## 5 Conclusions  
RC shear-controlled beams were tested under high levels of sustained load to evaluate their time-dependent strength and stiffness characteristics. The beams were tested at concrete ages of 67 to 543 days and exposed to high levels of sustained load ranging from 82% to 98% of the short-term capacity for time periods from 24 to 52 days. The experimental work yielded the following conclusions.  
* Even if plain concrete can experience tertiary creep and eventual failure under load levels near 75-80% of its compressive strength, the sustained load level for RC beams to cause failure is very close to their short-term capacity. Only one beam (B10-SL) failed under sustained load at a load level of 98%. Another beam (B9-SL) was able to carry a load level of 95% for 30 days without failure.
* Deflections increased during sustained loading for all specimens. At 24 days, the increase in deflection was on average 24% of the initial deflection. The increase in deflection was higher for specimens with higher levels of sustained load.
* On average 55% of the increase in deflection under sustained load took place in the first 24 h. The sharper increase in deflection early in the loading coincided with the primary stage of creep deformations and time-dependent bond slip. The secondary stage exhibited nearly a linear increase in deflection with time. Only specimen B10-SL experienced a tertiary stage that showed a sharp increase in deflection with time just 2 min before failure.
* Sustained load increased the deflection at shear failure for all specimens. For beams series I, the increase in deflection at shear failure compared to the control specimen was 230% while for beams series II, the increase in deflection was 150%. The large increase in deflection shows that sustained loading can significantly affect the failure behavior of a beam. This large increase in deflection would allow for load redistribution in redundant systems or provide warning signs of impending failure.
* Shear-controlled beams tested under sustained load showed a different failure behavior from the control specimen that was tested under monotonically increasing load to failure. Unlike the control specimen, which failed brittelly in shear, specimens tested  
Figure 23: Increase in deflection and strain under sustained load (beams series I).  
under sustained loads experienced significant increases in deflection and flexural cracking before ultimate shear failure.
* Both the tensile and compressive strains increased under sustained load. The increase in strain shows that the reinforcement took more of the loading as the concrete softened under the sustained load. The increase in the compression strain was higher than in the tension strain.  
Conceptualization, S.O. and Y.T.; methodology, S.O.; formal analysis, M.S.; investigation, M.S.; resources, S.O.; data curation, M.S.; writing--original draft preparation, M.S.; writing--review and editing, A.E.; supervision, S.O.; project administration, S.O.; funding acquisition, S.O. and Y.T. All authors have read and agreed to the published version of the manuscript.  
This paper is based on work supported by the National Science Foundation under Grant No. 1762362 and 1760915. The authors gratefully acknowledge the financial support from the National Science Foundation. The opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsor.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.