Keywords

1 Introduction

The construction industry has gradually expanded and has become an important pillar industry of the national economy. The traditional cast-in-place construction mode has large energy consumption, construction environment noise and dust pollution, which affect the industry’s construction prospects [1,2,3]. At present, the types of work in the construction industry are relatively old, and there are two challenges: sustainable development and labor shortage. Therefore, it is urgent to reform the construction mode and management mode, and upgrade the construction form of the construction industry. Fabricated building is a new construction method, which can decompose the whole structure into multiple prefabricated production components, which are uniformly manufactured and maintained by the factory, and connected and assembled at the construction site [2,3,4]. This method not only improves the efficiency of building construction, but also solves the problems of green environmental protection and labor shortage of buildings. It has become a widely used construction mode in the construction industry. As an indispensable component of buildings, reinforced concrete has opened up the industrial chain of component production in the construction industry through standardization, industrialization, assembly, informatization and other construction design forms in the factory, meeting the requirements of green development.

Earthquakes are disasters with large destructive power, explosive speed and impact range, which often cause casualties and property losses [5,6,7] to people in the area near the source center. The prefabricated building components are connected through dry connection and wet connection, and the connection assembly does not require post pouring concrete to ensure the integrity of the components, so as to meet the stability requirements of the building connection. However, in the process of building use, the reinforced concrete is affected by the weather environment, low temperature environment, high temperature environment, etc., which often leads to uneven corrosion, affecting the stability of building components [8,9,10]. In order to meet the overall stability of building components, flat steel reinforcement, CFRP reinforcement, tinplate reinforcement and other forms are often used to enhance the strength of modified reinforced concrete. In this paper, the seismic behavior of the non uniformly corroded RC column assembly joints strengthened with CFRP is studied.

2 Test Preparation

In this paper, before the test, a total of five reinforced concrete column specimens were made in the form of cast-in-place, and they were dissolved in Ca(OH)2 to form non-uniform corroded reinforced concrete columns. Four corroded reinforced concrete columns were strengthened with CFRP to form one corroded reinforced concrete column specimen and four corroded reinforced concrete column specimens strengthened with CFRP. Where, VB_1 is the cast-in-place reinforced concrete column specimen; SRTP_1, SRTP_2, SRTP_3, SRTP_4 is a fabricated test piece [11, 12]. A set of three 150 mm × 150 mm × 150 mm cube test blocks shall be reserved when pouring the test pieces, and a set of three 100 mm × 100 mm × 100 mm cube test blocks shall be reserved in the assembled test pieces. The compressive strength and tensile strength of cast-in-place reinforced concrete column specimens are analyzed. The retained test block and test piece shall be cured under the same conditions. After 28 days of curing according to relevant standards, the standardized test of compressive strength and tensile strength shall be carried out. In this test, C35 concrete is used for all test pieces except CFRP test pieces. The performance indexes of the test pieces are shown in Table 1 below.

Table 1. Performance Indexes of Reinforced Concrete Column Specimens.
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As shown in Table 1, the stiffness of CFRP specimens is evenly distributed, and the reverse bending point of the reinforced concrete column is located in the center of the column. The inverted “T” column shall be made below the reverse bending point and above the foundation. The section of non-uniform modified reinforced concrete column head strengthened with CFRP is increased, which is convenient for the installation of horizontal actuator of MTS machine. Considering that the axial compression ratio of reinforced concrete and the diameter of longitudinal reinforcement are different, the reinforcement ratio is the same. When the axial compression ratio is set to 0.3 and 0.6, the corresponding vertical axial forces are 1000 kN and 2000 kN respectively. The column section is symmetrically arranged with longitudinal reinforcement along the push and pull sides. The bottom of the longitudinal reinforcement is designed as an “L” shape, and 200 mm is arranged along the long side of the column bottom foundation beam to strengthen the CFRP reinforcement effect of the reinforcement.

3 Test Method

In order to study the damage degree of VB_1, SRTP_1, SRTP_2, SRTP_3, and SRTP_4 more comprehensively and reasonably, the ductility, stiffness and energy consumption are selected as the seismic performance analysis indexes. The damage index of the specimen is expressed as:

$$ D = f(x_1 ,x_2 ,..,x_n ) $$
(1)

In formula (1), \(D\) is the damage index of the test piece; \(f\) is the damage function; \(x_1\), \(x_2\), \(x_n\) is to reflect the change parameters of mechanical properties of the specimen damage. Usually, VB_1, SRTP_1, SRTP_2, SRTP_3, SRTP_4 test piece \(D\) are the value after multiple parameters are combined during the damage process of the test piece, which can represent the damage degree of the test piece. \(D\) reflects differently on VB_1 and test piece SRTP_1, SRTP_2, SRTP_3, SRTP_4. When 0 < \(D\) < 1, the nondestructive state of the specimen under non-uniform corrosion is 0, and the corresponding seismic failure state is 1. According to the seismic action form, under the condition of reciprocating load, the damage index of the corner is calculated as follows:

$$ \Delta D_i = A(\theta_o )^a $$
(2)

