Experimental analysis of historic masonry walls reinforced by CFRP under in-plane cyclic loading

External bonding of Fiber Reinforced Polymers (FRPs) has become a popular technique for strengthening historic masonry wall buildings in seismic area. Although FRPs seem to improve response of historic masonry walls under in-plane shear loading, further investigations must specify a number of aspects for both researchers and practitioners. This paper presents an experimental analysis pertaining to the response of unreinforced and reinforced historic masonry walls built using full clay bricks in scale 1/3rd. On the basis of previous experimental research carried out by shear tests on triplets and unreinforced walls, a shear criterion for historic unreinforced masonry (HURM) has been assumed. In this experimental analysis two HURM walls, characterized by double T shape, were subjected to in-plane cyclic loading to shear cracking. The damaged walls were reinforced using horizontal–vertical and diagonal Carbon FRP strips. An anchorage system was also put into place to improve the adhesion of the strips used; the historic reinforced masonry (HRM) walls were tested by cyclic loading until failure. The experimental results are illustrated and discussed taking into account the delamination failure of the CFRP strips. Finally, the increase obtained in walls’ shear capacity is analysed considering theoretical bilinear diagrams and the coefficient of ductility.     

Experimental analysis of historic masonry walls reinforced by CFRP under in-plane cyclic loading

External bonding of Fiber Reinforced Polymers (FRPs) has become a popular technique for strengthening historic masonry wall buildings in seismic area. Although FRPs seem to improve response of historic masonry walls under in-plane shear loading, further investigations must specify a number of aspects for both researchers and practitioners. This paper presents an experimental analysis pertaining to the response of unreinforced and reinforced historic masonry walls built using full clay bricks in scale 1/3rd. On the basis of previous experimental research carried out by shear tests on triplets and unreinforced walls, a shear criterion for historic unreinforced masonry (HURM) has been assumed. In this experimental analysis two HURM walls, characterized by double T shape, were subjected to in-plane cyclic loading to shear cracking. The damaged walls were reinforced using horizontal–vertical and diagonal Carbon FRP strips. An anchorage system was also put into place to improve the adhesion of the strips used; the historic reinforced masonry (HRM) walls were tested by cyclic loading until failure. The experimental results are illustrated and discussed taking into account the delamination failure of the CFRP strips. Finally, the increase obtained in walls’ shear capacity is analysed considering theoretical bilinear diagrams and the coefficient of ductility.     

藏式石砌体在剪-压复合作用下抗剪性能研究

为研究藏式石砌体抗剪性能,对4组共13个不同压力作用下的泥浆砌筑石墙试件进行双剪试验,分析藏式石砌体沿通缝抗剪强度和破坏机理,研究利用灰缝抗剪强度评估墙体整体抗剪性能的方法。试验结果表明,藏式石砌体灰缝受剪破坏为泥浆层自身变形与界面分离同步发展的延性破坏过程,最终达到完全的界面剪切滑移状态。通过对试验数据的统计回归提出藏式石砌体灰缝的抗剪强度计算公式,计算结果和试验值吻合程度较好。泥浆与水硬性砂浆的主要性能差别在于粘结强度和摩擦系数均较低。通过2种理论模型,分析了砌体灰缝抗剪强度与剪-压复合作用下抗剪强度表达式之间的关系,利用其他研究者的试验数据进行了验证。提出了基于双剪试验结果的藏式石砌体抗剪强度预测表达式,可供藏式石砌体抗剪性能研究参考。     

Experimental FRP/SRP–historic masonry delamination

The preservation of the architectural heritage presents one of the important challenges in civil engineering due to the complexity of the geometry of the structures, the variability of the materials used and the loading history of the buildings. This objective increases for existing constructions in the seismic area. External bonding of fiber or, more recently, steel reinforced polymer composites has become a popular technique for strengthening historic monumental masonry buildings. The performance of the interface between composites and masonry is one of the key factors affecting the behaviour of strengthened structures: shear walls, arches and vaults. This paper aims to present the results of an experimental study to evaluate the bond between fiber reinforced polymer (FRP) – glass and carbonFRP – and steel reinforced polymer (SRP) with historic masonry: pull–push shear tests on FRP/SRP-to-historic brick bonded joints specimens were carried out. Modes of failure are discussed in detail and analytical results are compared with experimental data. Experimental strains recorded on FRP/SRP strips were processed to evaluate shear–slip laws of tested specimens; energy fracture and failure load values are compared with theoretical values by simplified models for shear stress–slip. Finally, a simple model of FRP/SRP design suitable for practical application to historic masonry is proposed.     

