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.     

Robustness as a criterion for use of hollow clay masonry units in seismic zones: an attempt to propose the measure

In order to provide adequate seismic behavior of masonry walls, local brittle failure of masonry units in the most stressed zones of structural walls should be prevented. Although robust behavior is required by the code, no specifications are given regarding the criteria to fulfill this requirement. To propose such criteria, a series of 28 masonry walls, built with six different types of hollow clay masonry units, currently available on the market, have been tested by subjecting them to cyclic lateral load at two levels of constant precompression. Besides, the strength characteristics of the units, like compressive strength orthogonal and parallel to the bed joints and tensile and shear strength of the units have been determined by standardized and specifically designed testing procedures. By correlating the parameters of seismic resistance of the walls and strength characteristics of the units, no specific indicator for robustness could have been determined on the basis of the mechanical characteristics of the tested units. It has been found that in all cases the level of precompression, i.e. the ratio between the compressive stresses in the walls and the compressive strength of masonry, represents the governing parameter.     

Bond behaviour of a textile‐reinforced mortar for AAC masonry

The bond behaviour of a textile reinforced mortar (TRM) applied to autoclaved aerated concrete (AAC) masonry has been evaluated experimentally. The TRM is composed of a glass‐fibre mesh combined with a cementitious mortar and is intended to strengthen AAC masonry walls subjected to out‐of‐plane bending during an earthquake. The main components have been characterized with preliminary tests. Then, pull‐off and shear bond tests have been performed to determine the bonding properties of the TRM applied to the AAC substrate. Three types of AAC blocks have been used, which differ in the bulk density and compressive strength, to evaluate possible variation in the bond strength. The results of the experimental campaign have shown a good performance of the strengthening system. In most cases, the bonding between TRM and masonry was maintained up to tensile failure of the dry textile. As expected, the masonry samples realized using AAC blocks with a higher bulk density showed better performances. The paper presents and discusses main test results, providing background data for future recommendations for the use of the analysed strengthening system in AAC masonry structures.     

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。     

Behavior of unreinforced masonry prisms and beams retrofitted with engineered cementitious composites

The impact of a thin layer of a ductile fiber-reinforced concrete referred to as engineered cementitious composites (ECC) on unreinforced masonry (URM) prisms and beams has been evaluated. The objective of the research was to characterize the performance and potential benefits of using ECC to retrofit URM with eventual application to masonry infill walls in non-ductile reinforced concrete frames. Compression tests of masonry prisms and flexural tests of masonry beams with different ECC retrofit schemes were conducted. The variables studied were the use of wall anchors to improve the ECC-masonry bond and alternate steel reinforcement ratios within the ECC layer in the form of welded wire fabric. The ECC retrofit was found to increase the strength and stiffness of URM prisms by 45 and 53 %, respectively compared to those of a plain specimen. When wall anchors were installed on the masonry specimens, the bond between the ECC layer and the masonry surface was improved. Four-point bending tests indicated that the strength and more importantly the ductility of an ECC retrofitted brick beam are increased significantly, especially when light reinforcement is added to the ECC layer, relative to an URM beam. Analytical models for estimating the strength and stiffness of ECC retrofitted masonry specimens are proposed and evaluated.     

Longitudinal compressive strength of masonry – Simplified calculation approach / Längsdruckfestigkeit von Mauerwerk – Vereinfachter Berechnungsansatz

For the structural analysis of the bending compression zone of masonry building elements, which are loaded under bending compression parallel to the bed joints, the longitudinal compressive strength of the masonry is required. According to the code, this value has until now been determined in laborious tests on masonry building elements or calculated analogously to the approach for the determination of the compressive strength perpendicular to the bed joints. In such a way, the actual failure mechanisms of masonry under compression loading parallel to the bed joints and relevant influential parameters for the longitudinal compressive strength however remain unconsidered. The article presents a proposal for the determination of minimum values of the longitudinal compressive strength of masonry in a simplified form for structural design purposes, which include consideration of both the relevant failure cases and also the significant influential parameters.     

