Numerical derivation of homogenized dynamic masonry material properties with strain rate effects

Masonry is a composite material composed of bricks and mortar disposed in a regular arrangement. It is commonly used as load bearing or partition walls in building structures. Owing to limitations of computer power, detailed distinctive modelling of brick and mortar of a realistic masonry structure or a structure with masonry infilled walls is usually not possible. Moreover, no dynamic masonry material model can be found in the open literature. Dynamic masonry material properties are important for an accurate prediction of masonry failure and fragmentation under dynamic loads. In this paper, a continuum damage model with strain rate effect is developed for masonry materials based on the homogenization method. The equivalent elastic properties, strength envelope and dynamic increase factors (DIFs) of strength and moduli for the homogenized masonry material are numerically derived from the simulated responses of a representative volume element (RVE). A numerical model of an RVE is analyzed with detailed distinctive modelling of brick and mortar with their respective dynamic material properties obtained from laboratory tests. The homogenized material model can be used to analyse large-scale masonry structures subjected to dynamic loading.     

The effects of pre and post construction moisture condition on the in-plane and out-of-plane strengths of brick walls

An important factor affecting the strength of a masonry wall is the capacity of the bond between the mortar and the bricks to transfer the loads within the wall. The main parameter influencing the bond strength is the moisture content of the brick units and the consistency of mortar paste at the time of construction. Another important, but little known, factor is the post-construction moisture curing of the wall. In this paper, the results of a series of standard tests, aimed at determining the effects of moisture condition on the flexural strength, direct tensile bond strength, shear bond strength, compressive strength and diagonal shear strength of brick masonry are presented. The results indicate a substantial increase in the bond strength when saturated, surface dry, brick units are used and show the favorable effects of post-construction water curing of brickwork. Also, in-plane shear and out-of-plane bending tests are carried out on a number of brick wallets and infills, constructed with different pre and post construction moisture conditions and results are compared to drive at quantitative conclusions on the effects of pre-wetting the brick units at the time of construction and post-construction moisture curing on the behavior and strength properties of brick wall construction.     

Zerstörungsarme Bestimmung der Steindruckfestigkeit von Bestandsmauerwerk aus Hochlochziegeln

Non‐destructive determination of the compressive strength in existing masonry made of vertically perforated bricks Urban consolidation and the conservation of listed buildings often require measures to determine the structural stability of the existing masonry. The key parameter for the static proof is the compressive strength of the masonry, which consists of the compressive strength of the bricks and the compressive strength of the mortar bed. So far, no testing methods have been developed that do not significantly interfere with the static load bearing capacity of masonry made of vertically perforated bricks and which make it possible to determine the compressive strength by analysing parts of the bricks. This article presents a non‐destructive test method to determine the compressive strength of vertically perforated bricks of existing masonry. This test method only uses small test specimens taken from parts of the bricks. As a result, the static load bearing capacity of the existing masonry is hardly affected. The results of these tests show that it is possible to establish a plausible correlation between the comprehensive strength of the brick and the compressive strength of the small test specimens. On this basis, a concept for a non‐destructive testing method which makes the determination of the compressive strength of vertically perforated bricks in existing buildings possible is presented.     

Model validation and parametric study on the blast response of unreinforced brick masonry walls

Numerical simulations are carried out to estimate the response and damage of unreinforced brick masonry walls subjected to explosive blast loading based on the transient dynamic finite element program LS-DYNA. A previously developed dynamic plastic damage model was used for brick and mortar. A new model for strain rate effects of bricks and mortar is included in the numerical analysis. The results obtained from the numerical models are compared with field test data and good agreement can be found. Parametric studies are conducted to evaluate the effect of material strength, boundary conditions, and thickness of the wall on the blast response of unreinforced brick masonry walls. It was found that boundary conditions and wall thickness significantly affect the blast response, while the effect of material strength is relatively small.     

