Interface bond strength performance of overlap joints within the corrugated pipes used in prefabricated bridges

Abstract

This paper presents an experimental study of a novel composite structure used in prefabricated bridges. Corrugated pipes were used to improve the interface bond performance of the structure because of their excellent stiffening effect on the grouting material. Interface bond performance of overlap joints within corrugated pipes was explored by the load-displacement curve and load-strain curves. Ultra-High Performance Concrete (UHPC) and high-strength mortar were used as grouting materials. The diameter of steel bars, UHPC, high-strength mortar, strength grades of surrounded concrete, anchorage length, the diameter of the corrugated pipe, and lap length was taken as influential factors. Twenty specimens were designed for the pull-out test by using a larger cover thickness. The failure modes and the influence of different influential factors on the interface bond strength of each specimen were analyzed. The results show that the bond performance between UHPC and reinforcement was better than that of high-strength mortar and normal concrete, which can effectively improve the bond strength and reduce the basic anchorage length of reinforcement besides the design size of prefabricated members. In addition, the differences in anchorage length and lap length between the corrugated pipe grouting reinforcement were compared to the different specifications and prefabricated concrete members. Combined with the test phenomenon and analysis results, it is suggested that the anchorage length and lap length of connecting reinforcement should be reconsidered. Furthermore, the grouting effect under different diameters of corrugated pipe and reinforcement were compared. It is recommended that the corrugated pipe diameter should be four times that of the overlapping grouting reinforcement.

Keywords

bonding mechanism ultra-high performance concrete grouting high-strength mortar corrugated pipe grouting connection

Introduction

In recent years, prefabricated reinforced concrete structures have been widely promoted. The prefabricated structure has the advantages of short construction time, controllable construction quality, and less environmental pollution (Islamaj, 2018; Priya et al., 2018; Tomek, 2017). The sufficient bond strength between the connecting reinforcement and the high-strength grouting material is the key to ensuring the prefabricated structures’ safety and service performance (Haber et al., 2018b; Seibert et al., 2019; Semendary et al., 2019; Zhang et al., 2017). However, the prefabricated structure has different characteristics than the cast-in-place structure (Xia et al., 2021). The thickness of the protective layer of the prefabricated connection is generally more than 50 mm larger than that of the cast-in-place structure (Breccolotti et al., 2016; Hu et al., 2021a).

Only a few publications for grouting material reinforcement concrete used in the prefabricated structure are available. However, there is a series of research for interface bond strength of Ultra-High Performance Concrete (UHPC), normal strength concrete, and steel bars; the most relevant are mentioned. High-strength concrete and the diameter of steel bars significantly affect the mechanical property and bond strength of concrete. Pull-out resistance of steel can be improved by controlling the crack growth inside the concrete (Baran et al., 2012). Yazıcı and Arel (2013) carried out an experimental study to analyze the influence of the volume content of steel fibers and the thickness of the concrete protective layer on the bond and anchorage performance of steel fiber concrete and deformed steel bar.

The connection of structure for prefabricated components is an essential part of the design and construction. The joint connection forms of prefabricated structures mainly include wet connection and dry connection (Shah et al., 2021). The form of a wet connection is to weld, lap, or mechanically connect the reserved connecting steel bar at the connection part. The dry connection embeds the steel connecting parts in the prefabricated concrete components and connects them into one through bolt connection or welding a holistic approach (Sulaiman et al., 2017). Khabaz (2017) reviewed the influence of the past lateral pressure on the bond and anchorage performance of the interface between reinforcement high-strength concrete and normal concrete. Results show that the application of concrete lateral constraints can limit the radial deformation of the reinforced concrete protective layer.

Valikhani et al. (2020) conducted an experimental study to investigate the bond strength between UHPC and normal concrete and found that the interfacial surface with or without mechanical connectors transferred the failure mode to the concrete substrate, indicating high bond strength between the two materials. Yin et al. (2017) conducted an experimental study to analyze interfacial bond behavior between UHPC and normal. The test results show that different types of interfaces between UHPC and normal concrete have different bond performances. Concrete strength is the critical factor determining the shear performance of the interfaces between UHPC and normal concrete. Different casting sequences of UHPC and normal concrete significantly affect the shear performance of different interfaces.