Reinforced bending load‐stressed masonry – Suggestions for future designs / Bewehrtes biegedruckbeanspruchtes Mauerwerk – Vorschläge für zukünftige Bemessungen

European standardization bodies are currently working on the amendment to EN 1996‐1‐1, which will also affect the evaluation of reinforced masonry in Germany. For that reason, discussion suggestions are being made here for revisions to lay the groundwork for building materials evaluations and especially, evaluations of bending load‐stressed masonry walls or beams at their serviceability limit state (SLS) for load‐bearing capacities. Information already presented in E DIN 1053‐3:2008‐03 [N3] is being incorporated as well. Characteristic values for the compressive strength of the masonry parallel to the bed joints f_k,∥ are essential for the design of reinforced masonry, although they are currently not included in national application documents for Germany. For the time being, they can be mathematically calculated using conversion factors for the characteristic compressive strength values vertical to the bed joints f_k or by using the declared axial compressive strengths of the masonry units. The ultimate strains for masonry in general should be set consistently at ?_mu = ∣–0.002∣ as several masonry types do not exhibit higher compressive strain values. The use of steel strains higher than ?_su = 0.005 does not change any measurement results. Varying stress‐strain curves of the constitutive equations on masonry under compressive strain (parabolic, parabolic‐rectangular, tension block) lead to differing values of recordable bending moments despite having the same mechanical reinforcement percentage at higher normal forces. Therefore, clear guidelines should be made for the type of applicable constitutive equation for masonry walls under compressive strain. With the introduction of a tension block, the number values of the reduction factors λ for the compression zone height x, which is dependent on limit strains, and where applicable, reduced compressive strength, need to be determined, as with reinforced concrete construction. A modification of the bending moment based on the second order theory according to [N4] is presented for the calculation of reinforced masonry walls in danger of buckling. The use of reduction factors for the load capacity of the masonry cross section, such as for unreinforced masonry, does not appear to be appropriate as buckling safety evidence because here, the design task is the determination of a required reinforcement cross section.     

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.     

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.     

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.     

Behavior and modeling of strengthened three-leaf stone masonry walls

The application of three different intervention techniques on three-leaf rubble stone masonry walls are discussed here. Injections, repointing, and the placing of ties connecting the two external whytes were considered, both singularly and in combination. Lime-based products were chosen for injection grouts and repair mortars, to ensure better compatibility with the original materials. The experimental tests, performed on seventeen large scale samples under compressive loads, showed that: (i) injections are very effective to improve the mechanical characteristics of the walls; (ii) the other techniques have less influence on the strength but can operate in avoiding ‘brittle’ failure modes (ties placing) and in improving the durability of the masonry (repointing); (iii) the combination of the techniques ensures the enhancement of the global behavior of the walls. The integration of the experimental results with data available in literature allowed the calibration of an analytical model able to predict the compressive strength of injected walls, based on parameters given by simple experimental tests.     

In situ tests and seismic assessment of a stone-masonry building

The injection of grout into multi-leaf stone masonry walls with a sufficient amount of voids can be an effective technique for the seismic strengthening of such walls. In order to evaluate the effectiveness of different types of commercial grouts that fulfilled the adopted criteria, the stone masonry walls of an actual building were strengthened by means of grout injection, using cement and combined cement–lime grouts. The quality and effectiveness of grout injection technique was assessed by non-destructive tests (sonic and radar tests), minor destructive tests (surface and in-depth probing and coring, and the double flat jack test), and destructive test (shear-compressive test), all in situ. Preliminary laboratory tests were also performed on mortar and stone specimens, on the injection grouts, and on cylinders representing the inner core of the strengthened walls. Finally, the seismic resistance of the building was evaluated in non-strengthened and strengthened variants (i.e. after grout injection of the walls with cement or lime–cement grout) by means of non-linear static analysis, using the pushover method. Obtained results show that shear characteristics of the walls (tensile strength and stiffness) depend significantly on the type and properties of the injected grout, i.e. on the grout’s ability to achieve a solid bond between the stones and the leaves including the properties (strength and stiffness) of the grout itself. In the case of the type of masonry under consideration, an adequate level of seismic resistance can be achieved also by using combined cement–lime grouts, although cement grout can provide higher seismic resistance.     