A micromechanical model for linear homogenization of brick masonry

A micromechanical model, originally developed for long-fiber composites, is applied to determination of the overall linear-elastic mechanical properties of simple-texture brick masonry. The model relies upon exact solution after Eshelby and describes brickwork as a mortar matrix with insertions of elliptic cylinder-shaped bricks. The macroscopic elastic constants are derived from the mechanical properties of the constituent materials and the phase volume ratios. The ability of the suggested model to predict the behavior of real brickwork has been checked by performing uniaxial compression tests on brick masonry panels of two types, with cement mortar and lime mortar. The results obtained through the proposed model fit experimental data more closely than other models selected from the literature for the sake of comparison.     

Compressive strength and elasticity of pure lime mortar masonry

The procedure and findings of an experimental campaign for the mechanical characterization of brick masonry with lime mortar joints are presented. The campaign includes the determination of the properties of the constituent materials and of the resulting masonry composite. The masonry consisted of masonry stack bond prisms made of solid clay bricks and two types of pure lime/sand mortars, material combinations which correspond to the vast majority of historical and existing masonry structures. The paper includes a discussion on the ratio between the elastic modulus and the compressive strength of the masonry constituents and the comparison of these ratios with the ones suggested in design codes. The implications of this comparison are discussed in the context of interventions on historical masonry structures using modern and traditional materials.     

A numerical insight into the response of masonry reinforced by FRP strips. The case of perfect adhesion

In this study a heterogeneous model at the meso-scale is presented, suitable to interpret delamination phenomena of FRP strips glued to masonry. Both mortar and bricks are modeled independently in tension and compression, through different isotropic damage variables and activation criteria. Reference is made to a masonry pillar, constituted by three Italian standard bricks interspersed by two mortar joints, reinforced by a perfectly-adherent FRP external sheet. The overall response and the collapse mechanism are investigated, induced by different boundary conditions and mechanical properties of constituent materials. Numerical predictions are compared to those provided by the Italian design code, with the aim of assessing pros and cons of simplified formulae. As a valid alternative for practitioners, the recourse to a numerical database is proposed, generated by combining FE predictions and best fitting, suitable to specify closed-form interface laws with brick and mortar compressive strengths as entries.     

砌体结构的抗震研究现状

为研究打包带加固对砖砌体抗弯性能的增强效果,对216个砖砌体试件开展了弯曲抗拉试验,并结合数值模拟,探究了不同强度的砌筑砂浆和抹面砂浆,以及不同网格尺寸的打包带网对砖砌体抗弯能力的影响。结果表明：砌筑砂浆强度对砖砌体的抗弯峰值荷载影响最大,与M1强度相比,当砌筑砂浆强度增为M2.5、M5、M7.5和M10时,沿通缝截面砖砌体的抗弯峰值荷载增幅依次为15.87%、31.11%、37.60%和44.45%,沿齿缝截面砖砌体的增幅依次为28.95%、46.16%、55.19%和56.76%;抹面砂浆的强度对抗弯峰值荷载影响较小,其主要作用在于增强打包带网与砖砌体表面的粘结,高强度的抹面砂浆与打包带共同作用能够有效提高砖砌体的抗变形能力;打包带网格尺寸主要影响的是砖砌体破坏之后的荷载回升幅度,与12.5 cm网格尺寸相比,当网格尺寸减小为7.5 cm和5 cm时,沿通缝截面砖砌体的荷载回升峰值增幅可达63.42%和130.38%,沿齿缝截面砖砌体的增幅可达73.29%和127.66%。打包带加固可为砌体结构,特别是村镇砌体结构的抗震加固工程提供参考。     

Derivation of 3D masonry properties using numerical homogenization technique

Lots of research work has been conducted on homogenization technique, which derives global homogenized properties of masonry from the behaviour of the constitutive materials (brick and mortar). Such a technique mainly focused on two‐dimensional media in the previous studies with the out‐of‐plane properties of masonry material neglected. In this paper, homogenization technique and damage mechanics theory are used to model a three‐dimensional masonry basic cell to numerically derive the equivalent elastic properties, strength envelope, and failure characteristics of masonry material. The basic cell is modelled with distinctive consideration of non‐linear material properties of mortar and brick. Various displacement boundaries are applied on the basic cell surfaces in the numerical simulation. The detailed material properties of mortar and brick are modelled in a finite element program in the numerical analysis. The stress–strain relations of masonry material under various conditions are obtained from the simulation. The homogenized elastic properties and failure characteristics of masonry material are derived from the simulation results. The homogenized 3D model is then utilized to analyse the response of a masonry panel to airblast loads. The same panel is also analysed with distinctive material modelling. The efficiency and accuracy of the homogenized model are demonstrated. The homogenized material properties and failure model can be used to model large‐scale masonry structure response. Copyright © 2006 John Wiley & Sons, Ltd.     

