Retrofitting of Rectangular Columns with Deficient Lap Splices

The cyclic behavior of eight 0.4-scale reinforced concrete column specimens is investigated. The columns incorporated deficient design details to simulate bridge columns built in Washington State prior to 1971. Two columns were tested as reference specimens, five were tested after retrofitting using carbon fiber-reinforced polymer (CFRP), and one was tested after retrofitting using a conventional steel jacket. All the specimens were tested under constant gravity load and incrementally increasing lateral loading cycles. The specimens had rectangular cross sections with aspect ratios of 1.5 and 2.0. The parameters investigated included the amount of CFRP reinforcement, different retrofitting jacket configurations, and different retrofitting materials. For the as-built specimens, two modes of failure occurred, namely low cyclic fatigue of longitudinal reinforcement and lap splice failure. For the retrofitted specimens, no lap splice failure was observed. All the retrofitted specimens failed due to low cyclic fatigue failure of the longitudinal bars. The retrofitting measures improved the displacement ductility, energy dissipation, and equivalent viscous damping. In addition, increasing the amount of CFRP reinforcement improved the performance of the test specimens.     

Residual Performance of FRP-Retrofitted RC Columns after Being Subjected to Cyclic Loading Damage

Numerous recent research findings evidenced the success of retrofitting existing RC columns using fiber-reinforced plastic (FRP) jacketing. However, little is known about the residual performance of FRP-retrofitted RC columns following limited seismic damage. In this paper, the residual performance of FRP-retrofitted columns damaged after simulated seismic loading is studied. Eight model columns with a shear aspect ratio of 5.0 were tested first under cyclic lateral force and a constant axial load equal to 20% of the column gross axial load capacity. The main parameters considered were the type of FRP jacket and peak drift ratio where the lateral loading was interrupted. Glass fiber-reinforced plastic (GFRP) and carbon fiber-reinforced plastic (CFRP) were both used for retrofitting. Five of the model columns were subjected to long-term axial loading after being subjected to limited damage by lateral cyclic loading. From the results of long-term loading test, it was found that FRP-retrofitted columns had much smaller creep deformation than the counterpart as-built model. The deformation of retrofitted columns under long-term axial loading depended on the previous damage intensity and the modulus of elasticity of FRP. The effective creep Poisson’s ratios of the retrofitted columns were much smaller than the as-built column but identical for GFRP and CFRP retrofitted columns. Under the testing conditions of this study, the long-term axial deformation of retrofitted columns tends to be sufficiently stable, despite the simulated earthquake damage.     

Behavior of Composite Unreinforced Masonry–Fiber-Reinforced Polymer Wall Assemblages Under In-Plane Loading

An experimental investigation was conducted to study the in-plane behavior of face shell mortar bedded unreinforced masonry (URM) wall assemblages retrofitted with fiber-reinforced polymer (FRP) laminates. Forty-two URM assemblages were tested under different stress conditions present in masonry shear and infill walls. Tests included prisms loaded in compression with different bed joint orientation (on/off-axis compression), diagonal tension specimens, and specimens loaded under joint shear. The behavior of each specimen type is discussed with emphasis on modes of failure, strength and deformation characteristics. Results showed that the application of FRP laminates on URM has a great influence on strength, postpeak behavior, as well as altering failure modes and maintaining the specimen integrity. The retrofitted specimens reached compressive strength of 1.62–5.64 times that of their unretrofitted counterparts, depending on the bed joint orientation, and joint shear strength increased by eightfold.     

Experimental Response of Externally Retrofitted Masonry Walls Subjected to Shear Loading

Recent earthquakes have produced extensive damage in a large number of existing masonry buildings, demonstrating the need for retrofitting masonry structures. Externally bonded carbon fiber is a retrofitting technique that has been used to increase the strength of reinforced concrete elements. Sixteen full-scale shear dominant clay brick masonry walls, six with wire-steel shear reinforcement, were retrofitted with two configurations of externally bonded carbon fiber strips and subjected to shear loading. The results of the experimental program showed that the strength of the walls could be increased 13–84%, whereas, their displacement capacity increased 51–146%. This paper presents an analysis of the experimental results and simple equations to estimate the cracking load and the maximum shear strength of clay brick masonry walls, retrofitted with carbon fiber.     

