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.     

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.     

Shear Strengthening Masonry Panels with Sheet Glass-Fiber Reinforced Polymer

This paper investigates strengthening masonry walls using glass-fiber reinforced polymer (GFRP) sheets. An experimental research program was undertaken. Both clay and concrete brick specimens were tested, with and without GFRP strengthening. Single-sided strengthening was considered, as it is often not practicable to apply the reinforcement to both sides of a wall. Static tests were carried out on six masonry panels, under a combination of vertical preload, and in-plane horizontal shear loading. The mechanisms by which load was carried were observed, varying from the initial, uncracked state, to the final, fully cracked state. The results demonstrate that a significant increase of the in-plane shear capacity of masonry can be achieved by bonding GFRP sheets to the surface of masonry walls. The experimental data were used to assess the effectiveness of the GFRP strengthening, and suggestions are made to allow the test results to be used in the design of sheet GFRP strengthening for masonry structures.     

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.     

Nonlinear Analyses of Tuff Masonry Walls Strengthened with Cementitious Matrix-Grid Composites

Previous experimental studies, conducted by some of the authors, on in-plane response of tuff masonry walls strengthened with an innovative cementitious matrix composite grid (CMG) system confirmed that the CMG system could satisfy basic design requirements such as compatibility with the tuff masonry support (i.e., in terms of good bond properties), reversibility of the intervention and strengthening effectiveness. However, very large scatter was found in the experimental outcomes. Micromodeling and some parametric analyses were adopted to understand the contribution of basic material (mortar, tuff blocks and CMG strengthening) and the effect of the workmanship defects on the structural behavior of a natural stone wall. In order to conduct the analyses, finite-element method models of the elements have been compared to experimental data and they were found to be in good agreement with the test data. Significant improvements of strength and in the postpeak response were achieved installing different layouts of the CMG system. However the strengthening intervention had a negligible influence on the initial stiffness of the walls and this means that it has a reduced impact on the behavior of the existing structure.     

Behavior of Brick Masonry Vaults Strengthened by FRP Laminates

The results of experimental research on brick masonry vaults strengthened at their extrados or at their intrados by fiber-reinforced polymer (FRP) strips is presented here. The presence of the fibers prevents the typical brittle collapse that occurs in a plain arch because of the formation of four hinges; therefore, depending on the position and amount of the reinforcement in the strengthened vaults, three mechanisms are possible: (1) masonry crushing, (2) detachment of the fibers; and (3) sliding along a mortar joint due to the shear stresses. Some first theoretical approaches describing some of these mechanisms are discussed, and the formulation of further models based on the local interaction among the constituent materials is proposed. Six masonry vaults strengthened by glass FRPs or carbon FRPs have been tested. The results have pointed out the enhancement in strength and ductility of the strengthened vaults and the influence in the ultimate strength of the width of the strips and of the bond between the laminate and the masonry.     

Behavior of FRP Strengthened Infill Walls under In-Plane Seismic Loading

The present paper investigates the suitability and effectiveness of fiber-reinforced polymers (FRP) in strengthening and/or repairing unreinforced masonry infill walls in reinforced concrete frames which are subjected to in-plane seismic/cyclic loading. For this purpose, a detailed experimental program was conducted. Specimen geometry, test setup, instrumentation, and a loading procedure that simulates earthquake loading are presented in a detailed fashion. Results of experimental observations are discussed in the form of load-displacement hysteretic loops and envelopes; column profiles; strain diagrams, and wall shear distortion. The test results, in general, indicate that the use of glass FRP (GFRP) sheets as strengthening materials provides a degree of enhancement to the infill wall, upgrades its deformation capacity, and makes the wall work as one unit. These results thus show great potential for externally bonded GFRP sheets in upgrading and strengthening the infill walls under in-plane seismic loads.     

Experimental Bond Behavior of FRP Sheets Glued on Brick Masonry

This paper deals with the experimental characterization of the mechanical tensile and shear bond behavior of fiber-reinforced polymer (FRP) sheets externally glued on masonry prisms, in terms of load capacity and stress distribution along the bonded length. The brick masonry adopted tries to replicate ancient brick masonry, by using handmade low-strength solids bricks and low-strength lime-based mortar. Key parameters relative to the FRP-masonry interface response, particularly bonded length, FRP materials, anchor scheme adopted, and shape of masonry substrate, were studied. Finally, an analytical bond stress-slip formulation was developed, allowing deducing local bond stress-slip curves directly from the experiments.     

Bond Between Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Laminate Strips and Concrete

In recent years, a strengthening technique based on near-surface mounted (NSM) laminate strips of carbon-fiber-reinforced polymer (CFRP) has been used to increase the load-carrying capacity of concrete and masonry structures by introducing laminate strips into precut grooves on the concrete cover of the elements to be strengthened. The high experimentally derived levels of strength efficacy with concrete columns, beams, and masonry panels have presented NSM as a viable and promising technique. This practice requires no surface preparation work and, after cutting the groove, requires minimal installation time compared to the externally bonded reinforcing technique. A further advantage associated with NSM CFRP is its ability to significantly reduce the probability of harm resulting from fire, acts of vandalism, mechanical damage, and aging effects. To assess the bond behavior of CFRP to concrete, pullout-bending tests have been carried out. The influences of bond length and concrete strength on bond behavior are analyzed, the tests are described, and the results are presented and discussed in detail. Finally, a local stress-slip relationship is determined based on both experimental results and a numerical strategy.     

