## 1 Introduction
Under higher compressive stresses (greater than \(\sim\)0.4 f'\({}_{c}\)), the concrete undergoes additional damage processes as a result the propagation of microcracks initiated during loading [20; 21; 22]. Additional nonlinear creep strains are developed as the relationship between the linear creep strain and elastic strain is lost [16]. Tertiary creep is the last stage and generally occurs only when stress become greater than \(\sim\)0.75 f'\({}_{c}\). During this stage, concrete experiences coalescence of microcracks, which ultimately results in concrete failure [16]. The tertiary creep stage is signified by a rapid increase in the creep strain rate. Many studies [1; 12; 16; 17; 20; 23; 24; 25] found that plain concrete subjected to sustained load fails at about 70% to 80% of its short-term strength. Figure 2 shows the effect of level and duration of sustained load on concrete strain [1]. When the sustained stress is lower than 0.75 f'\({}_{c}\), microcracks grow slowly [20], creep deformations are limited under infinite loading, and concrete does not fail [1]. When the sustained stress is greater than 0.80 f'\({}_{c}\), microcracks grow rapidly [20] and concrete strains increase rapidly so that concrete fails at stresses lower than f'\({}_{c}\). [PERSON] and [PERSON] [12] demonstrated that as concrete strength increased, the ratio of sustained load strength to ultimate strength increased. Figure 1: Evolution of creep strains with time (primary, secondary, and tertiary stages of creep). [PERSON] et al. [16] experimentally and theoretically investigated concrete failure under varying strain and stress rates. They investigated the development of linear and nonlinear creep strains, as well as their relationship to material damage and eventual failure. [PERSON] et al. [16] concluded that the inelastic strain capacity of concrete governs failure under sustained load, which can be estimated as the difference between the instantaneous post- and pre-peak strains for a given stress level. Because stresses can be redistributed to the longitudinal reinforcement as the concrete creeps, RC members behave differently from plain concrete under sustained loading. Stress redistribution allows concrete stresses to be decreased, thereby reducing failure likelihood. Moreover, the bond between concrete and reinforcing bar is also affected by sustained load and may contribute to increased deformation or failure. While considerable research has been conducted on long-term deflection of RC beams under service-level sustained loads [26; 27; 28; 29; 30; 31] or corrosion [32; 33; 34; 35; 36], there has been little research on RC members subjected to high-level sustained loads. High sustained load has been found to negligibly impact the flexural capacity of simply supported beams [37; 38], likely because longitudinal bars, rather than concrete, control flexural strength. For shear-controlled beams, a study by [PERSON] et al. [39] showed failure in two beams tested under high sustained loads equal to 98% and 91% of their average short-time shear capacity after 2.5 and 44 h of loading, respectively; however, 16 other beams did not experience failure under sustained loads. The researchers also found that crack widths stabilized after 6 months and subsequent reloading to failure did show a reduction in shear strength due to sustained load. While this test provided valuable information, the beams did not contain compression reinforcement that is typical in many beams and may alter the sustained load behavior. [PERSON] et al. [40] tested shear critical beams under very slow loading (160 kN/1000 h) and found that the increase in crack width under sustained load was very small and creep had no effect on concrete shear strength. However, only two specimens were tested under the very slow load rate. [PERSON] and [PERSON] [41] investigated the deflection of eight cracked RC beams under sustained loads ranging from 48 to 88% of capacity.