In formula (2), \(\Delta D_i\) is the damage produced by the test piece under the condition of reciprocating load; \(A\), \(a\) is the test parameter; \(\theta_o\) is the plastic rotation angle generated by the reciprocating load. The ductility coefficient is introduced to represent the damage index of each specimen assembly node. The formula is as follows:

$$ d_y = \frac{\mu_m - 1}{{\mu_n - 1}} $$
(3)
$$ \mu_m = {{\Delta m} / {\Delta n}} $$
(4)
$$ \mu_n = {{\Delta n} / {\Delta y}} $$
(5)

In formula (3) (4) and (5), \(d_y\) is the displacement ductility coefficient; \(\mu_m\) is the maximum ductility coefficient of the specimen; \(\mu_n\) is the ultimate ductility coefficient of the specimen; \(\Delta m\) is the maximum elastic-plastic deformation of the specimen; \(\Delta y\) is yield deformation; \(\Delta n\) is the ultimate deformation. The damage index of test piece proposed according to the aging dissipation of structural energy is expressed as:

$$ D(t) = \frac{E(t)}{E} $$
(6)

In formula (6), \(D(t)\) is the seismic energy consumption of the test piece; \(E(t)\) is the energy dissipation value of the test piece at time t; \(E\) is the total cumulative hysteretic energy consumption of the test piece. In this paper, according to the actual situation of each test piece, the skeleton curve is analyzed, as shown in Fig. 1 below.

Fig. 1.
figure 1

Skeleton Curve of CFRP Specimen.

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As shown in Fig. 1, test piece VB_1 during the displacement process of −100 mm– +100 mm, the load presents the trend of descending ascending descending ascending descending, and the overall compressive load is relatively small. The load change of CFRP strengthened specimens is obvious. The order of compression load is: SRTP_4 > SRTP_3 > SRTP_1 > SRTP_2. When the structure or component is constantly stressed, it will continue to produce damage, and the stiffness and strength of the specimen will decrease. The seismic damage of the test piece is expressed by stiffness degradation, and the expression is:

$$ D(m) = \frac{K_0 }{{K_r }} $$
(7)

In Eq. (7), \(D(m)\) is the degree of stiffness degradation; \(K_0\) is the initial tangent stiffness of the test piece; \(K_r\) is the reduced secant stiffness at the maximum displacement of the test piece. According to test piece VB_1, SRTP_1, SRTP_2, SRTP_3, SRTP_4, draw the stiffness degradation curve of each test piece, as shown in Fig. 2 below.

Fig. 2.
figure 2

Stiffness Degradation Curve of CFRP Specimen.

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As shown in Fig. 2, test piece VB_1 the stiffness is within the displacement of 20 mm–80 mm, showing an overall decline trend. When the stiffness is 80 mm, it tends to be zero, and the stiffness is poor, so the seismic effect decreases. In the CFRP strengthened specimens, the stiffness tends to zero at 170 mm, which is relatively good, and the seismic effect increases accordingly. SRTP_1, SRTP_2, SRTP_3, SRTP_4 among the test pieces, the order of rigidity is SRTP_2 > SRTP_1 > SRTP_4 > SRTP_3. It can be seen from the figure that the yield displacement of the test piece is 40mm, and the stiffness degradation of each test piece gradually slows down after yielding. When the ultimate displacement reaches 170 mm, the stiffness degradation is almost flat, and the degradation amplitude is small. Therefore, the specimens strengthened with CFRP have higher stiffness and can dissipate more seismic energy.

4 Test Results

Earthquake is a disaster that has a great impact on the stability of buildings. There are three types of failure modes on building components, namely, the failure mode of structural integrity loss, the failure mode of insufficient bearing capacity of load-bearing structures, and the failure mode of foundation impact. Under the strong earthquake action, the internal force and deformation of the component increase, thus reducing the bearing capacity of the component. In this paper, ductility, energy consumption and other indicators are used to reflect the seismic capacity of CFRP strengthened specimens. Among them, ductility is a reflection of the elastic-plastic deformation capacity of CFRP strengthened specimens under non reciprocating loads. It is crucial to analyze the seismic performance of CFRP strengthened specimens by absorbing the energy generated by seismic loads through its own elastic-plastic deformation. In this paper, the ductility coefficient is used to express the seismic performance of the assembled joints, and the ductility variation law is judged in the form of forward loading and reverse loading. In general, the higher the ductility coefficient of CFRP strengthened specimens, the more seismic energy they absorb, the greater their ultimate deformation and yield deformation, and the better their seismic performance. When all the test pieces are damaged, the corner of the assembly node is between 0.09 rad and 0.12 rad; The corner of banana node is between 1/10–1/8, which is much larger than 1/30 in the specification. The ductility change of each test piece is obtained as shown in Table 2 below.

