Query: Concrete beams strengthened with prestressed near surface mounted CFRP

title: Concrete Beams Strengthened with Prestressed Near Surface Mounted CFRP
abstract: Retrofitting concrete structures with fiber reinforced polymer (FRP) has today grown to be a widely used method throughout most parts of the world. The main reason for this is that it is possible to obtain a good strengthening effect with a relatively small work effort. It is also possible to carry out strengthening work without changing the appearance or dimensions of the structure. Nevertheless, when strengthening a structure with external FRP, it is often not possible to make full use of the FRP. The reason for this depends mainly on the fact that a strain distribution exists over the section due to dead load or other loads that cannot be removed during strengthening. This implies that steel yielding in the reinforcement may already be occurring in the service limit state or that compressive failure in the concrete is occurring. By prestressing, a higher utilization of the FRP material is made possible. It is extremely important to ensure that, if external prestressing is used, the force is properly transferred to the structure. Most of the research conducted with prestressing carbon fiber reinforced polymer (CFRP) for strengthening has been on surface bonded laminates. However, this paper presents research on prestressed CFRP quadratic rods bonded in sawed grooves in the concrete cover. This method has proven to be an advantageous means of bonding CFRP to concrete, and in comparison to surface bonded laminates, the shear and normal stress between the CFRP and the concrete are more efficiently transferred to the structure. In the presented test, no mechanical device has been used to maintain the prestress during testing, which means that the adhesive must transfer all shear stresses to the concrete. Fifteen beams with a length of 4 m have been tested. The tests show that the prestressed beams exhibited a higher first crack load as well as a higher steel yielding load as compared to nonprestressed strengthened beams. The ultimate load at failure was also higher, as compared to nonprestressed beams, but in relation not as large as for the cracking and yielding. In addition, the beams strengthened with prestressed FRP had a smaller midpoint deflection. All strengthened beams failed due to fiber rupture of the FRP.

title: An experimental study on reinforced concrete beams strengthened with prestressed near surface mounted CFRP strips
abstract: Externally bonded prestressed FRP plates and near surface mounted (NSM) FRP strips are two important techniques of strengthening concrete structures with FRP materials. However, there are considerable limitations for these two techniques. Using prestressed NSM FRP strips can combine the advantages of these two strengthening methods and it thus becomes very attractive to further promote FRP applications in retrofitting concrete structures. In this paper, a total of 7 rectangular reinforced concrete beams were constructed for static load tests, of which 1 specimen was the reference beam and the other 6 beams were strengthened with FRP strips. The behavior and failure modes of these specimens were investigated, the difference of the mechanical performance due to various strengthening techniques was compared, and the effect of parameters, such as the bond length, prestressing force, and the anchorage, on the behavior and failure modes of specimens was analyzed. The testing results showed that the load carrying behavior of flexural members could be improved significantly by strengthening with prestressed NSM FRP strips, since the highest load carrying capacity of the beam with prestressed NSM strips is increased by 122% and 44.2% when compared to the beam without strengthening and with non prestressed FRP strips, respectively. While the prestressed FRP strips could be anchored by epoxy through the near surface mounting, the failure modes of beams strengthened with this technique might be the debonding of epoxy concrete interface and delamination of the concrete cover. It was proven that these failure modes can be avoided by extending the bond length of FRP strips or using U wraps of CFRP sheets. The stress transfer length of the prestressed FRP strengthened beam with U wrap anchorage is approximately 100 mm.

title: Effect of Partial Unbonding on Prestressed Near Surface Mounted CFRP Strengthened Concrete T Beams
abstract: The flexural behavior of RC T beams strengthened with prestressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) reinforcement was investigated. The specific objective was to study the effect of partial unbonding of the CFRP reinforcement on the beam flexural behavior to increase the deformability. A total of eight RC T beams were tested under four point monotonic loading. The main variables were the level of prestressing force in the CFRP bars and the unbonded length at the midspan of the beam. The test results showed that all of the prestressed strengthened beams effectively improved the ultimate load carrying capacity and the serviceability performance compared to the unstrengthened beam. The partially bonded prestressed beams exhibited an enhancement of the deformability compared to the fully bonded beams while minimizing the reduction of the load carrying capacity. Partial unbonding was more effective to improve the deformability at higher levels of prestressing force. The general behavior of the partially bonded beams was reasonably well predicted by an analytical model developed previously by the writers.