The configuration of UHPC significantly affects the failure modes and crack patterns of the composite UHPC-concrete structure. It shows that the thickness of the UHPC layer changes the failure mode from the brittle diagonal shear failure to ductile flexure failure, but there is no improvement observed in the ultimate strength (Haber et al., 2018a). Semendary et al. (2020) investigated the bond strength of UHPC and found that the UHPC age after casting resulted in a marginal increase in the bond strength for normal strength concrete and a significant increase in the bond strength of the high-strength concrete specimens.

Semendary et al. (2020) examined interface bond performance under direct shear and the contribution of the shear reinforcement to the load transfer before and after interface failure. The test results show that UHPC significantly increases the adhesion value of the monolithic surface, and the shear reinforcement placed across the interface reduces the slip at the interface before interface failure. Deng et al. (2020) studied two different shapes of joint performance from the comparative model test. It was observed that the T-shaped joint was superior to the traditional I-shaped joint in terms of crack resistance. The weak interfaces of the T-shaped joint were set in the areas with the relatively lower negative bending moment, which could significantly decrease the cracking risk of the joint interface.

The interface bond strength between UHPC and concrete depends on interface roughness. Surface roughness yields greater bond strength in shear-compression and shear-flexure loading. The bond strength behavior of steel bars in UHPC is different from that in normal strength concrete in many aspects. The diameter of steel bars and anchorage can be significantly reduced (Yuan and Graybeal et al., 2015). Alkaysi and El-Tawil (2017) studied the influence of plain and epoxy-coated steel bars with different diameters on the bond strength performance of reinforcement and UHPC. Results show that bond strength decreases with increased embedment, and steel fiber content differences yielded significant bond strength differences.

However, the interface bonding mechanism is complex, and there are many influential factors involved. It is concluded from the literature review that there is no research on the interface bonding performance of corrugated pipe grouting connections. There are few design specifications for grouting materials, and there are no provisions on bond strength, anchorage, and lap length of the reinforcement for corrugated pipe grouting connection, which to some extent, limits the application of grouting materials and development of prefabricated bridges.

Therefore, to further understand the interface bond strength and anchoring performance of corrugated pipe grouting connections within the prefabricated structure, 20 specimens were manufactured for the pull-out test using a larger cover thickness. The failure modes, load-displacement curves, and the influence of different influential factors on the interface bond strength of each specimen were analyzed. Furthermore, different specifications' differences in anchorage length and lap length of corrugated pipe grouting reinforcement were compared, and recommendations were made for the design standard of prefabricated concrete members.

Specimen design and fabrication

Qiaonancun flyover of Changjiu Expressway in Jiangxi province was taken as a prototype. In order to study the reliability of the splicing and anchoring connection mode of corrugated grouting reinforcement in a prefabricated bridge connection, the overlapping of reinforcement in the corrugated pipe was set. The overlapping reinforcement extends 200 mm at both ends, and the section size was 300 mm × 300 mm; the corrugated pipe was made of steel strip as shown in Figure 2(a). The design drawing cross-section layout and measuring point arrangement of corrugated pipe are shown in Figure 1.

Figure 1.

Drawing design of corrugated pipe grouting. (a) Side layout and (b) Cross-section layout.

The mold of the test specimens and the corrugated pipe were assembled, as shown in Figure 2. There are holes on both sides of the mold to fix the corrugated pipe at the center of the cross-section. The wood mold is used for pouring the specimen, and the manual vibration method is used. The specimens were cured in the standard way for 28 days after the completion of fabrication.

Figure 2.

Fabrication drawing of mold and corrugated pipe grouting. (a) Corrugated pipe fabrication, (b) Mold and (c) Pouring of concrete.

Parameters of the test specimen

Ultra-High Performance Concrete and high-strength mortar were used as grouting materials within the corrugated pipe. C30 and C50 were used as surrounded concrete, and the reinforcement diameter was 10 mm, 12 mm, 16 mm, 20 mm, and 22 mm. Anchorage length was 300 mm, 500 mm, and 600 mm, the diameter of the corrugated pipe was 35 mm, 50 mm, 60 mm, and 70 mm, and the lap length was 100 mm, 200 mm, and 300 mm. Design parameters of 20 specimens are shown in Table 1.

Table 1.

Design parameters of test specimens.

