Experimental Study on Shear Performance of Innovated Precast Shear Walls with Different Yield Strength Grades of Reinforcements

2.1. Test Specimen Information

Specimen size and situation of reinforcement are shown in Fig. (2), and the shadow parts in it represent precast concrete. Two new type precast concrete shear walls were designed, named 2-SW1 and 2-SW5-H. The specimen was composed of loading beam, middle shear wall and foundation beam. The size of loading beam was 290mm in width and 200mm in height; the foundation beam was 500mm in width and 650mm in height; the middle shear wall was 1600mm in width and 200mm in thickness which included precast two-way hollow slab, edge member and cast-in concrete in the hollows. The length of edge member was 210mm along the wall width. Their shear span ratios were 1.5 and the corresponding heights were 2400mm.

According to the design principle of strong bending and weak shear, the vertical reinforcement of edge member was high-tensile deformed steel bars of 25mm diameter respectively. Its stirrups were high-tensile deformed steel bars of 8mm diameter, and the distance was 100mm. The difference of specimen 2-SW1 and specimen 2-SW5-H is the yield strength grade of horizontal reinforcements which were set in the lateral holes. The yield strength grade of horizontal reinforcements was high-tensile in specimen 2-SW1, and was low-tensile in specimen. The diameter of the horizontal steel bar in the two specimens was 10mm.

2.2. Specimens Fabricating

When the specimens fabricating, the foundation beam was made at first. The longitudinal steel bars of boundary member and vertical inserted reinforcement were pre-buried in the foundation beam. There were 2C 8 in the vertical hole of precast hollow slab to connect the foundation beam and shear wall which elongated to 410mm from the top surface of foundation beam. When the concrete strength reached the design strength, the top surface of foundation would be roughened to make the coarse aggregate of foundation watched. It is shown in Fig. (3a). And then, the precast hollow slab was hoisted. In order to ensure the effective connection between the precast hollow slab and loading beam, the precast concrete of 100mm length was chiseled at the top of precast hollow slab and the vertical steel bars were inserted in loading beam. It is shown in Fig. (3b) that horizontal bars were replaced in the horizontal holes of precast hollow slab and the ends of them were inserted to boundary element to make the effective connection between precast hollow slab and boundary beam. At last, templates of loading beam and boundary elements were supported and the concrete was poured in the hole.

2.3. Material Characteristics

The concrete standard test cubes were reserved during making the walls. The cubic compressive strength of concrete [6] was measured at the test day, which is shown in Table 1. The measured yield strength, ultimate strength and the elongation of reinforcement can be observed in Table 2.

2.4. The Method of Loading and Measurement

The test setup is shown in Fig. (4). During the test, the vertical load was firstly applied by a vertical jack and the vertical load kept constant. At the same time, the horizontal load was applied by a horizontal jack. The loading method of horizontal load was firstly the load-displacement control [7], with the differential of 250kN, and each load was applied cyclically once. After the diagonal cracks occurred on the shear wall, displacement control was applied with every subsequent cycle repeated twice. The controlled displacement was chosen as the measured crack displacement value. After the peak load, the differential of the control displacement changed double after the peak load.

The measurement of displacements and strains is shown in Fig. (5). 12 displacement transducers were replaced to measure the horizontal displacement of the loading point, the horizontal displacements along different heights of the wall, and the rotation and translation of the foundation beam. Meanwhile, 27 electric resistance strain gauges were replaced to measure the strain’s variation of the horizontal bars, the vertical dowel bars, and the longitudinal bars in the boundary elements. All test data was timely monitored and collected by DH3816N system.

4.1. Top Lateral Load-displacement Hysteretic Loops of Specimens

The top lateral load-displacement hysteretic loops of specimens are shown in Fig. (7), where vertical axis is shear-compression ratio P/f_cbh₀, and lateral axis is the displacement angle of loading point. According to the curves, it can be concluded that:

1. At the initial stages of the test, the number of crack was less and the width was very tiny. Bending deformation could be recovered, with low residual deformation. Therefore, the hysteretic loops were described as straight lines.
2. With the increasing of lateral load, the number and width of diagonal cracks increased. Bending deformation could not be recovered and slip deformation in the root of shear wall increased to aggravate the narrow hysteretic characteristic.
3. The top lateral load-displacement hysteretic loop of specimen 2-SW1 was similar to specimen 2-SW5-H. The yield strength of horizontal steel bars had little influence on the characteristic hysteretic of the new-type shear wall.

4.2. Shear Capacity

The peak loads of specimen 2-SW1 and specimen 2-SW5-H were 1330kN and 1393kN respectively. The shear-compression ratio was 0.137 and 0.148 respectively. Fig. (8) shows the strain of horizontal steel bars of specimen in the position HR4. It can be gained that the strain of horizontal steel bar in specimen 2-SW5-H was obviously higher than specimen 2-SW1 in the same position.

4.3. Deformation Capacity

Ductility: The displacement of yield point, peak point and limit point and the displacement ductility coefficient are shown in Table 3, where yield point is selected by geometrograph, and limit point is the state point which is the horizontal load decreased to the peak load at 85% on the skeleton curve. It can be seen that, the displacement ductility coefficient of the two specimen was close to or more than 4, and the new-type shear walls have good ductility. The yield displacement and peak displacement of specimen 2-SW5-H were more than specimen 2-SW1 to prove that decreasing the yield strength of horizontal steel bar can raise the deformability.

Energy Dissipation: Equivalent viscous damping coefficients-displacement curves are shown in Fig. (9). By curves comparison, it can be seen that the curves of specimen 2-SW1 and specimen 2-SW5-H basically in coincidence to prove that the energy dissipation capacity of two specimens was equally and the yield strength of horizontal steel bars had little influence on the energy dissipation capacity of new-type shear wall.