This paper proposed a hybrid precast concrete shear wall emulating monolithic construction (HPWEM) that utilized grouted vertical connecting reinforcements and unbonded posttensioned high-strength strands across the horizontal joint for the lateral resistance. The grouted reinforcements with predetermined debond length were used to provide strength by tension and energy dissipation by yielding. The posttensioned strands were mainly employed to offer the restoring force to reduce the residual displacement by elastic extension. The overlapping welded closed stirrups improved the confinement property of the restrained concrete, avoiding the brittle failure. Six HPWEM specimens, considering variables including the amounts of strands and the debond lengths of grouted reinforcements, as well as one referenced cast-in-place monolithic wall specimen, were tested under the low-cycle reversed lateral load. The HPWEM specimens were capable of providing strength, stiffness, ductility, and energy dissipation equivalent to that of the monolithic wall specimen under certain variable condition.
Precast concrete walls have been continuously investigated by many researchers and the earliest research can trace to the PREcast Seismic Structural Systems (PRESSS) Research Program in the early 1990s [
In this paper, hybrid precast concrete shear walls emulating monolithic construction (HPWEM) are presented. The key features of HPWEM are as follows (as shown in Figure The unbonded posttensioned multistrand tendons are placed in the PVC duct embedded in the center of the precast wall panel to delay cracking at small lateral drift stage, decrease residual displacement, and enhance force capacity at large lateral drift stage. The vertical connecting reinforcements are protruded from the foundation or the lower-storey precast wall panel and grouted in the metal bellows embedded adjacent to vertical reinforcements of the reinforcement cage in the upper-storey precast wall panel and that formed overlapping connection of the vertical load-bearing reinforcements. The strength grade and diameter of the grouted reinforcements are identical with the vertical reinforcements of the reinforcement cage. In addition, a predetermined length of these reinforcements is debonded above the joints (by wrapping the reinforcements with plastic films) to limit the steel strains and prevent low-cycle fatigue fracture. These grouted connection reinforcements anchored on both sides of the horizontal joint are designed to yield in tension and compression and provide flexural capacity as well as energy dissipation under lateral load into the nonlinear range. The horizontal reinforcement in the precast wall panel is fabricated from single steel bar, enclosing the confinement cage at both wall toes, and overlap welded near the middle of the panel to ensure the reinforcement effectiveness and also strengthen concrete constraint property. The stirrups with limited length-to-diameter aspect ratio are placed in both wall toes. The stirrups are closed by overlapping weld, placed surrounding four vertical reinforcements and overlapped against each other through co-used two vertical reinforcements. The overlapped welded closed stirrups can improve the confinement property of the concrete more effectively.
General features of HPWEM.
With respect to HPWEM, the specific project objectives in this paper are mainly to develop (1) experimental evidence demonstrating the emulative monolithic performance of the specimens under cyclic lateral force; (2) validated analytical models by ABAQUS; (3) the impact of different amounts of strands and different debond lengths of grouted reinforcements on lateral performance of the specimens.
Precast concrete structure was wildly used due to its high-quality production, fast erection, and relatively economic benefit. Emulative and hybrid precast concrete wall system were continuously researched and developed in the last few decades. However, significant limitations existed in their practical use in seismic zones according to current standards. HPWEM combining emulative and hybrid wall concepts should be capable of performing exactly consistent with, even better than cast-in-situ monolithic wall. The authors believe that the proposed wall has competitive advantage and the research achievement will be very useful to precast concrete technology.
Seven full scale wall specimens consisting of one reference cast-in-place monolithic wall specimen and six HPWEM specimens were investigated in this paper. All the specimens were tested under reversed cyclic lateral loading. Hysteretic behavior, load-carrying capacity, stiffness degradation, ductility, energy dissipation, and residual displacement of HPWEM specimens were obtained by contract to the monolithic specimen. The design variables including the amounts of posttensioned strands and debond length of grouted mild steel reinforcements were considered in the experiment.
