Experimental Study on Flexural Performance of Screw Clamping and Welding Joint for Prestressed Concrete Square Piles

Abstract

To ensure the connection performance of precast concrete square piles, a screw clamping and welding joint connection is applied to the solid square piles. By conducting full-scale bending performance tests on six solid square pile specimens with cross-sectional side lengths of 300, 450, and 600 mm, including pile bodies, screw clamping joints, screw clamping, and welding joints, the bending load-bearing capacity, deformation capacity, and failure characteristics of the screw clamping–welding joint connection are compared and studied. The results show that the bending failure mode of the pile body specimens is shear failure in the flexural shear section and concrete crushing in the compression zone of the pure bending section; the bending failure mode of the screw clamping joint specimens are the pull-out of steel bar heads at the joint end plate; the bending failure mode of the screw clamping and welding joint specimens are concrete crushing in the compression zone of the pure bending section, steel bar breakage in the tension zone of the flexural shear section, and pull-out of steel bar heads at the end plate. It is worth noting that no significant damage occurred at the joints. The cracks in the pure bending section of the bending specimens mainly develop vertically and are evenly distributed, while some cracks in the flexural shear section develop obliquely towards the loading point, with branching. Compared to the pile body specimens, the cracking moment of the joint specimens is up to 16% higher, the ultimate moment is within 15% lower, and the maximum mid-span deflection is within 25% lower, indicating that the provision of anchorage reinforcement can increase the stiffness and cracking moment of the specimens.

1. Introduction

Precast concrete piles have the advantages of a high vertical bearing capacity of single pile, high industrialization degree, wide application range, and environmental friendliness, and have been widely used in pile foundation engineering in soft soil environments [1,2,3,4]. Limited by production and transportation conditions, a single section of pile is generally within 15 m; in order to meet the design requirements, it is necessary to conduct the pile grafting treatment on site. At present, there are two kinds of pile connecting methods of prefabricated piles in China, welding and mechanical connection [5]. Choosing to use the end plate welding connection means that the site construction workload is large, the construction conditions are poor, the weld quality is difficult to ensure, and the weld is easily affected by the surrounding corrosion environment. With mechanical joint connection, the integrity is poor, and the connection gap easily causes corrosion of the connector. Attention is given to the detection of joint quality and cracks by employing the low-strain reflected wave method [6] and micro magnification probe [7]. Although advancements have been made in machine learning approaches [8,9,10], their application in pile foundation engineering for crack identification remains limited.

The bearing capacity of the prefabricated pile is closely related to that of the joint. Liu Furong et al. [11] used welded joints for prestressed concrete hollow square piles and carried out full scale flexural tests, finding that both the end plate and the weld would affect the performance of the welded joints and the quality was difficult to control. Li Weixing et al. [12] adopted the improved treatment of external steel plate welding for prestressed high strength concrete pipe piles and conducted full-size axial center tension tests for improved welding and standard welding finding that improved welding has the advantages of higher tensile strength, faster construction, and stable weld quality compared with standard welding. Xu Quanbiao et al. [13,14,15] conducted full scale bending tests and finite element analyses on the prestressed reinforced solid square pile body and the joint with the gird-welded angle steel and found that the joint with the gird-welded angle steel had a better bearing capacity than the pile with the gird-welded angle steel. Guo Yang et al. [16] adopted hoop connection joints for prestressed high strength concrete pipe piles and conducted vertical pull-out load tests on single pile of hoop connection pipe piles and bored piles. They found that the hoop connection had a good pull-out performance. Wu [17] conducted full-scale flexural tests and finite element analyses of embedded anchoring joint of PRS square piles and found that the flexural bearing performance of the joint specimen was close to that of the pile body specimen. Lu Linhai et al. [18] carried out flexural bearing performance tests and finite element analyses of spigot and socket-connected PRS square piles and derived the calculation formula of the flexural bearing capacity of the pile joint. Wang Yunfei et al. [19] carried out tensile tests and finite element analyses on elastic-clip-connected prestressed reinforced solid square piles, and found that the elastic-clip mechanical connection has a good tensile bearing performance. Yang et al. [20,21] investigated the influence of deformed steel bars and concrete infilling on the seismic performance of prestressed high strength concrete pipe piles, and pointed out that both methods could enhance the bearing capacity of prestressed high-strength concrete pipe piles, while the use of deformed steel bars could also significantly improve the ductility capacity. Zhang et al. [22] through flexural tests on prestressed solid concrete piles, demonstrated that prestressed solid concrete piles with fewer prestressed tendons exhibit better deformation capacity and higher load-bearing capacity, outperforming prestressed high strength concrete piles. Tang et al. [23] conducted full-size bending tests and finite element analyses on large-diameter prestressed reinforced concrete pipe pile bodies and welded joints, and found that the cracking load of the welded joint was similar to that of the pile body, but the ultimate bending moment was lower and the anchorage reinforcement could be increased to improve the flexural performance of the joint.

