## 4 Discussion
### Deflection under Sustained Loading  
Figures 13 and 14 plot load and deflection time history under sustained loads for the two series of tests. During sustained loading, the deflection of all beams increased. All specimens had the same general behavior which matches the behavior of plain concrete as shown in Figure 1. In the first stage (primary) deflections increased rapidly, and in the second stage (secondary) the deflections increased at a linear rate. Jumps in deflections were due to load adjustments. During sustained loading, the deflection of B2-SL increased from 5.31 mm to 6.42 mm (21%). B3-SL increased in deflection by 38%. However, B4-SL with the highest sustained load level of 93% increased by only 20.5%. At 24 days, the deflection increases under sustained loading in B2-SL, B3-SL, and B4-SL were 21%, 33%, and 20%, respectively, of the initial deflection. B2-SL was near yield when sustained loading started. This may have led to higher deflections as the reinforcement yielded under sustained load. The post-yielding load-deflection behavior of B3-SL appeared to be softer than that of the other specimens, possibly leading to increased deflection under sustained load. The small difference (0.44 mm) in the total and 24-day deflection for B3-SL shows that the additional 18 days of testing did not result in significant increases in displacement. The deflection at  
Figure 12: Beam series II failures. the end of sustained loading of B6-SL, B7-SL, and B9-SL (beam series II) increased by 19%, 25%, and 29%, respectively. At 24 days, the increase in deflection was 18%, 25%, and 28%, respectively. The deflection increase was greater for shear-controlled beams with higher levels of sustained load and demonstrated a more consistent trend than for beam series L, as shown in Figure 15.  
Figures 16 and 17 describe the rate of deflection increase under sustained loading by depicting the additional deflection that occurred under sustained load (\(\delta_{\rm s}\)) divided by the sustained load deflection at 24 days (\(\delta_{\rm s@24\ days}\)) for series I and series II. Despite some differences, all specimens show a similar rate of deflection increase with time. Most of the deflection increase took place in the first few days. In the first 24 h for beam series I, 44%, 61%, and 65% of the deflection increase occurred in specimens B2-SL, B3-SL, and B4-SL, respectively. After 7 days, 67%, 72%, and 80% of the deflection increase occurred in these specimens. Moreover, higher sustained load induced greater deflections in the first few days. In the first 24 h for beam series II during the first stage of sustained loading, 35%, 72%, and 71% of the deflection increase occurred in specimens B6-SL, B7-SL, and B9-SL, respectively. Specim B6-SL experienced the slowest gain in deflection, most likely due to the much lower load level compared to the other specimens (84% vs. >91%). After 7 days, 71%, 85%, and 84% of the 24-day deflection had occurred in the specimens. Again, the results show that most of the deflection increase occurred in the first days of sustained loading. After about 2 days the rate of deflection increase was nearly constant at about 0.025 mm/day (0.001 in./day) for the specimens as expected in the secondary stage of creep. Only specimen B10-SL (series II) reached the third stage (ter  
Figure 14: Sustained load and deflection with the time for the first stage (series II). Figure 13: Sustained load and deflection with time for the first stage (series I). sustained load, as shown in Figure 18. The two earlier increases in deflection were due to load adjustment, while the last increase occurred during tertiary creep with no adjustment in the loading. This stage happened within 2 min before failure. Beam series 1 was loaded to failure after the period of sustained loading. The beams experienced a peak load at deflections 12%, 65%, and 47% greater than the failure deflection of BC1. The load level then dropped as the concrete crushed on the extreme compression face.