Interaction, instability, and simultaneity of debonding processes in flexure of masonry walls strengthened on two sides with FRP

This paper studies the interactive, simultaneous, and unstable multiple debonding mechanisms in masonry walls strengthened to resist out-of-plane flexure using composite materials bonded on their tensed and compressed faces. This structural form combines rigid blocks and brittle joints sandwiched between adhesively bonded composite layers. It characterizes masonry walls strengthened to resist dynamic and seismic out-of-plane loads, but also armor plates with ceramic tiles, electronic packaging, and sandwich panels with embedded inserts. The study focuses on the stability of the interactive debonding mechanisms and on the potential evolution of coupled and simultaneous debonding processes. It uses a cohesive interface approach and a geometrically nonlinear high order model. A numerical study that examines the coupled debonding processes in masonry wall samples strengthened with composite materials on the two outer faces is presented. The study examines the instabilities involved with the processes, the order in which the tension driven debonding and the buckling driven one nucleate, and the interaction between them. It also examines the impact of the properties of the cohesive interface, the presence of pre-existing debonded regions, the arching effect, and the accumulation of damage at the interfaces. This effort aims to gain insight into the debonding processes and their impact on the performance, functionality, and safety of the strengthened element.     

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.     

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。     

Effects of mortar layers in the delamination of GFRP bonded to historic masonry

Externally bonded fiber reinforced polymers (FRPs) used as strengthening for shear historic masonry walls increase tensile capacity so as to support combined compression and high shear forces released during earthquakes. The local and global capacity of FRP strengthened shear walls depends on delamination. This paper deals with the anchorage strength of GFRP strips bonded to historic masonry by analysing results obtained in pull–push shear tests carried out on GFRP-to-historic-brick bonded joints. The experimental research also foresaw the analysis of effects deriving from the presence of mortar layers in actual historic brickwork masonry, simulated through the creation of grooves on the surface of mortar filled clay bricks. The experimental results indicated brittle failure of joints due to delamination; results were processed to evaluate failure load values, strain vs. anchorage length diagrams and shear stress vs. slip relationships experimentally. Finally, the anchorage of GFRP-to-historic-brick was theoretically studied to improve the classic solution by incorporating adherent shear deformation. Discussion on the experimental and theoretical data was developed.     

钢纤维水泥砂浆加固砌体墙的平面外受力性能

用钢纤维混凝土加固修复工程结构是一种提高其极限承载力和延性的有效方法。该文研究了钢纤维水泥砂浆加固砌体墙的平面外受力性能,试验了4个1000mm×1000mm×115mm的砌体墙,采用30mm厚、纤维体积含量分别为1.0%、1.5%、2.0%和2.5%的钢纤维水泥砂浆加固。试验结果表明:用体积含量为1.5%的钢纤维水泥砂浆加固效果最好;同时该文数值模拟了钢纤维水泥沙浆加固砌体墙的平面外极限承载力,数值模拟结果与试验结果吻合较好。     

组合材料加固砖砌体的有限元模拟方法

基于ABAQUS有限元软件,提出了组合材料加固砌体数值建模方法。该方法是在未加固砌体整体式模型的基础上,结合分离式思想建立组合材料加固砌体模型。通过对8片采用粘钢-聚合物砂浆组合材料加固的砖砌体墙体（其中,4片采用粘贴正交钢片,4片采用粘贴斜撑钢片）的拟静力试验结果进行了数值模拟对比分析,结果显示:模拟所得墙体滞回、骨架及刚度退化曲线与试验曲线基本吻合;仿真破坏形态与试验现象一致;计算所得荷载、位移、延性和耗能等全部指标中有81%的误差在20%以内。     

Bond behaviour between textile reinforcement and mortar under tensile loading / Verbundverhalten zwischen textiler Bewehrung und Mörtel unter Zugbeanspruchung

