# Long-Term Behavior of RC Beams Strengthened with Hybrid Composite Beam
## 1 Introduction  
Since seismic records were first collected in South Korea in 1978, two powerful earthquakes of 5.1 and 5.8 magnitude were recorded in the country, without significant human injuries or serious damage to building structures in the past three years. They were the strongest earthquakes in the country so far, respectively. After that, there were big concerns about existing reinforced concrete school buildings that did not meet current seismic design codes. According to the data, more than 50% of school buildings in South Korea did not meet current seismic design codes. Thereafter, governments have conducted various strengthening works on school reinforced concrete (RC) buildings to make them stronger so that they can withstand more powerful earthquakes, by increasing the strength of rigid structures. Many different ways for strengthening existing school buildings were used, such as damping systems, bracing, external strengthening of steel plates or fiber reinforced polymer ( FRP) composites, section enlargement and a combination of several systems [1; 2; 3; 4]. At this time, a comprehensive understanding of the long-term behavior of FRP-strengthened RC members may be needed for analysis and developing further FRP strengthening systems in this country. Generally, the long-term behavior of structural members can be defined as a creep strain, and this creep strain is indicated as a function of time. It was found that there are several similar analytical and experimental investigations involving long-term tests of FRP-strengthened RC beams [5; 6; 7; 8; 9]. [PERSON] et al. [10] reported on the creep behavior of carbon fiber reinforced polymer (CFRP)-strengthened reinforced concrete beams from a series of experiments with twenty-six RC beams with and without bonded CFRP laminates. Different reinforcement ratios were used to evaluatethe contribution of the external reinforcement on the creep resistance of the beams. High levels of sustained load were used in order to determine the maximum sustained load that can be applied without any risk of creep failure. The applied sustained loads varied from 59% to 78% of the ultimate static capacities of the unstrengthened beams. This was done to account for the fact that strengthening is typically required when a structure is expected to carry increased service loads. It was found that CFRP-strengthening on RC beams is effective for increasing the ultimate capacities of the beams. However, test results showed there was no improvement with regard to the long-term deflections due to the addition of a very small area of CFRP compared to the tension steel reinforcement. [PERSON] et al. [11] investigated the time-dependent deformations of concrete beams reinforced with CFRP bars from four rectangular concrete beams, under four different uniform sustained loadings for nearly two years. Four rectangular concrete beams reinforced with CFRP bars were cast in two sizes: 152 mm \(\times\) 203 mm \(\times\) 2438 mm; 152 mm \(\times\) 152 mm \(\times\) 2438 mm. The beams were instrumented and monitored to observe the changes in behavior due to creep and shrinkage of concrete. Test results showed that time-dependent deflection, strain and curvature increased with the increase in the applied moment in the beams, and the rate of increase in strains and deflections was higher in the initial period of loading and tended to reduce with time under sustained loading. In this investigation, the age-adjusted elastic modulus method appears to be realistic in predicting the long-term behavior of concrete beams reinforced with CFRP. The time-dependent deflections are computed using a simple modified equation in terms of loading duration and compression CFRP reinforcement ratio. The proposed time-dependent factor due to reinforcing CFRP bars for 3, 6, 12, and 60 months showed slightly lower figures than those suggested by ACI for reinforced concrete beams. [PERSON] et al.