Masonry Substrate Research Articles

TRM strengthening as effective prevention against premature failure of masonry walls

Masonry structures are very prone to cracking and premature failure in the presence of shear loading. Therefore, effective prevention of their premature damage is needed. This paper presents laboratory tests of small models made of autoclaved aerated concrete (AAC) blocks, which were externally reinforced using two different strengthening sets consisting of mortar and mesh, and subjected to diagonal compression. The experiments used a commercialised system designed for the strengthening of typical masonry substrates (brick, stone) and a low-cost and lowstrength mesh and mortar set used as part of a system for insulating buildings. The analyses presented are of a comparative nature and describe changes in the behaviour of the models depending on the strengthening set used. The application of full-surface strengthening clearly changed the behaviour of AAC walls from brittle one to the ‘pseudo-ductile’ one observed after the appearance of the first cracks. In all strengthened models, a significant increase in the load capacity (from 3% to 62%) and a post-peak phase were reported. An interesting finding was the high efficiency of nonstructural materials (plastering system), comparable to the effects of the use of commercialised system solutions. This strengthening can be successfully considered as an effective preventive action. Finally, a simplified method is provided to calculate the contribution of the strengthening to the load bearing capacity of the strengthened models. The method defines the effect of the mesh and the mortar independently, based only on the basic material properties of these two components.

Development and performance evaluation of a novel high-ductility fiber-reinforced lime-pozzolana matrix for textile reinforced mortar (TRM) masonry strengthening applications

This paper discusses the development of a high ductility fiber reinforced lime mortar with deflection hardening behavior. In this study, polypropylene fibers have been incorporated in a cementless reference matrix comprising of Natural Hydraulic Lime, silica fume, siliceous aggregates and workability aid admixtures. The properties of the reference and fiber reinforced mortar compositions were assessed by means of standardized laboratory tests and scanning electron microscopy. The reference composition gave an average compressive strength of 12 MPa, rendering it suitable for structural applications. Fiber addition at a dosage of 0.76% by wt. of solids led to a reduction of the compressive strength; however, sufficient load bearing capacity was still achieved (8 MPa). More importantly, the fibers enabled the mortar to sustain useful load after damage initiation, improving post-peak ductility under compression and allowing the material to reach bending stresses up to 20% higher than the first-crack strength. To evaluate the practical application of the proposed materials, their use as matrices in textile reinforced mortar (TRM) strengthening systems was investigated through pilot applications on stone masonry. Diagonal compression tests were performed on 9 ashlar masonry wallettes, including both un-retrofitted and TRM-retrofitted specimens. Single-sided TRMs consisting of alkali resistant glass textiles embedded in either the reference or the fiber reinforced mortar were considered. Specimens retrofitted with TRM constructed using the fiber reinforced mortar, exhibited a remarkable ~ 270% increase in shear strength and > 50% higher deformation capacity, compared to un-retrofitted ones. The strength increment achieved with the reference mortar as the TRM matrix was 80%, with minimal impact on ductility. The TRM comprising the fiber reinforced mortar also showed superior performance in terms of ability to retain integrity at high levels of shear deformation. Overall, the results indicate that the use of fiber reinforced lime matrices in TRM systems applied to masonry substrates has the potential to substantially enhance mechanical performance under in-plane loading, even in cases where only single-sided retrofitting can be realized.

Seismic strengthening of partially grouted masonry walls with openings: Evaluation of ferrocement and BTRM solutions

This paper presents an experimental study on the seismic performance of partially grouted reinforced masonry (PG-RM) walls strengthened with two solutions: Ferrocement coating (welded wire steel mesh embedded in cementitious mortar, WWM) and Basalt Textile Reinforced Mortar (BTRM) layer. For this purpose, two full-scale walls were built with hollow concrete units, with identical geometry and construction details of a previously reported tested wall, which acts as the control specimen (unstrengthened case) in this study. The walls, with a window-type opening and another door-type opening, were strengthened on one of their sides and tested under the combined action of constant axial compression and cyclic lateral loading up to failure.The experimental results show that both strengthening solutions increased the investigated walls' initial stiffness and lateral capacity. The increase in lateral capacity of both solutions was similar, about 37%. The 2D-DIC analysis implemented during the tests was instrumental in highlighting the areas on the strengthened walls where high strains were developed. The damage registered on the BTRM layer was more consistent with what was observed on the surface of the unstrengthened wall. Meanwhile, the damage reported on the wall strengthened with WWM did not exhibit apparent damage at higher drift levels due to the detachment between the ferrocement coating and the masonry substrate. Finally, the performance of some procedures to compute the contribution of the studied strengthening solutions was also assessed.