External concrete	Reinforcement diameter/(mm)	Diameter of corrugated pipe/(mm)	Anchorage length/(mm)	Grouting material	Lap length/(mm)
C50	10	35	500	Mortar	200
C50	10	35	500	Mortar	200
C50	12	35	500	UHPC	200
C50	12	35	500	UHPC	100
C50	12	50	500	UHPC	300
C50	12	50	500	UHPC	100
C50	12	50	500	UHPC	200
C50	12	50	500	UHPC	200
C50	16	35	500	Mortar	-
C50	16	50	500	UHPC	100
C50	16	50	500	UHPC	200
C30	16	50	500	UHPC	200
C50	16	50	500	UHPC	200
C50	16	60	500	UHPC	200
C50	16	60	500	UHPC	200
C30	20	50	300	UHPC	-
C30	20	50	300	Mortar	200
C50	20	70	500	UHPC	200
C50	20	70	500	Mortar	200
C50	22	70	600	UHPC	100

Note: UHPC: Ultra-High Performance Concrete.

Loading device and site layout

The hydraulic oil pump was used as the loading device with a measuring range of load 0–500 kN and measuring range of displacement 0–50 mm. In order to eliminate the restraint effect of the pull-out device on the concrete around the steel bar at the loading end and to better analyze the bonding characteristics of the bond among the steel bar, grouting material, corrugated pipe, and external concrete, a transition device with a circular hole was set between the pull-out device and the loading end of the specimen. The load increment was 2 kN–5 kN at the initial stage and 5 kN–10 kN at the middle. Under the same load, the displacement increases, the anchorage reinforcement is considered to yield; when the value load no longer increases with the increase of displacement, the loading mode is changed from load control to displacement control. At the end of each loading stage, according to the principle of one-to-one correspondence, the load values of the pull-out device, the displacement values of the loading end, and the steel bar strain value inside the specimen were collected. Figure 3 shows the schematic diagram and site layout of the loading device.

Figure 3.

Schematic diagram and site layout of the loading device. (a) Schematic diagram and (b) Site layout.

Materials and properties

Relevant provisions (GB/T-2009) were used to measure the compressive strength and elastic modulus of C30 and C50. The average compressive strength of C50 and C30 was 50 MPa and 30.4 MPa, respectively. The prism blocks of 150 mm×150 mm×300 mm were made for elastic modulus tests of C30 and C50 referring code (GB 50010-2010).The average value of the elastic modulus for C50 and C30 was 34,613 MPa and 32,522 MPa, respectively.

The UHPC and high-strength mortar were independently developed by “Huaxin new building materials Co., Ltd.” UHPC was composed of premixed powder, admixture, steel fiber, and water, aggregates gradation of UHPC is provided in the author’s previous work (Shah et al., 2021). According to relevant provisions (CECS13-2009; GB/T 31387-2015, 2015). Three blocks of cubes with the length of 100 mm×100 mm ×100 mm and three prism specimens with the length of 100 mm ×100 mm ×300 mm were designed for compressive strength tests and elastic modulus test, respectively. The average value of UHPC compressive strength was 150 MPa, and the average value of elastic modulus was 54.7 GPa. Tensile test of steel bar was carried out according to the relevant provision (GB1499.2-2018). Hot-rolled Ribbed Bar (HRB400Φ16 and Φ12) steel bars with the yield strength of 570 MPa, 525 MPa, and ultimate strength of 705 MPa, 645 MPa, respectively were used in this study tensile test of steel bar is provided in the author’s previous work (Hu et al., 2021a, 2021b). The high-strength mortar used in this study had a compressive strength of 80.5 MPa, and the elastic modulus was 40 GPa.

Test criteria

The pull-out test of corrugated pipe grouting in a prefabricated concrete structure was carried out, and the ultimate load of the grouting anchorage specimen was obtained as shown in Figure 4. According to technical specifications for post anchorage of concrete structures (JGJ145-2004), the failure modes of grouting anchorage reinforcement connection mainly include steel bar failure, concrete cone failure, and cement-concrete interface failure, as shown in Figure 4(a), 4(b), and 4(c) respectively. Among them, the interface failure of cement concrete can be divided into three types: pull-out along with the interface between reinforcement and grouting material, pull-out along with the interface between grouting material and corrugated pipe, and pulling out at the interface between corrugated pipe and concrete. If the reinforcement failure occurs, the connection is reliable; if the other two forms of failure occur, the connection is not reliable.