On the in-plane shear response of the high bond strength concrete masonry walls

Conventional unreinforced masonry walls subject to in-plane shear loading fail due to exceedance of shear and tensile bond strengths. This paper examines whether or not the in-plane shear capacity of masonry walls would increase with the increase in the bond strengths through experimental and numerical investigations. For these investigations, shear walls were built with high bond strength polymer cement mortar; they were applied in thin layers of 2 mm thickness each. Material tests were carried out to characterise the bond and the compressive strengths of the high bond strength thin layer mortared masonry; the bond strengths were found approximately double that of the conventional 10 mm thick cement mortars. The shear walls, however, exhibited significantly lower capacity (contrasting the expectation) and displayed base course sliding mode of failure. To ascertain the validity of the experimental results, a combined surface contact—interface element micro finite element (FE) modelling technique was formulated; the results adequately reproduced the experimental datasets. The validated FE model was then applied to examine the effect of the aspect ratios and pre-compression levels to the failure modes, deformation and strength of the high bond strength shear walls and is shown that once the pre-compression exceeds 15% of the masonry compressive strength, the base sliding failure mode changes to the diagonal cracking mode with corresponding increase in in-plane shear capacity. Therefore, it is concluded that the increase the bond strength without regard to pre-compression could adversely affect the safety of the high bond strength unreinforced masonry shear walls.     

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.     

基于Cohesive单元的石拱桥主拱圈二相数值模拟方法

为解决石砌体材料非均质的描述问题,提出一种基于Cohesive单元的石拱桥主拱圈两相数值模拟方法。视石砌体为两相材料(砌块和砌缝),采用实体单元模拟砌块并引入非线性本构描述其破坏行为,在相邻砌块间插入Cohesive单元考虑砌缝砂浆的剪切和拉伸破坏。通过室内试验与数值模拟对比验证方法的有效性及适用性,分析了砌缝抗剪摩擦系数μ、加载位置等敏感参数对拱桥承载力的影响。结果表明：基于Cohesive单元的石砌体两相数值模型,可以有效描述石砌体材料的非均匀性及石拱桥的破坏过程(尤其是砌缝剪切滑移破坏行为),可为石拱桥极限承载力评估提供重要信息,如荷载-位移曲线、破坏模式等。此外,研究结果还发现主拱圈破坏机制由拱的受弯、受剪特性决定,并与砌缝抗剪摩擦系数μ强相关。     

机器切割条石砌筑石墙灰缝抗剪性能试验研究

随着石材开采和切割技术的发展,建筑砌筑条石正从传统手工粗细料石逐步向机器切割条石过渡。机器切割条石砌筑石墙灰缝界面性能与传统石墙灰缝存在较大差异,开展其灰缝抗剪性能研究是机器切割条石砌筑石墙推广应用的关键。该文通过15 片新型机器切割条石无垫片砌筑石墙灰缝双剪试验,研究这种新型石墙灰缝的抗剪性能。主要的研究参数包括砌筑灰浆强度、压应力水平和界面处理方式。根据试验结果,分析各因素对灰缝的受力特点、破坏模式、界面抗剪强度和变形性能的影响,同时基于正交试验分析各因素对灰缝抗剪性能的影响程度。研究结果表明:新型机器切割条石砌筑石墙灰缝的破坏呈现脆性破坏,灰缝开裂即达最大承载力,同时灰缝迅速滑移;各研究参数对灰缝抗剪性能的影响程度依次为界面处理方式、压应力水平和灰浆强度。通过对试验数据的统计回归,提出机器切割条石砌筑石墙灰缝的抗剪强度计算公式,计算结果和试验值吻合程度较好,可以用于工程设计和实践。     