我国农村典型砖砌体墙片拟静力试验研究

农村房屋抗震是我国防震减灾工作的重要内容。砖砌体结构作为农村房屋的主要结构形式,如何提高其抗震能力具有重要研究意义。为研究砂浆强度、构造措施和窗洞口对砖砌体抗震性能的影响,该文首先对农村砖砌体房屋常用的几种砂浆和砖砌体进行了抗压强度试验,得到了相应材料及构件的强度参数;然后对不同砂浆强度、不同构造措施和开洞的12个墙片进行了拟静力试验,对比分析了不同因素对墙体力学性能影响。试验结果表明,砂浆强度影响最大,构造措施和开洞影响次之。最后依据试验分析结果为我国农居防震减灾工作提出建议。该文相关研究可为农居的抗震设防、标准制定以及施工提供参考。     

砌体结构双轴拉压应力强度试验研究

为弥补砌体结构双轴拉压应力强度理论研究的不足,以多孔砖砌体为研究对象,基于材料主应力与平面应力的对应关系,考虑不同灰缝角度及拉压主应力比的影响,重点开展了砌体双轴拉压应力宏观单元试件的强度试验。从中揭示了砌体双轴拉压应力状态下的破坏特征与规律,分析得出双轴拉压应力强度破坏曲线及对应平面应力状态的强度破坏准则函数表达式。试验结果验证了砌体结构材料各向异性的力学特性,试验方法为砌体结构抗拉强度的测试提供借鉴,试验数据为丰富砌体结构强度理论提供参考,研究成果为建立完善的砌体结构破坏准则提供依据。     

Composite modelling of masonry deformation

Based on the measured properties of individual bricks, blocks and mortar specimens, composite model expressions are presented for elasticity, creep and moisture movement of masonry in two directions. The model demonstrates the influence of creep in horizontal moisture movement, and allows for anisotropy of brick or block. Simulation of the role of moisture diffusion in masonry is a requirement for statisfactory predictions by the model, this being achieved by testing specimens having the same volume/exposed surface area ratio as the masonry component phases. Experimental verification is shown for clay brickwork walls and piers, and for calcium silicate walls.     

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.     

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.     

Flexural bond strength of masonry using various blocks and mortars

This paper deals with an experimental study on flexural bond strength of masonry using various blocks in combination with different mortars. Flexural bond strength of masonry has been determined by testing stack-bonded prisms using a modified bond wrench test set-up. The effect of mortar composition and strength on the masonry's flexural bond strength using three types of masonry units (stabilized mud blocks, stabilized soil-sand blocks and burnt brick) has been examined. The effect of the masonry unit's moisture content on flexural bond strength has also been studied. Increases in mortar strength lead to increased flexural bond strength for cement mortar, irrespective of the type of masonry unit. It has been found that combination mortars, such as soil-cement mortar and cement-lime mortar, lead to better bond strength compared to cement mortars. The moisture content of the masonry unit at the time of casting has displayed significant influence on the flexural bond strength of the masonry. It has been found that for each type of masonry unit, an optimum moisture content exists, beyond which the flexural bond strength falls off quickly.     

