) is the deformation of the specimen when the fiber fails in tension; \(X^{T}\) is the longitudinal tensile strength; \(\alpha\) is the influence coefficient of shear strength on fiber stretching; \(S^{L}\) is the transverse shear strength. Compression failure is expressed as:

$$ F_{f}^{c} = \left( {\frac{{\delta_{c} }}{{X^{C} }}} \right)^{2} $$
(3)

In the formula (3), \(F_{f}^{c}\) is the deformation of the specimen when compression fails; \(X^{C}\) is the longitudinal compressive strength. Under the condition of basic tensile failure, the deformation of the specimen is expressed as:

$$ F_{m}^{t} = \left( {\frac{{\delta_{c} }}{{Y^{T} }}} \right)^{2} + \alpha \left( {\frac{{\varepsilon_{c} }}{{S^{L} }}} \right)^{2} $$
(4)

In the formula (4), \(F_{m}^{t}\) is the deformation of the specimen under the condition of basic tensile failure; \(Y^{T}\) is the transverse tensile strength. When \(\delta_{c}\) < 0, the compression failure is expressed as:

$$ F_{m}^{c} = \left( {\frac{{\delta_{c} }}{{S^{T} }}} \right)^{2} + \left[ {\left( {\frac{{Y^{C} }}{{2S^{T} }}} \right)^{2} - 1} \right]\frac{{\delta_{c} }}{{Y^{T} }} + \left( {\frac{{\varepsilon_{c} }}{{S^{L} }}} \right)^{2} $$
(5)

In the formula (5), \(F_{m}^{c}\) is the basic tensile failure and compression failure at the same time, the deformation of the specimen at this time; \(S^{T}\) is the vertical shear strength; \(Y^{C}\) is the transverse compressive strength. Under the compression limit condition, the variable of CFRP- reinforced concrete specimen is expressed as:

$$ D = 1 - (1 - d_{f} )(1 - d_{m} )\delta_{c} \varepsilon_{c} $$
(6)

In the formula (6), \(D\) is an elastic damage variable; \(d_{f}\) Is the fiber damage variable; \(d_{m}\) is the matrix damage variable. The load-displacement change, specimen plastic deformation, fiber tensile failure deformation, compression failure deformation, elastic damage, fiber damage, matrix damage and other variables are comprehensively analyzed, so as to obtain the ultimate bearing capacity change of CFRP- reinforced concrete under compression.

4 Test Results

Under the condition of reciprocating load, the load applied in CFRP-reinforced concrete changes constantly, and the deformation changes with the increase of load, so the specimen is in the linear elastic stage. Under the condition of increasing load, the specimen enters the stage of elastic-plastic deformation, and the deformation increases, and the load-deformation curve changes nonlinearly. When the load is close to the ultimate load, the deformation increases rapidly, and after reaching the ultimate load, the load begins to decrease, but the deformation is still increasing, and the specimen has a large deformation, that is, instability and failure. In this paper, multi-layer CFRP is added to CFRP-reinforced concrete, and the decline of load-deformation curve is not obvious, which proves that CFRP can enhance the ductility of reinforced concrete. After the surface CFRP is damaged by compression, the load that concrete can bear reaches the limit, then decreases and the deformation increases. It is proved that CFRP can strengthen and restrain the deformation of reinforced concrete, which has a great influence on its ultimate bearing capacity under compression. In this paper, several specimens were randomly selected and numbered as CFRP_0, CFRP_15, CFRP_30, CFRP_45, CFRP_015, CFRP_030, CFRP_045, CFRP_090, CFRP_L8_090, CFRP_L10_090 and CFRP_L12_090. The compression deformation state of the specimen is analyzed, as shown in Table 2 below.

