## 1 Introduction
Recently, in June 2021, a 12-story RC flat-plate building in the Miami suburb of Surfside, Florida, collapsed. While the investigation is ongoing, early results show that long-term degradation of RC structural support in the ground-level parking garage under the housing units may have been a contributing factor [8]. As a result, the structural components in the building may have been exposed to high sustained loads and their time-dependent effects may have contributed to the eventual collapse. In a recent experiment to investigate the progressive collapse resistance of RC flat-plate buildings, a full-scale test was conducted in 2016 by the Defense Threat Reduction Agency (DTRA) [9]. Following the instantaneous removal of the front center column, the test structure was seemingly safe and obtained an alternative load path to carry the sustained gravity loads. However, the slab continued to deform. After four hours, punching failures occurred at two neighboring slab-column connections. The system at this stage was still able to tolerate the local failures and maintain stability. Overnight, the test structure completely collapsed. All these aforementioned examples show that sustained load can lead to local component failure and, if the failure is not mitigated timely and properly, time-dependent effects can cause structural collapse. Although the cited in-service structures had additional factors for failure such as material degradation or poor-quality concrete, they also had concrete members loaded close to their capacity. The exact mechanism of collapse evolution from highly stressed concrete to structural collapse is largely unknown. Currently, very few tests have been conducted on RC members under high sustained loads. Failure of RC structures under high sustained load is most directly related to concrete creep behavior. Slip between the reinforcement and concrete due to creep in the concrete can also occur. Creep is defined as the time-dependent increase in strain of a solid body subjected to sustained stress. Many parameters influence concrete creep, including stress level relative to short-term compressive strength, age, temperature, aggregate type and size, water-cement ratio, geometry, and humidity [10; 11; 12]. This research is concerned with collapse of older concrete structures where the effects of shrinkage and changes in moisture content are minimal after a high sustained load has been applied. As a result, basic creep (time-dependent deformations at a constant temperature caused by load) in concrete with a nearly constant moisture content is considered. In terms of creep strain kinematics under high constant stresses, concrete experiences three stages of creep: primary, secondary, and tertiary creep, see Figure 1[1; 13; 14; 15; 16]. Stress level dictates whether the concrete will experience all the three stages, or only a subset of them. The primary stage of creep occurs early in the loading history. It is characterized a high early creep rate that steadily decreases with time. Primary creep is generally associated with low stress up to about 40% of short-term compressive strength (f'\({}_{c}\)). Creep under this level of stress has been the focus of most past research [13; 17; 18]. The stress level (\(\sigma_{\rm c}\)) falls within the elastic domain of concrete response and creep deformations are caused by the breaking and reformation of atomic bonds at various highly stressed sites within the colloidal microstructure of the calcium silicate hydrate gels in the hardened cement paste [19]. At sustained stress levels \(\leq\) -0.4 f'\({}_{c}\), there is a nearly linear relationship between the short-term strains (strain under short-term loading) and delayed creep strains (strains developed with time) and the rate of creep strain development decreases with time [16]. The secondary stage of creep occurs later when creep strains increase with a relatively constant creep strain rate.