Waves S1 Stereo Imager Crack !!BETTER!! [EXCLUSIVE]
Figure 16 presents the tensile and compressive strains of the longitudinal reinforcement. The maximum tensile and compressive strains occur at the bottom of the longitudinal reinforcement. Both the tensile and compressive strains in the longitudinal reinforcement are insignificant before the concrete cracks. As the applied load increases, the tensile strains at T3, T2 and T1 develop inflection points in turn, after which they increase at a faster rate. The inflection points mean that the tensile stress provided by the concrete is transferred to the longitudinal tensile reinforcement due to the occurrence of flexural cracks. Strain gauge T3 at the bottom of the longitudinal reinforcement is easily damaged, caused by the generation of cracks. Therefore, the strain gauges T2 and B3 are selected to analyze the contribution level of longitudinal reinforcement of the columns with different parameters (see Figure 17). It can be observed from Figure 17a,b that the tensile and compressive strains increase by increasing the applied load, and this increase is mitigated after the occurrence of flexural cracks when a longitudinal reinforcement ratio of 2.25% and a stirrup reinforcement ratio of 0.47% are used. Figure 17b indicates that the contribution level of the longitudinal reinforcement after the occurrence of flexural cracks is weakened by increasing the stirrup reinforcement ratio. As shown in Figure 17c, the tensile strain in the longitudinal reinforcement at loads greater than 60 kN is greater in column G1 than column S1. Compared with GFRP spiral stirrups, the steel stirrups with higher elasticity modulus can better restrain the development of diagonal cracks and reduce the lateral deformation.
Waves S1 Stereo Imager Crack !!BETTER!!
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In this study, several smart aggregate (SA) transducers were used to monitor the crack development and evaluate the structural damage of the concrete short columns. The active sensing technology that is, using a pair of SAs, one as the actuator and the other as the sensor, was adopted [40,41]. The stress waves were generated by the SA acting as actuator and are received by the SA acting as sensor. As shown in Figure 25, SA1 acts as an actuator and SA3 acts as a sensor, which aims to monitor the overall structural damage of the column. In addition, using SA1 as the actuator and SA2 as the sensor can detect the damage of the lower part of the column. Similarly, the damage of the upper part of the column can be detected by using SA2 which functions as an actuator and SA3 which functions as a sensor.
Time-domain signals of the overall structure, that is, the stress wave generated by SA1 and received by SA3, are presented in Figure 26. After the column is cracked, the increased load results in a decrease in the amplitude of the time-domain signal. This is caused by the generation and development of cracks that hinder the propagation of stress waves. It also indicates that the damage level of the columns is constantly increasing. When the columns are close to the failure status, the received time-domain signals are roughly on the level of the noise in the signals. This indicates that the propagation of the stress wave is mostly impeded due to cracks penetrating the entire column cross-section (see Figure 21).
The variation in ERI with load is presented in Figure 29. Among them, SA1-SA3 refers to the stress wave generated by SA1 and received by SA3. It can be seen that the ERI values barely vary before the onset of cracking of the columns, but they then drop significantly afterward. The change in ERI values relates to the damage zones, that is, the generation and propagation of cracks, as shown in Figure 19, Figure 20 and Figure 21. Take the test specimen G1 as an example: (i) The first crack is observed at the bottom of the column with the load at about 24 kN, as shown in Figure 19a. Correspondingly, the ERI values for SA1-SA3 and SA1-SA2 begin to drop at about 21 kN, see Figure 29a, with the appearance and development of the first crack. Meanwhile, the first crack at the bottom of the column has scarcely an effect on the ERI values for SA2-SA3. (ii) As the load increases to 51 kN, the second crack is observed in the upper part of the column; see Figure 19c. Correspondingly, the ERI values for SA2-SA3 decrease significantly from 40 kN for SA2-SA3 in Figure 29a. (iii) Further increase in the load leads to the rapid development of the first and second cracks and even the appearance of a third crack. The ERI values decline to very small, since the propagation of stress waves is mostly impeded by the long and wide cracks. Note that for the test specimen S1, the ERI values for SA1-SA2 and SA2-SA3 are almost zero after the second crack forms. This is owing to the fact that the second crack formed just at the position of SA2. The above analysis illustrates that the loads corresponding to the first crack in the upper and lower parts of a column can be identified using the ERI values for SA1-SA2 and SA2-SA3, respectively. The identification results of crack formation are consistent with those observed by the naked eye (see Table 3) and monitored with the DIC (see Figure 19, Figure 20 and Figure 21), though both generally lag in ERI identification. This is because the micro-cracks may not be visible to the naked eye and the sides of a column are blind spots for DIC measurement (see Figure 9).