Specimens with high strength material

As the advances of material science and triggered by the demand in the construction industry, the use of high strength material would become a future trend in the future.

However, up to the author’s best knowledge, no tests are currently available to study the applicability of high strength material in the coupling beam specimens. In this five-year project, some specimens were constructed using this high strength material as discussed below.

3.7.1 Test specimens

Specimen CB20-7 was dedicated for ACI 318-14 diagonal reinforcement layout using high strength material. Four Grade 100 D25 bars were used as the diagonal bars with concrete compressive strength of 70 MPa (Fig. 3.34a).

Specimen CB30-17 and CB30-18 was designed exactly similar to CB30-14, in which two D25 bars were used in one group of diagonal bar and three D29 bars were used as longitudinal bars; but high strength material was used instead. In CB30-17 (Fig. 3.34b), the designed concrete compressive strength was 70 MPa and the nominal yield strength of the diagonal bars was 685 MPa. The other parts (longitudinal and vertical shear reinforcement) remain using normal strength material. Meanwhile, in specimen CB30-18 (3.34c), only effect of high strength material as vertical stirrups was evaluated. The nominal yield strength of the vertical reinforcement was 785 MPa, while the concrete and diagonal bars remain using normal strength material.

3.7.2 Test results of specimens with high strength material

The use of high strength material for coupling beam detailed according to the ACI 318-14 diagonal layout as in CB20-7 did not show significant difference from the one with

normal strength material (CB20-3). Robust hysteretic loop was observed for this specimen (Fig. 3.35a). The maximum lateral load of 1238 kN occurred at DR of 2.1%

where the crack width was as wide as 0.25 mm. The strength degradation of this specimen occurred mildly and concrete crushing began to appear at DR of 5.4% (Fig.

3.36a). At the last drift ratio, i.e. DR of 9.7%, the test was terminated after severe crushing at the upper and lower part of the beam as indicated in Fig. 3.36a.

The hysteretic behavior of CB30-17 and CB30-18 was in general similar to that of CB30-14. Specimen CB30-17 reached the maximum lateral load of 942.6 kN at DR of 2.7% (Fig. 3.35b), while specimen CB30-18 reached the maximum lateral load of 826.0 kN at DR of 2.7% (Fig. 3.35c). The maximum flexural crack width for these two specimens at DR of 2.7% was 1.5 mm and 2.0 mm for CB30-17 and CB30-18, respectively. The drastic reduction of lateral load started to occur after a severe crushing of concrete at the beam ends. For specimen CB30-17, the severe crushing of concrete occurred at DR of 5.7% (Fig. 3.36b), while for CB30-18, it occurred at DR of 4.6% (Fig.

3.36c). Finally, specimen CB30-18 was terminated at DR of 5.9% after severe crushing of core concrete, while CB30-17 went to DR of 7.9% before the test was fully terminated. The failure modes of these two specimens were flexural-shear failure with UDR of 5.7% and 4.3% for CB30-17 and CB30-18, respectively.

3.7.3 Discussion on effects of high strength material On deformation capacity

The behavior of ACI diagonal specimen with high strength material (CB20-7) seems to have no difference compared to the one using normal strength material (CB20-3). The UDR achieved by CB20-7 reached a DR of 7.7%.

With a similar amount of diagonal reinforcement, the UDR of CB30-17 with high strength material had no difference compared to CB30-14 with normal strength material.

The UDR achieved by CB30-14 was 5.5%, while the UDR achieved by CB30-17 was 5.7%. The reason for this similar deformation capacity might come from the fact that the diagonal bar contributed to both flexural and shear strength. While we might expect that a coupling beam with high strength steel for diagonal bars would result in better shear strength and hence better deformation capacity, but the reality is not. The roles of diagonal bar as flexural and shear strength are coupled. So, at the same time, the same high strength steel diagonal bar would also increase the flexural strength and hence, larger shear demand.

However, specimen CB30-18 with the use of high strength vertical shear reinforcement seems to have less deformation capacity compared to CB30-14 with normal strength vertical shear reinforcement. The UDR of CB30-18 was only as much as 4.3%, while that of CB30-14 reached 5.5%. The reason behind this finding is still unknown yet.

On stiffness degradation

The stiffness degradation curve of CB20-3 (normal strength) and CB20-7 (high strength) as shown in Fig. 3.37a indicates no difference behavior. Similar observation also applies when comparing CB30-14 and CB30-17. The reason for this finding is similar to that described in the previous point on the deformation capacity.

However, worse behavior was observed for CB30-18 (Fig. 3.37b), which reason is still unknown.

On the effective stiffness

The effective stiffness of CB30-17 and CB30-18 is 0.11 EcIg and 0.14 EcIg , respectively.

On cumulative energy absorption

Figure 3.38a showed that the use of high strength material (CB20-7) has less capacity in absorbing energy as compared to CB20-3. However, comparing CB30-14 and CB30-17,

no significant difference can be observed. However, by detailing high strength stirrups (CB30-18), the absorbed energy is less as shown in Fig. 3.38b.

3.8 Specimen with discontinuous diagonal bars