Despite extensive research on the performance of various pile types and different connections, there is still a lack of studies on the performance of new types of connections that combine welding and mechanical joining. A new type of screw clamping mechanical connection and welding concrete precast square pile joint is designed and developed. The screw clamping joint can be fastened by an internal thread and arc body design, and the pile circumference is welded, which can withstand complex loads. The construction environment is less affected, the construction quality is controllable, and the integrity is good. The crack development, distribution, flexural bearing capacity, deformation ductility, and failure characteristics of concrete prefabricated square piles, screw clamping joints, and screw clamping and welding joints were studied using comparative tests, which provided a theoretical basis for the design and engineering application of screw clamping and welding joints in the field connection of concrete prefabricated square piles.

2. Test Setup

2.1. Specimen Design

In order to study the bending bearing performance of concrete precast square piles with snap-welded joints, solid square piles with three cross-section sizes of 300 mm, 450 mm, and 600 mm were selected. According to steel rods for prestressed concrete (GB/T5223.3-2005) [24], the prestressed steel reinforcement tensile control stress is 0.7 times the standard tensile strength value of 1420 MPa, which equals 994 MPa. Pile type numbers are PSP-300AB, PSP-450AB, and PSP-600AB, where “PSP” indicates prestressed concrete solid square pile, the number indicates the pile’s side length, and “AB” indicates the level of effective prestressed applied to the pile. In order to facilitate the identification of pile body test pieces and joint test pieces, the letter ZS is added before the pile type number to indicate pile body test pieces. Add the letter LK for the screw joint specimen and add the letter LKHJ for the screw-welded joint specimen. The corresponding geometric dimensions and structural reinforcement of pile type are shown in Figure 1 and

Table 1. Geometric sizes and reinforcements of specimens.

Specimen	B/mm	BP/mm	Longitudinal Reinforcement	Stirrup	Anchorage Reinforcement
LKHJ-PSP-300AB	300	208	8ΦD10.7	Φb4@50/100	8C14
ZS-PSP-450AB	450	357	16ΦD10.7	Φb5@50/100	\
LK-PSP-450AB	450	357	16ΦD10.7	Φb5@50/100	8C16
LKHJ-PSP-450AB	450	357	16ΦD10.7	Φb5@50/100	8C16
ZS-PSP-600AB	600	506	20ΦD12.6	Φb6@50/100	\
LKHJ-PSP-600AB	600	506	20ΦD12.6	Φb6@50/100	12C18

. B is the section side length of hollow square pile, and B_p is the distribution side length of prestressed reinforcement. To ensure the connection strength and reliability between the pile end plate and the pile body, a connection method involving drilling holes in the end plate and then sealing welding is used to add hot-rolled ribbed thread steel bars at the joint end plate. For the PSP-300AB pile, 8C14 threaded steel bars were used with an anchorage length of 500 mm. For the PSP-450AB pile, 8C16 threaded steel bars were used with an anchorage length of 600 mm. For the PSP-600AB pile, 12C18 threaded steel bars were used with an anchorage length of 650 mm.

