Masonry Members Research Articles

Strengthening Effect Evaluation of Developed Stiff-Type Polyurea Sprayed on Masonry Beam Surface Under Static Loading in Experimental and Numerical Tests

Recently, deteriorated masonry structures aged over 30 years have shown serious structural problems. Simple and rapid maintenance plans are urgently needed for aging masonry structures. Polyurea (PU) is an effective retrofitting material for aging structures due to its easy spray application. This process saves time, reduces costs, and allows the structure to remain in use during retrofitting. However, a general PU is not suitable for retrofitting aged masonry and concrete structures due to its low stiffness. In this study, stiff-type polyurea (STPU) was selected as the reinforcement material for masonry structures. It was developed by modifying the chemical mix of general PU to improve stiffness. To evaluate the strengthening effect of STPU on masonry members under static loading, tests were conducted. The flexural load capacity of masonry beams with STPU-sprayed surfaces was assessed. Three different types of STPU applications were used to select the most efficient strengthening method. Reinforcing masonry structures with STPU allows brittle failure modes to achieve ductile behavior. This improves their structural performance under lateral stresses. The experimental data were used to calibrate FEM models for simulation. These models can be used for future parametric studies and masonry structural design.

Alvenarias de concreto não-armadas sob compressão: estatísticas brasileiras, análises de confiabilidade e calibração de norma

Abstract Although the new Brazilian code is considered an advancement, it still lacks a reliability-based calibration of partial safety factors. Therefore, this paper presents a comprehensive set of reliability analyses oriented to structural masonry members under compression in Brazil. This study was based on structural reliability theory, which allowed a safety assessment through the reliability index (β). Walls with hollow concrete blocks of three classes (A, B and C) and two modular sizes (M-15 and M-20), two ratios between the prism strength and the block compressive strength ( η = 0.5 and 0.6), and three ratios between effective height and effective width (λ = 14, 19 and 24) were evaluated. Considering that each one of the 36 elements is described by five ratios between live and dead loads and the five ratios between wind and dead loads, 900 configurations were analyzed. Reliability procedures, the structural configurations and the calibration methodology were implemented and solved in the StRAnD – Structural Reliability Analysis and Design software. Results showed that the reliability indexes are greater for unreinforced masonry walls under compression with modular size M-20 than modular size M-15. It can be noted that the block compressive strength not only represents the greatest impact on the masonry’s safety level, but is also the most relevant random variable. Based on the reliability-based calibration of the Brazilian code, it is recommended that unreinforced masonry presents different partial safety factors ( ɣ m) than reinforced masonry. This work represents a stepping stone in evaluating the safety of Brazilian masonry structures, indicating a possible path in terms of achieving a more uniform reliability in masonry design.

Experimental investigation on a FRCM bio-composite for sustainable retrofitting of masonry buildings

Strengthening masonry structures by using green and natural materials has received increasing attention. Natural fibers can be obtained from vegetables, animals or mineral sources and can be used to meet modern buildings safety standards and to reduce carbon dioxide emissions. This paper presents a novel method to strengthen masonry buildings through the application of a mortar coating reinforced with natural fibers made of a jute or a basalt grid. This class of materials is known as Fiber Reinforced Cementitious Matrix (FRCM). FRCM composites can produce an increment in the mechanical properties of masonry members and contribute to the thermal-acoustic insultation of the building envelope. The mortar coating is made of a hydraulic lime: to increase thermal properties this was admixed with short spelt fibers. Both matrix (lime mortar) and fibrous material were initially mechanically and thermally characterized. Laboratory tests have been carried out to study the bond characteristics between the FRCM composite and brickwork masonry. It was found the low mechanical properties of jute fibers are aggravated by a low FRCM-to-masonry bonding, making difficult to use this material for structural retrofit. On opposite, basalt fibers exhibit high mechanical properties and their use in conservation is surely promising and interesting.

