Strengthening of Brick Masonry Arches with Externally Bonded Steel Reinforced Composites

The objective of this study is to investigate the efficiency of an innovative technique for strengthening masonry arches, based on the use of high strength steel cords embedded in either an epoxy (steel reinforced polymer) or mortar matrix (steel reinforced grout). Ten prototypes of brickwork arches strengthened by composite laminates were tested under a monotonic vertical load applied at the quarter-span. Load tests were performed to compare the behavior up to collapse of strengthened masonry arches; the influence of the types of reinforcement (steel and carbon fibers) and matrices (epoxy and cementitious), as well as location of the strengthening layer (intrados, extrados, and both) and the presence of anchorage systems has been investigated. The experimental results highlight the enhanced strength of the arches reinforced with steel cords, as well as the role of the mechanical anchoring with regard to the resulting final strength.     

Lateral Out-of-Plane Strengthening of Masonry Walls with Composite Materials

The structural behavior of masonry walls laterally strengthened with externally bonded composite materials to resist out-of-plane loads is theoretically and experimentally studied. Hollow concrete block masonry walls and solid autoclaved aerated concrete (AAC) block masonry walls are examined. A theoretical model that accounts for the cracking and the physical nonlinear behavior, the debonding of the composite layers, the arching effect, the interfacial stresses, and the unique modeling aspects of the laterally strengthened wall is presented. The experimental study includes loading to failure of 4 laterally strengthened masonry walls and 2 control walls. The experimental and analytical results point at the unique aspects of the lateral strengthening of masonry walls with composite materials. In particular, they reveal and explain the premature shear failure in laterally strengthened hollow concrete blocks walls and, on the other hand, demonstrate the potential of lateral fiber-reinforced polymer strengthening of AAC masonry walls. The laterally strengthened AAC masonry walls reveal improved strength, deformability, and integrity at failure characteristics.     

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%.     

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.     

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.     

Evaluation of Masonry Deep Beams Externally Strengthened with CFRP Sheets

In this study, carbon fiber-reinforced polymer (CFRP) sheets were examined as a means to strengthening existing masonry walls allowing for efficient creation of doors, windows, and passage openings. The research reported here deals with eight masonry walls made with concrete blocks, subjected to three-point quasistatic loading. The parameters examined include the reinforcement configuration and their amount. While CFRP sheets were used as external reinforcement, companion studies were carried out with conventional steel rebars. Test results indicate an increase of 180% in shear strength of the reinforced walls as compared to reference unreinforced walls. Load-deflection relationships indicate that the combined plain masonry and CFRP laminate system possessed some nonlinear deformability. The use of CFRP laminates on the walls was found to have an influence on the mode of failure. Anchoring the CFRP laminates at both support regions helped in using a larger portion of the strength of the laminates. The reinforced walls exhibited diagonal shear cracks that developed at a much slower rate and were ultimately accompanied by the peeling off of the CFRP laminates.     

Timber Beams Strengthened with GFRP Bars: Development and Applications

Repair and rehabilitation of infrastructure is becoming increasingly important for bridges due to material deterioration and limited capacity to accommodate current load levels. An experimental program was undertaken to study the flexural behavior of creosote-treated sawn Douglas fir timber beams strengthened with glass fiber-reinforced polymer (GFRP) bars. Twenty-two half-scale and four full-scale timber beams strengthened with GFRP were tested to failure. The percent reinforcement ratios were between 0.27 and 0.82%. Additional unreinforced timber beams were tested as control specimens. The results have shown that using the proposed experimental technique changed the failure mode from brittle tension to compression failure, and flexural strength increased by 18 to 46%. Research findings indicate the use of near-surface GFRP bars overcomes the effect of local defects in the timber and enhances the bending strength of the members. Based on the experimental results, an analytical model is proposed to predict the flexural capacity of both unreinforced and GFRP-reinforced timber beams. The article also reviews implementation of the proposed technique for strengthening a timber bridge near Winnipeg, Manitoba, Canada.     

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.     

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.     