Analytical Investigation of Lateral Strength of Masonry Infilled RC Frames Retrofitted with CFRP

In this study, two reinforced concrete frames with hollow clay tile masonry infill walls, retrofitted with diagonally applied carbon fiber-reinforced polymer (CFRP), which were tested previously, were analytically investigated. A simple material model for the masonry infill wall strengthened with CFRP is suggested. The lateral strength of each rehabilitated frame was obtained by pushover analysis of four different models using a commercially available finite-element program, and the results were compared with the test results. We also determined the lateral strength of the CFRP-applied masonry infill walls, and compared the results with the results obtained from existing analytical models. Drift capacity of the masonry infill walls strengthened with CFRP was also investigated, and the drift capacity of the masonry infill walls strengthened with diagonally applied CFRP was recommended. It is concluded that the strength of the masonry infilled frames strengthened with diagonally applied CFRP can be satisfactorily predicted with the suggested procedure. The ultimate drift capacity of the masonry infill walls strengthened with diagonally applied CFRP strips was conservatively predicted to be 1.0%.     

Flexural Capacity of Glass FRP Strengthened Concrete Masonry Walls

Fiber-reinforced polymers (FRP) can provide a strengthening alternative for unreinforced and underreinforced masonry. The ease with which FRP can be installed on the exterior of a masonry wall makes this form of strengthening attractive to the owner, considering both reduced installation cost and down time of the occupied structure. Six unreinforced concrete masonry walls (four at 1.8 m tall and two at 4.7 m tall) were tested in out-of-plane flexure up to capacity. The walls were strengthened with glass FRP composite composed of unidirectional E-glass fabric with an epoxy matrix. The composite was adhered to the surface of the masonry using the same epoxy with the fibers oriented perpendicular to the bed joints. General flexural strength design equations are presented and compared with the results of the testing. It was found that the equations overpredicted the actual capacity of the test specimens by no more than 20%.     

Confinement Effectiveness of FRP in Retrofitting Circular Concrete Columns under Simulated Seismic Load

The results of a research program that evaluated the confinement effectiveness of the type and the amount of fiber-reinforced polymer (FRP) used to retrofit circular concrete columns are presented. A total of 17 circular concrete columns were tested under combined lateral cyclic displacement excursions and constant axial load. It is demonstrated that a high axial load level has a detrimental effect and that a large aspect ratio has a positive effect on drift capacity. Compared with the performance of columns that are monotonically loaded until failure, three cycles of every displacement excursion significantly affect drift capacity. The energy dissipation capacity is controlled by FRP jacket confinement stiffness, especially under a high axial load level. The fracture strain of FRP material has no significant impact on the drift capacity of retrofitted circular concrete columns as long as the same confining pressure is provided, which differs from the common opinion that a larger FRP fracture strain is advantageous in seismic retrofitting. The amount of confining FRP greatly affects the length of the plastic hinge region and the drift capacity of FRP-retrofitted columns. A further increase in confinement after a critical value causes a reduction in the deformation capacity of the columns.     

Tests on Seismically Damaged Reinforced Concrete Structural Walls Repaired Using Fiber-Reinforced Polymers

This paper presents the results of an experimental study on the seismic performance of axially loaded reinforced concrete (RC) walls with boundary elements confined by limited transverse reinforcement. These specimens were initially subjected to axial compression loading and cyclic lateral loading to failure, and subsequently repaired and subjected to loading again. The test specimens include two low-rise walls of aspect ratio 1.125 and two medium-rise walls of aspect ratio 1.625. Results show that significant drift capacities were achieved from the strengthened walls. The performance of the repaired walls was similar to the original walls before repair in terms of the flexural behavior, shear strength, and ductility capacities. While the fiber-reinforced polymer (FRP) anchorage may undergo premature failure, it however failed only after the peak lateral strength of the repaired wall was attained. This paper demonstrates that repair of damaged RC walls using FRP is able to restore the performance of damaged RC walls while also serving as repair method of relative ease.     

Rehabilitation of Masonry Walls Using Unobtrusive FRP Techniques for Enhanced Out-of-Plane Seismic Resistance

Earthquake damage to unreinforced masonry buildings has shown the vulnerability of perimeter walls to out-of-plane failure. This paper describes a study that was carried out to develop and test innovative fiber reinforced polymer (FRP) rehabilitation techniques that meet the stringent requirements for strengthening historical buildings and to be cost-effective alternatives applicable to other existing masonry structures. Unobtrusive FRP rehabilitation techniques that utilize flexible carbon fiber composite cables, mounted near the surface of the fa?ade walls in epoxy-filled grooves in the bed and head joints, were developed. Ten full size walls were constructed of clay bricks and retrofitted using the developed FRP rehabilitation techniques. The test results demonstrated the high efficiency of the rehabilitation techniques under both monotonic and quasistatic cyclic loadings. Significant increases in ultimate capacities, energy absorption, and deformability were achieved for various reinforcing schemes compared to the behavior of the unreinforced walls.     