The screw clamping and welding joint achieves mechanical connection through a screw clamping connector, while E4303 type welding rod is used for circumferential welding around the pile. The screw clamping and welding joint includes the connecting male head and the connecting female head, as shown in Figure 2. The connecting male head is composed of a small nut and an inserting rod. The small nut is stuck to the steel bar pier head and connects to the prestressed main bar. The inserting rod is connected to the small nut through the end thread. The connecting female head is composed of a large nut, a #1 fixing sleeve, a #2 fixing tensioning sleeve, and a spring, a #1 fixing sleeve is connected with a large nut through an external thread, and a #2 fixing tensioning sleeve is composed of 3 arc-shaped bodies with internal tapping threads. The card is located in the #1 fixing sleeve, and the spring is set under the #1 fixing sleeve to buffer and tighten the effect. When connecting the male head to the female head, insert the #1 fixing sleeve through the insert rod, and the #2 fixing tension sleeve corresponding clamping, to achieve close connection.

The summary of material properties is shown in

Table 2. Mechanical properties of materials.

Material Specification	fck/MPa	fpu/MPa	fsu/MPa	Ep/GPa
C65 concrete	91.9	\	\	\
ΦD10.7 prestressed steel bar	\	1508.1	\	201.3
ΦD12.6 prestressed steel bar	\	1457.8	\	201.7
Φb4 stirrups	\	\	553.0	203.8
Φb5 stirrups	\	\	550.6	202.6
Φb6 stirrups	\	\	574.9	202.6

. The concrete strength grade of prestressed concrete square pile is C65, and 9 standard concrete cube test blocks are poured at the same time. The curing conditions are the same as those of the test pieces, and the material performance compressive test is carried out on them. The average compressive strength of the concrete cube f_ck measured is 91.9 MPa. The prestressed steel bar is made of a spiral channel steel bar with low relaxation and with a diameter of Φ^D10.7 and a Φ^D12.6 prestressed steel bar in the same batch as the specimen for material property tensile test. The average values of tensile strength f_pu measured are 1508.1 MPa and 1457.8 MPa, respectively. The mean values of the elastic modulus E_p are 201.3 GPa and 201.7 GPa, respectively. The spiral stirrups are made of cold-drawn low-carbon steel wire for concrete products, and 3 stirrups with diameter of Φ^b4, Φ^b5, and Φ^b6 in the same batch as the specimens are selected for material property tensile test. The average values of tensile strength f_su measured are 553 MPa, 550.6 MPa, and 574.9 MPa, respectively. The mean values of the elastic modulus E_p are 203.8 GPa, 202.6 GPa, and 202.6 GPa, respectively.

2.2. Test Apparatus

Specimens of 300 mm, 450 mm, and 600 mm were selected with lengths of 6 m, 8 m, and 10 m, respectively, and the bending tests of pile body, screw clamp joint, and screw clamp-welded joint were carried out. The design was carried out with reference to the pretensioned prestressed spun concrete piles (GB13476-2023) [25] and the test method standard for concrete structure (GB50152-2012) [26]. The four-point loading method was adopted for the test. The length of the pure bend section in the span was 1.0 m, and the two supports are fixed pivot supports, with a spacing of 0.6L (L is the length of the specimen). YAW-10000F microcomputer-controlled electro-hydraulic servo multi-functional testing machine was used to load the specimen. Displacement meters were arranged to measure pile displacement and support settlement, and strain gauges were arranged to measure strain. The test loading device and measuring point layout are shown in Figure 3. The loading process is divided into pre-loading and formal loading.

Pre-loading Process: The pre-load is controlled to within 70% of the specimen’s cracking moment, applied in three stages, with a stabilization period of 1 min for each stage, followed by gradual unloading through three stages. During pre-loading, check whether all the measuring instruments and equipment are functioning properly. After the pre-loading is completed, adjust the instruments and record the initial readings.

Formal Loading Process: (1) Load from zero to 80% of the cracking moment with increments of 20% of the cracking moment, holding each load level for 3 min; then, continue loading to 100% of the cracking moment with increments of 10% of the cracking moment. Observe for any cracks, measure, and record the crack widths, with each load level held for 3 min. (2) If no cracks appear when reaching 100% of the cracking moment, continue loading with increments of 5% of the cracking moment until cracks occur, holding each load level for 3 min, and measure and record the crack widths. (3) After the specimen cracks, load with increments of 5% of the ultimate moment to 100% of the ultimate moment, holding each load level for 3 min, and observe and record all readings. (4) Switch to displacement-controlled loading until the specimen fails, holding each load level for 3 min.

