Experimental Study on Shear Capacity of Waste Fiber Recycled Concrete Shear Wall

In this paper, the shear capacity of waste fiber recycled concrete shear wall (FCSW) is studied by cyclic loading tests on 9 specimens. The failure of the specimens under low cycle cyclic loading is analyzed. The influence of waste fiber content, recycled aggregate content and axial compression ratio on the shear capacity of shear wall is studied. The results show that: the larger the volume ratio of waste fiber is, the higher the shear bearing capacity of shear wall is; the addition of recycled coarse aggregate will reduce the shear capacity of shear wall; the shear capacity of shear wall increases with the increase of axial pressure. On the basis of relevant specifications, the calculation formula of in-plane shear capacity of waste fiber recycled concrete shear wall is proposed considering the influence of waste fiber on shear capacity. By comparing with the test results, it can be found that the two formulas are in good agreement. The conclusion of the study can provide reference for the design of waste fiber recycled concrete shear wall.


Introduction
With the rapid development of Chinese urbanization, a large number of waste buildings have been demolished. It resultied in more and more construction waste such as waste concrete. At present, there are serious waste of land resources and environmental pollution in the treatment of construction waste in China [1]. In order to make full use of waste concrete and other construction waste, domestic and foreign scholars began to use waste fiber concrete to replace part or all of the concrete coarse aggregate to make recycled concrete, and studied its mechanical properties and components [2~5]. Due to the decrease of strength, toughness and crack resistance of recycled aggregate concrete, its application in practical engineering is limited. According to the research results at home and abroad, the incorporation of fiber can improve the mechanical properties of recycled aggregate concrete [6~8]. The recycled waste fiber is processed, manufactured and mixed into recycled aggregate concrete for reuse, which not only solves the problems of resource waste and environmental pollution, but also produces better economic and social benefits.
At present, many scholars at home and abroad began to study the waste fiber recycled concrete, mainly focusing on the material and structural performance. Zhou[9~12] studied the constitutive problems such as splitting tensile strength and compression damage of waste fiber recycled concrete, and the flexural performance of waste fiber recycled concrete beam. The test results show that the splitting tensile strength of concrete can be improved to a certain extent by adding appropriate amount of waste fiber; the uniaxial compression damage test of prism specimen is carried out. In the test, with the increase of strain, the damage variable of waste fiber recycled concrete also increases. In the bending process of waste fiber recycled concrete beam, the ductility and bearing capacity of the beam are improved. Wang [13] studied the seismic performance of fiber reinforced recycled concrete frame joints, and the results show that: the incorporation of fiber improves the bearing capacity and seismic performance of frame joints. Wang [14] studied the carbonation performance of waste fiber recycled concrete through rapid carbonation test. The results show that: the addition of waste fiber can reduce the carbonation depth. At present, the research on the waste fiber recycled concrete shear wall is still relatively few at home and abroad. In this paper, the shear capacity of waste fiber recycled concrete shear wall under low cycle cyclic load is obtained through theoretical analysis and test. The shear bearing capacity formula of waste fiber recycled concrete shear wall is proposed by theoretical analysis method. The research results can provide reference for the design of waste fiber recycled concrete shear wall, and lay the foundation for the future application of waste fiber recycled concrete shear wall in practical engineering.

General situation of constitutive test
Recycled coarse aggregate takes concrete with original strength of C40 as raw material. The concrete is artificially crushed and then made into continuously graded coarse aggregate with a maximum diameter of 25 mm. The waste fiber is made from waste fabric as shown in Figure 1.

Figure 1. Coarse aggregate and waste fiber
The test scheme of concrete test block is determined according to the type of shear wall. Three cubes with dimensions of 150 mm × 150 mm × 150 mm, 3 prism blocks with dimensions of 150 mm × 150 mm × 300 mm and 3 tensile test blocks with dimensions of 100 mm × 100 mm × 370 mm are made for each type of concrete specimens. The strength, uniaxial tensile stressstrain curve and uniaxial compression stress-strain curve of concrete were measured. The test loading device is shown in Figure. 2. The values of measured test data after processing are shown in Table 1.

Specimen design
In this paper, nine specimens of waste fiber recycled concrete shear wall are designed. Taking the common C40 reinforced concrete shear wall as a reference, the influence of waste fiber volume ratio, recycled aggregate volume ratio and axial compression ratio on the shear capacity of shear wall is studied. The test parameters are shown in Table 2. The reinforcement is HRB335, the diameter of longitudinal reinforcement is 16mm, and the diameter of stirrup is 8mm. The shear wall specimens are mainly composed of loading beam, shear wall and foundation. The size of the loading beam (length × width × height) is 1000mm × 230mm × 200mm, the size of shear wall is 1000mm × 130mm × 1900mm, and the size of foundation is 2000mm × 500mm × 500mm. In order to compare and study the changes of various factors, the size and reinforcement of each specimen are the same, as shown in Figure 3.

