3.1 Test Observations
After the quasi-static tests, Fig. 9 shows the damage status of the specimens. As shown in Fig. 9(a), for specimen S1, the UHPC retaining block had no visible damage and it tended to have strong self-resetting ability when the horizontal load increased from 0 to 90 kN. The first micro-crack appeared in the block body as the applied horizontal load exceeded 90 kN, but the visible micro-crack was closed completely after unloading. Afterwards, the first micro-crack did not expand significantly and in the meantime no new cracks were observed. However, as the applied load increased from 130 kN to 170 kN, the initial micro-crack began to further expand and extended to the main crack, which cannot be closed after unloading. Also, in this stage, many new developed micro-cracks were observed around the main crack, and their widths increased rapidly. In addition, in the subsequent loading stage, as the applied load increased to 190 kN, many visible cracks appeared on the surface of the block body and a few vertical cracks began to appear in the compression zone. Finally, the loading process was stopped as the tensile force of the upper prestressing tendon reached 190.5 kN, which was close to 80% of the yield strength (260 kN). Moreover, as seen from the final failure mode of specimen S1 given in Fig. 9(a), specimen S1 could basically return to the initial position, but the block body and cap beam could not fully fit together completely. It should be noted that the appeared cracks in specimen S1 were concentrated around the prestressing ducts of UHPC retaining block. This is mainly because the prestressing ducts of the UHPC-PRB structure were large, leading to the width of the UHPC-PRB structure was very narrow.
Similarly, as shown in Fig. 9(b), the damage mode of specimen S2 in the initial loading stages was similar to that of specimen S1, but the first micro-crack appeared as the applied horizontal load exceeded 110 kN. When the applied load increased from 170 kN to 210 kN, the main crack formed and extended gradually to the compression zone and its width increased rapidly. This developed main crack could not be closed completely after unloading. Subsequently, as the load increased to 240 kN, many visible cracks appeared on the surface of the block body and a few vertical cracks were also observed in the compression zone. Then, as the tensile force of the upper prestressing tendon reached 200.8 kN, which was close to 80% of the yield strength (260 kN), the loading test was stopped for safety. In addition, as observed from the final failure mode of specimen S2 as shown in Fig. 9(b), specimen S2 could also basically return to the initial position, but UHPC retaining block could not fully fit together with the cap beam. The locations and cause of the formed cracks in specimen S2 were similar to that of specimen S1.
As shown in Fig. 9(c), for specimen S3, UHPC retaining block had no damage and exhibited strong self-resetting ability before the applied horizontal load increased to 150 kN. As the load exceeded 150 kN, the first visible micro-crack appeared on the surface of the block. During the subsequent process, no other new visible micro-cracks appeared except for the further development and propagation of the first visible crack, and it did not extend to form the main crack. The first visible crack could be completely closed after unloading. Then, as the load increased to 210 kN, the tensile forces of these two upper prestressing tendons reached 213.6 kN, which exceeded 80% of the yield strength (260 kN), specimen S3 only had the relatively slight damage, so it could return to its initial position after unloading. Thus, the loading test continued to the next stage of load, but the corresponding load increment interval was changed to 10 kN. As the applied horizontal load increased to 218.2 kN, the loading was stopped for safety because the upper prestressing tendons fractured with a big sound of “bang” and the load dropped significantly to 198.3 kN. Then, in this case, specimen S3 could not return to the initial position after unloading, but fell downward slightly.
Likewise, as seen from Fig. 9(d), for specimen S4, the damage in the earlier loading stages was basically similar to that of specimen S3. As the applied load increased to 230 kN, the tensile forces of two upper prestressing tendons were 217.6 kN, which exceeded 80% of the yield strength;however, specimen S4 only exhibited slightly damage mode, it could return its original position after unloading. Then, the loading test for this specimen continued to proceed the next loading stage, but the corresponding load increment interval was changed to 6 kN. Finally, as the load reached 236.2 kN, the loading test was stopped for safety because the upper prestressing tendons fractured with a big sound of “bang”, and the applied load dropped sharply to 221. 2 kN. These observations were basically the same as those of specimen S3. However, compared with that of specimen S3, due to the larger initial tensile forces of these four prestressing tendons in specimen S4, the two lower prestressing tendons with elastic tension force could still provide enough resistance to overcome the self-weight of UHPC retaining block. Therefore, specimen S4 did not fall down after unloading, as shown in Fig. 9(d).