title: Flexural Strengthening of Reinforced Concrete Beams Using Prestressed, Near Surface Mounted CFRP Bars
abstract: This study investigates the effectiveness of using prestressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) to strengthen reinforced concrete beams. Five large scale reinforced concrete beams were tested: one control, one strengthened with nonprestressed NSM CFRP bars, and three strengthened with NSM CFRP bars prestressed to 20% 40% and 60% of the ultimate strength of the CFRP. The flexural behavior of the prestressed concrete beams was compared with the nonprestressed beam and the control. Results indicate that all prestressed concrete beams failed due to rupture of the CFRP bars without any premature or debonding failure. Prestressing the embedded NSM CFRP reinforcement enhanced the overall performance of the beams by increasing the loads at cracking, yielding, and ultimate. When compared with the control beam, prestressed CFRP enhanced overall performance by decreasing crack widths and deflections, as well as delaying the formation of new cracks. In general, increasing the prestressing levels of the NSM CFRP reinforcement improved the overall flexural behavior of the beams at service and ultimate conditions, while decreasing the ductility of the beams.

title: Reseach on Flexural Behavior of Reinforced Concrete Beams Strengthened with Prestressed Near Surface Mounted CFRP Tendons
abstract: The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods has been investigated. The RC beams are tested under monotonic loading. The stressing process, failure capacity, flexural capaticy, displacement ductility and deformation are systematically studied. The text results show that the new strengthening technique with prestressed NSM CFRP rods can enhance the first crack load, steel yielding load of beams compared to the control beam. It is worth noting that the prestress can limit the crack width all the way up to failure, reduce the midpoint deflection, and improve the service performance of the strengthened beams. The strengthening method offers an alternative way to existing strengthening method. The results provide a reference for engineering application.

title: Mechanism of Bond Behavior of Concrete Beams Strengthened with Near Surface Mounted CFRP Rods
abstract: Bond tests were conducted on concrete beams strengthened with near surface mounted (NSM) nonprestressed and prestressed carbon fiber reinforced polymer (CFRP) rods under static loading. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. Six concrete beams were tested. The test variables included presence of internal tension steel reinforcement (unreinforced and reinforced) use of NSM CFRP strengthening (nonprestressed and prestressed) and type of CFRP rod (spirally wound and sand blasted) The beams were tested statically in four point bending. Based on the test results, the transfer length for the prestressed CFRP rod in epoxied groove was 150 and 210 mm for the sand blasted and spirally wound rods, respectively. The main failure mode was debonding between the CFRP rod and the epoxy that starts at sections close to the midspan then, as the load increases, it propagates toward the supports. At failure, the beams strengthened with a given

title: Efficient near surface mounted CFRP shear strengthening of high strength prestressed concrete beams An experimental study
abstract: Shear behavior of prestressed concrete members and its strengthening is complex due to its brittle nature of failure and a number of variables affecting its behavior. The main goal of this investigation is to assess the efficiency of different near surface mounted (NSM) strengthening configurations using CFRP (Carbon Fiber Reinforced Polymer) laminates on the shear behavior of prestressed concrete beams. The parameters considered in this study are (i) the presence of vertical stirrups and (ii) different NSM FRP shear strengthening configurations. Twelve prestressed concrete beams with and without vertical stirrups were cast and strengthened using different NSM CFRP configurations and tested under shear. All the beams were tested under three point bending at a shear span to depth (a/d) ratio of 2.5 to simulate the shear dominant behavior. Test results revealed that NSM strengthening of prestressed concrete beams using CFRP laminates at 45deg was more efficient in improving the shear capacity of beams with and without vertical stirrups. It was also observed that the shear mode of failure could be converted to the flexural mode with improved ductility by designing a suitable NSM shear strengthening scheme. Shear strength calculations of NSM strengthened beams were made using ACI codes and other analytical models. ACI calculations were found to be conservative in estimating the shear contribution of NSM laminates.