Table 2. Lithologic coefficient of assembly node displacement.
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As shown in Table 2, the ductility coefficient of cast-in-place reinforced concrete column specimens is smaller than that of fabricated reinforced concrete column specimens. The order of ductility coefficient of fabricated test piece is SRTP_2 > SRTP_3 > SRTP_1 > SRTP_4. The larger the ductility coefficient is, the stronger the plastic deformation capacity of the reinforced concrete column structure can withstand, and the better the seismic performance of the structure is. Under earthquake action, CFRP strengthened specimens absorb energy through cracking and yielding, and the energy absorbed under repeated earthquake action is the seismic energy dissipation capacity of the specimens. In general, the energy dissipation capacity is measured by the surrounding area of the hysteresis curve. The larger the load envelope area of the hysteresis curve is, the stronger the energy dissipation capacity of CFRP strengthened specimens is; The smaller the area of the load envelope of the hysteresis curve, the weaker the energy dissipation capacity of the CFRP strengthened specimens. In this paper, cumulative energy consumption and equivalent viscous damping coefficient are used to express the energy consumption of CFRP strengthened specimens, and the first cycle energy of each horizontal displacement loading step is taken as the single cycle energy consumption of CFRP strengthened specimens, and the single cycle energy consumption of loading under seismic action is accumulated. The energy consumption change of the test piece is obtained as shown in Table 3 below.

Table 3. Lithologic coefficient of assembly node displacement.
Full size table

As shown in Table 3, this paper selects VB_1, SRTP_1, SRTP_2, SRTP_3, SRTP_4, for test pieces such as, three assembly nodes X, Y and Z are selected for each test piece. Among them, the test piece SRTP_1, SRTP_2, SRTP_3, SRTP_4 is the CFRP strengthened specimen; Test piece VB_1 is the test piece not strengthened by CFRP. In case of positive loading, the test piece SRTP_1, SRTP_2, SRTP_3, SRTP_4 has a high energy consumption capacity. And the energy consumption capacity is sorted as SRTP_1 > SRTP_4 > SRTP_3 > SRTP_2. Test piece VB_1’s energy consumption capacity is low. It can be seen that after CFRP strengthening and repair, the energy dissipation capacity of uniformly corroded reinforced concrete column specimens can be restored to the energy dissipation state when they are undamaged, even stronger than the energy dissipation state when they are undamaged, and the seismic performance is better. SRTP of test piece under negative loading SRTP_1,SRTP_2,SRTP_3,SRTP_4 The energy consumption capacity of is also high. And the energy consumption capacity is ranked as SRTP_3 > SRTP_4 > SRTP_1 > SRTP_2. The energy consumption capacity of the test piece VB_1 is also low. It can be seen that the non-uniform corrosion damage of reinforced concrete variable section under negative loading has a greater impact on the energy dissipation capacity of the specimen, and the cumulative energy dissipation changes more obviously. The specimens strengthened with CFRP can enhance the energy dissipation capacity of integral lifting joints of reinforced concrete columns under positive loading. The shorter the non-uniform corrosion damage part, the weaker the energy dissipation capacity of the specimens. The more the cumulative energy dissipation increases under positive loading, the better the seismic performance.

5 Conclusion

In recent years, prefabricated building structures have been widely used to meet the development needs of building construction. Prefabricated buildings significantly reduce process requirements and speed up construction efficiency. At the same time, building components can be mass-produced in the factory in advance, the construction quality and cost have been improved, the loss of building materials has also been reduced, and the demand for saving building construction resources has been met. Therefore, the prefabricated building structure is replacing the traditional cast-in-place concrete building structure in recent decades. Under the action of earthquake, the assembly nodes of prefabricated buildings are relatively stable, and the stability requirements of building components can be guaranteed only by strengthening or widening a few components. However, with the increase of application time of reinforced concrete, the problem of uneven corrosion on its surface becomes more serious, which affects its seismic performance. Therefore, this paper uses CFRP strengthening method to analyze the seismic performance of assembled joints of reinforced concrete columns with uneven corrosion. In the form of experiments, the stiffness, ductility, strain and energy consumption of assembly joints are analyzed to provide data support for the stability of building components.