Extensive experimental investigations are currently being carried out on various selected materials covering a wide range of properties to achieve a deeper knowledge about the bond performance of textile reinforced mortar (TRM) for masonry strengthening. The objective of the tests includes investigations of the bonding behaviour between alkali‐resistant glass textile reinforcement and mortar under tensile loading to determine the required anchorage and overlapping lengths of the reinforcement in the mortar‐based material. This article describes the test methods used as well as the results obtained so far. This research will also examine debonding of the mortar‐based reinforcement system and the masonry surface under shear load. The definition of these bond parameters is necessary for the design of textile‐reinforced masonry components, which will be developed in the near future. The research is also intended to contribute to the finding or even designing of matching alkali‐resistant glass textiles specifically for use in masonry.     

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.     

Experimental and analytical investigation on bond between Carbon-FRCM materials and masonry

Historical masonry constructions often need to be strengthened and upgraded to satisfy current seismic code requirements. Recently many interventions have been done bonding composite materials to the surface of existing masonry elements. The effectiveness of these interventions strongly depends on the bond between the strengthening material and the masonry and on the mechanical properties of the masonry substrate. In this paper the bond between fiber reinforced cementitious matrix (FRCM) materials made out of a Carbon net embedded in a cement based matrix and the masonry is experimentally and analytically investigated. Experimental results of double shear tests involving different bond lengths are presented. The results evidence that the debonding occurs at the fibers/matrix interface after a considerable fibers/matrix slip. They also confirms the effectiveness of the Carbon-FRCM materials as external reinforcements for masonry structures. The obtained experimental results are used to calibrate a local bond-slip relation that is essential in the modeling of the structural behavior of masonry elements strengthened with Carbon-FRCM.     

高延性混凝土面层加固受弯无筋砌体墙抗震性能试验研究

为研究高延性混凝土（HDC）加固无筋受弯砌体墙的抗震性能,设计了1片未加固砖墙、1片钢筋网水泥砂浆面层加固砖墙和3片HDC面层加固砖墙进行拟静力试验,研究其破坏机理、滞回特性和耗能能力。结果表明:1）HDC面层和砖墙具有良好的粘结性能,可有效抑制墙体的开裂和破坏,能改善墙体的脆性破坏特征;2）HDC面层对砖墙形成良好的约束作用,提高了墙体的抗震承载力;3）HDC面层可显著提高砖墙的变形和耐损伤能力,减小或免去震后修复费用,其加固效果优于传统的钢筋网水泥砂浆面层。根据统计分析,该文给出了HDC面层加固墙体基于位移角的易损性曲线。     

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

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 out-of-plane loading is experimentally investigated in this study. The effectiveness of TRM overlays is evaluated in comparison to the one provided by fiber reinforced polymers (FRP) in the form of overlays or near-surface mounted (NSM) reinforcement. TRM systems may be considered as alternative to FRPs, tangling with some of the drawbacks associated with the application of the latter without compromising performance. Medium-scale tests were carried out on 12 masonry walls subjected to out-of-plane bending. The parameters under investigation comprised mortar-based versus resin-based matrix materials, the number of layers, the orientation of the moment vector with respect to the bed joints and the performance of TRM or FRP jackets in comparison to NSM strips. It is concluded that TRM jacketing provides substantial increase in strength and deformability. Compared with their epoxy-resin counterparts (FRP), TRM may result in generally higher effectiveness in terms of strength and deformability. NSM strips offer lower strength but higher deformability, due to controlled debonding. From the results obtained in this study it is believed that TRMs comprise an extremely promising solution for the structural upgrading of masonry structures under out-of-plane loading.     