Ultimate in-plane shear behaviour of clay brick masonry elements strengthened with TRM overlays

This paper studies the response of unreinforced masonry (URM) members made of hydraulic lime mortar and fired clay bricks, commonly found in heritage structures, strengthened with textile reinforced mortar (TRM) overlays. The investigation includes URM and TRM-strengthened diagonal compression tests on square panels, and relatively large-scale wall specimens subjected to combined gravity and lateral cyclic loads. Complementary compression, tension, and interface material tests are also carried out. The diagonal panel tests show that the TRM effectiveness depends in a non-proportional manner on the overlays, render thickness, and substrate strength. The enhancement in stiffness, strength, and ultimate shear strain, using one to four mesh layers on each side, is shown to vary in the range of 49–132%, 102–536%, and 300–556% respectively. It is shown that strut crushing typically governs the response of such low-strength URM masonry elements confined by TRM overlays. The cyclic tests on the comparatively larger walls show that the TRM is effective, shifting the response from URM diagonal tension to rocking, and enhancing the stiffness, strength, and ultimate drift capacity by more than 160%, 30%, and 130%, respectively. It is shown that analytical assessment methods for predicting the response of TRM-strengthened and URM members in terms of stiffness, strength and load-deformation can be reliably adapted. The cumulative contribution of the URM and TRM components, in conjunction with a suitable fibre textile strain, is also found to offer an improved prediction of the shear strength compared to codified procedures. The findings enable the evaluation and improvement of analytical models for determining the main inelastic response parameters of TRM-strengthened masonry and provide information for validating future detailed nonlinear numerical simulations.

Prediction of the additional compressive strength due to PBO-FRCM-confinement of brick-masonry depending on the stiffening effect caused by different discontinuous wrappings: new design-oriented perspective

Fabric Reinforced Cementitious Mortar (FRCM) is an innovative composite system able to strength or rehabilitate masonry members with an efficacy, which is affected by the mechanical and geometrical properties of its constituents: matrix and fabric. Since one of the most appreciated benefits of FRCM is the breathability, the discontinuous application is commonly not considered in the retrofitting. On the other side, the thickness of the matrix may be considerable (ranging between 10 and 30 mm) implying an over-weight and over-stiffness which may influence the seismic response of the structure in a detrimental manner; especially when dealing with column confinement. Consequently, the experimental evidence and the proper design for the partially wrapping with FRCMs is currently lacking.For this reason, the present research reports on an experimental and analytical investigation about PBO (polybenzoxazoles) FRCM-confinement of clay-brick based masonry columns by varying the confinement scheme. The aim is to evaluate the effectiveness of the confinement in terms of axial bearing load capacity and ductility and, at the same time, focusing on the parameters related to the confining scheme showing how these affect the results. Two columns were tested unconfined as lower-bound reference; six columns (i.e. three couples) were partially confined respectively with 3, 4 and 5 wraps having the same width and lastly, a further couple was fully-jacketed as upper-bound reference. The findings evidenced that an appreciable gain in axial strength and deformability was ensured. On the other side, the axial stiffness increased proportionally to the number of confining wraps.Thus, a new analytical model was herein proposed considering the geometry of the confining scheme, as well as the stiffening effect related to it. In particular, a novel effectiveness coefficient (namely KE) is evaluated affecting the theoretical confining pressure based on the axial stiffness of the FRCM confined masonry. The comparison with the own and other available experimental data confirmed the reliability of the proposal which wants to offer a new perspective for the design of the FRCM partial confinement of masonry that needs further investigations to be confirmed by including a variety of masonry and FRCM typologies.

