Journal of Composites for Construction

Reinforced concrete beam-column joints are commonly used in structures such as parking garages and road overpasses, which might be exposed to extreme weathering conditions and the application of deicing salts. The use of the noncorrodible fiber-reinforced polymer (FRP) reinforcing bars in such structures is beneficial to overcome the steel-corrosion problems. However, FRP materials exhibit linear-elastic stress-strain characteristics up to failure, which raises concerns on their performance in beam-column joints in which energy dissipation, through plastic behavior, is required. The objective of this research project is to assess the seismic behavior of concrete beam-column joints reinforced with glass (G) FRP bars and stirrups. Five full-scale exterior $T$-shaped beam-column joint prototypes were constructed and tested under simulated seismic load conditions. The longitudinal and transversal reinforcement types and ratios are the main investigated parameters in this study. The experimental results showed that the GFRP-reinforced joints can successfully sustain a 4.0% drift ratio without any significant residual deformation. This indicates the feasibility of using GFRP bars and stirrups as reinforcement in the beam-column joints subjected to seismic-type loading. It was also concluded that, increasing the beam reinforcement ratio, while satisfying the strong column-weak beam concept, can enhance the ability of the joint to dissipate seismic energy.

In this research, concrete beams were strengthened with carbon-fiber-reinforced polymer (CFRP) laminate strips that were placed in precut grooves on the concrete surface. Five simple-span RC beams were tested. Among these specimens, one served as a reference beam and was not strengthened, one was tested after being strengthened with nonprestressed near-surface-mounted (NSM) CFRP laminates, and the remaining three were strengthened with NSM CFRP laminates that had been prestressed to 5, 20, and 30% of the nominal ultimate strain of the strips. The test results indicate that the prestressed beams more effectively improved the cracking and yielding loads than the nonprestressed strengthened specimen. In addition to a higher cracking load, the prestressed strengthened specimens had cracks with a more limited distribution and width. Furthermore, the prestressed strengthened beams had up to a 15% higher ultimate load-carrying capacity than the nonprestressed specimen, and their failures corresponded to lower deflections.

Nonlinear finite-element (FE) analysis was carried out to investigate the structural response of prestressed concrete (PC) beams strengthened with fiber reinforced polymer (FRP) composites in shear. Development of the FE model for concrete structures dominated by shear, in particular for FRP-strengthened concrete structures, requires careful consideration in choosing the appropriate elements and constitutive laws to simulate the proper failure mode. The paper presents a new three-dimensional technique to model FRP-strengthened prestressed concrete beams. Most previous work has dealt with two-dimensional analysis of FRP-strengthened reinforced concrete beams. The novel attributes of the proposed model lie in the description of the three-dimensional constitutive material models for each component. The model accounts for the softening behavior of concrete under a triaxial state of stress. A new three-dimensional interface element necessary to capture the debonding failure was proposed. In particular, the effect of the out of plane stress behavior of the FRP-concrete interface, which is currently unavailable through experimental measurements, was carefully evaluated. The use of mechanical anchors to improve the bond behavior was properly simulated. In addition, the prestressing operation and precracking effects were also simulated in the model through a phased analysis technique, which makes it possible to predict the response with respect to the time-dependent behavior of elements and materials. The FE model developed in this study was calibrated through comparison with test results. This paper reports not only on the fundamental investigation of the strengthening effect of FRP composites for PC beams but also on the identification of the damage mechanisms and the progression of failure. The model was shown to provide a good level of correlation with experimental data, and could therefore be used to conduct extensive parameter studies.

The aim of this experimental investigation is to study the effect of the ratio of carbon fiber-reinforced polymer (CFRP) precured laminate length to shear span and different end plate anchorage systems on the flexural behavior of reinforced concrete (RC) beams cast with self-consolidating concrete (SCC). SCC with a grade of 54 MPa has been used throughout this research to ensure consistent high quality and high concrete compressive strength in all beams and to eliminate the need of any compaction. Ten RC beams strengthened with CFRP plate lengths to shear span ratio of 0, 25, 70, and 85% with and without end anchorages, were tested under monotonic loading. In particular, a single layer of U-wrap sheet and two layers of U-wrap sheets with one layer in the longitudinal direction and the other in the transverse direction were used as end anchorages (double wrap). The results were compared with each other and with those of the same test conducted on an unstrengthened control beam specimen. The load-deflection response of the tested specimens was recorded along with the associated failure modes. In addition, predicted results for the ultimate load capacity computed according to the ACI 440.2R-08 and FIB-TG9.3-01 documents were compared with those of the measured experimental results and other models from the literature. It is concluded from this study that, depending on the ratio of CFRP plate length/shear span and also on end anchorage arrangement, the increase in load capacity of the strengthened beams range from 5% to 80% of the unstrengthened beam. Therefore, the selection of the optimum combination of plate length and end anchorage arrangement can lead to an overall saving in the cost of the strengthening of beams. Other conclusions and observations as related to the effect of CFRP plate length/shear span ratio and end anchorage on ductility and mode of failure were also drawn.