The formula [22] for calculating the loading bending moment test value of the pure bending section of the specimen is as follows:

$$M={\displaystyle \frac{P}{4}}\left({\displaystyle \frac{3}{5}}L-\mathrm{a}\right)+{\displaystyle \frac{\mathit{WL}}{40}}$$

(1)

where P is the test load, L is the length of the specimen; a is a constant, equal to 1 m; and W is the gravity load of the specimen.

3. Analysis of Bending Test Results

3.1. Evaluation of Flexural Capacity

During the bending evaluation of prestressed concrete solid square pile, the crack control grade is level 2 [27], and the calculation formula of the cracking bending moment of solid square pile body is as follows:

$$M_{\mathrm{cr}}=({\sigma }_{\mathrm{ce}}+\gamma f_{\mathrm{tk}})W_0$$

(2)

where $M_{\mathrm{cr}}$ is the cracking moment of pile body; ${\sigma }_{\mathrm{ce}}$ is the effective precompression stress of concrete in pile section; γ is the plastic influence coefficient of cross section resistance, according to the formula $\gamma =\left(0.7+120/h\right){\gamma }_{\mathrm{m}}$; h is the side length of solid square pile, and h = 400 when h < 400; ${\gamma }_{\mathrm{m}}$ is the basic value of γ, and the rectangular cross-section is 1.55; $f_{\mathrm{tk}}$ is the standard value of tensile strength of pile concrete. $W_0$ is the elastic resistance moment of pile section.

When the square pile is bent, the bending capacity of the normal section of the pile body is calculated by the following formula:

$$M_{\mathrm{u}}={\displaystyle \sum \left[f_{\mathrm{ptk}}{A}_{\mathrm{pi}}\left(h_{\mathrm{pi}}-\frac{x}{2}\right)\right]}$$

(3)

$${\alpha }_1{f}_{\mathrm{ck}}\mathit{Bx}={\displaystyle \sum f_{\mathrm{ptk}}}{A}_{\mathrm{p}}+{\displaystyle \sum \left({\sigma }_{\mathrm{p}0}^{\prime }-f_{\mathrm{py}}^{\prime }\right)}{A}_{\mathrm{p}}^{\prime }$$

(4)

where $M_{\mathrm{u}}$ is the flexural capacity of normal section of pile body; $f_{\mathrm{ptk}}$ is the standard value of tensile strength of prestressed reinforcement; $A_{\mathrm{pi}}$ is the section area of the i row of tension prestressed bars; $h_{\mathrm{pi}}$ is the distance between the i row of tension prestressed steel bars and the compression edge of concrete; x is the height of the concrete compression zone of the equivalent rectangular stress figure, and when x is less than 2a’, it is 2a’; a’ is the distance between the joint force point of the longitudinal reinforcement in the compression zone and the compression edge of the section; ${\alpha }_1$ is the ratio of the stress value of the concrete rectangular stress diagram to the design value of the axial compressive strength, C65 is 0.97; $f_{\mathrm{ck}}$ is the standard value of axial compressive strength of concrete; B is the side length of the square pile section; $f_{\mathrm{py}}^{\prime }$ is the compressive strength of the prestressed steel bar; $A_{\mathrm{p}}$, $A_{\mathrm{p}}^{\prime }$ is the section area of the prestressed reinforcement in the tension zone and the compression zone; and ${\sigma }_{\mathrm{p}0}^{\prime }$ is the stress of the prestressed reinforcement when the normal stress of the concrete is equal to zero at the joint force point of the prestressed reinforcement in the compression zone.

The bending capacity test results of each specimen and the calculation results of the standard formula are shown in

Table 3. Comparison of flexural bearing capacity between test results and calculated values of code formulas.