Textile-reinforced mortar (TRM) versus FRP as strengthening material of URM walls: in-plane cyclic loading

In this study the application of a new structural material, namely textile-reinforced mortar (TRM), as a means of increasing the load carrying capacity and deformability of unreinforced masonry walls subjected to cyclic in-plane loading is experimentally investigated. The application of externally bonded TRM is considered in this work as an alternative method to the application of fiber-reinforced polymers (FRP). Hence, the effectiveness of TRM overlays is evaluated in comparison to the one provided by FRPs. Medium-scale tests were carried out on 22 masonry walls subjected to in-plane cyclic loading. Three types of specimens were used: (a) shear walls; (b) beam-columns; and (c) beams. The parameters under investigation included the matrix material (mortar versus resin), the number of textile layers and the compressive stress level applied to shear walls and beam-columns. Compared with their resin-impregnated counterparts, mortar-impregnated textiles may result in generally lower effectiveness in terms of strength, but in much higher in terms of deformability. From the results obtained in this study it is believed that TRMs hold strong promise as a solution for the structural upgrading of masonry structures under in-plane loading.     

Numerical and experimental study of GFRP-masonry interface behavior: Bond evolution and role of the mortar layers

Bond behavior between masonry and FRP was investigated in the present paper; in particular, its numerical modeling was carried out considering both single bricks and masonry prisms (with presence of mortar joints between the bricks) as possible substrate. The numerical approach was presented and results compared with those obtained from a recent extensive experimental campaign carried out on GFRP sheets bonded to bricks and prisms, and by considering four different types of clay bricks. Was presented an investigation of the role of the non-linear behavior of the substrate on the bond behavior when using an interface law, and a deep analysis of the debonding process leading to the observation of two different interface mechanisms, properly captured by introducing two separate interface laws and their effectiveness discussed in terms of force-elongation curves or strain, shear stress distributions along the bonded part. Numerical results showed that the adopted interface model is in good agreement with the experimental results. Moreover, the effect of discontinuity represented by the presence of weak mortar layers between bricks (inside prisms) was discussed and clarified.     

Tragfähigkeitserhöhung bei Mauerwerk durch textilglasgitterverstärkte Mörtelfugen

Increase of the vertical load carrying capacity of masonry due to mortar bed joints with textile glass mesh reinforcement From a structural point of view, one of the most important material parameters in the construction sector is the vertical compressive strength of masonry, which consists of the compressive strength of the bricks as well as of the mortar bed. The interaction between the bricks and the mortar beds is the main reason for compression failures of masonry walls. A close analysis of the deformation behavior of the two components shows that different transverse strains in the contact surface between the bricks and the mortar are the main cause for compression failures. However, the load‐bearing capacity of masonry walls can be increased by using some reinforcement in the mortar beds which counteracts lateral expansion. The impact of textile glass mesh reinforcement on the load‐bearing capacity of masonry was analyzed in a test program on masonry columns with different numbers of textile glass mesh reinforced mortar beds. The results of the analyses show that the load‐bearing capacity of the columns rises with an increased ratio of reinforcement, regardless of the type of bricks used. From the ratio of the height of the reinforcement layers to the thickness of the wall it can be deduced that a higher degree of reinforcement has a positive effect on the load‐bearing capacity of the masonry. On this basis, an increase of the strength and load‐bearing capacity of masonry walls is formulated to be on the safe side.     

Durability of steel FRCM-masonry joints: effect of water and salt crystallization

Fiber-reinforced cementitious matrix (FRCM) composites have been recently employed as a strengthening system to retrofit historic masonry structures, since they are more compatible with masonry, less subject to ageing, and compliant with conservation criteria for cultural heritage than fiber-reinforced polymer composites. However, the durability of FRCMs, such as the resistance to salt attack, has not been deeply investigated yet. In this paper, the mechanical behavior of steel FRCM strips bonded to fired-clay brick masonry blocks is studied experimentally. Single-lap shear tests are conducted in laboratory on control FRCM-masonry joints and on additional joints that underwent artificial weathering cycles in a solution of sodium sulfate decahydrate. Peak loads, slips at failure at the loaded end as well as failure modes are investigated. Pore size distribution of the constituent materials and the amount and distribution of salt within the specimens are reported and discussed in order to evaluate the transport and crystallization mechanisms of sulfate in masonry elements strengthened with externally bonded FRCM strips.     