Experimental test.
In the experiment, the electro-hydraulic servo loader online system was used to carry out low cycle cyclic loading on specimens of FCSW. The loading diagram of the test device is shown in Figure 4.

Experimental phenomena and failure modes
The low cycle cyclic loading test of FCSW specimens is similar, which can be roughly divided into four stages: elastic stage, crack development stage, yield stage and failure stage. The crack development and failure mode of shear wall specimens are shown in Figure 5. Crack development stage: with the increase of load, cracks first appear at the root of the wall tensile zone and continue to develop. When the loading continues, the inclined cracks appear at the position near the top of the foundation and extend to the middle of the wall.
Yield stage: the cracks in the corner concrete become wider, and the fibers in the concrete begin to play a role. But under the same load, the displacement variable increases. The specimens yield obviously. The yield phenomenon of shear wall specimens without waste fiber is not obvious.
Failure stage: the corner concrete is crushed by large blocks, and the fiber is broken. The reinforcement at the corner of the wall yielded and the bearing capacity of the wall decreased sharply.
It can be seen from Figure 5 that when the shear failure occurs in the shear wall specimen FCSW6, the number of inclined cracks is more, and the through cracks are relatively short. When the shear wall bottom is damaged, the concrete spalling is serious and the failure is ductile; the distribution of cracks in the shear wall specimen FCSW1 is relatively sparse, but the through cracks are relatively long. Through comparison, it can be found that the incorporation of waste fiber can prevent the formation of cracks to a certain extent.

Influence of various factors on shear capacity of FCSW
Based on the test results of specimens FCSW2, FCSW3, FCSW4 and FCSW5, Figure 6 (a) shows the change of shear capacity of shear wall specimens with volume fraction of waste fiber when the replacement rate of recycled aggregate is 50%. It can be seen from the figure that the more the waste fiber volume content, the greater the shear bearing capacity of the shear wall specimen.
Comparing the test results of FCSW6, FCSW3 and FCSW7, Figure 6 (b) shows the change of shear capacity of shear wall specimens with recycled coarse aggregate content when the volume ratio of waste fiber is 0.08%. It can be seen from the figure that when the content of recycled aggregate increases from 0% to 50%, the shear capacity of shear wall specimen decreases by 4.85%; when the content of recycled aggregate increases from 50%, the shear bearing capacity of shear wall specimen decreases by 3.69%.
Comparing the test results of FCSW4, FCSW8 and FCSW9, Figure 6 (c) shows the variation of shear capacity of shear wall specimens with axial compression ratio. It can be seen from the figure that the shear bearing capacity of shear wall specimens increases with the increase of axial compression ratio. When the axial compression ratio changes from 0.2 to 0.3, the shear capacity increases by 10.4%; when the axial compression ratio changes from 0.2 to 0.3, the shear bearing capacity increases by 13.5%. Figure 6. Influence of various factors on shear capacity

Calculation of shear capacity of FCSW
The incorporation of waste fiber and replacement of natural coarse aggregate with recycled fiber make the material performance of recycled concrete with waste fiber different from that of ordinary reinforced concrete. Therefore, the structural performance of FCSW is different from that of ordinary reinforced concrete shear wall. According to the provisions of code for design of concrete structures (GB 50010-2010) and technical specification for fiber reinforced concrete structures (CECS 38:2004), and considering the influence of waste fiber and recycled concrete on shear capacity of shear wall, the shear capacity of FCSW u V can be divided into two parts: Shear capacity of concrete c V and horizontal reinforcement concrete resistance s V . The concrete strength is the recycled aggregate concrete strength considering the recycled aggregate content and multiplied by the influence coefficient of waste fiber. Therefore, the shear capacity of shear wall can be calculated according to the following formula: Where  is shear span ratio, According to the test data [15], it can be found that the ultimate shear capacity increases with the increase of waste fiber volume content in a certain range. Therefore, the influence coefficient of waste fiber on shear capacity of inclined section v  is introduced. The formula of influence coefficient is obtained by fitting test data and applied to the design of recycled concrete shear wall. The shear capacity of FCSW can be expressed by formula (4):

Comparison of calculated and measured shear capacity
The calculated and measured values of each specimen of waste fiber recycled concrete shear wall are shown in Table 3. It can be seen from table 3 that the average value of the ratio between the measured value and the calculated value of the waste fiber recycled concrete shear wall is 1.101, and the calculated value is conservative.

Conclusion
In this paper, nine FCSW specimens are tested under low cyclic loading test, and the method of theoretical analysis is used to study the calculation of shear capacity of FCSW.The following conclusions can be drawn.
(1) The influence of various factors on the shear capacity of FCSW: waste fiber delays the development of cracks to a certain extent, the shear capacity increases with the increase of waste fiber volume ratio, and decreases with the increase of recycled aggregate volume ratio.
(2) Based on the code for design of concrete structures, combined with the influence of volume ratio of waste fiber on shear capacity, the calculation formula of shear capacity of FCSW is proposed. By comparing with the test results, it can be found that the two formulas are in good agreement.