Toughness of Natural Hydraulic Lime Fibre-Reinforced Mortars for Masonry Strengthening Overlay Systems

Masonry structures are susceptible to damage and collapse due to seismic actions, a problem in many urban areas. To address this issue, researchers are studying the use of fibre-reinforced mortars as overlay strengthening systems. This study assessed the use of synthetic polyacrylonitrile (PAN) fibres as reinforcement of natural hydraulic lime mortar, focusing on their influence on fresh behaviour and mechanical properties. Natural hydraulic lime (NHL) was chosen for its compatibility with typical older ceramic and natural stone structural masonry and contemporary ceramic brick infill masonry substrates, as well as for the sustainability benefits. The study also assessed the contribution of the PAN fibres to toughness enhancement in the developed formulations. The fresh behaviour of fibre-reinforced mortar (FRM) was found to be adequate for applications with fibre volume fractions below 0.50%. The compressive and flexural strengths were affected differently by the increase in fibre volume fraction, with compressive strength decreasing and flexural strength increasing. The maximum compressive strength of 13.3 MPa was obtained for 0.25% of fibres, while for flexural strength a maximum of 6.70 MPa was achieved with 1.00% of fibres. The compressive and flexural toughness, related to the post-cracking responses, increased with the fibre fraction, and even for fractions as low as 0.25%, an important increment of the capacity to dissipate energy was achieved.

Influence of spike anchors in the bond behaviour of FRCM systems applied on concave masonry substrates

One of the methods that can be used for strengthening masonry arch structures is through the use of composite systems bonded to the external surfaces, preferably to the intrados, such as textile reinforced mortar systems (FRCM, TRM, SRG etc). The application of such systems to the intrados allows the ultimate load to be increased while keeping the structure in service. The load increment depends on the performance of the bond between the matrix and the masonry substrate and between the matrix and the fabric, and also the radial detachment stresses (normal for the strengthening system) that are generated when applying this technology to concave surfaces must also be taken into account, as they have a negative effect on the load transfer mechanism. Spike anchors are generally used to reduce the negative effect of curvature in order to postpone the debonding process of the strengthening system. This paper presents the results obtained from a direct shear test campaign on curved masonry specimens strengthened with a fiber-reinforced cement-based matrix (FRCM) with pozzolanic reaction and a carbon fiber fabric. In particular, the effects of spike anchors on the load transfer mechanism between FRCM and the masonry substrate are shown. The use of such spike anchors produced, on average, a higher maximum ultimate force than specimens without spike anchors.

Effect of Spike Anchors in the Bond Behavior of FRCM Systems Applied onto Curved Masonry Substrates

Effect of Spike Anchors in the Bond Behavior of FRCM Systems Applied onto Curved Masonry Substrates

Influence of severe thermal preconditioning on the bond between carbon FRCM and masonry substrate: Effect of textile pre-impregnation

Fabric-reinforced cementitious matrix (FRCM) composites often include polymer-impregnated bundles to improve the exploitation of the textile mechanical properties. However, organic components may degrade when exposed to elevated temperature. In this paper, the bond behavior of a carbon FRCM applied to a masonry substrate and exposed to a thermal preconditioning up to 300 °C for 250 min is investigated. Tensile tests on the textile and flexural and compression tests on the mortar matrix, as well as single-lap direct shear tests of FRCM-masonry joints with bare and impregnated textiles, are performed. Results show that the polymeric impregnation improves the mechanical properties of the FRCM even after thermal preconditioning.

