In-plane shear performance of masonry panels strengthened with FRP

The opportunities provided by the use of Fiber Reinforced Polymers (FRPs) composites for the shear strengthening of tuff masonry structures were assessed on full-scale panels subjected to in-plane shear-compression tests at the ENEL HYDRO S.p.A. laboratory, ITALY. Tuff masonry specimens have been arranged in order to simulate both mechanical and textural properties typical of buildings located in South-Central Italian historical centres. In this paper, the outcomes of the experimental tests are presented. The monotonic shear-compression tests were performed under displacement control and experimental data have provided information about in-plane behaviour of as-built and FRP strengthened tuff masonry walls. Failure modes, shear strength, displacement capacity and post-peak performance are discussed.     

Experimental bond tests on masonry panels strengthened by FRP

The results of an experimental campaign on bond between Glass Fiber Reinforced Polymer (GFRP) sheets and single clay brick or masonry panel is presented. Four different types of clay bricks (new and ancient) are considered, where the difference between bricks is not only due to their mechanical properties but also to their surface texture. Another focus point of the experimental campaign is the effect of mortar joints on the GFRP-masonry panel bond. Moreover, the effects of different surface preparations on the debonding load were investigated, concerning both bricks and masonry panels. A total number of 38 specimens was tested and results in terms of debonding force, strain along the GFRP and failure modes are here reported. The experimental results were also compared to design formula proposed by the new version of Italian Guidelines. Furthermore, in order to numerically describe the bond behaviour of the specimens tested, non-linear interface laws were calibrated starting from the debonding load and the measured strains along the GFRP for various loading levels.     

Out-of-plane behavior of unreinforced masonry walls strengthened with FRP strips

The out-of-plane behavior of unreinforced masonry walls strengthened with externally bonded fiber reinforced polymer (FRP) strips is analytically studied. The analytical model uses variational principles, equilibrium requirements, and compatibility conditions between the structural components (masonry units, mortar joints, FRP strips, and adhesive layers) and assumes one-way flexural action of the strengthened wall. The masonry units and the mortar joints are modeled as Timoshenko’s beams. The FRP strips are modeled using the lamination and the first-order shear deformation theories, and the adhesive layers are modeled as 2D linear elastic continua. The model accounts for cracking of the mortar joints and for the development of debonding zones near the cracked joints. Numerical and parametric studies that reveal the capabilities of the model, throw light on the interaction between the variables, and quantitatively explain some aspects of the behavior of the strengthened wall are also presented.     

Pull out of FRP reinforcement from masonry pillars: Experimental and numerical results

In this paper, debonding phenomena between carbon fiber reinforced polymer (CFRP) strips and masonry support were investigated on the basis of single-lap shear tests, considering different dimensions of the bond length. To capture the post-peak response of the CFRP–masonry joint, the slip between the support and the reinforcement strip was controlled using a clip gauge positioned at the end of the reinforcement. The tests were simulated by means of a finite element model able to capture the post-peak snap-back behavior due to the failure process. The numerical model is based on zero-thickness interface elements and on a proper non-linear cohesive law. The comparison between experimental and numerical results was performed in terms of overall response, measured by both the machine stroke and the clip gauge positioned at the free end of the reinforcement. The cases of effective bond length greater and lesser than the minimum anchorage length, suggested by the CNR Italian recommendation, were considered.     

Experimental and analytical study of flat-plate floor confinement

Currently available analytical models were developed for homogeneous concrete and are therefore inapplicable to specimens cast with concretes of different strengths. The present study examines such composite structures, and more especially normal-strength floor concrete sandwiched between columns of high-strength concrete, as well as the aspect ratio (ratio of slab thickness to column dimension) and closely spaced slab reinforcement. The effect on column–slab joint strength of confinement by rectangular hoops and slab portions extending in all directions from the joint are investigated. Three series of experiments on column specimens were carried out and the experimental results compared with the analytical ones. The experimental results conform to the predictions made by the theoretical models. The same models were used to evaluate the effects on slab concrete behavior of confinement by lateral reinforcement and by a slab surrounding the column–slab joint. A surrounding-slab confinement factor was defined and developed for use in analysis. This study represents a first attempt to evaluate confinement effects in column–slab joints in the presence of surrounding slab. Application of the types of structure investigated here could yield improved strength and ductility, enabling smarter design.     

Influence of free edge stress concentration on effectiveness of FRP confinement

The use of fiber-reinforced polymers (FRP) composite materials have been widely used and have been extensively studied in the last decades in the form of jacketing to enhance axial strength as well as ductility of confined columns. Their effectiveness has been extensively proven in many research programs investigating confined concrete column behavior. Despite a large research effort, a proper analytical tool to predict the behavior of FRP-confined concrete has not yet been established due to the many factors jeopardizing the reliability of such tools. The aim of the present study is to analyze the effect of interlaminar stresses at the free edge of the FRP jacket to determine a potential reason for the premature failure of the strengthening system. Numerical examples and a parametric study are presented to illustrate the governing parameters that control the stress concentrations at the edge of the FRP jacket in the overlapping zone.     