Experimental Investigation on Structural Performance Enhancement of Brick Masonry Member by Internal Reinforcement

This study focused on perforated bricks, and the structural performance enhancement of brick members by internal reinforcement was experimentally investigated. As a new reinforcing material, screw iron (SI) rods were selected for the internal reinforcement, and they were inserted into the perforated brick. To investigate the most-effective reinforcing method as well as to understand the fundamental structural behaviors, four specimens with different variables were fabricated, and three-point bending tests were carried out. From the experiments, it was found that the maximum strength of the specimen increased by more than two times with internal reinforcement. The internal reinforcement, fixed by nuts at both ends of the specimen to enhance integrity, increased the maximum strength by nine times. Moreover, the deformation capacities of the specimens were also greatly enhanced. The case where the internal SI rods were fixed by nuts without mortar also showed a similar structural performance to the case with mortar. The estimation methods of the maximum strength of the specimens were also discussed, and they showed reasonable agreement with the test results. It was proved that the proposed material and methods enabled effective utilization of the internal reinforcement, and they could contribute to the improvement of structural performances in masonry construction.

A reassessment of the CSA S304 chi-factor for design of reinforced concrete masonry members: large-scale experimental investigation

An experimental investigation was conducted that consisted of 12 pairs of reinforced masonry specimens in which the two specimens were constructed with their longitudinal axes oriented parallel and perpendicular to the laboratory floor, respectively. This work was conducted to reassess the need for the χ-factor accounting for the direction of compressive stresses in masonry members in relation to the direction used to establish their masonry assemblage strength included in CSA S304-14— Design of Masonry Structures. Parameters investigated included block size and reinforcement ratio. Test results suggest that the χ-factor can be eliminated from future editions of CSA S304 but should be considered with caution: a review of the literature and the current tests show that, in some cases, voids may develop between the exterior webs of adjacent blocks and so may be the source of the reduction in the magnitude of the equivalent rectangular compressive stress block.

Symbolic Regression Model for Predicting Compression Strength of Prismatic Masonry Columns Confined by FRP

The use of Fiber Reinforced Polymer (FRP) materials for the external confinement of existing concrete or masonry members is now an established technical solution. Several studies in the scientific literature show how FRP wrapping can improve the mechanical properties of members. Though there are numerous methods for determining the compressive strength of FRP confined concrete, no generalized formulae are available because of the greater complexity and heterogeneity of FRP-confined masonry. There are two main objectives in this analytical study: (a) proposing an entirely new mathematical expression to estimate the compressive strength of FRP confined masonry columns using symbolic regression model approach which can outperform traditional regression models, and (b) evaluating existing formulas. Over 198 tests of FRP wrapped masonry were compiled in a database and used to train the model. Several formulations from the published literature and international guidelines have been compared against experimental data. It is observed that the proposed symbolic regression model shows excellent performance compared to the existing models. The model is easier, has no restriction and thereby it can be feasibly employed to foresee the behavior of FRP confined masonry elements. The coefficient of determination for the proposed symbolic regression model is determined as 0.91.

Compressive Strength of Masonry Confined by FRCM Systems: Assessment of Existing Models and New Proposals

Fabric-reinforced cementitious matrix (FRCM) composites have emerged as a viable solution for the external confinement of masonry members. However, what still discourages the use of these composites in the civil engineering field is the lack of analytical models capable of estimating the compressive strength of FRCM confined members with appreciable accuracy.This paper contributes to filling this knowledge gap by presenting an analytical study on the compressive strength of masonry confined by FRCM systems. The goal is twofold: a) to assess existing formulae published in the literature or in some international guidelines, and b) to provide new and more accurate proposals. To this purpose, a large database including results of compression tests on masonry members wrapped with FRCM systems was compiled from the literature. The database was organized by considering some relevant parameters, such as: type of fiber and geometry of the mesh, number of employed layers, mechanical properties of the inorganic matrix, compressive strength of the masonry. The accuracy of the strength models is examined by both considering all FRCM confined members together and treating natural and artificial masonry separately.