Figure 4.

Schematic diagram of failure mode. (a) Interface failure between cement and concrete, (b) Tensile failure of steel bar and (c) Cone failure.

Test results

Failure mode and ultimate load of 20 corrugated pipe grouting specimens were recorded, and the load-displacement and load-strain curves were analyzed. The influence of different grouting materials on bond performance was compared. The failure mode of each specimen is shown in Table 2. The failure load of the specimen increases with the increase of reinforcement diameter, and the strength grade of surrounded concrete had little effect on the failure load of the specimens. Under the different diameters of the corrugated pipe, the failure load of the specimens was almost the same. The failure load of the specimen was almost similar when the anchorage length was not more than 22d.

Table 2.

Failure mode of specimens.

External concrete	Reinforcement diameter/(mm)	Diameter of corrugated pipe/(mm)	Anchorage length/(mm)	Grouting material	Lap length	Load/(kN)	State of failure
C50	10	35	500	Mortar	200	42.8	Reinforcement failure
C50	10	35	500	Mortar	200	44.3	Reinforcement failure
C50	12	35	500	UHPC	200	71.8	Reinforcement failure
C50	12	35	500	UHPC	100	69.4	Reinforcement failure
C50	12	50	500	UHPC	300	69.6	Reinforcement failure
C50	12	50	500	UHPC	100	68.5	Reinforcement failure
C50	12	50	500	UHPC	200	69.8	Reinforcement failure
C50	12	50	500	UHPC	200	72.2	Reinforcement failure
C50	16	35	500	Mortar	-	128.7	Reinforcement failure
C50	16	50	500	UHPC	100	126.2	Reinforcement failure
C50	16	50	500	UHPC	200	128.6	Reinforcement failure
C30	16	50	500	UHPC	200	130.3	Reinforcement failure
C50	16	50	500	UHPC	200	108.1	Thread failure
C50	16	60	500	UHPC	200	116.34	Reinforcement failure
C50	16	60	500	UHPC	200	125.5	Reinforcement failure
C30	20	50	300	UHPC	-	191	Reinforcement failure
C30	20	50	300	Mortar	200	138.82	Thread failure
C50	20	70	500	UHPC	200	193.2	Reinforcement failure
C50	20	70	500	Mortar	200	215.02	Reinforcement failure
C50	22	70	600	UHPC	100	225.57	Reinforcement failure

Note: UHPC: Ultra-High Performance Concrete.

The pull-out results of all specimens were the tensile failure of reinforcement, and there was no cone-shaped failure and cement-concrete interface failure. Therefore, the connection mode of corrugated pipe reinforcement anchorage was adopted. The anchorage length is taken as 53% of the anchorage length in the current specification, which can still ensure that the corrugated pipe grouting lap reinforcement can give full play to its strength function.

Failure mode of grouting anchorage

In order to explore the reliability of the lap joint connection of corrugated pipe grouting reinforcement in prefabricated concrete members, the corresponding specimens were made for the steel bar pull-out test. The failure mode of the specimens is shown in Figure 5. The results show that both the lap joint specimens Figure 5(a), Figure 5(b), and the non-lap joint specimens Figure 5(c) has a tensile failure.

Figure 5.

Failure diagram of corrugated pipe grouting specimen. (a) Tensile failure of reinforcement at loading end, (b) Free end and (c) Tensile failure of non-lap reinforcement.

There was no splitting failure, and cementation surface failure occurred. It shows that the anchorage performance of the overlapped connection of corrugated pipe grouting reinforcement was consistent with that of the non-lap reinforcement, and the reliability of the lap connection is high. Under the pull-out load, the cone-shaped failure of concrete does not occur at the loading end of specimens. The corrugated pipes have an excellent stiffening effect on the grouting material, which improves the bond performance between the reinforcement and the grouting material to a certain extent. There was no obvious damage or slippage among the free end reinforcement, grouting material, corrugated pipe, and surrounded concrete. The strength of grouting material is far greater than that of normal concrete. The tensile failure of reinforcement still occurs after the anchorage length of reinforcement specified in the specification was reduced, which indicates that the anchorage length of reinforcement and grouting material was sufficient in the test.