Bond behaviour of Fibre Reinforced Polymers applied on masonry substrate: Analysis based on acoustic emission, digital image correlation and analytical modelling

Fibre-reinforced polymers (FRP) are effective for masonry reinforcement, but performance depends on bonding capacity, but performance depends on the bonding capacity. This paper analyses the shear behaviour of the FRP-to-masonry interface and uses the synergistic strengths of different investigation tools to characterise the debonding process. Twelve shear tests were carried out, using FRPs with three different fabrics. Using digital image correlation and Wu’s model, the parameters of the bond–slip laws are determined. α is constant. α/β is a function of stiffness at power −0.5. The cumulative acoustic amplitudes increase significantly for a slip inversely proportional to α (R2=0.926) and with a slope proportional to the fracture energy (R2=0.810). The high frequency partial power increases sharply at a load for which the model predicts debonding at the sensor, or because of a textile tensile failure. The linear location obtained with AE makes it possible to differentiate these two failure modes.

Bond behaviour of light and heavy carbon fibre TRM to masonry interfaces

We present the results of an experimental campaign on the bond behaviour between carbon fibre textile reinforced mortars and masonry substrate. The campaign involved 54 single lap direct shear tests on masonry wallettes reinforced with a single TRM layer. The key investigated parameters were the type of the textile fibre material (light carbon and heavy carbon), the coating of the textile reinforcement with epoxy resin, and the bond length (100, 150, 200, and 250 mm). The results highlight the beneficial effect of the epoxy resin coating that practically doubles the maximum attained loads. This increase is shown to be associated to a consistent shift of the failure mode from textile slippage to detachment at the matrix to matrix interface. In the uncoated specimens, the maximum load is practically identical for heavy and light carbon. Conversely, in the coated specimens the maximum load attained by the heavy carbon fibre textiles is 65% higher than that of the light carbon coated fibre textiles. However, both coated textiles demonstrate the same exploitation ratio. Cost-wise, this renders light carbon an appealing choice in practical applications. The experimental observations are further examined in view of analytical modelling.

Estimation of the Ultimate Strength of FRP Strips-to-Masonry Substrates Bond

Fiber-Reinforced Polymers (FRP) were developed as a new method over the past decades due to their many beneficial mechanical properties, and they are commonly applied to strengthen masonry structures. In this paper, the Artificial Neural Network (ANN), K-fold Cross-Validation (KFCV) technique, Multivariate Adaptive Regression Spline (MARS) method, and M5 Model Tree (M5MT) method were utilized to predict the ultimate strength of FRP strips applied on masonry substrates. The results obtained via ANN, KFCV, MARS, and M5MT were compared with the existing models. The results clearly indicate that the considered approaches have better efficiency and higher precision compared to the models available in the literature. The correlation coefficient values for the considered models (i.e., ANN, KFCV, MARS, and M5MT) are promising results, with up to 99% reliability.

Non-linear analytical model for FRCM coupons in tension

Externally bonded FRCM (Fiber Reinforced Cementitious Matrix) materials are nowadays quite diffused to reinforce existing structures: taking as reference FRP (Fiber Reinforced Polymer), the replacement of an organic matrix with mortar is more compatible with masonry substrates, and the reversibility of the reinforcement is more appealing for the architectural heritage conservation. At the same time, due to the low strength of the mortar layers, complex failure mechanisms are observed for FRCMs, which still need comprehensive theoretical investigations. Indeed, not only the mortar-fiber interface may behave non-linearly, but also the mortar may crack and affect the overall behavior. The tensile test on coupons is commonly used as a direct approach to characterize the properties of the reinforcing package, and there is a wide consensus on approximating the global behavior with a trilinear law; however, the different test set-ups and the inhomogeneity of the material itself bring about large dispersion of the experimental results. This paper aims at proposing a simplified analytical model to consider the damage mechanisms in FRCM coupons in tension, including interface slippage, mortar cracking, and their simultaneous occurrence. The accuracy of the model is then verified by comparing the results with experimental data. The conditions under which such failure mechanisms occur and the effects of material properties are also investigated with a view to complement the understanding of experimental observations from a theoretical perspective.