Mechanical response of solid clay brickwork under eccentric loading. Part I: Unreinforced masonry

The compressive strength of eccentrically loaded masonry, affecting the strength of arches, vaults, pillars and out-of-plane loaded masonry panels, is addressed in this paper both from the experimental and numerical point of view. The aim is that of relating the eccentric compressive strength to the concentric value, to the mechanical characteristics of the constituents,i.e. mortar and bricks, and to the brickwork bond. In the paper, displacement controlled compression tests on solid clay brick and cement-lime mortar masonry prisms, under concentric and moderate-to-highly eccentric loading, are presented and discussed. The experimental outcomes and the results of FEM models give a preliminary insight in the mechanical response of masonry up to collapse. It is found that edge effects may affect the load carrying capacity of the brickwork, while detailed measurements on the mortar joints show that the plane section assumption, typical of many design procedures, is reasonably verified up to the limit load, giving was to simplified but reliable design procedures.     

Mechanical behavior of masonry assemblages manufactured with recycled-aggregate mortars

Mortars containing recycled aggregate, instead of quartz sand, were characterized to find an alternative application for the fine recycled-aggregate fraction coming from building debris processing. Tests on bond strength of mortar to masonry units were carried out, as well as tests on compressive and shear strengths of masonry assemblages. The results obtained were related to the mechanical properties of mortars and brick. On the basis of the characterization results and performance evaluations, recycled-aggregate mortar appears to be superior to ordinary mortars in terms of mortar–brick bond strength and shear strength of masonry assemblages. This improved performance is of particular interest for the masonry structures in zones of seismic activity. In addition, the use of fine recycled aggregate is in accordance with the sustainable development concept, where recycling of building rubble plays a key role in ending the building life cycle.     

Nonlinear homogenization techniques to solve masonry structures problems

The behaviour of masonry material subjected to different in-plane loading combination is studied in this work. The masonry is considered as a periodic composite material composed by a regular distribution of brick and mortar and it is analyzed using a homogenization technique. The mechanical properties of the masonry, as an orthotropic homogeneous material, depend on the geometrical and mechanical properties of the components based on the study of the equilibrium and compatibility of a basic cell. The masonry is a frictional material and its behaviour depends on the loading direction, for these reasons, a unilateral damage model is chosen for the analysis. This model describes the behaviour of brittle materials subjected to tension-compression cyclic loads based on the introduction of two damage variables and it assumes that the damage is due to the beginning and growth of cracks only in the mortar joints. It is considered that the bricks have a linear elastic constitutive relationship. Numerical applications are performed with a nonlinear finite element code in order to test the proposed procedure by comparing the results with those available in the literature and also with experimental data.     

Experimental research of masonry compressive strength in the Auschwitz II - Birkenau former death camp buildings

The paper presents the results of strength tests of brick masonry in 14 prisoner barracks located at the Auschwitz II-Birkenau former death camp. These buildings were constructed in a few months in 1941, that is in the initial phase of the extermination camp. Strength tests were conducted as part of a broad program whose purpose was to document the condition of the camp buildings being invaluable historical material objects. Due to the nature of the historical buildings a broad program has been consulted with the services of conservation. Strength tests of masonry and masonry materials (bricks and mortars) were performed on small-diameter core samples cut from the original structures of buildings in the camp. In addition, comparative tests were carried out on samples made in the laboratory. Results and their analysis made it possible to estimate the compressive strength of masonry, bricks and mortars in the brick buildings erected more than 70 years ago. The performed documentation of brick walls and identification of strength parameters are an important stage of the planned preservation process, the ultimate goal is to preserve the historic buildings for future generations.     

Thaumasite swelling in historic mortars: field observations and laboratory research

The formation of thaumasite in historic mortars was found to be a recurrent problem in cases of conservation of historic masonry in the Netherlands. Several case studies in which mortar swelling occurred were performed. In this paper two case studies concerning thaumasite formation are briefly described, focusing on the damage patterns observed and the analyses performed in order to arrive at the diagnosis of thaumasite formation. Sulfate may come from inside (the historic brick) and from outside (combustion processes from industry or traffic). The second part of the paper deals with laboratory research aimed at provoking the formation of thaumasite in hydraulic lime mortars. A further aim of the laboratory research was to develop an adequate test procedure in order to evaluate the vulnerability of mortars to be used in conservation. A test procedure on mortar, cured between bricks in order to have realistic mortar properties, appears adequate.