Load carrying capacity of 2D FRP/strengthened masonry structures

An adaptive discontinuous finite element model is formulated for limit analysis of masonry structures strengthened by fiber composites. The model is able to predict the effects of fracture damage and delamination on the load carrying capacity of the reinforced structures. A numerical investigation on the collapse mechanisms of masonry structures under plain strain/stress is presented, accounting for different mechanical properties of FRP–masonry interface and different placements of the reinforcement in the masonry structures.     

FRP strengthening of steel and steel-concrete composite structures: an analytical approach

A recent technique for strengthening steel and steel-concrete composite structures by the use of externally bonded Fiber Reinforced Polymer (FRP) sheets, to increase the flexural capacity of the structural element, is described. Several researches developed FRP strengthening of reinforced concrete and masonry structures, but few experimental studies about steel and steel-concrete composite elements are available. Some examples of guidelines for the design and construction of externally bonded FRP systems for strengthening existing metal structures are available, but the method used to predict the flexural behaviour of FRP strengthened elements is usually based on the hypothesis of elastic behaviour of materials and FRP laminate is mainly considered only under the tensile flange. In this paper, an analytical procedure to predict the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, based on cross-sectional behaviour and taking into account the non-linear behaviour of the materials with any configuration of FRP reinforcement, is given. Analytical predictions are compared with some experimental results available in the literature on the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, showing good agreement of the results, even in the non-linear phase, until failure.     

Mechanical response of anchored FRP bonded joints: A nonlinear analytical approach

This article presents a nonlinear analytical solution for the prediction of the full-range debonding response of mechanically anchored, fiber-reinforced polymer (FRP) composites from the substrate. The nonlinear analytical approach predicts, for any monotonic loading history or bonded length, the relative displacements (or slips) between materials, the strains in the FRP composite, the bond stresses within the interface, and the stresses developed in the substrate. The load-slip responses of FRP-to-substrate interfaces with short and long bonded lengths are motives of analysis and discussion. The solutions obtained from the proposed approach are also compared with other experimental results found in the literature.     

Limit analysis of FRP strengthened masonry arches via nonlinear and linear programming

The collapse load of masonry arches strengthened with FRP materials is determined. The arch is made of quadrangular blocks and the nonlinearity of the problem (no-tension material, frictional sliding and crushing) is concentrated at the interface between the blocks. Two methods are used to solve the problem. In the first method, a nonlinear programming problem (NLP) is formulated and is solved by using the successive quadratic programming algorithm (SQP) and combinatorial analysis. This method finds the optimal solution in the analysed cases. In the second method, a linear programming problem (LP) is formulated and is solved with classical techniques. LP approximates the optimal solution to any desired degree of accuracy. Although the number of variables of LP is much larger than that of NLP, LP process time can result much lower than NLP process time. Numerical examples are provided in order to show the advantages of the two methods and the effectiveness of FRP strengthening for different arch geometries.     

An approach to the positioning of FRP provisions in vaulted masonry structures

In the paper, one starts from a theoretical formulation aimed at analysing masonry vaults by selecting, in an inverted approach, families of load shapes that may be equilibrated by sets of admissible solutions, in order to develop an operative method for the positioning of FRP reinforcements in masonry vaulted constructions. On the basis of this premise a strategy is outlined for identifying the areas of the vault to be selected for introducing the FRP provisions. As shown in the numerical investigation, higher intensities of the stress state are then allowed by the introduction of the reinforcement and the local relaxation of some of the constraints of the problem is possible.     

L-shaped piezoelectric motor--part II: analytical modeling

This paper develops an analytical model for an L-shaped piezoelectric motor. The motor structure has been described in detail in Part I of this study. The coupling of the bending vibration mode of the bimorphs results in an elliptical motion at the tip. The emphasis of this paper is on the development of a precise analytical model which can predict the dynamic behavior of the motor based on its geometry. The motor was first modeled mechanically to identify the natural frequencies and mode shapes of the structure. Next, an electromechanical model of the motor was developed to take into account the piezoelectric effect, and dynamics of L-shaped piezoelectric motor were obtained as a function of voltage and frequency. Finally, the analytical model was validated by comparing it to experiment results and the finite element method (FEM).     

Strengthening of masonry arches with Textile-Reinforced Mortar: experimental behaviour and analytical approaches

The continuous and growing interest in the conservation of historical heritages requires easy to use and reliable strengthening systems with related calculation methods that allow evaluating the capacity of existing and strengthened masonry structures. However, the analytical models applicable to retrofitted masonry structures have not been developed at the same level as those of other modern construction materials. In particular, there is a gap between the experimental results of masonry elements strengthened with innovative systems and the predicted structural behaviour provided by analytical models. This can hinder exhaustive analysis of experimental results and potentially led to over conservative design methods for innovative strengthening solutions. The present work investigated the performance of the basalt textile reinforced mortar (BTRM) strengthening system, applied to stone masonry arches, and evaluated the applicability of three different analytical approaches for design purposes. The basic materials and the BTRM composite were tested for the definition of the main constitutive laws. Three unreinforced stone masonry arches and nine arches retrofitted according to three different layouts were tested under vertical monotonic load. The experimental and analytical results were compared for the identification of the more suitable analytical approach for design purposes.     