Specimen	${\mathit{M}}_{\mathbf{cr}}^{\mathbf{t}}$/kN·m	${\mathit{M}}_{\mathbf{cr}}^{\mathbf{c}}$/kN·m	${\mathit{M}}_{\mathbf{u}}^{\mathbf{t}}$/kN·m	${\mathit{M}}_{\mathbf{u}}^{\mathbf{c}}$/kN·m	${\mathit{f}}_{\mathbf{u}}$/mm
LKHJ-PSP-300AB	56.6	49.3	254.8	106.3	55.8
ZS-PSP-450AB	145.9	152.0	611.8	506.9	58.6
LK-PSP-450AB	144.4	152.0	532.0	506.9	43.3
LKHJ-PSP-450AB	176.7	152.0	545.3	506.9	41.0
ZS-PSP-600AB	327.8	345.1	1121.8	898.2	63.9
LKHJ-PSP-600AB	381.1	345.1	1107.1	898.2	47.8

, where $M_{\mathrm{cr}}^{\mathrm{t}}$ is the cracking moment measured using the test. $M_{\mathrm{u}}^{\mathrm{t}}$ is the ultimate bending moment measured by experiment, and $M_{\mathrm{cr}}^{\mathrm{c}}$ is the cracking moment calculated by standard formula. $M_{\mathrm{u}}^{\mathrm{c}}$ is the ultimate bending moment calculated by the standard formula, and $f_{\mathrm{u}}$ is the maximum mid-span deflection measured in the test.

The cracking moment test values of 2 pile specimens are 4% and 5% smaller than those calculated by the standard formula, the cracking moment test values of the screw clamping joint specimens is 5% smaller than calculated by the standard formula, and the cracking moment test values of 3 screw clamping and welding specimens are 15%, 16%, and 10% larger than those calculated by the standard formula, respectively. The cracking moment test value of LKHJ-PSP-450AB specimen is larger than those of ZS-PSP-450AB specimen and PSP-450AB specimen and the cracking moment test value of LKHJ-PSP-600AB specimen is larger than that of ZS-PSP-600AB specimen, indicating the cracking moment test value of the screw clamping and welding joint specimen is larger than those of the screw clamping joint specimen and the pile body specimen.

The ultimate bending moment test values of 2 pile body specimens are 21% and 25% larger than those calculated by the standard formula, the ultimate bending moment test values of the screw clamping joint specimens are 5% larger than those calculated by the standard formula, and the ultimate bending moment test values of 3 screw clamping and welding joint specimens are 140%, 8%, and 23% larger than those calculated by the standard formula, respectively.

3.2. Load-Mid-Span Deflection Curve

The load-mid-span deflection curve of the specimen measured by bending test is shown in Figure 4. The whole process of bending loading can be divided into the following four stages: elastic stage, crack development stage, steel rod yield stage, and failure stage. At the initial stage of loading, all specimens were in the elastic stage, the flexural stiffness remained basically unchanged, the load was basically linear with the mid-span deflection, and the mid-span deflection was small. When the cracking load is applied to the specimen, vertical cracks appear in the pure bend section of the specimen span, the bending stiffness decreases, and the deflection increases rapidly. When loaded to yield load, the bottom tension steel rod gradually yields, and the vertical crack develops continuously. As the load continued to increase, when the pile body and joint specimens reached the ultimate load, the specimens immediately broke and the load dropped rapidly. After the LKHJ-PSP-300AB specimen reached the ultimate load, the concrete in the mid-span compression zone was gradually crushed, the load slowly decreased, and the mid-span deflection increased rapidly, until the concrete in the compression zone was crushed and the load dropped sharply. The specimen is damaged.

It can be seen from Figure 4 that before vertical cracks appear in the mid-span pure bend section, the stiffness and cracking load of the screw clamping joint specimen are similar to that of the pile body specimen, while the stiffness and cracking load of the screw clamping and welding joint specimen are larger than that of the pile body specimen, indicating that the screw clamping and welding joint has better initial stiffness. After the concrete cracking of the pile body to the steel rod yielding, the cracks of the specimens continue to develop, and the stiffness of the specimens decreases gradually. The stiffness of the specimens of the screw clamping joint is smaller than that of the pile body, and the stiffness of the specimens of the screw clamping and welding joint is close to or larger than that of the pile body. After yielding to the ultimate load, the crack width of the steel bar increases gradually, the load increases slowly, the mid-span displacement increases rapidly, and the stiffness decreases obviously. The ultimate load value and the ultimate load point displacement of the screw clamping joint specimen and the screw clamping and welding joint specimen are close to each other, and the joint specimen is smaller than the ultimate load value and the ultimate load point displacement of the pile body specimen. The deformation ductility and ultimate load of the screw clamping and welding joint specimen is similar to that of the screw clamping and welding joint specimens, while the deformation ductility and ultimate load of the screw clamping and welding joint specimens is smaller than that of the pile body specimens, which is due to the increased stiffness of the joint specimens provided by the anchorage reinforcement, indicating that the screw clamping and welding joint can effectively ensure the flexural performance of the pile body connection.