When mortar grouting was used in corrugated pipe, the grouting material at the loading end appeared cone-shaped damage phenomenon as shown in Figure 6(a). When UHPC grouting was used, as shown in Figure 6(b), there was no cone-shaped damage phenomenon at the loading end, which indicates that the grouting effect and bonding performance of UHPC was better than that of mortar. The cross-sectional of each specimen was 300 mm × 300 mm, and the diameter of the anchor reinforcement was 16 mm.

Figure 6.

Comparison of failure phenomena of grouting materials. (a) Mortar grouting specimen and (b) Ultra-High Performance Concrete Grouting specimen.

Grouting effect analysis

Figure 7 shows that the grouting material inside the corrugated pipe is not damaged, and the grouting material has a good bond with the corrugated pipe, indicating that the bonding between the corrugated pipe and the grouting material is reliable. From the appearance and compactness of grouting material, it can be seen that the grouting effect of corrugated pipe with a diameter of 70 mm is better, and there is no phenomenon of insufficient grouting. The results show that the bond surface between the corrugated pipe and normal concrete does not slip obviously. The steel bar failure indicates that corrugated pipe and normal concrete’s bonding effect was better than the semi-grouted sleeve. It has the advantages of convenient construction, low cost, large allowable assembly error, and good connection effect. It can be applied to the connection of members, mainly bearing vertical force and bending load.

Figure 7.

Cutting diagram of corrugated pipe specimen. (a) Grouting material cutting and (b) Corrugated pipe cutting.

Test results analysis

Load-displacement curve

Figure 8(a) shows the load-displacement curve of corrugated pipe specimens under multiple loading. It shows that reinforced concrete pull-out specimens have a stiffness reduction stage before yielding at the first loading stage. When the load value reaches the maximum value of the first loading, the reinforcement begins to yield, and there is a long yield platform. At the third stage of loading, there was no obvious strength after the yield transformation stage. The ductility of the specimen decreases after repeated loading, and the tensile failure occurs in a short deformation after reaching the yield load of reinforcement.

Figure 8.

Load-displacement curve of corrugated pipe specimen. (a) Multiple loads and (b) Single load.

By comparing the load-displacement curves from multiple loading to specimen failure and single loading to specimen failure, it can be seen that the cumulative deformation from multiple loading to steel bar failure was more significant than a single loading specimen.

Corresponding to the actual engineering, the ductility of the component under repeated load may be a sudden brittle failure. However, the cumulative deformation under repeated loading was more significant than that under a single overload condition. Therefore, in structural detection, 50% of the ultimate deformation capacity of the member under single loading should be used for early warning control to ensure the structure’s safety.

Load-strain curve

Figure 9 shows the load-strain curve of the corrugated pipe under the pull-out load, the stress mode of the two-lap bars in the corrugated pipe is similar, and the force transmission performance of the overlapping connection mode the corrugated pipe grouting reinforcement is better. As shown in Figure 9, the strain of loaded end reinforcement is about 20 times that of free end reinforcement. It is found that the strain of corrugated pipe grouting reinforcement increases slowly under the restraint and reinforcement effect of corrugated pipe grouting structure, which indicates that the bonding performance between grouting material and reinforcement is good.

Figure 9.

Load-strain curve of 500-70-16-Ultra-High Performance Concrete specimen.

The basic anchorage length of reinforcement in this structure is small. Under the pull-out load, the reinforcement in the grouting material is still in the elastic stage, and the bonding effect between deformed reinforcement and grouting material is good. When the anchorage length is sufficient, the tensile capacity of the joint is mainly determined by the tensile capacity of the reinforcement. The bearing capacity of the fabricated structure can be improved by increasing the diameter of the connecting reinforcement.

Analysis of influencing factors

Reinforcement diameter

It was found that the ultimate load of the specimens increases gradually with the increase of reinforcement diameter, as shown in Figure 10. Compared with 500-50-12-UHPC and 500-16-20-UHPC. The ultimate load of 500-50-20-UHPC was increased by 1.8 times and 50.2%, respectively. The specimens with different diameters of steel bars have a tensile failure, and there is no obvious slip at the free end.

Figure 10.

Comparison of reinforcement diameter.