Modeling of shear-lap tests of flat and curved masonry specimens strengthened by FRCM

Nowadays Fiber Reinforced Cementitious Matrix (FRCM) systems play a relevant role in the context of innovative interventions for the seismic rehabilitation of masonry structures. Their capacity in improving the strength of masonry components is comparable with the one observed in case of Fiber Reinforce Polymer (FRP) systems, but with additional advantages (lower costs, environmental compatibility, removability, etc.). Nevertheless, although a relevant number of applications concerns curved structures (arches, vaults, domes, etc.), the majority of studies available in literature provide a contribution mainly concerning the specific case of applications on flat masonry substrates. The present paper is part of a research activity carried out by the Authors with the main goal to provide a contribution toward the study of the bond behavior of FRCM systems externally applied to curved masonry elements. In particular, the results of numerical analyses carried out by means of a simple modeling approach proposed by the Authors are here carried out by considering as case studies specimens object of a recent experimental investigation. Specific aspects influencing the local bond behavior of FRCM systems applied on curved masonry substrates are then analyzed by opportunely introducing them into the numerical model. The obtained results allow for understanding the effect of important features, experimentally observed in terms of global response, and here assessed in terms of local bond behavior, by particularly emphasizing the role of the curvature and the strengthening position.

Blast-Resistant Window Anchors. I: Experimental Investigation

A comprehensive experimental investigation was conducted on blast-resistant window anchors involving 46 tests of double-pane insulated glass units (IGUs) anchored to structural steel, reinforced concrete, concrete block masonry, and stone masonry substrates. The tests were conducted using a shock tube. The windows were glazed with security films of different thickness. Different number and spacing of steel anchors were used to secure the window frames to substrates. Each window was subjected to two levels of blast loads, consisting of 28 kPa, 207 kPa-ms, and 69 kPa, 621 kPa-ms reflected pressure–impulse combinations. The windows were instrumented to measure anchor forces. The anchors developed out-of-plane shear forces and in-plane axial tension associated with postbreak membrane action. The results indicate that anchor shear forces show variations with window stiffness, substrate type, and anchor arrangements, often developing lower forces than those computed based on the static application of blast loads, indicating significant inertia resistance. Rigid substrates produce higher anchor forces. In addition to the experimental results, the paper presents single-degree-of-freedom analysis results for anchor force computation, indicating good correlations with experimental data.

Behavior and Design of Masonry Strengthened with Steel-Wire–Reinforced Cementitious Matrix under Flexure

Overlays consisting of welded steel-wire mesh embedded in an inorganic matrix have shown promising results in improving the flexural behavior of masonry in terms of strength, ductility, and energy dissipation. The flexural capacity of masonry strengthened with steel-wire–reinforced cementitious matrix (SWRCM) can be influenced by various factors, such as the percentage of reinforcement, strength of cementitious matrix, and masonry compressive strength. To understand the influence of these parameters, the results of past experimental studies were carefully examined, and parametric finite-element analyses were performed. The analytical equations available in the literature were used for the flexural capacity estimation of masonry reinforced with SWRCM; however, the majority of them could not provide accurate and reliable predictions. This may be due to neglecting the contribution of the cementitious matrix and debonding at the interface of the masonry substrate and composite overlay. Thus, a new equation has been proposed in this study for estimating the flexural capacity of masonry strengthened with SWRCM, which can consider the contribution of the inorganic matrix and the debonding at various interfaces through an effective tensile strain parameter. In addition, a stepwise procedure has been provided to estimate the effective tensile strain of SWRCM composite. Results of the analytical investigation showed that the proposed method can effectively provide a reliable and consistent prediction of the flexural capacity of masonry strengthened with SWRCM composites.

Lime-based mortar reinforced with randomly oriented polyvinyl-alcohol (PVA) fibers for strengthening historical masonry structures

