Issue 46

S.Y. Jiang et alii, Frattura ed Integrità Strutturale, 46 (2018) 275-284; DOI: 10.3221/IGF-ESIS.46.25 276 (GFRP) and unreinforced concrete beams. During the experiment, each kind of concrete beams was piled up to withstand the external load. El-Sayed A.K. et al. [8] performed a bearing capacity test on two flat beams and a 600-day creep test on three CFRP-reinforced flat beams. Tan K.H. [9] carried out a 2 year-long creep test on six reinforced concrete beams partially reinforced by the GFRP and three reinforced concrete beams. During the test, the load was adjusted to different levels, and the beams were reinforced to varied degrees. Kim [10] carried out a 300-day load test on a GFRP-reinforced beam and a CFRP-reinforced beam, and measured the beam deflection, concrete strain, internal rebar strain, and FRP strain during the loading. All the above long-term load tests show that the ultimate deflection of directly reinforced beam with no cracks directly hinges on the level of external load and the degree of reinforcement, and that the long-term deflection can be partially constrained by the reinforcement (e.g. CFRP and GFRP). In practice, directly reinforced intact beam is very rare. Most beams are already cracked before reinforcement due to previous use. Nevertheless, there is very limited report on the long-term deformation behaviours of reinforced beams with cracks prior to reinforcement (pre-cracked reinforced beam). Through a relaxation test on three pre-cracked reinforced concrete beams, M. Muller [11] disclosed the negative correlation between the number of initial cracks and the additional beam deflection after the reinforcement. Rao Xinpin [12] from Hunan University was the only scholar in China that explored the long-term deflection of initially deformed reinforced concrete beams. During Rao’s research, two reinforced concrete beams were loaded to the same level; one of them was reinforced with the CFRP after its deformation, while the other was directly taken as the control. Then, long-term observation was made without changing the load. The research shows that the CFRP has little impact on long-term beam deflection, but fails to describe the crack condition of the beams. To sum up, the research into the long-term deformation of reinforced beams, especially pre-cracked ones, is severely insufficient. To make up for the gap, this paper attempts to disclose the effect of cracks on instantaneous and long-term deflections of reinforced beams. For this purpose, four concrete beams were created, pre-loaded to varied degrees of cracking, and reinforced with the CFRP. Then, a long-term load test was carried out on these samples. The innovation of this research is based on practical engineering, and the cracks in concrete beams are considered in the test. In order to explore the effects of damage of concrete beams on the instantaneous and long-term deformation of reinforced beams. The research findings provide a valuable reference for actual reinforcement projects. E XPERIMENTAL S TUDY Test parameters n our research, the CFRP plates are 100mm-wide and 1.4mm-thick adhesive plates with a tensile strength of 2,482MPa, an elastic modulus of 174GPa, and an ultimate elongation of 1.68%. The elastic modulus of the structural adhesive is 3.5GPa. The reinforcements are all made of HPB400 steel. Specifically, the diameter, tensile strength and elastic modulus are respectively 28mm, 456MPa and 200GPa for compressive bars, 14mm, 461MPa and 200GPa for tensile bars, and 8mm, 457MPa and 200GPa for stirrups. The concrete for the test beams was supplied by a local commercial concrete mixing station. It was casted into a batch of 150mm×150mm×300mm (L×W×H) standard prismatic specimens, turned into test beams, and cured together before the experiment. On the day of experiment, the compressive strength and elastic modulus of the prismatic concrete specimens were measured as 41.9MPa and 33.38GPa, respectively. Sample design Four identical reinforced concrete beams, denoted as B-1, B-2, B-3 and B-4, were prepared for the experiments. All four beams are 3,300mm long, 250mm wide and 400mm tall. The details of the beams and reinforcement are given in Fig. 1. Note that a high reinforcement ratio was adopted for the beams so that the concrete edge can withstand the compressive strain under heavy external load (maximum compressive strain ≤ (0.4~0.5) fc). In this case, the creep of the concrete always falls in the range of linear creep [13]. The stirrups were dandified at both ends of the support to eliminate shearing failure in the long-term experiment. To prevent delamination of CRFP plates, the key anchoring length of these plates were kept constant during the reinforcement, and stirrups were provided at both ends of each CFRP plate [14]. Experimental procedure Let Fu be the ultimate bearing capacity of the test beams before reinforcement. After curing, the four-point bending method was adopted to apply 15% Fu, 45% Fu and 65% Fu onto B-2, B-3 and B-4, respectively (pre-loading), The B-1 beam is not preloaded as a contrast beam for the test, and the midspan deflection and sectional crack development were recorded for each beam. Then, the load was removed (unloading), and the four test beams were reinforced and cured as per the above I