Furthermore, Fig. 9(e) shows the final damage modes of several other components of the specimens, including the failure modes of the upper prestressing tendons and that of the roughness construction joint connection contact surfaces between the vertical UHPC retaining blocks and the NC cap beams. As seen from Fig. 9(e), several steel wires of the upper prestressing tendons of specimens S3 and S4 were observed to be pulled off at the end sections. Moreover, all the contact surfaces of these four test specimens were undamaged and the cap beams were always well protected. Overall, the proposed modified UHPC-PRB structures were demonstrated to have small residual displacements and excellent self-centering ability. Additionally, the new structural form with the straight joint connection would not cause serious damage to the cap beam during the process of rotation, which is beneficial and convenient to the design and retrofit of UHPC retaining blocks.
3.2 Strains of the Vertical Reinforcements
Figure 10 shows the obtained maximum strains of the vertical reinforcements in different specimens. As seen from Fig. 10, the maximum strain of all vertical reinforcements monitored in the test did not exceed the yield strain. It should be noted that, since the reinforcements with the greatest tensile stress were the first row of vertical rebars, the other ones should be elastic, and the measured results were not given in this figure. On the other hand, as shown in Fig. 10, the maximum strain of the rebar appeared in specimens S1, and its strain was about 1500 µε, which is very close to the yield strain (1675 µε). In addition, the maximum strain of the reinforcements in other specimens were much smaller than the yield strain, especially these two thicker specimens (S3 and S4). Moreover, the strain situation of the vertical reinforcements of different specimens were consistent relatively well with their corresponding damage modes. For example, as shown in Fig. 9, there were many cracks in specimens S1 and S2, but the UHPC retaining blocks of specimens S3 and S4 were always in elastic during the loading test.
3.3 Stress of Prestressing Tendons
As illustrated in Fig. 5, four pressure sensors were installed between the anchorage plate and the side of the cap beam to monitor the stress states of different prestressing tendons during the test. Since the tensile force of the lower prestressing tendon was smaller than that of the upper one, Fig. 11 shows the relationship between the applied horizontal load and tensile force of the upper prestressing tendon monitored by the pressure sensors. It can be seen that the tensile forces of the prestressing tendons increased significantly with the increase of the applied horizontal load. When the horizontal load was less than the critical rotational load of UHPC retaining block, the tensile forces of the prestressing tendons tended to increase at a relatively small rate, and the corresponding residual tensile forces remain unchanged after unloading. However, as the applied horizontal load exceeded the critical rotational load of the retaining block, the increase rate of the tensile forces of the prestressing tendons increased significantly. Meanwhile, the corresponding residual tensile forces of the tendons gradually decreased after unloading. When the tensile forces of the prestressing tendons were greater than the initial prestress, prestress loss caused by the anchorage retraction would occur. As seen from Fig. 11, the greater the applied horizontal load, the greater prestress loss in the prestressing tendons would occur in the test specimens. As a result, the final tensile forces of the upper prestressing tendons for specimens S3 and S4 were zero after the loading test. This could be mainly attributed to the prestress loss and fractured of steel wires in the upper prestressing tendons of specimens S3 and S4. In addition, as seen from Fig. 11(a), the increase rates of the tensile forces of the upper prestressing tendons decreased with the decrease of the loading height. Moreover, as shown in Fig. 11(b), when other conditions were the same, variations of the tensile forces of the prestressing tendons were independent from the block thickness. In other words, thickness of the retaining block did not affect variations of the tensile forces of the prestressing tendons.
Furthermore, as shown in Fig. 6, some strain gauges were attached on the prestressing tendons to monitor the strain states of the prestressing tendons during the test. Figure 12 shows the measured strain time-histories of the prestressing tendons for different specimens. As seen from Fig. 12, for all the test specimens, strains of the upper two prestressing tendons were greater than that of the lower ones, and the strain variations of the prestressing tendons in the same row under the applied horizontal load were almost the same. According to the obtained strain time-histories of the prestressing tendons as shown in Fig. 12(a) and (b), the maximum strains of the prestressing tendons in specimens S1 and S2 were smaller than the yield strain (around 9000 µε). However, the maximum strains of the prestressing tendons in specimens S3 and S4 were larger than the yield strain, as observed from Fig. 12(c) and (d). Meanwhile, strains of all prestressing tendons would increase with the extension of the loading time due to the increase of the applied horizontal load. However, the residual stresses or residual strains of the prestressing tendons after unloading would decrease gradually with the increase of the applied horizontal load, which was consistent with the monitored tensile force variations. Additionally, as shown in Fig. 12, owing to the prestress loss effect, the upper two prestressing tendons of specimens S3 and S4 were completely relaxed after the loading test.