Numerical simulation of a timber retrofit solution for unreinforced masonry buildings

This paper presents the results of a numerical study about the performance of a new timber retrofit solution for unreinforced masonry buildings in seismic regions. The proposed retrofit consists of timber frames fastened to the internal surface of masonry piers, with oriented-strand boards nailed to the frames. This technique aims at improving both the in-plane and out-of-plane capacities of masonry piers as well as wall-to-diaphragm connections. Quasi-static cyclic shear-compression tests were first conducted on two isolated piers to assess the in-plane response improvement. Dynamic shake-table tests were then performed on two full-scale building specimens representing the end-unit of a cavity-wall terraced house, in bare and retrofitted conditions, where also the timber diaphragms were stiffened. Numerical models were built and calibrated against the experimental data adopting an equivalent-frame approach, where masonry members were discretized into multiple nonlinear macroelements while timber components were represented by equivalent elasto-plastic truss elements. Results in terms of hysteretic lateral force-displacement responses from nonlinear dynamic analyses are discussed to validate the proposed modeling strategy.

Global modeling strategies for masonry buildings with timber diaphragms under seismic actions

This paper discusses the effects of different modeling strategies on the simulated global response of masonry buildings with timber diaphragms under earthquake excitation. The benchmark for this study was provided by a unidirectional dynamic shake-table test on a three-story, half-scale natural stone masonry building aggregate, incorporating the main architectural and structural features of the historical center of Basel (Switzerland). The global response of the specimen was simulated through nonlinear static analyses using an equivalent frame approach, with nonlinear macroelements for masonry members and linear orthotropic membranes for diaphragms. Three modeling strategies were followed. First, an unconventional modeling strategy was adopted, to implement explicitly the out-of-plane stiffness of walls orthogonal to the shaking direction through a particular combination of equivalent frames and membranes. The second option consisted of a 3D model with walls in both directions and finite-stiffness diaphragms, however neglecting the out-of-plane overturning response of walls. In the third case, given their low stiffness compared to masonry walls, the timber diaphragms were considered infinitely flexible and single-wall 2D models were analyzed in the shaking direction. Numerical capacity curves were compared to experimental backbone curves, showing satisfactory accuracy even when diaphragm and out-of-plane wall stiffness were neglected.

Experimental and numerical investigation on the tensile behavior of PBO FRCM composites with textile lap splice

Fiber-reinforced cementitious matrix (FRCM) composites are comprised of high strength textiles embedded within inorganic matrices. Due to their peculiar mechanical and physical properties, they represent a valid alternative to fiber-reinforced polymer (FRP) composites for strengthening existing reinforced concrete and masonry members. Depending on the textile and matrix adopted, FRCM composites provide a peculiar mechanical behavior, which shall be assessed to obtain the mechanical properties to be used in the design of FRCM-strengthened members. Since FRCM can be applied to large surfaces, reinforcing textile lap splice is needed to guarantee the continuity of the composite mechanical performance. Thus, the behavior of FRCM with textile lap splice should be properly investigated. In this paper, the tensile behavior of a polyparaphenilene benzobisoxazole (PBO) FRCM with continuous and overlapped textiles is studied using clevis-grip tensile tests. The experimental results are modeled using two finite element approaches, which accounted for the non-linear behavior of materials and for the interface between matrix and textile fiber. The results obtained showed that clevis-grip tests can be used to determine the minimum lap splice length needed to obtain the full composite load-carrying capacity and confirmed the validity of the method previously proposed by the authors to estimate such lap splice length.

Analytical study of the bond behavior of fiber reinforced cementitious matrix (FRCM)-substrate joints based on a two-stage nonlinear cohesive material law

Externally bonding fiber-reinforced cementitious matrix (FRCM) composite to the surface of concrete or masonry members is an effective technique to improve the performance of existing structures. In many cases, interfacial debonding between the FRCM fiber textile and embedding matrix governs the capacity of FRCM-strengthened structures. The interfacial debonding process can be studied analytically by assuming a cohesive material law (CML), which represents the relationship between interfacial shear stress and slip. In this study, a two-stage function with an exponential stage and a constant stage is proposed to describe the CML associated with the matrix-fiber interface. The latter stage is characterized by a constant shear stress to account for the friction/interlocking between the matrix and fiber observed in experimental tests. With the assumed CML, the full-range loading response was obtained. Additionally, the interfacial slip, fiber axial strain, and interfacial shear stress were analytically derived. The parameters of the two-stage CMLs for PBO, glass, and carbon FRCM were inversely determined by matching the analytical relationships of peak applied axial stresses associated with different bonded lengths with the experimental ones. Considering the inversely determined CMLs, the predicted load responses and strain profiles showed good agreement with the measurements of direct shear test specimens.