title: Experiment and calculation on crack of reinforced concrete beams strengthened with near surface mounted CFRP PCPs reinforcement
abstract: Based on the flexural experimental research of five reinforced concrete beams strengthened with near surface mounted carbon fibre reinforced plastics(CFRP) prestressed concrete prisms(CFRP PCPs composite reinforcement)of different control tension stress,the characteristics of mean crack width and maximum crack width were acquired with comparison and analysis of crack distribution and development under static loading.The results indicate that this type of the unique composite reinforcement has an inhibitory effect on development of crack width and crack height.Based on the calculation method for crack width in the current national standard and experimental data,the suggested maximum crack width of the reinforced concrete beams strengthened with near surface mounted CFRP PCPs reinforcement under static loading was given.And the calculated values are revealed to be in reasonable matching the tested value.

title: Evaluation of the transfer length of prestressed near surface mounted CFRP rods in concrete
abstract: The transfer length of a prestressed near surface mounted (NSM) fiber reinforced polymer (FRP) rod is the distance over which the rod must be bonded to the epoxy to develop the prestressing force in the rod. The transfer length is intended to provide bond integrity for the strengthened concrete member. This paper presents experimental results and an empirical equation to estimate the transfer length of prestressed NSM Carbon FRP (CFRP) rod in concrete beams. Twenty two reinforced concrete specimens were strengthened with NSM CFRP rods. Two types of CFRP rods were used: spirally wound and sand blasted rods. Four prestressing levels were used: 40% 45% 50% and 60% of the tensile strength of the CFRP rod. The strain behavior in the CFRP rod was monitored by gauges mounted on the CFRP rod along the length of the beam. The test results showed that the transfer length of the prestressed NSM CFRP rod was about 35 times the diameter of the CFRP rod. The maximum bond stress of the CFRP rod in epoxy was found to range from 11 to 16 MPa for the sand blasted rods and from 12 to 23 MPa for the spirally wound rods. An empirical expression based on curve fitting of the measured data was proposed to predict the prestressing stress in the CFRP rod along the length of the beam.

title: Flexural fatigue behavior of large scale beams strengthened with side near surface mounted (SNSM) CFRP strips
abstract: The effectiveness of the fatigue behavior of the large scale reinforced concrete (RC) beams strengthened with side near surface mounted (SNSM) carbon fiber reinforced plastics (CFRP) was assessed. Six RC beams (three static beams and three fatigue beams, strengthened with SNSM non prestressed and prestressed CFRP strips) were tested to investigate the effects of the different loading and strengthening methods on the flexural fatigue performance of the beams. In order to simulate the existence of a certain degree of initial damage when the service beams were strengthened, the fatigue test beams were strengthened after they had been subjected to 2 million fatigue cycle loading, and then continued to be subjected to 500,000 fatigue cycle loading. The results showed that the flexural capacity and ductility of static strengthened beams were obviously higher than that of the un strengthened beam, and the residual flexural capacity of the fatigue test beams was significantly lower than those of the static test beams. In terms of fatigue resistance, the prestressed CFRP strengthened beam was superior to the non prestressed CFRP one; the tests beams were better after being strengthened than before. The damage of the strength of the concrete and steel bars was mainly accumulated at the initial stage of fatigue cycle loading, where the stiffness of the test beams decreased rapidly. The steel bar strain, concrete strain and mid span deflection of strengthened beams after 2.5 million loading of fatigue cycles were basically the same as those of 2 million one, which indicated that the fatigue flexural behavior of the strengthened beams was enhanced significantly. Finally, the predicted models of static mid span deflection and fatigue residual flexural capacity for the SNSM strengthening beams were derived and validated.