The plain concrete of strength grade C35 (i.e., the cubic compressive strength was 35 MPa [5.08 ksi]) were specified for all specimens. Three groups of nine cubes with side length 150 mm (5.91 in.) were fabricated and tested 28 days later. The statistic compression strength from the test results was 35.38 MPa (5.17 ksi).
The nonshrink, high-strength, and cement-based grout was used in the metal bellows and grout pad. The measured compressive strength of cylinders with 100 mm (3.94 in.) side length and 200 mm (7.87 in.) high was 60 MPa (8.77 ksi). Moreover, in order to ensure the quality of grouting construction, the flow ability of the grout is required to be no less than 290 mm (11.42 in.) and the measured fluidity was 300 mm (11.81 in.).
Three samples of both steel strands and reinforcements incorporated in specimens were pulled and the average tensile properties are summarized in Table
Tensile properties of steel materials.
Characteristics | 15.24 mm diameter posttensioned strand | 16 diameter bar | 10 diameter bar | 8 diameter stirrup |
---|---|---|---|---|
Area, mm2 (in.2) | 140 (0.22) | 201.1 (0.31) | 78.5 (0.12) | 50.3 (0.08) |
Yield strength, MPa (ksi) | 1650 (239.25) | 438 (63.51) | 430 (62.35) | 451 (65.4) |
Strain at yield strength, % | 0.85 | 0.31 | 0.36 | 0.43 |
Peak strength, MPa (ksi) | 2050 (297.25) | 620 (89.9) | 557 (80.77) | 618 (89.61) |
Strain at peak strength, % | 5.6 | 15 | 12 | 7.7 |
Elongation, % | 5.6 | 19 | 14.3 | 10.8 |
One monolithic and six HPWEM specimens were investigated. All specimens were tested at full scale with consistent wall geometry of thickness 200 mm (7.87 in.), length 1700 mm (66.93 in.) and height 3620 mm (142.52 in.). A foundation beam of height 640 mm (25.2 in.), width 700 mm (27.56 in.) and length 2200 mm (86.61 in.) was used to simulate structural base. In addition, a capping beam of height 320 mm (12.6 in.), width 240 mm (9.45 in.) and length 1700 mm (66.93 in.) were fabricated on the top of the wall for connecting the hydraulic jack.
The reinforcement details of the monolithic wall specimen (referred to as Specimen MW) are shown in Figure
Details of the monolithic wall specimen.
The reinforcement details of HPWEM specimens (referred to as Specimens HPWEM1–5) were generally identical to Specimen XJ, except for the following as shown in Figure The vertical reinforcements crossing the horizontal joint were anchored in grouted metal bellows embedded adjacent to the vertical reinforcements in the wall and that provided lap splice connection for continuity of the reinforcements as shown in Figure Certain amounts of high-strength strands of diameter 15.2 mm (0.6 in.) were placed in embedded PVC duct of diameter 75 mm (2.95 in.) and passed through the wall panel, external beam, and the foundation beam. Stirrups with 135 hooks were replaced by overlapping welded closed stirrups as shown in Figure
Details of HPWEM specimens.
The construction progress of HPWEM specimens was as follows: The steel shims with 20 mm (0.79 in.) thickness were uniformly placed on the top of the foundation beam along the cross section of the shear wall to adjust the construction elevation and form the gap for the grout layer. The precast wall panel was then erected and temporarily fixed on the foundation beam and the connection reinforcements were also inserted into the metal bellows. The wood molding was set up around the grout layer. The grout was poured into the metal bellows and the grout layer under the effect of gravity from the location about 1000 mm (39.37 in.) higher than the grout inlet. The bellows were grouted one by one while the grout outflowed from the working inlet. Once all the bellows were grouted, the supplying grout was essential to ensure the compactness of the grout. Once the grout was maintained to its design strength, the high-strength strands were placed into the duct and then posttensioned and anchored on the specimen. Thus, the fabrication of HPWEM specimen was finished.
Moreover, different amounts of posttensioned strands as well as different debond lengths of grouted reinforcements were considered in the test. The list of important features of all specimens is provided in Table
Parameters of specimens.