Failure Modes of RC Structural Elements and Masonry Members Retrofitted with Fabric-Reinforced Cementitious Matrix (FRCM) System: A Review

Much research has been conducted and published on the examination of the behavior of reinforced steel and concrete structures with a FRP system. Nevertheless, the performance of FRP differs from that of FRCM, particularly at high temperature and ultimate strength. The present study provides a review of previous research on structural elements (viz. beams, columns, arches, slabs, and walls) retrofitted with FRCM systems, taking account of various parameters, such as layers, composite types, configurations, and anchors for controlling or delaying failure modes (FMs). Additionally, this paper discussed the details of different FMs observed during experimental tests, such as crushed concrete or bricks, fiber debonding from substrate materials, slippage, fiber rupture, and telescopic failure for strengthened specimens. Moreover, this paper investigated where and how fractures may develop in structural elements retrofitted with the FRCM system under various retrofit scenarios. To this end, in addition to the review of the relevant literature, a large dataset has been compiled from different (RC) structural elements and masonry members. Next, a relationship is developed between failure modes (FMS) and influential parameters, i.e., the number of layers and the type of composite, based on this dataset. This can be used as a benchmark example in future studies, as there is no such basis available in the literature, to the best of the authors’ knowledge.

Design of FRCM Strengthened Masonry Walls Subjected to Out-of-Plane Loading According to CNR-DT 215: Discussion of the α Coefficient

Fiber-reinforced cementitious matrix (FRCM) composites are largely employed in Italy to improve the mechanical behavior of masonry members. Many different matrices and fiber textiles are available on the market, which entails for a large number of available composites, each with peculiar mechanical and physical properties. Among the possible applications, FRCM are often externally bonded to masonry walls to increase the wall shear capacity or to prevent possible wall out-of-plane failure. Up to date, only two guidelines are available for the design of FRCM strengthened masonry members, namely the American ACI 549.6R and the Italian CNR-DT 215. In the Italian guideline, the bending strength of an FRCM strengthened masonry wall is associated with the performance of the composite - which is investigated by FRCM coupon tensile tests and FRCM-masonry joint bond tests - through a cross-sectional equilibrium that assumes perfect bond among each material.In this paper, a database comprising 90 experimental tests on FRCM-strengthened masonry walls subjected to out-of-plane loading is collated from the available literature. The experimental results are used to compute the composite effective strain, which is then compared with the corresponding composite maximum strain obtained by characterization tests according to the CNR-DT 215 procedure. The comparison sheds light on the role of coefficients employed in the analytical procedure and helps understanding the influence of the FRCM on the wall bending capacity.

Calibration of a Rigid-Trilinear Cohesive Material Law to Describe the Matrix-Fiber Bond Behavior in FRCM Composites

Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly adopted as externally bonded reinforcement (EBR) of existing concrete and masonry members. Being debonding at the matrix-fiber interface one of the most frequent failure mechanisms of externally bonded FRCM, the matrix-fiber bond behavior represents a fundamental aspect for the effectiveness of the external reinforcement. A cohesive material law (CML) that describes the interface where debonding occurs can be used to model the bond behavior observed. In this paper, a rigid-trilinear CML is used to solve the differential equation that governs the bond problem at the matrix-fiber interface of an FRCM composite. The CML adopted has peculiar characteristics that entail for a finite length of the bond stress transfer zone (BSTZ). Furthermore, it allows for a simple and accurate analytical solution of the bond problem. The analytical solution obtained is compared with the results of an experimental campaign comprising single-lap direct shear tests of a polyparaphenylene benzobisoxazole (PBO) FRCM composite specifically designed for masonry substrates. Different calibrations of the rigid-trilinear CML are proposed, also considering the matrix-fiber free end slip.