The seismic vulnerability of unreinforced masonries underlines the importance of developing new methods and materials for the retrofitting of historical structures. Nowadays, two are the most widely diffused strengthening systems for masonry structures, i.e. fiber reinforced polymers (FRP) and fabric reinforced cementitious matrix (FRCM) composites. Despite some degree of effectiveness, FRPs show poor chemical compatibility with masonry supports, that makes them unsuitable for intervention on historical heritage structures. FRCMs frequently consists of a cement-based mortar and, because of this, the cementitious nature of the inorganic matrix can not ensure the chemical compatibility with the historical masonry substrate; also, being FRCMs composites made out of fiber fabrics embedded in an inorganic matrix, the mono- or bi-directionality characterizing the fiber fabric makes this type of reinforcement ineffective out of the fiber axial direction. This paper aims to overcome the listed drawbacks presenting a systematic study on a newly developed lime-based mortar, reinforced with randomly oriented polyvinyl alcohol (PVA) fibers: the lime-based nature of the mortar meets the requirement of chemical compatibility with the historical substrate, that is fundamental for the restoration of heritage buildings, and the random orientation of the PVA fibers makes them globally effective, being the stress state induced by seismic action directionally unknown. Specimens measuring 160 mm × 40 mm x 40 mm characterized by six different fiber contents and two different fiber lengths, namely 6 mm and 12 mm, in addition with plain mortar samples, are tested in three-point-bending configuration to compute both flexural strength and fracture energy. Then, the two broken pieces resulting from the flexural tests, each one measuring 80 mm × 40 mm x 40 mm, are tested in splitting configuration and in compression, and the resulting tensile strength and compressive strength are computed. An analytical formulation is finally proposed by fitting the experimental data with linear, parabolic and exponential regression models. The results related to averaged values of mechanical properties show that, for both short and long fiber lengths, the flexural strength and the fracture energy linearly increase with the fiber content and that the tensile and the compressive strengths parabolically increase with the fiber content. Overall, the newly proposed mortar stands as a valid system for the strengthening of masonry structures, being especially suitable for intervention on cultural heritage buildings.

FRCM-to-masonry bonding behaviour in the case of curved surfaces: Experimental investigation

Fabric-reinforced cementitious matrix (FRCM) are composite materials more and more used for the reinforcement of masonry structures. The combination of high tensile strength fabrics (or meshes) with cementitious matrices, having good thixotropic capabilities and vapour permeability, makes such composites suitable for reinforcing a large number of masonry structures, including the one belonging to the historic heritage. FRCMs are bonded to the outer surfaces of structural masonry elements and, thanks to their adhesive capacity, bear much of the tensile stresses that unreinforced masonry cannot withstand. The effectiveness of such reinforcements, which is highly dependent on their ability to adhere to the masonry substrate, is generally investigated throughout specific experimental investigations (shear tests). Almost all the papers in the literature devoted to bond-slip analysis refer to the case of flat bonding surfaces, although these reinforcements are also widely used on curved structural elements such as arches and vaults. Therefore, this paper reports and examines the results of an extensive experimental program concerning the behavior of FRCM systems applied on curved masonry specimens. The results point out the influence of both curvature and reinforcement position (intrados or extrados) on the response of specimens in terms of bearing capacity, failure mode and post-peak response.

Prediction of compressive strength and evaluation of different theoretical standards and proposed models of brick columns confined with FRP, FRCM, or SRG system

The strength capacity of confined masonry column is one of the topics that need to be studied. In this study, the efficiency of using different types of advanced composite (non-corrosive materials) such as fiber reinforced polymer (FRP), fiber reinforced cementitious matrix (FRCM), or steel reinforced grout (SRG) in confining masonry columns is investigated. A wide range of experimental database of masonry column specimens has been collected from the results that available in scientific literatures. Different theoretical standards and proposed models that used to predict the capacity of masonry columns confined with FRP and FRCM are evaluated based on collected experimental database. Since there is no standard code or specific proposed model for SRG system, the confined capacity of this system is predicted and evaluated using the FRCM proposed models. The justification of using these models is that both FRCM and SRG systems have the same concept of using inorganic material as a paste material. An index named “equivalent fiber reinforcement index (EFRI)” is proposed to capture the key factors that control the behavior of the confined masonry columns with different advanced composite. This index is used as reference parameter for the purpose of the comparison between different strengthening systems. As a result, all types of advanced composite presented a significant increase in ultimate capacity. Also, the behavior of the masonry columns was significantly dependent on the type of fabric used. Different modes of failure were reported, including crushing of masonry block, as well as a deboning of FRP from the masonry substrate and deboning or slippage of fabric within inorganic paste matrix. Compared with other models and standards, the American Concrete Institute Committee 440 (ACI 440) and American Concrete Institute Committee 549 (ACI 549) shows very good predictions for the confined capacity of masonry columns strengthened by FRP and FRCM or SRG respectively.