In-Plane Shear Behavior of Masonry Panels Strengthened with NSM CFRP Strips. I: Experimental Investigation

An experimental investigation was conducted to study the in-plane shear behavior of masonry panels strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer strips (CFRP). As part of the study four unreinforced masonry panels and seven strengthened panels were tested in diagonal tension/shear. Different reinforcement orientations were used including vertical, horizontal, and a combination of both. The effect of nonsymmetric reinforcement was also studied. The results of these tests are presented in this paper, and include the load-displacement behaviors, crack patterns, failure modes, and FRP strains. The results showed that the vertically aligned reinforcement was the most effective, with significant increases in strength and ductility observed. The dowel strength of the vertical reinforcement did not likely contribute significantly to the shear resistance of the masonry. Instead, it was likely that the vertical reinforcement acted in tension to restrain shear induced dilation and restrain sliding. In some panels cracking adjacent to the FRP strip, through the panel thickness was observed. This type of cracking reduced the bond between one side of the FRP strip and the masonry, and led to premature debonding. A comparison of the test results with the results of other tests from the literature is also presented in this paper.     

Strengthening of Unreinforced Masonry Walls Using FRPs

An experimental program conducted at the University of Alberta showed that externally applied fiber reinforced polymers (FRPs) are effective in increasing the load-carrying capacity of unreinforced masonry walls that are subjected to out-of-plane flexural loads. Ten walls with a height of 4 m were used to conduct 13 tests in two series. Both undamaged and slightly damaged walls were tested. The following experimental parameters were investigated: (1) type of fiber reinforcement; (2) amount of fiber reinforcement; (3) layout of fiber reinforcement; (4) effects of moderate compressive axial load; and (5) cyclic behavior. This paper briefly reviews the existing rehabilitation methods available and explains why the use of FRPs as external reinforcement is a possible alternative. The test setup and instrumentation of the specimens are described followed by a discussion of the results. The general behavior of the specimens is discussed with emphasis on the load deflection and strain characteristics. The modes of failure are identified and categorized. Finally, a simple analytical model is proposed and compared with the test results followed by a summary of the major results.     

Modeling Out-of-Plane Behavior of URM Walls Retrofitted with Fiber Composites

Although masonry is one of the oldest construction materials, its behavior has not been investigated as extensively as other construction materials. Out-of-plane failures are common in unreinforced masonry (URM) buildings constructed in seismic regions. Seven half-scale brick masonry walls were constructed, externally strengthened with vertical glass-fabric composite strips, and subjected to static cyclic out-of-plane loading. The flexural behavior of the tested specimens is characterized by three main stages corresponding to the first visible bed-joint crack, the first delamination, and the ultimate load. The main parameters being investigated in this study are the amount of composite, the height-to-thickness ratio h∕t, the tensile strain in composites, and the mode of failure. Based on the trends observed in the experimental phase, it was concluded that the behavior of the walls is best predicted with a linear elastic approach. It was also concluded that the ultimate strength method overestimates the flexural capacity and the ultimate deflection of the wall. Preliminary design recommendations are also proposed for tensile strain in the composite, maximum deflection, and maximum reinforcement ratio.     

In-Plane Lateral Response of a Full-Scale Masonry Subassemblage with and without an Inorganic Matrix-Grid Strengthening System

A full-scale unreinforced masonry (URM) wall with an opening was tested under in-plane lateral loading. The wall was first subjected to monotonically increasing displacements until a moderate damage level was reached. The damaged specimen was then cyclically tested up to almost the same maximum drift attained during the monotonic test to investigate the effects of previous damage on its nonlinear response. Finally, the masonry wall was repaired with inorganic matrix-grid (IMG) composites and subjected to a cyclic displacement-controlled test up to a near-collapse state. Most of the observed damage developed in the spandrel panel affecting both lateral resistance and strength degradation. Rocking of piers governed lateral stiffness and hysteretic response, which was characterized by low residual displacements and recentering behavior. The comparison between the experimental force-displacement curves demonstrated that the IMG strengthening system was able to provide energy dissipation capacity to the spandrel panel, restoring load-bearing capacity of the as-built wall, and delaying strength degradation that was indeed observed at larger displacements. Bilinear idealizations of force-displacement curves allowed the identification of displacement ductility, global overstrength, and strength reduction factor of the tested wall systems.     

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.     

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.     