FRP-Retrofitted URM Walls under In-Plane Shear: Review and Assessment of Available Models

In the last two decades, several seismic retrofitting techniques for masonry structures have been developed and practiced and fiber-reinforced polymer (FRP) material has been increasingly used owing to its high strength/stiffness to mass ratio and easy application. Although much research has been carried out on FRP strengthening of unreinforced masonry (URM) structures, most of it has been experimental studies to investigate the effectiveness of retrofitting techniques rather than the development of a rational design model. In addition, more research has been conducted on FRP-retrofitted URM walls under out-of-plane loads where flexural behavior dominates, the research on the shear strength of FRP-retrofitted URM walls has been limited. This paper presents a review of research in this area. Existing retrofitting techniques are overviewed, followed by a detailed discussion of experimental results of failure modes as they are directly related to the design model. The available design models are then assessed based on a test database collected from the available literature. Limitations of each model are addressed.     

Out-of-Plane Strengthening of Masonry Walls with Reinforced Composites

This paper presents an investigation into the effectiveness of using fiber-reinforced composite overlays to strengthen existing unreinforced masonry walls to resist out-of-plane static loads. A total of fifteen wall panels [1,200 × 1,800 × 200 mm (4 ft × 6 ft × 8 in.)] were tested. Twelve panels were assembled with fiber-reinforcing systems attached to the tension side, and the remaining three control walls were left without any external reinforcement. Two configurations of external reinforcement were evaluated. The first reinforcement configuration consisted of two layers of fiber-reinforced plastic webbing and the second consisted of vertical and horizontal bands of undirectional fiber composites. The three wall specimens without external reinforcement were tested to evaluate the change in the system strength and behavior with application of the external reinforcing systems. In addition to the two fiber configurations, the testing program also evaluated two methods of surface preparation of the walls, sand blasting, and wire brush. All specimens were thoroughly washed by water jet, 48 hours prior to application of the fiber-reinforcing systems. Three specimens were tested for each variable. A uniformly distributed lateral load was applied to each panel using the procedures described in the ASTM Standard E-72 Test Method (airbag). Failure loads, strains in the external reinforcement (FRP), out-of-plane deformations, and failure modes were recorded. Recommendations on the usefulness of the proposed technique as a means of strengthening masonry walls for out-of-plane loads are presented. In general, flexural strength of masonry walls can be increased if the shear failure is controlled.     

Normal- and High-Strength Concrete Circular Elements Wrapped with FRP Composites

Reinforced concrete columns usually have a minimum amount of transverse steel reinforcement this transverse reinforcement can have non negligible effects on the response of columns retrofitted with fiber-reinforced polymers (FRP). This paper presents a test program that was designed to study the behavior of small- and large-scale normal- and high-strength concrete circular columns confined with transverse steel reinforcement, FRP, and both transverse steel reinforcement and FRP under concentric loading. The effect of the main variables—such as the unconfined concrete strength, the volumetric ratio, the type and the yield strength of the transverse steel reinforcement, the concrete cover, and the number of FRP layers—are studied in this research program. The test results show that the enhancement of the confined concrete strength and strain is more pronounced in specimens with normal-strength concrete. It is also shown that the rupture of the FRP in the specimens with higher volumetric transverse steel reinforcement ratios corresponds to larger axial compressive strength and strain and that the postpeak behavior of these specimens is more ductile.     

Rehabilitation of Cracked and Corroded Reinforced Concrete Beams with Fiber-Reinforced Plastic Patches

The behavior under static loading of fiber-reinforced plastic (FRP) retrofitted reinforced concrete beams, possessing a high chloride content and rebar corrosion, was studied both experimentally and analytically. The test beams were characterized as falling into three different groups according to the state of their corrosion damage: (1) natural corrosion, (2) cathodic protection, and (3) accelerated corrosion. The load carrying capacities of the beams, with or without FRP patching, were tested in the laboratory. The experimental results show that the state of corrosion of the steel, the water/cement ratio of the concrete material, and the arrangement and the number of FRP patches all affect the strength as well as the failure mechanisms of retrofitted RC beams. Some simple analytical models and a design concept for retrofitting cracked and corroded RC beams with FRP sheets are also presented and discussed.     