Table 2. Specimen Parameter Table.
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As shown in Table 2, the compressive state of CFRP-reinforced concrete specimens has two forms: positive symmetric pressure and anti-symmetric pressure, and the compressive state of CFRP-reinforced concrete specimens is closely related to its initial pressure. The initial pressures of CFRP_0, CFRP_15, CFRP_45, CFRP_015, CFRP_045, CFRP_090, CFRP_L10_090 and CFRP_L12_090 are all antisymmetric, so the final compression forms of the above specimens are also antisymmetric. The initial pressures of CFRP_30, CFRP_030 and CFRP_L8_090 are close to positive symmetry, and their pressure forms are also positive symmetry. It can be seen that winding CFRP and changing the strength of steel tube can not change the compression state of CFRP-reinforced concrete specimens. The smaller the overall deformation of specimens, the maximum load that CFRP-reinforced concrete specimens can bear, and the ultimate bearing capacity also increases. Under the condition of constant compression state, the load and its proportion of CFRP-reinforced concrete are analyzed, as shown in the following Table 3.

Table 3. CFRP-reinforced concrete load and its proportion table.
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As shown in Table 3, when the CFRP-reinforced concrete specimen is in the position of maximum load, the load-bearing positions include concrete, steel skeleton and CFRP pipe. It can be seen from the table that the bearing capacity of concrete is relatively large, ranging from 800 kN to 1820 kN, and the proportion of its load in the whole specimen varies from 73% to 86%. The bearing capacity of steel skeleton is inferior to that of concrete, and the bearing capacity varies from 235 kn to 400 kn, and the proportion of steel skeleton in the whole specimen varies from 15% to 23%. The bearing capacity of CFRP tube is inferior to that of steel skeleton, and it has great axial stiffness under compression. Its bearing capacity varies from 9 kN to 265 kN, and its load ratio varies from 0.9% to 12% in the whole specimen. As far as the ultimate bearing capacity of CFRP-reinforced concrete specimens is concerned, it is concrete bearing capacity + steel skeleton bearing capacity + CERP pipe bearing capacity. Among them, the ultimate bearing capacity of specimen CFRP_L12_090 is the smallest, and that of specimen CFRP_090 is the largest. When CFRP-reinforced concrete is compressed, the ultimate bearing capacity decreases with the increase of slenderness ratio. At the same time, the ultimate bearing capacity increases with the increase of the number of layers of outsourcing CFRP; The bearing capacity of two-layer CFRP strengthened by circumferential winding and two-layer CFRP strengthened by longitudinal tiling is higher than that of four-layer CFRP strengthened by longitudinal tiling. It can be seen that the ultimate bearing capacity of CFRP-concrete filled steel tube increases with the increase of circumferential fiber filaments. The ultimate compressive bearing capacity of CFRP-reinforced concrete can be changed by adjusting the slenderness ratio, the number of CFRP layers and the circumferential fiber yarn, which plays an important role in improving its strength.

5 Conclusion

Under the condition of reciprocating load, CFRP-reinforced concrete transforms in elastic stage and elastic-plastic deformation stage. Under compression, the ultimate bearing capacity of CFRP-concrete filled steel tube is related to the slenderness ratio, the number of CFRP cladding layers, circumferential fiber filaments and other indicators. With the increase of slenderness ratio, the ultimate bearing capacity decreases gradually. With the increase of the number of layers of outsourcing CFRP, the ultimate bearing capacity is continuously improved. At the same time, the compressive state of CFRP-reinforced concrete is closely related to its initial pressure. Winding CFRP and changing the strength of steel tube can not change the compression state of CFRP-reinforced concrete. The smaller the overall deformation, the maximum load that CFRP-reinforced concrete can bear, and the ultimate bearing capacity also increases. Therefore, adjusting the slenderness ratio of CFRP-reinforced concrete, the number of CFRP layers and the circumferential fiber yarn can change its ultimate compressive bearing capacity and play an important role in improving the strength of CFRP-reinforced concrete.

In the bearing capacity analysis of CFRP-reinforced concrete, the load-displacement and load-strain at the compression position are very important. In the fracture process of CFRP-reinforced concrete, the fracture process zone, as a key feature, is determined based on linear elastic fracture mechanics. Under the action of reciprocating load, the stress of reinforced concrete is concentrated, and CFRP material is wrapped around it, which can avoid structural cracking and even protect the integrity of CFRP-reinforced concrete. In this paper, the ultimate bearing capacity of CFRP-reinforced concrete under reciprocating load is studied. In the form of experiment, the load change of concrete is analyzed in all directions, which makes the ultimate compressive bearing capacity analysis more accurate and guarantees the construction stability of building components.