ECC面层加固砖墙抗震性能试验研究

高延性纤维增强水泥基复合材料(ECC)是一种高强度、高韧性的生态建筑材料,在土木工程领域具有广泛的应用前景。为充分发挥ECC的韧性和良好的抗剪性能,该文提出一种采用ECC面层加固砖墙的方法,对2片未加固砖墙和2片ECC面层加固的砖墙进行了拟静力试验。试验结果表明:ECC面层与砖墙具有良好的粘结性能,采用ECC面层加固砖墙能有效限制墙体的开裂和破坏,提高砖砌体墙的延性和耗能能力,改善砖墙的剪切脆性破坏模式,提高墙体的损伤容限,减小砌体结构的震后加固费用。     

The bending strength of masonry / Biegezugfestigkeit von Mauerwerk

Bending strength of masonry becomes an important design aspect especially when the walls subjected to lateral loads like, cellar walls which are subject to earth pressure, and façades/ infill walls which are exposed to the wind actions. Bending strength is required wherever the applied load is perpendicular to the wall. It is also required in non‐load‐bearing partition walls, where the load applied in both the normal and perpendicular directions. Besides, the tensile properties of the brick/block and lengthwise parameters related to geometry and materials technology also influence the bending strength of masonry. These include the thickness of the wall, the extent of overlap, and both the shear and the tensile strength of the bond. Consideration must also be given to the possible presence of mortar in the head joints, which can significantly increase bending strength, especially where joints fail. In addition to these materials technology factors, it is very important to observe the realistic influence of the boundary conditions. The degree of fixity is among the factors to be considered in calculation models. In this contribution, two aspects are going to be observed and analysed, namely: the principles of bearing capacity under lateral loading alongside the models derived from these for cellar walls and areas of infill; and the influence of materials technology/geometric parameters on the bending strength of masonry.     

The effect of damage and creep interaction on the behaviour of masonry columns including interface debonding and cracking

Creep can produce significant effects on the structural behaviour of composite quasi-brittle systems, such as masonry, by altering the stress distribution between and within structural elements. The failure of a masonry element can be accelerated through damage incurred by weathering or degradation from creep effects. In this study, a three-dimensional finite element model of a grouted masonry column is used to evaluate the significance of the interaction of creep and damage on the structural behaviour of the column. The effects of Poisson??s ratio in producing differential out-of-plane constraint stresses can be simulated using this model. By utilizing a cracking criterion and incorporating a cohesive zone material (CZM) model for the brickwork-grout interface, the sequence and the patterns of cracking of the masonry column, debonding of the interface and local failure are examined. It is shown that debonding of the brickwork-grout interface occurs prior to cracking of the outer shell of brickwork. Case studies are presented to demonstrate the significance of the interaction of creep and damage on local failure, cracking and debonding. It is shown that cracking and debonding can result in a stability failure of a masonry column that was originally in a stable condition. Further work on local buckling and post-buckling analysis seems crucial to explain composite masonry behaviour.     

Experimental assessment of an innovative strengthening material for brick masonry infills

The vulnerability of masonry infill walls has been highlighted in recent earthquakes in which severe in-plane damage and out-of-plane collapse developed, justifying the investment in the proposal of strengthening solutions aiming to improve the seismic performance of these construction elements. Therefore, this work presents an innovative strengthening solution to be applied in masonry infill walls, in order to avoid brittle failure and thus minimize the material damage and human losses. The textile-reinforced mortar technique (TRM) has been shown to improve the out-of-plane resistance of masonry and to enhance its ductility, and here an innovative reinforcing mesh composed of braided composite rods is proposed. The external part of the rod is composed of braided polyester whose structure is defined so that the bond adherence with mortar is optimized. The mechanical performance of the strengthening technique to improve the out-of-plane behaviour of brick masonry is assessed based on experimental bending tests. Additionally, a comparison of the mechanical behaviour of the proposed meshes with commercial meshes is provided. The idea is that the proposed meshes are efficient in avoiding brittle collapse and premature disintegration of brick masonry during seismic events.     

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.     