Diameter of corrugated pipe

As shown in Figure 11(b), the tensile failure of reinforcement occurs when the corrugated pipe diameter is 35 mm, 50 mm, and 70 mm, and the diameter of the steel bar is 16 mm. The ultimate load of each specimen was basically the same, and the change of corrugated pipe diameter has little effect on the bearing capacity of the specimen. The ratio of the corrugated pipe diameter to the steel bar was used as the restraint size of the corrugated pipe to grouting material and internal reinforcement. If the ratio were large, the restraining effect of the corrugated pipe would be small. To enhance the anchorage performance of the overlapped reinforcement in the grouting connection of the corrugated pipe, the diameter of the corrugated pipe should not be too large. The grouting effect under different diameters of corrugated pipe and reinforcement are compared and analyzed, and the diameter of the corrugated pipe was recommended to be four times the diameter of the overlapping grouting reinforcement.

Figure 11.

Comparison of the corrugated pipe diameter. (a) Reinforcement diameter 12 mm and (b) Steel bar diameter 16 mm.

Lap length

Figure 12 shows the comparison of load-displacement curves under different lap lengths. The ascending section of the load-displacement curve for lap lengths of 100 mm and 200 mm shows that the stiffness of the component increases as the overlapping length of the reinforcement in the corrugated pipe increases.

Figure 12.

Comparison of lap length.

Comparing the load-displacement curves of the specimens with 200 mm and 300 mm lap lengths, it was found that the stiffness of the specimens remained unchanged when the lap length was sufficient. At the same time, it was found that under the test design parameters, the specimens with different lap lengths have a tensile failure, which indicates that the reliability of the grouting reinforcement lap connection was high.

Types of grouting materials

Figure 13 shows the comparison of the load-displacement curves of different grouting. The normal strength concrete specimen (without grouting material C50-10-300) was slightly greater than the grouting material; the bond strength between reinforcement and normal concrete and the bonding interface was less than the grouting material. Therefore the displacement of normal concrete specimens increases, and the strength of anchoring reinforcement reflects the joint action of reinforcement and concrete; In contrast, the bond strength of corrugated pipe grouting material and reinforcement was significant.

Figure 13.

Comparison of grouting material load-displacement curves.

Therefore, to improve the connection effect of corrugated pipe grouting, high-strength steel bars should be used for high-strength grouting materials. From the ascending section of the load-displacement curve, bond stiffness between UHPC and reinforcement was better than mortar and normal concrete, and the bond stiffness between concrete and reinforcement was the smallest. The bond strength between reinforcement and three kinds of materials from high to low is UHPC, high-strength mortar, and normal concrete.

Surrounded concrete strength

As shown in Figure 14, when the corrugated pipe’s surrounding concrete strength was C30, the failure mode of the specimen was still the tensile failure of the reinforcement, and there was no apparent difference between the specimens with the surrounded concrete strength of C50. Because the diameter of the corrugated pipe was larger than the diameter of the steel bar, the bonding area between the corrugated pipe and surrounded concrete was larger, and the wave crest height was greater than the height of the transverse rib of the steel bar, so the bond strength of the corrugated pipe and grouting material was much greater than that of the normal steel bar. According to the previous analysis, when the diameter of the corrugated pipe was four times to the diameter of internal lap reinforcement, the formula of interface bearing capacity is $F = π d l \times τ_{u} = A \times τ_{u}$ (A is the area of the bonding surface) can be obtained when the anchorage length was the same; the bond bearing capacity of the interface between the corrugated pipe and the normal concrete was four times that of the reinforcement.

Figure 14.

Strength grade comparison of surrounded concrete of corrugated pipe.

Therefore, when the anchorage length of the corrugated pipe in the normal concrete can be taken as 25% of the anchorage length of the reinforcement l_a, the anchorage requirement can be met, and the anchorage length of the overlapping reinforcement in the corrugated pipe was more significant than 0.25l_a.

Anchorage length and lap length of reinforcement

Anchorage length of reinforcement

In the corrugated pipe grouting connection of prefabricated concrete members, the anchorage length of connecting reinforcement was the critical parameter of component size design. At the same time, due to the application and development of new materials in engineering practice, it was essential to study the anchorage performance of the new materials and reinforcement. Because the tensile and compressive strength of UHPC grouting material was significantly higher than that of high-strength mortar and normal concrete, its bond performance with reinforcement is better. The anchorage length of connecting reinforcement in UHPC grouting material will also be reduced. Simultaneously, JTG3362-2018 stipulates that the reinforcement ratio of all longitudinal reinforcement of axial compression or eccentric compression members should be between 0.5%-0.6% when the concrete strength grade was C50 or above.