Specimen number | Posttensioned tendons | Mild steel bars | Confinement stirrups | ||||||
---|---|---|---|---|---|---|---|---|---|
Number of strands | | Posttensioned force, kN (kips) | Wrapped length, mm (in.) | Bonded length, mm (in.) | | | | | |
MW | — | — | — | — | — | 400 (15.75) | 100 (3.94) | 50 (0.98) | 1.61 |
HPWEM1 | 2 | 0.6 | 312.5 (70.25) | 200 (7.87) | 400 (15.75) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
HPWEM2 | 3 | 0.4 | 312.5 (70.25) | 200 (7.87) | 400 (15.75) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
HPWEM3 | 4 | 0.3 | 312.5 (70.25) | 200 (7.87) | 400 (15.75) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
HPWEM4 | 4 | 0.5 | 520.8 (117.08) | 250 (9.84) | 350 (13.78) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
HPWEM5 | 4 | 0.5 | 520.8 (117.08) | 200 (7.87) | 400 (15.75) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
HPWEM6 | 4 | 0.5 | 520.8 (117.08) | 150 (5.91) | 450 (17.72) | 400 (15.75) | 100 (3.94) | 25 (0.98) | 1.27 |
Note:
The overall test setup is shown in Figure
Test setup.
The load history consisted of force and displacement cycles as shown in Figure
Load history. (1 kN = 0.22 kips, 1 mm = 0.04 in.)
The measurement system used in this experiment included 54 channels of data with 6 displacement transducers, 4 load cells, and 44 strain gauges. The displacement transducers were used to measure the in-plane displacements of the wall panel. The load cells were used to measure the applied lateral load, the vertical force simulating service-level gravity loads on the wall, and the force in unbonded posttensioned strands. The strain gauges were used to measure the strain of the grouted reinforcements.
The finite element model (FEM) was established using ABAQUS to capture the general behavior of HPWEM specimens as can be seen in Figure
Finite element modeling.
FEM
Constitutive model for concrete
Constitutive model for mild steel
Constitutive model for strand steel
The observations made during the test sequence are summarized in Table
Test observations of the specimens.
Specimen number | Crack initiate | Concrete over spalling | Concrete crushing | Crack | Failure |
---|---|---|---|---|---|
MW | | | | Ordinary flexural-shear crack | Reinforcement fracture |
HPWEM1 | | | | More inclined flexural-shear crack | Reinforcement fracture |
HPWEM2 | | | | More inclined flexural-shear crack | Reinforcement fracture |
HPWEM3 | | | | More inclined flexural-shear crack | Reinforcement fracture |
HPWEM4 | | | | More inclined flexural-shear crack | Reinforcement fracture |
HPWEM5 | | | | More inclined flexural-shear crack | Reinforcement fracture |
HPWEM6 | | | | More inclined flexural-shear crack | Reinforcement fracture |
Note: 1 kN = 0.22 kips.
Failure modes.
MW
HPWEM1
HPWEM2
HPWEM3
HPWEM4
HPWEM5
HPWEM6
The
Hysteretic curves.
MW
HPWEM1
HPWEM2
HPWEM3
HPWEM4
HPWEM5
HPWEM6
Besides, the predicted
The ultimate load-carrying capacities of the specimens in the positive and negative directions are shown in Figure
Measured strength.
The lateral stiffness was defined as the secant stiffness which was calculated as the slope of the line connecting the peak load response in the positive and negative directions during a loading cycle. The stiffness changes throughout the test are plotted in Figure
Stiffness degradation of the specimens.
The average displacement ductility in the positive and negative directions is listed in Table
Displacement ductility of the specimens.