Local phenomena around a steel dowel embedded in a stone masonry wall

The study of the local behaviour of steel dowels used to transfer horizontal shear loads between concrete slabs and stone masonry walls has been carried out. Monotonic loading tests have been performed on six specimens of stone masonry made in the laboratory concerning three different embedding techniques for dowels. The geometric interferometry technique was adopted to survey the surface displacements, in the load direction, of the loaded stone block and surrounding mortar joints. For applying the shear load to the connection, a special experimental apparatus has been designed so as to allow the arrangement of moiré gratings as well as to capture the sequence of the fringe pattern.     

高延性混凝土加固无筋砖墙抗震性能试验研究与承载力分析

为研究高延性混凝土(HDC)加固无筋砖墙的抗震性能,设计制作了3片HDC面层加固砖墙、1片钢筋网水泥砂浆面层加固砖墙和1片作为对比试件的未加固砖墙,通过拟静力试验,研究了HDC面层加固砖墙的破坏形态、滞回性能及耗能能力。试验结果表明:HDC面层可对墙体形成约束作用,延缓墙体开裂并改变墙体的破坏模式,提高墙体的承载力和延性;与钢筋网水泥砂浆面层加固相比,单面HDC加固的墙体开裂荷载与耗能能力明显提高,承载力下降缓慢。针对试件的破坏形态,考虑未开裂区加固面层对墙体水平承载力的贡献,提出了加固墙体的承载力计算方法,并根据试验结果进行了验证。     

Hazard mitigation and strengthening of unreinforced masonry walls using composites

An experimental investigation was conducted to study the behavior of unreinforced masonry (URM) walls retrofitted with composite laminates. The first testing phase included testing 24 URM assemblages under different stress conditions present in masonry walls. Tests included prisms loaded in compression normal and parallel to bed joints, diagonal tension specimens, and specimens loaded under joint shear. In the second testing phase, five masonry-infilled steel frames were tested with and without retrofit. The composite laminates increased the stiffness and strength and enhanced the post-peak behavior by stabilizing the masonry walls and preventing their out-of-plane spalling. Tests reported in this paper demonstrate the efficiency of composite laminates in improving the deformation capacity of URM, containing the hazardous URM damage, preventing catastrophic failure and maintaining the wall integrity even after significant structural damage.     

FE homogenized limit analysis model for masonry strengthened by near surface bed joint FRP bars

A homogenized limit analysis model for the prediction of collapse loads and failure mechanisms of masonry walls reinforced with near surface bed joint GFRP bars is presented. Reinforced masonry homogenized failure surfaces are obtained by means of a compatible identification procedure, where each brick is supposed interacting with its six neighbors by means of finite thickness mortar joints, filler epoxy resin and FRP rods.In the framework of the kinematic theorem of limit analysis, a simple constrained minimization problem is obtained on the unit cell, suitable to estimate – with a very limited computational effort – reinforced masonry homogenized failure surfaces.A FE strategy is adopted to solve the homogenization problem at a cell level, modeling joints, bricks, filler and FRP rods by means of eight-noded infinitely resistant parallelepiped elements. A possible jump of velocities is assumed at the interfaces between contiguous elements, where plastic dissipation occurs. For mortar and bricks interfaces, a frictional behavior with possible limited tensile and compressive strength is assumed, whereas for epoxy resin and FRP bars some formulas available in the literature are adopted in order to take into account in an approximate but effective way, the delamination of the bar from the epoxy and the failure of the filler at the interface with the joint.In order to validate the model proposed, two meaningful examples are critically analyzed. The first relies on a reinforced masonry beam in four-point bending, whereas the second is a full scale wall constrained at three edges and loaded until failure with a distributed out-of-plane pressure. While the first example is useful to test the model at a cell level, since only horizontal ultimate bending moment is involved in the failure mechanism, the second provides a full assessment of the procedure proposed at a structural level. In both cases, very good agreement is found with literature data, meaning that the model proposed may provide useful information for all practitioners interested in the design of masonry walls reinforced with bed joint FRP bars.     