With the increase of concrete strength, the reinforcement ratio required to use its strength will also increase. When the thickness of the corrugated pipe was c (about 1 mm), and the cross-sectional area of each corrugated pipe was $π c D$ , the reinforcement ratio of the component was $(n π c D + n π d^{2} / 4) / A$ . Taking the reinforcement of a section with a diameter of 20 mm as an example, when the corrugated pipe diameter was two times the reinforcement diameter, the corrugated steel pipe accounts for 28.6% of the total reinforcement ratio of the section. When the corrugated pipe diameter was four times the reinforcement diameter, the corrugated steel pipe accounts for 44% of the total reinforcement ratio. Therefore, the influence of steel the corrugated pipe on reinforcement ratio and structural performance should be considered in prefabricated components.

Simultaneously, the anchorage length of connecting reinforcement should be reconsidered, and the effective anchorage of reinforcement in the corrugated pipe should be taken as the design standard.

According to the current code of GB 50010-2010, the basic anchorage length of UHPC grouting material was 47.9% and 25% of that of high-strength mortar and normal C50 concrete, respectively. According to ACI 318-11, the basic anchorage length of UHPC grouting material was 53.7% and 26.2% of that of high-strength mortar and normal C50 concrete, respectively, as shown in Figure 15. The anchorage length of reinforcement in UHPC grouting material was far less than that of high-strength mortar and normal concrete. The corrugated pipe and UHPC grouting material can enhance the restraint of reinforcement, increase the mechanical interlocking force and interfacial bond strength, and reduce the basic anchorage length. Therefore, in the grouting connection of prefabricated corrugated pipe UHPC, the anchorage length of connecting reinforcement can be multiplied by the corresponding correction coefficient based on existing specifications to guide the design and construction.

Figure 15.

Comparison of different grouting anchorage lengths. (a) GB50010-2010 and (b) ACI318-11.

The test results show that there was no reinforcement anchorage failure in all the specimens. The failure mode of the specimens was the tensile failure of the reinforcement, which indicates that the design anchorage length of the connecting reinforcement in the corrugated pipe of the prefabricated bridge was sufficient. As shown in Figure 16, the test specimens with commonly used reinforcement diameters of 16 mm, 20 mm, and 22 mm for prefabricated bridges are considered the analysis objects. The anchorage length between connecting reinforcement and corrugated pipe grouting reinforcement in different specifications and prefabricated concrete members were compared, where the basic anchorage length was calculated in the specification.

Figure 16.

Comparison of anchorage length of corrugated pipe grouting reinforcement.

Because the reinforcement and grouting material in the overlap section was not fully anchored, the anchorage length of reinforcement was considered the superposition of 0.5 times of overlapping length and non-overlapping length. When the diameter of steel bars was 16 mm, the anchorage length of the corrugated pipe lap reinforcement was the same as that of the prefabricated concrete member. When the reinforcement diameter is 20 mm and 22 mm, the anchorage length of the corrugated pipe grouting lap reinforcement was about 0.72 times that of the normal connecting steel bar.

It is recommended that the anchorage length of corrugated pipe grouting lap reinforcement should be 15d when the diameter of the reinforcement is not greater than 16 mm, which is about 0.53 times of the anchorage length specified in GB 50010-2010 or 0.49 times of JTG 3362-2018. When the diameter of the reinforcement is not less than 20 mm, it is recommended that the anchorage length of the corrugated pipe grouting lap reinforcement should not be less than 12d, which is about 0.42 times of the anchorage length specified in GB 50010-2010 or 0.39 times of JTG 3362-2018.

Lap length of reinforcement

For prefabricated construction, the original longitudinal full-length reinforcement will be cut off and connected by connecting components or structures, which will inevitably affect the mechanical performance of the structure. Moreover, the existing sleeve connection directly connects the reinforcement with high precision, which was assembled on-site. The construction workability is low, and the cost is high. The corrugated pipe grouting connection has the advantages of convenient construction, reliable connection, and cost-saving.