Experimental tests on the shear behaviour of dowels connecting concrete slabs to stone masonry walls

An experimental investigation has been conducted on the behaviour of dowels used to connect concrete slabs to stone masonry walls in order to transfer horizontal shear forces. A technique for embedding the dowels in the stone block without injection of grouting material or resin has been developed. Special experimental equipment has been designed in order to allow the execution ofin situ load tests on representative ancient buildings. Monotonic loading tests have been carried out on eight specimens with the purpose of measuring both stone block displacement and dowel deformation     

基于装配式技术加固的砌体墙片的力学性能研究

基于装配式技术的砌体结构加固方案可以有效提高结构的抗震能力。采用预制钢筋混凝土墙板对原有的砌体墙片进行双面外贴加固,并用销键、灌浆、后浇带等连接方式保证砌体墙片与预制钢筋混凝土墙板的共同工作。该文通过非线性有限元模型分析了未加固砌体及加固砌体结构在循环往复加载下的力学行为,探讨混凝土墙板和砌体砖墙之间的传力途径,验证加固方法和三种连接措施的有效性。分析证明:采用预制钢筋混凝土贴墙墙板的加固方案可以显著提高砌体结构的抗震性能,强度提高3倍~4倍,刚度提高2倍~3倍,满足现行抗震规范的要求。三种连接方式中,后浇带最有效,在弹性阶段传递了70%的荷载,在后期传递了80%的荷载。     

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.     

基于极限分析的三叶墙抗压强度预测模型研究

该文以极限分析法为基础,将有限元塑性极限分析下限法运用于砌体结构三叶墙的抗压强度预测。结合常见受压三叶墙破坏模式,基于材料试验和小型砌体试件试验赋予了本构模型参数。在此基础上,考虑了不均匀压应力边界条件,提出了三叶墙抗压强度预测模型。根据文献的试验数据,利用该模型预测了多个三叶墙的抗压强度,并与试验结果和抗压强度预测公式的结果进行了对比,说明了该模型的准确性。     

Textile reinforcement in the bed joint of basement masonry and infill walls subjected to high wind loads

The successful structural verification of basement walls under earth pressure loading with light vertical loading is often difficult. This situation is often encountered for external basement walls under terrace doors, stairs, masonry light wells, etc., where the vertical loading that is theoretically necessary is absent. This makes it impossible to resist the acting flexural forces from earth using a vertical arch model alone. In such cases the basement wall must also resist the earth pressure in a horizontal direction. However, due to the fact the bending moment capacity of unreinforced masonry parallel to the bed joint is low you have the option here of using a textile‐reinforced bed joint with longitudinal fibres of alkali‐resistant glass or carbon fibre. With an appropriately adapted textile reinforcement in the bed joints, the masonry can fulfil the requirements for load‐bearing capacity against earth pressure with a horizontal load transfer, even under a small vertical load. The same applies to infill walls subjected to high wind loads the bending moment capacities of which are also slightly parallel to and vertically to the bed joint and cannot be provably demonstrated on large infill surfaces and strong wind loads. The load‐bearing can also be increased by improving the flexural strength parallel to the bed joint. The Chair of Structural Design in the Faculty of Architecture of the Technical University (TU) Dresden was carrying out extensive numerical and experimental studies for this purpose. In the journal Mauerwerk 01/2018 [1] first findings from small trial series have already been presented. In the meantime, a series of large‐scale tests have additionally been performed to check the promising results of the small‐scale tests with respect to their real applicability. This report should provide a combined insight into the work of the concluded research project.     

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.     

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。