FRP Confinement of Tuff and Clay Brick Columns: Experimental Study and Assessment of Analytical Models

In recent years, fiber-reinforced polymer (FRP) wrapping effectiveness has been clearly confirmed especially with reference to concrete structures. Despite evident advantages of FRP based confinement on members subjected to compressive overloads due to static or seismic actions, the use of such technique in the field of masonry has not been fully explored. Thus, to assess the potential of confinement of masonry columns, the present paper shows the results of an experimental program dealing with 18 square cross sections (listed faced tuff or clay brick) masonry scaled columns subjected to uniaxial compression load. In particular, three different confinement solutions have been experimentally analyzed in order to evaluate and compare the effectiveness of uniaxial glass FRP, carbon FRP, and basalt FRP laminates wrapping. The main experimental outcomes are presented and discussed in the paper considering mechanical behavior of specimens, axial stress-axial strain relationships, and effective strains at failure on the reinforcement. Test results have showed that the investigated confining systems are able to provide significant gains both in terms of compressive strength and ductility of masonry columns. Results of the presented experimental activity along with data available in the literature have been finally used to assess the reliability of the main existing analytical models; refined equations have been then proposed to minimize the scattering between theoretical predictions and experimental available data.     

Experimental Behavior of FRP Strengthened Masonry Arches

This paper deals with the experimental behavior of solid clay brick masonry arches strengthened with glass fiber-reinforced polymer composites. Twelve half-scaled segmental masonry arches subjected to a load applied at the quarter span were tested under displacement control up to failure. The arches were built using handmade low strength bricks and a commercial lime-based mortar, trying to mimic ancient structures. Besides reference unreinforced arches, five different strengthening arrangements, including the use of spike anchors, were studied. The experimental results provide significant information for validation of advanced numerical models and analytical tools and for code drafting. The experimental results also show that (1) only continuous strengthening strategies are able to prevent typical local failure mechanisms of unreinforced arches; (2) strengthening at the intrados is the most effective option to increase strength; and (3) strengthening applied at the extrados provides the higher deformation capacity prior to failure, endowing arches with considerable ductility behavior.     

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.     

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.     

Suitability of Micromechanical Model for Elastic Analysis of Masonry

A micromechanical model is proposed for determining the overall linear elastic mechanical properties of simple-texture brick masonry. The model, originally developed for long-fiber composites, relies on the exact solution due to Eshelby and describes brickwork as a mortar matrix with insertions of elliptical cylinder-shaped bricks. Macroscopic elastic constants are derived from the mechanical properties of the constituent materials and phase volume ratios. Conformity of the suggested model to real brickwork behavior has been verified by performing uniaxial compression tests on masonry panels composed of fired bricks and mud mortar. Composite masonry panels of varying phase percentages were then constructed and tested by replacing several of the fired bricks with mud bricks. Comparison of experimental results with theoretical predictions demonstrates that the model is suitable even in the presence of strongly differentiated phases, and is moreover able to predict different behavior as a function of phase concentration. The model fits experimental results more closely than the micromechanical models previously reported in the literature.     

Monte Carlo Simulation of Shear Capacity of URM Walls Retrofitted by Polyurea Reinforced GFRP Grids

A model proposed in the literature for the evaluation of the in-plane shear capacity of unstrengthened and strengthened concrete and clay brick unreinforced masonry (URM) walls was modified and calibrated following the results from an experimental research program. The tested walls were strengthened with grids made from glass fiber-reinforced polymer (GFRP) embedded within a rapid-setting sprayed polyurea. Various GFRP grid reinforced polyurea layouts were investigated, and consisted of strips oriented in either the vertical or horizontal direction and installed on one or both faces. The prediction models proposed in this paper were subsequently evaluated using a probabilistic Monte Carlo simulation (MCS) by considering the uncertainty and variability of the independent variables, which were assumed to follow a truncated normal distribution. Corroborated by the MCS, test results clearly show that the failure modes of the strengthened URM walls were affected by the strengthening scheme. Experimental and simulation results are presented and discussed in this paper.     

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.     

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.     

Load Rating of Masonry Arch Bridges: Refinements

In a paper previously published by the first writer, a procedure for load-rating masonry arch bridges was introduced. The procedure uses the Load Factor Method of the 1994 American Association of State Highway and Transportation Officials Manual for the Condition Evaluation of Bridges, applied to a frame analysis model of a masonry arch spanning from abutment to abutment. The procedure is based on the assumption of the arch barrel having no tensile strength. The objective of this technical note is to complement the initial procedure by enabling the assessing engineer to exercise discretion in deciding whether or not a small value of tensile strength should be allowable in determining a suitable rating for masonry arch bridges. In addition the initially proposed strength values, which are considered overly conservative, are increased. The introduction of these refinements will allow a more accurate assessment of the nation’s stock of stone masonry bridges.     

Experimental In-Plane Behavior of Tuff Masonry Strengthened with Cementitious Matrix–Grid Composites

Tuff buildings are a significant part of the Mediterranean area and are to be preserved from a structural viewpoint especially in seismic areas. Over the past few decades, the interest in strengthening of historical tuff masonry structures has led to developing specific and noninvasive architectural and engineering strategies. In the present paper, a comprehensive experimental program on tuff masonry panels is presented; the results are intended as a contribution to the knowledge of in-plane behavior of tuff masonry strengthened with composite materials. Particularly, a cement based matrix-coated alkali resistant glass grid system (CMG) was used to strengthen tuff masonry walls; different CMG layouts were selected, and overall performances were compared with those of as-built ones. The characterization of base materials was carried out first, followed by uniaxial tests of masonry and shear tests on triplets. Finally, tuff masonry panels were subjected to diagonal compression loading under displacement control in order to measure their in-plane deformation and strength properties, including the postpeak softening regime in view of seismic applications.     

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.