However, the existing corrugated pipe grouting connection was mostly a single steel bar anchoring connection. To ensure the normal transmission of load between the connecting reinforcement, the grouting anchorage length of the two connected components should be at least two times the anchorage length of the reinforcement in the grouting material.

Due to the large size requirement of prefabricated components, the self-weight of the structure will increase, which may have adverse effects on the stress of some structures, and increase the production, transportation, and construction costs. The reinforcement lap connection method can effectively utilize the anchoring effect of corrugated pipe grouting on the reinforcement, significantly reduce the size of prefabricated components, improve the structure’s overall performance, and save costs based on meeting the normal load transfer in the reinforcement.

The regulation of the overlapping length of reinforcement was to meet the demand for force transmission between overlapping bars and ensure the reliability of the structure. According to the domestic code GB 50010-2010, the lap length of longitudinal tensile reinforcement binding lap joint should be based on the percentage of joint lap area in the same connection section according to the formula $l_{l} = ζ_{l} \times l_{a}$ .

The calculation, JTG3362-2018, stipulates that when the concrete strength grade is more than C30, the lap length of HRB400 reinforcement should be 45d, and both of them stipulate that the lap length of tensile reinforcement should not be less than 300 mm in any case.

In the corrugated pipe grouting connection of prefabricated concrete members, due to the structural requirements, the percentage of joint lap area is 100%, and the lap length of longitudinal tensile reinforcement needs to be multiplied by a correction factor of 1.6 $ζ_{l}$ . That is, the overlapping length of reinforcement in corrugated pipe shall not be less than 480 mm.

Considering the binding and strengthening effect of corrugated pipe and grouting material on the connecting reinforcement, the overlapping length of reinforcement in the existing code is reduced. In this paper, the overlapping length of reinforcement used in the partial lap test of corrugated pipe grouting reinforcement is 200 mm, which is about 0.67 times to the minimum lap length specified in the specification is 0.26 times, 0.16 times, and 0.33 times of that in GB 50010-2010, JTG3362-2018 and JGJ107-2016, as shown in Figure 17.

Figure 17.

Comparison of overlapping length of grouting reinforcement for corrugated pipe.

Simultaneously, the control group with a lap length of 100 mm and 300 mm was set as a reference. The lap length of reinforcement designed in the test was not more than the minimum value specified in the specification. The results show that the structural parameters of the corrugated pipe grouting connection in the test can meet the mechanical requirements of the prefabricated bridge member connection.

Therefore, considering the safety factor of 1.2 times, it is suggested that the lap length of corrugated pipe grouting connection reinforcement of prefabricated components should be 12d, which is about 0.21 times the design value of the existing JTG3362-2018, 0.34 times of the design value of the GB 50010-2010, and the length of the connecting member was 0.71 times that of the single bidirectional anchorage with corrugated pipe grouting.

Conclusion

Through the experimental study of 20 corrugated pipe grouting specimens, the failure modes, load-strain curves, load-displacement curves, and the influence of different influential factors on the interface bond strength of each specimen were analyzed. The main conclusions are as follows.

1. Test results show the tensile failure of reinforcement; there was no reinforcement pull-out failure, concrete cone failure, and adhesive concrete interface failure which indicates that the connection mode of the corrugated pipe grouting reinforcement overlapping was more reliable in the connection of prefabricated concrete components.

2. The bond performance between UHPC and reinforcement was better than that of high-strength mortar and normal concrete, which can effectively improve the bond strength and reduce the basic anchorage length of reinforcement and the design size of prefabricated members.

3. To make full use of the restraining and strengthening effect of the corrugated pipe on the grouting connection reinforcement under the condition of ensuring the grouting effect, the diameter of the corrugated pipe should be four times than the overlapping grouting reinforcement.

4. Combined with the test phenomenon and analysis results, considering the safety factor of 1.2 times, it is suggested that the lap length of the corrugated pipe grouting connection reinforcement is 12d.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financially supported by “The Project of National Key Research & Development (2017YFC0806000),” “Transportation Science and Technology Project of Jiangxi Province (2017C0005)" and the “5511” Innovation Driven Project of Jiangxi Province (20165ABC28001).

Data availability

All data, models, and code generated or used during the study appear in the submitted article.

ORCID iD

Yasir Ibrahim Shah

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