## 3 Experimental Results
### Strengthened Beams
Although the decrease in the load was not evident after the maximum load, the experiment was terminated when the measured deflection was 87 mm because brittle failure by concrete crushing was predicted. The initial strain of the post-tensioning rod (\(\varepsilon_{se}\) = 2153 \(\mu\varepsilon\)) increased to 3141, 3805, and 5411 \(\mu\varepsilon\) at \(\varepsilon_{cu}\) = 0.003, \(\varepsilon_{cu}\) = 0.005, and \(P_{2 nd}\) = 419.0 kN, respectively. The external rod yielded at the load and deflection of 417 kN and 62.6 mm, respectively; this occurred immediately prior to attaining the maximum load. The measured load-deflection and rod strain curves for S2-R28 are shown in Figure 10b. The stiffness prior to post-tensioning of 13.8 kN/mm increased to 19.8 kN/mm after post-tensioning, which resulted in a deflection recovery of approximately 8.5 mm. The initial strain in the external rod was 2035 \(\mu\varepsilon\). Reloading was applied at a deflection of 8.2 mm, which was less than the deflection 11.3 mm of the unstrengthened specimen owing to the deflection recovery. In this beam, a maximum load and the corresponding strain of 513.8 kN and 61.1 mm, respectively, were recorded after post-tensioning and reloading. The deflections at the measured strains of 0.003 and 0.005 were 26 and 29 mm, respectively. The compressive strain at the concrete extreme fiber reached 0.005 prior to the maximum load. Although the strain of the external rod reached 4592 \(\mu\varepsilon\) at the maximum load, the yield strain was not attained at this point. Figure 9: Deflection and rod strain with respect to applied load: (**a**) S1-R22 and (**b**) S1-R28. The changes in the deflection and strain of the external rod with increasing applied load for the S3-R22 beam are plotted in Figure 11a. The stiffness of 10.4 kN/mm prior to post-tensioning reduced to 17.5 kN/mm afterwards. The first maximum load prior to post-tensioning was 182.4 kN, and the post-tensioning was conducted after unloading to 120 kN. Post-tensioning achieved a deflection recovery of 6.0 mm, and the initial strain of the steel rod was 2148 \(\upmu\)\(\upmu\) at the end of post-tensioning. In this beam, the load marginally decreased after the maximum load of 409 kN and, thereafter, the compressive strain of the concrete extreme fiber reached 0.003. Figure 11b shows the load-deflection and load-rod strain plots of S3-R28. In this specimen, because of an operational error that caused the UTM to act abruptly on the beam immediately prior to the post-tensioning, the initial strain of the rod and the deflection recovery were zero. The stiffnesses before and after post-tensioning were calculated as 10.4 kN/mm and 18.0 kN/mm, respectively. The load was plotted as 281.9 kN at the concrete extreme fiber strain of 0.005, and the maximum load was recorded as 436.9 kN. Table 4 summarizes the test results of the strengthened specimens. The stiffness-strengthening effects for the S1-, S2-, and S3-type specimens were approximately 30, 43, and 70%, respectively. The specimens with a higher number of tensile rebars exhibited less stiffness-strengthening effects. Additionally, based on the maximum load, the strengthening effect was observed to be approximately two times for the S3-type specimens, approximately 40 and 55% for the S1-type specimens, and 64 and 101% for the S2-type specimens. The strengthening effect was more than 37% at the compressive concrete strain of 0.003 or 0.005 except for S3-28, which was not post-tensioned owing to the aforementioned operational error. Figure 11: Deflection and rod strain with respect to applied load: (**a**) S3-R22 and (**b**) S3-R28. Figure 10: Deflection and rod strain with respect to applied load: (**a**) S2-R22 and (**b**) S2-R28.