In this paper, efficiency and effectiveness of textile-reinforced mortars (TRM) on upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint has been studied. The results are then compared with that of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP)-strengthened joint specimens. Five as-built joint specimens were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing an extreme case of preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these five as-built specimens, two specimens were used as baseline specimens (control specimens) and the other three were strengthened with TRM, CFRP, and GFRP sheets, respectively. All five subassemblages were subjected to quasi-static cyclic lateral load histories to provide the equivalent of severe earthquake damage. The response histories of control and strengthened specimens were then compared. The test results demonstrated that TRM can effectively improve both the shear strength and deformation capacity of seismically deficient beam-column joints to an extent that is comparable to the strength and ductility achieved by well-established CFRP, and GFRP-strengthening of joints.

Bond tests were conducted on 28 concrete beams strengthened with near-surface mounted (NSM) carbon-fiber-reinforced polymer (CFRP) rods under different fatigue load levels. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. The test variables included: presence of internal tension steel reinforcement (unreinforced or reinforced), type of CFRP rod (spirally wound or sandblasted), and the fatigue load level. The beams were tested in four-point bending. Bond failure was by debonding between the CFRP rod and the epoxy that started at sections close to the beam midspan. As the load was cycled further, debonding propagated toward the supports. At failure, the beams strengthened with a given rod type (sandblasted or spirally wound) showed the same CFRP strain at sections close to the support. The sandblasted rods showed a better bond fatigue performance than the spirally wound rods, which, at a given load level, failed at a higher number of cycles.

The out-of-plane bending resistance of a fiber-reinforced polymer (FRP)-strengthened masonry wall is generally governed by debonding failure mechanisms. The behavior at the FRP-to-masonry interface is a key factor affecting the flexural capacity as it is the means for transfer of stress between the FRP and the substrate to develop composite action. Although the beneficial effects of strengthening masonry structures with FRP materials have been demonstrated in the literature, the knowledge of the bond at the FRP-to-masonry interface is still relatively limited. To improve this understanding, this paper presents the results of a numerical investigation into the local behavior at the FRP-masonry interface. Two analytical procedures (i.e., a new generic numerical procedure and a closed-form mathematical solution) were developed to predict the global load-slip response of FRP-to-masonry pull tests by using various local bond-slip relationships. This study compares the effects of homogeneous and heterogeneous representations of the masonry substrate on the prediction of this global response and the influence of different bond-slip models and bond-slip parameters. The analytical results were validated against test data with both methods predicting the experimental behavior well. The paper concludes with recommendations regarding the influence of the heterogeneous nature of masonry and the effect of the bond-slip parameters on the load-slip response of FRP-to-masonry pull tests.

This paper presents an experimental program to study the behavior of notched steel beams repaired with carbon-fiber-reinforced polymer (CFRP) sheets. Particular attention is paid to examining the interaction between the level of initial damage (i.e., notch depth) and CFRP repair. Multiple stages of fatigue-crack-propagation in a steel beam are simulated by various notch sizes, including $a_0/h\mathrm{ratios}=0.1$, 0.3, and 0.5 in which $a_0$= notch depth; and $h$ = beam height. A modeling approach is proposed to simultaneously take into account crack-propagation across the steel section and debonding of the CFRP. Efficacy of the CFRP repair is more pronounced when the damage level increases. CFRP sheet stabilizes the crack mouth opening displacement of the repaired beams until significant debonding takes place. The level of initial damage affects the behavior of a plastic region above the notch tip, the rate of web fracture, and the initiation of CFRP debonding. The damage, however, does not influence the failure mode of the repaired beams. A crack-path independent fracture mechanics method is used to evaluate the energy release rate of the repaired beams.