Mechanical behaviour of ancient masonry

The aim of this research was to build a behaviour law for ancient masonry made during the nineteenth century with bricks and lime mortar bonds. This work should be of interest to researchers involved in the study of ancient masonry structures like arch bridges built in this period. To assess the masonry capacity vaults to support service loads and to determine their collapse loads, engineers need mechanical behaviour laws for their component parts. This experimental research was performed to explore the behaviour of the bricks, of the lime mortar, and of a wall until their failure in compression. In parallel the bricks / mortar interface criterion failure under shear and tensile load is characterised. After laboratory tests, numerical simulations were carried out using a finite element method (FEM) to define an homogenised behaviour law for a macro element including bricks and lime mortar bonds. In this goal, a behaviour law was firstly found for each component and then for the masonry as a whole by a FEM homogenisation process, including the non-linear behaviour domain up to the compression failure. The tension failure being reported into an interface element for which the failure criterion was adjusted on specific tests.     

An experimental analysis about the effects of mortar joints on the efficiency of anchored CFRP-to-masonry reinforcements

Fiber reinforced composite materials are widely used for structural rehabilitation and retrofitting of existing buildings; recent studies, devoted to Carbon Fiber Reinforced Polymers (CFRP) reinforcements of concrete structural elements [1–5], demonstrated that spike anchors are able to effectively increase the load carrying capacity and the ductility of CFRP bonded joints. However, application to masonry structures is disregarded by research since few experimental results are available. One of these, described in Refs. [6,7], compares the efficiency of a CFRP strengthening system provided with one or more CFRP spike anchors, also in dependence of some geometrical parameters; reinforcement sheets and spike anchors were applied only on the brick surface in order to evaluate the effects due to anchors only. In this paper the authors investigate the influence of mortar joints on the efficiency of anchored CFRP reinforcements on brick masonry. For this reason, an experimental campaign was planned on masonry pillars built with the same materials employed in Refs. [6,7], subjected to Near End Supported Single Shear Tests. Masonry pillars were built according to two different patterns, in order to detect the influence of both bed and perpend joints. The results are compared with results obtained from previous experimental campaign.     

Heterogeneous upper-bound finite element limit analysis of masonry walls out-of-plane loaded

A heterogeneous approach for FE upper bound limit analyses of out-of-plane loaded masonry panels is presented. Under the assumption of associated plasticity for the constituent materials, mortar joints are reduced to interfaces with a Mohr–Coulomb failure criterion with tension cut-off and cap in compression, whereas for bricks both limited and unlimited strength are taken into account. At each interface, plastic dissipation can occur as a combination of out-of-plane shear, bending and torsion. In order to test the reliability of the model proposed, several examples of dry-joint panels out-of-plane loaded tested at the University of Calabria (Italy) are discussed. Numerical results are compared with experimental data for three different series of walls at different values of the in-plane compressive vertical loads applied. The comparisons show that reliable predictions of both collapse loads and failure mechanisms can be obtained by means of the numerical procedure employed.     

Micro VS. Macro Reinforcement of Brickwork Masonry

In this paper the effect of different modifications of mortar on the behaviour of brick masonry has been experimentally studied. Two modifications of ordinary cement lime rich mortar were made by introducing polypropylene fibres (micro reinforcement) and by reinforcing mortar bed joints by means of non-metallic meshes (macro reinforcement). Experimental tests were carried on the constituents of the masonry, as well as on masonry specimens consisting of wallettes and panels. Following different methodologies of testing masonry by means of compressive, diagonal and shear tests, the results of the investigation revealed that by reinforcing masonry material both compressive and tensile (shear) strength could be improved. Also from the results of the different test methods, it has been concluded that diagonal tests are not suitable for investigations of reinforced masonry.