Structural Upgrading of Masonry Columns by Using Composite Reinforcements

Emerging techniques that use fiber-reinforced polymer (FRP) composites for strengthening and conservation of historic masonry are becoming increasingly accepted. In the last decades steel plates or wood frames were used for external confinement in containing the lateral dilation of masonry columns subjected to axial loads. In the last years FRP epoxy bonded strips or jackets were also employed to increase strength and ductility with encouraging results in terms of mechanical behavior and cost effectiveness. The behavior of masonry columns confined with FRP and subjected to axial compression is studied in this paper. An extended experimental investigation is presented in order to show the mechanical behavior of circular masonry columns built with calcareous blocks that may be commonly found in Italy and all over Europe in historical buildings. Different stacking schemes were used to build the columns, aiming to simulate the most common situations in existing masonry structures. Carbon FRP sheets were applied as external reinforcement; different amounts and different schemes of confining reinforcement were studied. The experiments include a new reinforcement technique made by using injected FRP bars through the columns cross section. Such a solution can be considered in place of a more traditional confinement, when external reinforcement must be avoided, or in addition to external reinforcement when an improved confinement effect is required. The structural behavior of masonry columns damaged under different levels of load and strengthened by using FRP reinforcements, was also investigated. Experimental results revealed the effectiveness of the FRP confinement for masonry columns, also for columns that were strongly predamaged before strengthening. A computation of the ultimate load was conducted using the Italian National Research Council recommendations to show an application of the design approach recently proposed in Italy. An existing analytical model, previously developed by the writers, was applied for computation of expected experimental values.     

Performance of Unreinforced Masonry Walls Retrofitted with Externally Anchored Steel Studs under Blast Loading

Six full-scale concrete masonry walls were tested under free-field blast loading using different charge sizes up to 250?kg of ammonium nitrate/fuel oil (ANFO) and at a constant stand-off distance of 15.0?m to cover a wide range of expected damage levels. Five walls were retrofitted with cold-formed steel studs anchored to the wall backs and were compared to the remaining as-built wall. Significant enhancement to the out-of-plane blast resistance of the retrofitted walls, compared to the as-built wall, was observed. This enhancement is attributed to the development of a tied-arch action in the retrofitted walls in which the masonry forms a compression strut while the steel studs serve as the tie. A simplified single-degree-of-freedom model was used to analyze the experimental results, and the model results agreed well with the observed damage levels and the resistances of the walls. In addition, the effectiveness of the proposed retrofit technique was evaluated in terms of strength enhancement and wall deflection reduction. The test results were also compared with those predicted by available blast damage assessment models for unreinforced masonry walls. However, it was found that available models, which do not account for the tied-arch mechanism, greatly underestimate the actual blast capacity of the retrofitted walls because of the assumption of a tensile flexural failure mode. Additionally, the proposed retrofit technique shifts the mode of failure from flexure to shear.     

Strengthening of Masonry Walls against Out-of-Plane Loads Using Fiber-Reinforced Polymer Reinforcement

Thirty masonry walls strengthened using three different fiber-reinforced polymer (FRP) systems, with three anchorage methods, were fabricated and tested under a concentrated load over a 100 mm square area or a patch load over a 500 mm square area. The test results indicated a significant increase in the out-of-plane wall strength over the unstrengthened wall. While failure occurred in the unstrengthened wall by bending, four different modes of failure, that is, punching shear through the bricks, debonding of FRP reinforcement from the masonry substrate, crushing of brick in compression, and tensile rupture of the FRP reinforcement, were observed in the strengthened walls, depending on the types and configurations of FRP and anchorage systems. With appropriate surface preparation and anchorage systems, premature failure due to FRP debonding is prevented. Based on the principles of strain compatibility and force equilibrium, simple analytical models are presented to predict the ultimate load-carrying capacity of the strengthened walls. The test results compared well with the analytical predictions.     

Load-Displacement of Masonry Panels with Unbonded and Intermittently Bonded FRP. I: Analytical Model

An experimental study was carried out to develop and test innovative fiber-reinforced polymer (FRP) rehabilitation techniques that meet the stringent requirements of restoration of historical buildings and are cost-effective alternatives applicable to existing masonry structures. In these techniques, FRP reinforcement was either unbonded or intermittently bonded to the masonry wall. In order to analyze performance, extend the range of the investigated parameters, and define limitations, a simplified analytical model was developed to predict the postcracking lateral load-displacement response under biaxial bending. The response of the retrofitted walls cannot be modeled by conventional approaches. The proposed model is based on balancing internal and external work and rigid body mechanics. It is assumed that all postcracking deformations take place at cracks between wall subpanels. Postcracking displacements are calculated from rotation rather than curvature. The adequacy of the model was verified by comparisons with the experimental results and a good agreement was found. The model could be used as the basis for a design method.     