3.3. Test Failure Pattern

The failure morphology of the bending specimen is shown in Figure 5. The bending failure mode of the LKHJ-PSP-300AB specimen is that the concrete in the upper part of the pure bending section is crushed, the bending failure mode of the ZS-PSP450AB specimen is shear failure in the right-bending shear section, the surface concrete falls off and the hoop is pulled off, and the bending failure mode of the LK-PSP-450AB specimen is that the steel bar pier on the right side of the joint end plate is pulled off. The bending failure mode of the LKHJ-PSP-450AB specimen is that the steel rod is broken in the tension zone at the bottom of the pile body in the left-bending shear section, the bending failure mode of the ZS-PSP-600AB specimen is that the concrete is crushed in the pressure zone at the upper part of the pure bending section, and the bending failure mode of the LKHJ-PSP-600AB specimen is that the steel rod pier head on the left side of the joint end plate is pulled off. When the four joint specimens were finally destroyed, the joints were intact and the direct gaps between the end plate and the concrete of the pile body were small, which indicates that the installation of anchorage reinforcement can adequately strengthen the integrity of the specimens and effectively ensure the flexural bearing performance of the pile body connection.

3.4. Specimen Crack Development

The crack distribution of the specimen under bending tests is shown in Figure 6. The cracks in the pure bending section of the specimen are mainly vertically developed, evenly distributed, and bifurcated. The cracks in the flexural shear section are inclined to the loading point and have bifurcation. The concrete in the tension area at the bottom of the span of the specimen is cracked, and the vertical cracks continue to develop until the specimen is destroyed. When the mid-span bending moment of LKHJ-PSP-300AB specimen reached the peak load of 254.8 kN∙m, the pile body cracks were mainly distributed in the range of −1000 mm~1000 mm, with a total of 14 major cracks (8 cracks in the pure bend section). The maximum vertical crack width was 1.02 mm, and the maximum development height was about 260 mm. When the mid-span bending moment of the ZS-PSP450AB specimen reaches the peak load of 611.8 kN∙m, the pile body cracks are mainly distributed in the range of −1300 mm~2000 mm, with a total of 16 main cracks (7 cracks in the pure bend section). The maximum vertical crack width is 2.12 mm, and the maximum development height is about 350 mm. When the mid-span bending moment of the LK-PSP-450AB specimen reaches the peak load of 532 kN∙m, the pile body cracks are mainly distributed in the range of −1500 mm~1500 mm, and there are 22 main cracks (8 cracks in the pure bend section). The maximum vertical crack width is 2.02 mm, and the maximum development height is about 350 mm. When the mid-span bending moment of LKHJ-PSP-450AB specimen reaches the peak load of 545.3 kN∙m, the pile body cracks are mainly distributed in the range of −1300 mm~1200 mm, with a total of 16 main cracks (7 cracks in the pure bend section). The maximum vertical crack width is 1.40 mm, and the maximum development height is about 350 mm. When the mid-span bending moment of ZS-PSP-600AB specimen reaches the peak load of 1121.8 kN∙m, the pile body cracks are mainly distributed in the range of −1800 mm~1500 mm, with a total of 18 main cracks (7 cracks in the pure bend section). The maximum vertical crack width is 1.46 mm, and the maximum development height is about 500 mm. When the mid-span bending moment of LKHJ-PSP-600AB specimen reaches the peak load of 1107.1 kN∙m, the pile body cracks are mainly distributed in the range of −1500 mm~1800 mm, with a total of 15 major cracks (5 cracks in the pure bend section). The maximum vertical crack width is 1.26 mm, and the maximum development height is about 520 mm.

The crack development in the flexural test is shown in

Table 4. Comparison of crack development in flexural test.

