Issue 46

S.Y. Jiang et alii, Frattura ed Integrità Strutturale, 46 (2018) 275-284; DOI: 10.3221/IGF-ESIS.46.25 282 R ESULTS ANALYSIS nder the action of sustained load, the additional deflection falls into two parts: the deflection increment arising from concrete creep, and the deflection increment resulted from the decrease in sectional stiffness The deflection increment arising from concrete creep When a beam is under a sustained load, the creep of its material may magnify its component deflection over the time. Here, concrete is the main creep material of the test beams. Previous research [15] has shown that concrete is restricted by steel bars, supports and other components, and the deflection of reinforced beam is affected by the creep of both compressive concrete and tensile concrete. In the same conditions, a CFRP-reinforced beam faces more stringent restrictions (e.g. CFRP plates) than unreinforced beams. The additional restrictions have an impact on long-term deflection, especially when the beam is pre-cracked. The higher the degree of pre-cracking, the less concrete will creep under sustained loading. Hence, the additional deflection decreases with the rise of the pre-cracking degree. Meanwhile, a high degree of pre-cracking means lots of stress will be released and redistributed to steel bars and CFRP plates. In this case, the mean stress level of the concrete becomes smaller (Fig. 10), leading to a small creep. Therefore, the number of pre-cracks is negatively correlated with the long-term additional deflection. Figure 10 : The stress distribution of tensile concrete under different pre-cracking degrees. Deflection increment resulted from the decrease in sectional stiffness With the decrease in sectional stiffness, there is an instantaneous increase in the deflection of the beams. Under the sustained loading, the bending stiffness of the beams will be reduced despite the constancy of the external load [16]. The stiffness reduction under sustained load can be explained by two main reasons: (1) the concrete shrinkage weakens the bond between the concrete and the steel bars; (2) the cracks continue to grow under sustained load. The four test beams were made of the same concrete and steel, and casted at the same time. The experiment was carried out indoors in the same environment. Therefore, the four beams must have the same shrinkage and the same effect on sectional stiffness under sustained load. Due to concrete creep, all beams undergo the increase in curvature and deflection. The underdeveloped cracks will grow in height. The height growth keeps reducing the sectional stiffness under sustained load and bolstering the deflection of components in the sustained loading phase until the crack propagation is restricted by compressive bars. In essence, concrete creep is the ultimate cause of the reducing sectional stiffness under sustained loading. Pre-crack Immediate crack Total-crack Additional crack B-1 -- 3939 6862 2923 B-2 -- 4024 6724 2700 B-3 4276 4276 6403 2127 B-4 6285 6989 7411 1125 Table 4 : Total height of cracks at different moment during the test (mm). U