This paper presents the results of an experimental study on the behavior of concrete confined by fiber reinforced polymer (FRP) jackets with a large rupture strain (LRS). The FRP composites considered herein are formed by embedding polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) fibers in a suitable epoxy resin matrix. The PEN and PET fibers are usually made from recycled materials (e.g., PET bottles) and have a strain capacity greater than 5%. They are ideal for use in seismic retrofit applications where increases in ductility and energy absorption capacity are of prime concern. The present study has two specific objectives: (1) to develop a good understanding of the compressive stress-strain behavior of concrete confined with LRS FRP; and (2) to examine whether existing confinement models developed for conventional FRPs are applicable to LRS FRPs. As the existing models have been developed and verified mainly based on test data for CFRP and GFRP, which have a jacket hoop rupture strain of less than 2%, their accuracy in the hoop/lateral strain range beyond 2% is unclear. Results presented in this paper indicate that the two LRS FRPs made from PEN and PET fibers possess a bilinear tensile stress-strain relationship, which has a significant effect on the axial compressive stress-strain behavior of FRP-confined concrete. A recent confinement model for conventional FRPs is compared with the present test results, indicating that the model significantly overestimates the ultimate axial strain. A modified version of the model is then presented to provide more accurate predictions of the test results.

This paper presents the results from a combined experimental and advanced computational study to understand the dynamic response of a pultruded fiber-reinforced polymer (FRP) sheet pile of 9 m length that is installed into the ground near Venice, Italy. The peak embedment force of 10 kN is applied at the top as a sinusoidal compression force having a maximum frequency of circa 760 Hz. Physical measurements from accelerometers are reported for the lateral deformation response of a single sheet pile and of a unit restrained by an installed waterfront barrier. A finite-element modeling methodology for the two test configurations is developed by using the Strand7 code, so that advanced computational results can be compared against the field application measurements. Closed-form equations for the fundamental frequency are developed, with one accounting for the presence of rotary inertia and shear deformation. Dynamic responses at different embedment lengths (1–7 m) are examined, and a very good correlation is found between theory and practice. Numerically, the performance of the FRP sheet pile is compared with the response of a fictitious sheet pile of steel and with two new FRP geometries that increase stiffness to minimize flexure about the minor axis of bending. By increasing the mass by 10%, the maximum lateral displacement can be the same as the steel unit and $1/20$ of the tested FRP unit. Findings of the research demonstrate that the FRP unit can be installed by using the same pile driving rig and procedure for steel sheet piling.

This paper presents the experimental study about the behavior of honeycomb fiber-reinforced polymer (HFRP) sandwich structure with corrugated cores under the combined effects of service load and low-temperature cycling (ranging from 24°C to $-35°\mathrm{C}$). The potential debonding at the interfaces between the corrugated cores and face sheets because of the service-load condition specific to bridge engineering at low temperatures and its impact on stiffness are studied in this paper. The finite-element analysis (FEA) was utilized to determine the load in the experiment. The experiment consisted of tests conducted at four different temperatures. The load-strain responses were monitored to study the behaviors of HFRP sandwich panels. On the basis of the observation and results from the experiment, this paper concludes that the deflection limit span over 400 can potentially be adopted in practice without incurring stiffness degradation because of interface debonding. The paper also shows that it is essential to study the tolerable size of cracks or defects at the interfaces before applying this deflection limit as a design criterion. The experimental results in this study serve as a necessary supplement to the material tests and confirm that the stiffness of the HFRP sandwich panels at the structural level will also increase when the temperatures are decreased at least up to the service-limit state. Finally, this paper verifies the important role that tensile stress plays in interface debonding and suggests that previous tests may underestimate the actual shear strength of the interfaces.

Introducing openings in existing reinforced concrete (RC) slabs can severely weaken the slabs because of the cut out of the concrete and reinforcing steel. This paper reports field tests on the use of carbon fiber-reinforced polymer (CFRP) composite strengthening techniques to restore the flexural capacity of RC slabs after having openings cut out in the positive moment region. The uniqueness of this study is that the tests were performed on an existing multistory RC building that was scheduled for demolition. Five tests on five different slabs were conducted using three different strengthening techniques—namely, externally bonded (EB) CFRP plates, EB CFRP plates with CFRP anchors, and near-surface mounted (NSM) CFRP strips—to determine the most effective system for strengthening. Test results showed that the three strengthening techniques increased the load-carrying capacity of the slabs with openings, with the NSM technique being more effective than the EB technique. However, the use of CFRP anchors to mechanically anchor the EB plates prevented complete detachment, and hence enabled the restoration of the slab to its full flexural capacity.