Masonry elements strengthened through Textile-Reinforced Mortar: application of detailed level modelling with a free open-source Finite-Element code

The paper concerns the modelling of masonry elements strengthened through Textile Reinforced Mortar (TRM), a near surface system made of fiber-based grids or textiles embedded in mortar layers. Recently, the author, focusing on the mechanical characterization of TRM composites, developed a detailed level modelling approach by using the free, open-source Finite-Element code OOFEM, for the simulation of experimental tests on TRM coupons (pull-out tests, tensile tests, shear bond tests and in-plane shear tests). The model was capable to account for the failure of single components (e.g. the fibers tensile failure, the mortar cracking and crushing), as well as of their interactions (the debonding of the fibers from the mortar and of the mortar from the masonry substrate). In this paper, the detailed-level modelling approach is applied to the simulation of TRM strengthened masonry elements subjected to diagonal compression, in-plane and out-of-plane bending tests, investigating on the typical failure modes of masonry. Non-linear static analyses are performed, with nonlinearities of materials and interfaces deduced from experimental evidences. The comparison with some experimental results and a parametric study allowed to evidence the reliability of the models and their sensitivity to the main components characteristics.

A novel parallel wire steel-reinforced mortar (PW-SRM) method versus textile reinforced mortar (TRM) for out-of-plane strengthening of masonry walls

Lateral loads cause extensive damage to buildings due to the weakness of bearing out-of-plane un-reinforced masonry (URM) walls. Strengthening URM walls is consequently favored by researchers aiming at achieving building and respective occupant safety. The strengthening of masonry structures with textile reinforced mortar (TRM) or steel-reinforced mortar (SRM) has become prevalent in the last two decades. This study presents an innovative method of strengthening masonry walls using mortar reinforced with high tensile strength steel (HTSS) parallel wires. For the first time, an unprecedented investigation as to the effects of the grooving method improving the load-carrying capacity and failure mechanism of masonry walls under out-of-plane loading was performed. In addition, the parallel wires steel-reinforce mortar (PW-SRM) was compared with the textile-reinforced mortar (TRM) reinforcement method. Nine walls with dimensions of 1400 × 440 × 100 mm each were reinforced through different techniques and tested under four-point out-of-plane bending. The implications of the number of glass textiles with open mesh layers, one-sided and two-sided reinforcement, engineered cementitious composites (ECC), grooves on the masonry substrate, and, the volume of HTSS wires in increasing out-of-plane loading were compared and discussed. The findings demonstrated that the best performance is related to the specimens reinforced by the PW-SRM method. Similarly, the amount of out-of-plane load-bearing capacity and energy absorption increased more than 24 and 5300 times compared to the control specimen, respectively. It was further discovered that the grooving method transferred the tensile stress created in the TRM composite to the depth of the masonry substrate, which enhanced the failure mechanism from debonding to textile rupture and increased the strength by 88% compared to the corresponding specimen without grooves.

A Multi Pier (MP) method for the evaluation FRP delamination on flat and curve masonry substrates

In this paper a truss based approach called Multi Pier (MP) method is developed to FRP delamination on masonry substrate. According to the method, a (2D) continuous masonry element is represented via trusses assemblage consisted of two vertical elements (piers) and two diagonal rods (braces). Both axial and shear stiffness of the substrate are provided by piers and braces, respectively. Bi-linear axial behavior is dedicated to trusses based on the masonry stress–strain curve. Coupled shear-normal interaction due to curvature is considered by appropriate forces distribution among the braces. In order to validation, the approach was benchmarked against six experimental strengthened masonry pillars via C/G FRP sheets from the literature. The specimens include four flat and two curve substrates with various properties and boundary conditions under single lap shear test. For each specimen global load-slip behavior, failure mechanism, fiber tensile stress and strain profile, bond and normal (in the case of curve pillars) stresses profile during various phases of the delamination process were compared with the available experimental results. Satisfactory accuracy was obtained despite very limited computational cost due to substitution of complicated (2D) or (3D) continuous elements by simplified (1D) trusses with bi-linear axial behavior.