Specimen	wmax/mm	h/mm	r/mm	n	n1
LKHJ-PSP-300AB	1.02	260	−1000~1000	14	8
ZS-PSP-450AB	2.12	350	−1300~2000	16	7
LK-PSP-450AB	2.02	350	−1500~1500	22	8
LKHJ-PSP-450AB	1.40	350	−1300~1200	16	7
ZS-PSP-600AB	1.46	500	−1800~1500	18	7
LKHJ-PSP-600AB	1.26	520	−1500~1800	15	5

, where w_max represents the maximum crack width of the pile body before specimen failure; h represents the crack development height; r represents the distribution range of pile cracks; n represents the total number of cracks in the final pile body; and n₁ represents the number of cracks in the pure bend section.

3.5. Weld Strain Development

The load–weld strain curves for the bending test of the screw-clamped welded joint specimens are shown in Figure 7. Four strain gauges are evenly distributed along the height direction of the weld seam, with the LKHJ-PSP-600AB specimen having an additional gauge, making it five in total. The arrangement of the weld strain gauges is referred to in Figure 3d.

The joint weld seam is welded using E4303 type welding rods. According to the “Covered electrodes for manual metal arc welding of non-alloy and fine grain steels” (GB/T 5117-2012) standard [28], the yield strength of the weld seam is 330 MPa, and the yield strain is 1650 με.

Table 5. Comparison of crack development in flexural test.

Specimen	ε1/10−6	ε2/10−6
LKHJ-PSP-300AB	477	381
LKHJ-PSP-450AB	460	364
LKHJ-PSP-600AB	914	259

presents the weld strain of the screw clamping and welding prestressed concrete solid square pile connection joint specimens at ultimate failure, where ε₁ and ε₂ represent the compressive weld strain values on the upper surface of the joint specimen and the tensile weld strain values on the lower surface, respectively. The results indicate that the weld strains of the three screw clamping and welding prestressed concrete solid square pile connection joint specimens are all relatively low, with both the compressive and tensile weld strains being significantly less than the yield strain.

4. Summary

(1) The bending failure patterns of the pile body specimens are concrete crushing in the compression zone of the pure bending section or shear failure in the flexural shear section. The bending failure pattern for the screw clamping joint specimens is the pull-out of the steel bar head on the right side of the joint end plate. The bending failure pattern for the LKHJ-PSP-300AB specimen is concrete crushing in the upper part of the pure bending section, while the LK-PSP-450AB and LKHJ-PSP-600AB specimens both exhibited failure with the steel bar head on one side of the joint end plate being pulled out. It is worth noting that during failure, the connections of the joint specimens remain relatively intact, indicating that the screw clamping and welding joint can ensure the effective connection of the prestressed solid square piles.

(2) The crack development in the pile body specimens, screw clamping joint specimens, and screw clamping and welding joint specimens are similar, with vertical cracks in the pure bending section developing evenly and branching; cracks in the flexural shear section developed obliquely towards the loading point and also branched. Compared to the pile body specimens and the screw clamping and welding joint specimens, the screw-clamped joint specimens have more cracks and a wider distribution.

(3) The cracking moment of the LKHJ-PSP-450AB and LKHJ-PSP-600AB joint specimens are 21% and 16% higher than that of the pile body specimen, respectively. The ultimate moment of the LK-PSP-450AB and LKHJ-PSP-450AB specimens are 13% and 11% lower than that of the pile body specimen, respectively, and the maximum mid-span deflection are 26% and 30% lower than that of the pile body specimen, respectively. The bending capacity of the LKHJ-PSP-600AB specimen is slightly lower than that of the pile body specimen, and the maximum mid-span deflection is 25% lower than that of the pile body specimen. The joint specimens equipped with anchorage reinforcement can effectively increase the cracking moment and flexural stiffness of the connection joints of the prestressed concrete solid square piles.

5. Limitations and Prospects

In this paper, the sole focus is on the flexural bearing capacity of the screw-clamped welded joint. However, in practical situations, the loads are more complex, and it is also necessary to consider the shear and tensile load conditions. In some regions, the effects of seismic loads [29,30,31,32,33] must also be taken into account. Future research can continue to explore the shear, tensile, and seismic performance of screw-clamped welded joints.