Specimen | Yield displacement | Ultimate displacement | Displacement ductility factor ( | Average displacement ductility factor ( | |||
---|---|---|---|---|---|---|---|
Positive | Negative | Positive | Negative | Positive | Negative | ||
MW | 27.3 | −39.7 | 92.4 | −94.2 | 3.38 | 2.37 | 2.875 |
HPWEM1 | 27.5 | −32.9 | 94.2 | −94.0 | 3.43 | 2.86 | 3.145 |
HPWEM2 | 28.5 | −34.1 | 93.3 | −94.3 | 3.27 | 2.77 | 3.02 |
HPWEM3 | 32.4 | −30 | 92.2 | −92.2 | 2.85 | 3.07 | 2.96 |
HPWEM4 | 31.6 | −44.2 | 93.5 | −92.5 | 2.96 | 2.09 | 2.525 |
HPWEM5 | 31.2 | −44.5 | 92.9 | −93.1 | 2.98 | 2.09 | 2.535 |
HPWEM6 | 31.0 | −37.7 | 94.3 | −91.2 | 3.04 | 2.42 | 2.730 |
Note: 1 mm = 0.04 in.
The equivalent viscous damping ratio, defined as the ratio of energy dissipated in a half-cycle to the strain energy of an equivalent linear system divided by the constant 2
Energy dissipation curves.
The measured residual displacements (i.e., the remaining lateral displacement at the load point upon the lateral force being zero) at the last cycle of each displacement loading increment of the specimens were shown in Figure
Residual displacement curves.
The ultimate stress in the unbonded tendons is the key parameter to the strength of the specimen. The measured and analytical ultimate stresses are given in Table
The ultimate stress of the unbonded strands.
Ultimate stress | HPWEM1 | HPWEM2 | HPWEM3 | HPWEM4 | HPWEM5 | HPWEM6 |
---|---|---|---|---|---|---|
Measured, MPa (ksi) | 1586.1 (230) | 1132.1 (164.2) | 767.5 (111.3) | 1120.0 (162.4) | 1105.7 (160.3) | 1142.5 (165.7) |
Analytical, | 1628.8 (236.2) | 1180.0 (171.1) | 812.0 (117.7) | 1152.6 (167.1) | 1149.5 (166.7) | 1151.1 (166.9) |
| 2.69% | 4.23% | 5.80% | 2.91% | 3.96% | 0.75% |
The yielding of grouted reinforcements was effectively delayed due to the locally debond configuration. The yielding loads (the tensile strain of the exterior connecting reinforcement exceeded 0.31%) of Specimens HPWEM1–6 were 500 kN (112.4 kips), 525 kN (118 kips), 550 kN (123.6 kips), 575 kN (129.3 kips), 550 kN (123.6 kips), and 500 kN (112.4 kips), respectively, and Specimen MW otherwise yielded at 475 kN (106.8 kips).
Quasistatic experimental investigations were conducted on HPWEM, considering design variables including the amount of strands and the debond length of grouted reinforcements. The results were then compared with the performance of a reference cast-in-place monolithic wall. The summary of the observations are as follows: HPWEM specimens (such as Specimens HPWEM1–3) were capable of providing strength, stiffness, energy dissipation, and ductility better than that of the monolithic wall specimen. The increase of the amount of the strands (i.e., enlargement of the area of the unbonded tendon) enhanced strength of the specimen and otherwise weakened energy-absorbing capacity as well as ductility of the specimen. The shortening of the debond length of grouted reinforcements enhanced the strength, initial stiffness, and ductility of the specimen. The increase of the posttensioned force of the unbonded tendons enhanced initial stiffness and self-centering capacity of the specimen and otherwise reduced ductility and energy dissipation capacity of the specimen. Overlapping welded closed stirrups with smaller length-to-width ratios improved the confinement property of the concrete, and thus the damage of the concrete at wall toes was obviously lightened. The yielding of grouted reinforcements was effectively delayed with wrapped length, and the minimum bond length of 350 mm (13.9 in., about 22 times the reinforcement diameter) in the high-strength grout was sufficient for the connecting reinforcements to develop the tensile strength. Analytical models by ABAQUS well replicated the skeleton curves of the specimens and the strand strain behavior.
The authors declare that they have no competing interests.
This work was funded by National Natural Science Foundation of China (no. 51308289). The authors wish to acknowledge the funding agency for their support. Additional support provided by the ZHONGNAN Group, Inc., the Nanjing Tech University, and the Southeast University was also gratefully appreciated.