Studying Damaged Moment Beams Reinforced With Carbon Polymer

Author Name(s): *Behzad Dezhkam
Author Email: behzaddejkam@yahoo.com

Abstract

Generally, passing of time and environmental factors such as oxidation and corrosion in some parts of the structure, has a significant effect on decreasing load capacity and creating a defect in structure members. Because of resistance and hardness of carbon fiber reinforced polymer (CFRP), they are suggested as the best materials for repairing the structure. In this paper, for investigating the effect of CFRP sheets for reinforcing steel beam which has initial defect, seven beams were studied for modeling by ABAQUS software. It should be noted that in all these beams, initial defect was created in middle tension flange area of beam bilaterally. This study determines the appropriate performance of CFRP sheets in increasing load capacity and preventing the spread of defect in steel beam.

Introduction

In 1967, once imperative auditing and control was done on bridges in the US, American Association of State Highway and Transportation Officials (AASHTO) and Federal Highway Station (FHWA) determined to develop programs for six month inspection. As a result, it was determined that more than one-third of a highway bridge in US were under standard significantly (1). Klaiber et al (1987) suggested that more than 43% of bridges in the US were built by steel. Based on NBI report, metal bridges are part of the category which are suggested for improvement more than others that their main problem related to the fatigue and exhaustion are needed for increasing service load, corrosion and inappropriate maintenance. Also, repairing and reinforcing before making decision for bridge displacement is recommended. Repairing and rebuilding in most of the cases has lower cost than displacement and it takes less time. Considering limitation of available resource for decreasing problems related to the steel bridges, there’s a need for adopting new material and thrifty method (2).

Lorenzo (1986) suggested that CFRP sheets with fatigue sensitive features in the metal member causes increasing resistance and life cycle of fatigue. Mechanical features and the superior fatigue of fiber polymer reinforced by carbon fiber can introduce them as the best option for repairing and reinforcing steel girders of bridges. CFRP sheets guarantee one million cycle from the fatigue load and tension suffering 1.5 times more than final resistance(3). Tavakolizade and Saadat manesh (2003) proposed results of research which was done about repairing Slotted steel beams with CFRP pieces for the fatigue load with average cycle of (R=0.1) (4). A set of 21 steel beam samples built from A36, S127×4.5 were tested. Unreinforced beams were tested as control samples. Steel beams were tested under four moment points with load ratio between 5 and 10 Hertz. Consistent tension with various domains were considered between 69 and 379 MPa. Length and thickness of pieces were considered equal for all repaired samples. Additionally, number of rupture cycle, hardness changings and fracture reopening and its spreading along each sample also were considered. Results show that CFRP piece not only tends to extend fatigue life of one piece more than 3 hours but also decreases the speed of fracture extension significantly. The importance of life restoration and delaying deterioration of steel structures, motivates researchers for easy and useful progress of rehabilitation techniques of structures. One of the development techniques is adhering CFRP to the wing of steel beams.

Colombi et al (2003) done analysis of fracture in the metal members reinforced with thepre-stressed composite and the fatigue failure. They used 3-layer technique for studying rupture extension in the metal system. They found that in sheet samples, detachment is not the dominant mode and using very hard metal sheets has not considerable increasing in the effectiveness of retrofitting(5). Kim and Harris (2011) investigated moment behavior of damaged metal beams reinforced by CFRP sheets. This damage was created in high tensile area. For evaluating fatigue, repaired beam was tested with emphasis on plastic area of six-beam. The amount of damage extension, fatigue life and plastic strain in repaired beams had improved. The amount of observed fatigue response in contact surface of CFRP and beam is related to the number of fatigue cycles and applied tension. Except in the damage area, in which strain focus is significant, strain along the CFRP sheet increases gradually. Damage degree was not influenced by range and tension amount till 60% of fatigue life and with increasing fatiguecycles, damage degree influenced by tension amount. In the beam web, fatigue life was not considerable by 40-50% of vertical fracture extension. But with increasing life, fatigue had increased highly and suddenly a fragile rupture was happened in web (6).

Ghafouri et al (2012) investigated beam behavior containing reinforced damage with two types of CFRP (pre-stressed and without pre-stressed). Effects of fatigue loads, humidity and water going of FRP and metal were considered. In a PBR system in which adhesive was used for adhering CFRP, local strain was centralized while in PUR system, the regular local strain was observed along the FRP sheet and also it can show better fatigue performance with a high degree of pre-stressed that strain extension along the CFRP was regular. Beam with reinforced damage has equal load capacity both in PUR and PBR systems while failure modes in two systems were different.  Beam reinforced with CFRP has pre-stressed 5 times more than fatigue life. Different modes of fracture including passive, semi-active and activewere introduced. Whenever fracture is passive, it has no considerable effect on structural behavior. In semi-active mode, with increasing external bending moment, bending hardness begin to decrease until fracture becomes active suddenly (7).

In this paper, effect of adhering CFRP sheets on structural behavior of steel beam and recovering hardness of damaged beam load capacity were investigated. Different sizes of CFRP sheets were used for reaching an appropriate length of modeling.

Conclusion

  • This research shows the possibility of using CFRP sheets in repairing thedefect beam and restoring lost strength because of the defect. Tension level and plastic strain had been improved in repaired beam.
  • Defect expansion in non-reinforced beam happens quickly, the level and amount of defect are influenced by applied load amount.
  • Local debonding in CFRP sheets resulted from defect, causes a sudden increase in adhesive tension, and respectively, CFRP slips. With increasing load amount, the strain of CFRP sheet in defect area and debonding strain is considerable.

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