A Multiplex Conversion of a Historical Cinema

The cultural building Heritage is a valuable trace of the past social life in a city. One peculiar typology of building is the theatre or, more recently, the cinema. Within this indoor space, the community of people actively live an emotional feeling over the time causing memories linked to not only the show itself, but also to the place. Furthermore, cinemas are often located in city centers where buildings are historical‐oriented form the architectural point of view and, for this reason, their added‐value needs to be recognized. Currently, the modernization and the improvement of old cinemas is becoming an interesting topic in the field of civil engineering. In order to empathize this aspect, the structural strengthening for a multiplex conversion of an historical cinema is herein reported. The studied structure required a seismic joint (separating the masonry to the reinforced concrete structural bodies) and a severe strengthening of a very long span beam due to a relevant overloading (in seismic loads combination) caused by the new architectonical project. The numerical simulations demonstrated the validity of two bending strengthening systems (i.e. Fiber Reinforced Polymer‐plate and Beton Plaquè) in terms of both load bearing capacity and maximum deflection. This overview is a part of a larger study in the way of a global structural interventions which will also involve the masonry members.

Low- and High-Cycle Fatigue Behavior of FRCM Composites.

This paper describes methods, procedures, and results of cyclic loading tensile tests of a PBO FRCM composite. The main objective of the research is the evaluation of the effect of low- and high-cycle fatigue on the composite tensile properties, namely the tensile strength, ultimate tensile strain, and slope of the stress–strain curve. To this end, low- and high-cycle fatigue tests and post-fatigue tests were performed to study the composite behavior when subjected to cyclic loading and after being subjected to a different number of cycles. The results showed that the mean stress and amplitude of fatigue cycles affect the specimen behavior and mode of failure. In high-cycle fatigue tests, failure occurred due to progressive fiber filaments rupture. In low-cycle fatigue, the stress–strain response and failure mode were similar to those observed in quasi-static tensile tests. The results obtained provide important information on the fatigue behavior of PBO FRCM coupons, showing the need for further studies to better understand the behavior of existing concrete and masonry members strengthened with FRCM composites and subjected to cyclic loading.

FRP confined masonry under compression: database collection and design proposals

Fiber Reinforced Polymer (FRP) materials have now reached the status of well-established technical solution for the external confinement of existing concrete or masonry members. Within this context, the scientific literature collects a number of experimental investigations proving the capability of the FRP wrapping in improving the mechanical behavior of members.However, from an analytical point of view, while several formulations are available for evaluating the compressive strength of FRP confined concrete, no general accredited formulae can be found for masonry columns, also due to the higher complexity and heterogeneity of this material with respect to concrete.With the aim to provide an updated contribute on this topic, this paper presents an analytical study on the FRP confinement of masonry columns with the twofold objective to: a) propose new models for the estimate of the compressive strength, and, b) assess existing formulae.A wide database including results of compression tests on over 230 masonry members externally wrapped with FRP was assembled from the literature.The experimental results were employed to develop new relationships for the estimate of the compressive strength of the FRP confined masonry through best-fit analyses. Comparisons with some formulations available in the literature and international guidelines were performed.

SEISMIC DESIGN OF MASONRY-INFILLED FRAMES: A REVIEW OF CODIFIED APPROACHES

This paper reviews the approach of eleven national codes on the analysis and design of masonry-infilled frames. It is shown that, in general, codes can be divided into two groups. The first group isolates the masonry and frame members by providing gaps to minimize the interaction between them. This method ensures that the complexities involved in analyzing the structure is avoided. However, the width of the gaps recommended is different for each of the codes. The second group takes advantage of the presence of high stiffness and strength masonry infill. In this technique, an equivalent-strut modeling strategy is mostly recommended. It is shown that the strut model suggested in each of the codes is different. An attempt to obtain a generic model for masonry-infilled frame failed largely due to the existence of many behavior-influencing parameters. Finally, it is suggested to have a paradigm shift in the modeling strategy where the masonry-infilled frames are classified into different categories and a model is suggested for each of them.