Flax and polyparaphenylene benzobisoxazole cementitious composites for the strengthening of masonry elements subjected to eccentric loading

To address the structural problems caused by eccentric loads in unreinforced masonry, three different types of masonry were prepared based on clay bricks bonded with a natural hydraulic lime mortar combined with a flax or polyparaphenylene benzobisoxazole (PBO) fabric-reinforced cementitious matrix (FRCM) composite. The mechanical behaviour when subjected to concentric and eccentric loads was studied by performing axial compression tests, with eccentric load tests only carried out in instances of large eccentricities. Analysis of the load–displacement and moment–curvature response revealed that both the flax- and PBO-based strengthening systems improve the strength and deformability of masonry. However, compared to the PBO fabric composite, the use of flax fabric provides a greater deformability that helps prevent the composite and substrate debonding.     

高延性混凝土加固蒸压加气混凝土砌体墙抗震性能试验研究

蒸压加气混凝土(AAC)砌块砌体墙自重轻,但其抗震性能较差,为提高该类墙体的抗震性能,提出采用高延性混凝土(HDC)面层和条带对其进行加固。设计制作了4个无筋砌体墙和2个构造柱约束墙体试件,其中2个试件采用HDC面层加固,2个试件采用HDC条带加固,通过拟静力试验,研究AAC砌体墙的破坏形态、滞回性能、承载力及变形能力等性能。试验结果表明:HDC面层可改变AAC墙体的破坏模式;对于无筋砌体墙,加固后试件的承载力、变形及耗能能力均得到了不同程度的提高,墙体裂缝数量明显减少,刚度退化较为平缓;对于构造柱约束墙体,单面HDC面层使加固试件的侧向刚度、水平承载力及耗能能力均大幅提高,且加固试件具有较高的残余承载力,墙体的开裂和损伤程度较小。基于试件的破坏形态,提出加固墙体的水平承载力计算方法,其计算结果与试验结果吻合较好,可为HDC加固AAC砌块墙体的承载力计算提供参考。     

Flexural strength characteristics of non-load bearing masonry walls in Kuwait

The controlling factor in designing non-load bearing masonry walls, such as those used in Kuwait, is the lateral resistance to wind loads. To ensure safety of the walls, data is needed on the flexural strength characteristics of walls constructed with locally-available materials. The flexural strength of masonry walls constructed with autoclaved aerated-concrete blocks, sand-cement concrete blocks or calcium silicate bricks was evaluated in a test program that involved testing small-scale walls or wallettes. The tests were performed in accordance with the British Standard for unreinforced masonry. The autoclaved aerated-concrete block wallettes were constructed with epoxy glue mortar, whereas the concrete block and calcium silicate brick walletters were constructed with sand-cement mortar. Two stages of testing were undertaken to evaluate bending parallel to bed joints and bending perpendicular to bed joints. The flexural strengths required by British and American codes exceed the strengths of the concrete block and calcium silicate brick walls used in Kuwait, implying that the allowable tensile stress requirements of these codes are not safe for assessing the lateral resistance of the walls. The format used for the autoclaved aerated-concrete block wallters, which is identical to the standardized format for concrete block wallettes in the British standard, is suitable for determining the flexural strength of full-size autoclaved aerated-concrete block walls.     

Out-of-plane flexural behavior of unreinforced red brick walls strengthened with FRP composites

This paper presents the results of a study focused on evaluating the out-of-plane flexural behavior of two fiber reinforced polymer (FRP) composite systems for strengthening unreinforced red brick masonry walls. The full-scale tests followed the International Code Council Evaluation Service (ICC-ES) AC 125 procedure. In the experimental program, a total of four full-scale destructive tests were conducted on UMR red brick walls. One wall specimen was used as control (as-built) specimen without composites, and the remaining three wall specimens were strengthened with either E-glass/epoxy or carbon/epoxy composite systems with different fiber architecture. The effect of applying a cross-ply laminate on the ultimate failure mode has been investigated. Full-scale experimental results confirmed the effectiveness of the FRP composite strengthening systems in upgrading the out-of-plane flexural structural performance of URM walls. In addition, an analytical model was developed to predict the ultimate load capacity of the retrofitted walls. The analytical modeling is based on deformation compatibility and force equilibrium using simple section analysis procedure. A good agreement between the experimental and theoretical results was obtained.