Ductile Anchorage for Connecting FRP Strengthening of Under-Reinforced Masonry Buildings

Fiber-reinforced polymer (FRP) composites have been examined as a convenient and cost-effective means of strengthening unreinforced masonry structures. Seismic design in the United States is almost entirely based on the assumption that the structural system provides a ductile failure mode. FRP strengthened masonry walls inherently have brittle failure modes due to the nature of the strengthening system. The concept explored in this article is the introduction of ductility using a hybrid strengthening system. This involves the placement of structural steel or reinforcing steel at critical locations in the lateral force resisting system. This article presents the testing and analysis of a ductile structural steel connection that can be used to strengthen the connection of FRP strengthened shear walls to the foundation. The connection also increases energy dissipation. Results indicate that a ductile failure mode can be attained when the connection is designed to yield prior to the failure of the FRP strengthening.     

Simple Model for Bond Behavior of Masonry Elements Strengthened with FRP

The aim of the present paper is the development of a simple procedure for the analysis of the bond behavior of fiber-reinforced polymer (FRP) sheets or plates externally applied to masonry supports for the strengthening or repair of masonry constructions. The procedure allows evaluation of the bond strength and the fracture energy developed during the debonding process through simple formulas based on a few parameters, evaluated either by standard tests performed on the materials making up the support and the strengthening system or by theoretical considerations. A brief discussion on the main experimental evidence and the theoretical models provided by the literature is also reported in this paper. The comparison between the theoretical results obtained by applying the proposed procedure and the experimental data deduced from literature is carried out.     

Unreinforced Concrete Masonry Walls Strengthened with CFRP Sheets and Strips under Pendulum Impact

Impact tests using drop-weight pendulum on nine 1.2-m-high full-scale concrete masonry block walls were conducted to investigate the out-of-plane impact behavior of unreinforced masonry (URM) walls externally strengthened with carbon-fiber-reinforced polymer (CFRP) composites. Three strengthening schemes on one side of the wall were studied: continuous unidirectional and continuous woven sheets, discrete strips in a vertical pattern, and discrete strips in orthogonal and diagonal patterns. All walls were vertically positioned resting on a knife-edge support with one face leaning against two steel rollers close to the upper and lower edges of the wall. The impact load was applied at the wall center through a drop-weight pendulum impact tester with various drop heights. Test results revealed that using composite laminates or strips could significantly improve the impact performance of URM walls. The wall strengthened with continuous woven sheets performed better than the one with unidirectional sheet. With the same amount of fiber-reinforced polymer strip material, the wall with narrower but more closely spaced strips performed slightly better than the one with wider strips.     

Assessment of Design Formulas for In-Plane FRP Strengthening of Masonry Walls

Masonry structures have demonstrated their seismic vulnerability during recent world seismic events. This paper investigates in-plane seismic performance of unreinforced masonry (URM) walls before and after they are retrofit using fiber-reinforced polymer (FRP) materials. An assessment of available design formulas for evaluating both the in-plane performance of URM walls and the contribution of FRP strengthening systems was performed. Walls with two configurations of the FRP reinforcement have been analyzed: one based on FRP strips installed parallel to the mortar joints, the other characterized by FRP strips arranged along the diagonals of the wall. Based on shear–compression tests carried out on FRP-strengthened masonry walls available in the literature, a comparison between theoretical and experimental data is performed. A discussion about the FRP strains at failure of the walls is provided and values of effective FRP strains to be used for design purposes are proposed.     

Nonlinear Behavior of a Masonry Subassemblage Before and After Strengthening with Inorganic Matrix-Grid Composites

Past experimental tests on a full-scale masonry wall with an opening evidenced the key role of the spandrel panel in the in-plane nonlinear response of the system. Recent seismic codes do not provide specific criteria to assess and to strengthen existing masonry spandrel panels with inorganic matrix-grid (IMG) composites. Numerical finite-element (FE) analyses are used to deepen the knowledge about the nonlinear response of masonry walls and the role of the IMG strengthening system. The comparison of experimental and numerical results contributes to the development of a simplified analytical model to assess the influence of the external reinforcement system on the in-plane seismic response of masonry wall systems. Some hints about the strengthening design that could change the failure mode from brittle shear to ductile flexure are given. Finally, a further enhancement of the IMG strengthening system is proposed to avoid the undesirable splitting phenomena attributable to compression forces and to exploit the full compressive strength of masonry against bending moments.