Assessment of bonding stresses between FRP sheets and masonry pillars during delamination tests

Under the simplifying assumption of perfect adhesion, motivated by the wide adoption of highly resistant glues, delamination of FRP-reinforced masonry pillars turns out to be governed exclusively by the non-linear behavior of the quasi-brittle, heterogeneous support. However, at the structural level, the macroscopic delamination response can be described effectively by concentrating all the sources of dissipation and non-linearity at the masonry-FRP interface, and assuming the support to behave as a linear-elastic body. In this paper, a detailed and critical comparison between two different fully three-dimensional finite element models is developed: (i) a model in which only masonry (i.e. brick and mortar independently) is damageable whilst the FRP reinforcement adheres perfectly to the support (namely, exclusively bulk damage is accounted for), and an alternative model (ii) in which a cohesive, zero thickness interface between the FRP and the support is considered (i.e. interface damage), whilst masonry behaves as a heterogeneous linear elastic material. The overall response during delamination and local stress distributions at the interface are critically investigated, varying the FRP reinforcement width.     

Effect of thermal ageing and salt decay on bond between FRP and masonry

An experimental research on of masonry has been carried out. Thermal cycles and salt crystallization test were carried out on solid brick samples and small masonry assemblages, using bricks and mortars produced in Italy and in Poland. The specimens were strengthened with CFRP textiles or laminates in different configurations. To perform thermal accelerated ageing tests, specimens were subjected to a temperature variation ranging between ?10 and +70 °C, applied cyclically. The procedure was validated during testing. The results showed the influence of the properties of the adhesive and of the strength of the brick in the failure of specimens. As for salt decay tests, a RILEM pre-standard procedure was followed to evaluate the resistance of tested materials to sulfates. Damage evolution was monitored by visual observation and by quantification, at each 4-week cycle, of material loss by a laser profilometer. The results showed the rising of salt from the uncovered surface as from the first week of observation, and also a concentration of stresses underneath the fibres. The pull-off test was chosen as reference test, in order to the loss of bond. The durability was also checked on reference unreinforced specimens. Pull off tests were carried out on the surviving specimens at the end of the tests. The results among the various series of specimens are compared. This costly repair technique can show adhesion problems due to humidity and high temperature.     

Bond between FRP composites and concrete: Assessment of design procedures and analytical models

The use of fibre reinforced polymers (FRPs) to strengthen reinforced concrete (RC) structures has gained a wide popularity in the last decades. Although many experimental and analytical studies are available in literature, some issues are still under discussion in the research communities. Since the typical failure mode of FRP–concrete joints is reported to be debonding of the composite from the concrete substrate [1], the estimation of the bond strength between FRP and concrete substrate represents a key issue for the proper use of this technology. For this reason, several analytical models for the evaluation of the FRP–concrete bond strength and few models for the estimation of the effective bond length were proposed (some of them are included in design codes/recommendations/guidelines); however they were not assessed by means of an appropriate experimental database.This work shows an assessment of twenty analytical models for the evaluation of the FRP–concrete bond strength. The assessment is based on the analysis of a wide experimental database collected from the literature. The results are provided distinguishing between the test setup adopted (single or double shear test, bending test) and the material used (post impregnated sheets or pre impregnated laminates). The accuracy of each model was evaluated by means of a simplified statistical analysis. The influence of the test setup and basic material on the accuracy of the model used was analysed as well. Lastly, the accuracy of twelve available models in providing an estimation of the effective bond length was also assessed.     

Passive and SMA-activated confinement of circular masonry columns with basalt and glass fibers composites

Fiber Reinforced Polymer (FRP) composites are widely used for strengthening and conservation of historic masonry, even if research problems are still open. The mechanical behavior of masonry columns having a circular cross section, confined with glass and basalt FRP systems was studied in this paper. An extended experimental investigation is presented in order to show the results of axial compression tests on circular masonry columns built with natural blocks (calcareous stone). Active confinement was also studied by using a novel technique that employs Shape Memory Alloys (SMA). Totally twenty-four masonry columns were built, instrumented and tested. Different fibers, strengthening schemes and matrix/adhesive were used for the confinement of the columns.Unstrengthened columns were tested as reference specimens. Axial strain of the columns and tensile strain of the fibers in the direction perpendicular to the primary axis of the columns were measured with the applied load. Experimental results revealed the effectiveness of the FRP-confinement for masonry columns. Active confinement was found to be effective at early loading stages since an increased stiffness of the SMA/GFRP-confined columns was measured.A prediction of the compressive strength was obtained by using the model of the Italian guidelines CNR DT 200 (National Research Council) in order to compare the experimental results with the design approach, also for new types of fiber like basalt which were not included in the technical codes. Finally, the experimental results were compared with theoretical values calculated according with to two existing analytical models in order to test their effectiveness for the analyzed configurations.