In-Plane Seismic Response of URM Walls Upgraded with FRP

Recent earthquakes have shown the vulnerability of unreinforced masonry (URM) buildings, which have led to an increasing demand for techniques to upgrade URM buildings. Fiber reinforced polymer (FRP) can provide an upgrading alternative for URM buildings. This paper presents results of dynamic tests investigating the in-plane behavior of URM walls upgraded with FRP (URM-FRP). These tests represent pioneer work in this area (dynamic and in-plane). Five half-scale walls were built, using half-scale brick clay units, and upgraded on one face only. Two moment/shear ratios (1.4 and 0.7), two mortar types (M2.5 and M9), three composite materials (carbon, aramid, and glass), three fiber structures (plates, loose fabric, and grids), and two upgrading configurations (diagonal “X” and full surface shapes) were investigated. The test specimens were subjected to a series of synthetic earthquake motions with increasing intensities on a uniaxial earthquake simulator. The tests validate the effectiveness of the one side upgrading: the upgrading technique improved the lateral resistance of the URM walls by a factor ranging from 1.3 to 2.9; however, the improvement in the lateral drift was less significant. Moreover, no uneven response was observed during the test due to the single side upgrading. Regarding the upgrading configurations, the bidirectional surface type materials (fabrics and grids) applied on the entire surface of the wall (and correctly anchored) can help postpone the three classic failure modes of masonry walls: rocking (“flexural failure”), step cracking, and sliding (“shear failures”). Additionally, in some situations, they will postpone collapse by “keeping the bricks together” under large seismic deformations. On the other hand, the diagonal “X” shape was less successful and premature failure was developed during the test.     

Strengthening of Infill Masonry Walls with FRP Materials

This paper evaluates the effectiveness of different externally bonded glass fiber–reinforced polymer (GFRP) systems for increasing the out-of-plane resistance of infill masonry walls to loading. The research included a comprehensive experimental program comprising 14 full-scale specimens, including four unstrengthened (control) specimens and 10 strengthened specimens. To simulate the boundary conditions of infill walls, all specimens consisted of a reinforced concrete (RC) frame, simulating the supporting RC elements of a building superstructure, which was infilled with solid concrete brick masonry. The specimens were loaded out-of-plane using uniformly distributed pressure to simulate the differential (suction) pressure induced by a tornado. Parameters investigated in the experimental program included aspect ratio, FRP coverage ratio, number of masonry wythes, and type of FRP anchorage. Test results indicated that the type of FRP anchorage had a significant effect on the failure mode. Research findings concluded that GFRP strengthening of infill masonry walls is effective in increasing the out-of-plane load-carrying capacity when proper anchorage of the FRP laminate is provided.     

FRP Confinement of Square Masonry Columns

The problem of masonry columns subjected to structural deficiency under axial load was studied and reported in this paper. The results of an extensive experimental campaign are presented in order to show the behavior of columns built with clay or with calcareous blocks, commonly found in southern Italy, especially in historical buildings. Rectangular masonry columns were tested for a total of 33 specimens; uniaxial compression tests were conducted on columns taking into account the influence of several variables: different strengthening schemes (internal and/or external confinement), curvature radius of the corners, amount of fiber-reinforced polymer (FRP) reinforcement, cross-section aspect ratio, and material of masonry blocks. Materials characterization was preliminarily carried out including a mechanical test on plain masonry. For all cases the experimental results evidenced a significant increase in load carrying capacity and ductility after FRP strengthening, which identified the columns as ductile elements despite the brittle nature of the unconfined masonry. Differences in mechanical behavior, due to the geometry of the columns, to the nature of different materials, to different strengthening schemes, and to the amount of reinforcement, are presented and discussed in the paper. The calibration of design equations recently developed by Italian National Research Council, CNR was conducted to compare analytical prediction and experimental results. The same procedure was applied to calibrate an analytical model recently published, in which the existing coefficients are related only to clay. Here the model is applied to limestone for the first time. Thus, new important information is furnished to researchers and practitioners involved in structural assessment and strengthening of compressed elements in historical buildings.