Textile reinforcement in the bed joint of basement masonry – First findings from a current research project

The successful structural verification of basement walls under earth pressure loading with light imposed loading is often difficult. This situation is often encountered for external basement walls under terrace doors, stairs, masonry light wells etc., where the theoretically necessary imposed loading is missing. This makes it impossible to resist the acting bending forces from earth pressure using a vertical arch model. In such cases, the earth pressure has to be resisted in a horizontal direction. Since however the bending moment capacity of unreinforced masonry parallel to the bed joint is low, another possibility is to use 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 horizontal load transfer, even under a small imposed load. Textile reinforcement has the advantage above all of corrosion resistance compared to conventional steel reinforcement, and textiles can also be inserted into thin bed joints. The Chair of Structural Design in the Faculty of Architecture of the TU Dresden is currently carrying out extensive numerical and experimental studies for this purpose. The objective is to develop an optimal configuration of material and textile form for use as bed joint reinforcement. The investigations are concentrating on the tension strength, bonding and durability of the composite material ”textile mortar“. This report should give a brief overview of the state of the work in the currently running 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.     

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.     

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.     

Analysis of steel‐reinforced masonry walls regarding maximum shear loads / Traglastberechnung von vertikal bewehrten Mauerwerksscheiben

Loadings on masonry for the earthquake case pose particular challenges for the material. In order to improve the load‐bearing and deformation behaviour, masonry building elements can be strengthened with reinforcement. This article presents an analytical model for the calculation of the load‐bearing capacity of vertically reinforced masonry panels. The masonry is modelled as a homogeneous and anisotropic material and failure conditions are based on the plastic theory. Using uniaxially loaded stress fields and considering the structural constraints, a lower load‐bearing threshold can be given. In order to verify the model, three shear tests on reinforced sand‐lime block masonry were recalculated regarding their load‐bearing capacity. The test panels each contained vertical steel reinforcement in the blocks. The blocks were laid in thin bed mortar.     

Experimental determination of the load‐bearing capacity of a reinforced perforated brick facade

For a new multi‐storey car park over the Central Bus Station (ZOB) in Kiel, a perforated clinker brick veneer facade not conforming to standards was planned. The design and technical characteristics of the facade have already been described in the article by Medzech and Schrade in this issue [1]. This article deals with the experimental investigations carried out to obtain a project‐related one‐off approval (ZiE). These experiments contain in particular large tests on storey‐height wall sections, which were subjected to eccentric compressive loading and partly to horizontal loads representing wind action. Supplementary small tests on unreinforced and reinforced masonry served to determine the bending capacity, the anchoring capacity of the reinforcement and the load‐bearing capacity of the wall anchors in the masonry. Due to the special facade construction with special bricks for the project, wall anchors, reinforcement bar couplers and unique test set‐ups had to be developed for the specific project.     

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.     

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.     

New approach for spacing of movement joints in reinforced and unreinforced masonry veneer walls / Neue Bemessungsmethode für die Abstände von Dehnungsfugen in bewehrten und unbewehrten Verblendmauerwerksschalen

The first part of the paper, published in issue 4 [3] and dealing with the spacing of movement joints, described the design method for unreinforced masonry veneer walls. This paper focuses on increasing the spacing of movement joints by applying bed joint reinforcement. The proposed approach enables manufacturers of bed joint reinforcement to provide recommendations for the spacing of movement joints for bed joint reinforced veneer walls dependent upon the allowable crack width, the reinforcement type, the diameter and the vertical spacing of the reinforcement.     

Design according to Eurocode 6 in practice

For the structural design of masonry according to Eurocode 6 with the associated German national annex, the simplified method and the further simplified calculation method in Annex A are available. These procedures provide tools that can be used in practice to design standard cases quickly and easily. One feature of the verification of masonry walls under compressive loading is that no bending moments in the walls have to be determined as part of the determination of section forces and moments since the verification of the load‐bearing capacity of the wall is based solely on the acting vertical force. The effects of floor end restraint and buckling are dealt with by simple equations. One new feature of verification according to Eurocode 6 is that the effect of partially supported floors on the load‐bearing capacity of the wall can be included directly. The code is compact and simple to use and the further simplified calculation method is predestined for verification by manual calculation.     

Simplified design concept for slender masonry walls / Vereinfachter Stabilitätsnachweis knickgefährdeter Mauerwerkswände

The verification of safety against buckling of unreinforced masonry walls according to the accurate design procedure of EN 1996‐1‐1 Appendix G is based on semi‐empirical approaches, which do not always realistically describe the load‐bearing behaviour. This statement is also supported by an objection of the country Denmark concerning the load capacity function which is regulated in Appendix G. Using new findings about the effects of non‐linear material behaviour in case of stability failure this article investigates fundamental questions about the buckling behaviour of masonry walls and transfers these into a simple practical structural design proposal. As a result, the load capacity function can be considerably simplified, the influence of creep can be integrated and the number of input parameters can be reduced.     

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.     

Tragfähigkeitstafeln für unbewehrtes Mauerwerk nach DIN EN 1996‐3/NA:2019‐12

Load‐bearing capacity tables for unreinforced masonry according to DIN EN 1996‐3/NA:2019‐12 Practical design aids are important tools in the day‐to‐day business of structural design. The design of primarily vertically loaded masonry walls in usual building construction can be carried out with the help of so‐called load‐bearing capacity tables. A table value is read off exclusively as a function of the geometric conditions, which – multiplied by the masonry compressive strength – results in the load‐bearing capacity of the wall for cold design and in case of fire. By comparing the acting and resisting force, the verification of structural design can be provided in a simple and yet economical form. The bearing capacity tables based on the simplified calculation methods according to DIN EN 1996‐3/NA:2019‐12 [1], [2] and DIN EN 1996‐1‐2/NA:2013‐06 [3], [4] are presented in this paper. Compared to the previous edition of Part 3 of Eurocode 6, the extended scope of application is taken into account, as well as the normative changes to the construction method with partially supported slabs.     

Experimentelle und analytische Untersuchung des out‐of‐plane‐Verhaltens von unbewehrten Mauerwerkswänden unter Erdbebenbelastung

Experimental and analytical investigation of the seismic out‐of‐plane behavior of unreinforced masonry walls In addition to the vertical and horizontal load‐bearing in‐plane, masonry must also withstand out‐of‐plane loads that occur in earthquake scenarios. The out‐of‐plane behavior of unreinforced masonry walls depends on a variety of parameters and is very complex due to the strong non‐linearity. Current design methods in German codes and various international codes have not been explicitly developed for out‐of‐plane behavior and contain considerable conservatism. In the present work, shaking‐table experiments with heat‐insulating masonry walls have been conducted to investigate the out‐of‐plane behavior of vertical spanning unreinforced masonry walls. As shown in previous numerical investigations, important parameters are neglected in existing design and analysis models and the out‐of‐plane capacity is underestimated significantly. In the conducted experiments the results of these numerical investigations are verified. Furthermore, the development of an analytical design model to determine the force‐displacement relationship and the out‐of‐plane load‐bearing capacity considering all significant parameters is presented.     

Investigation of the seismic out‐of‐plane behaviour of unreinforced masonry walls

The structural stability of unreinforced masonry (URM) walls has to be guaranteed not only under static (permanent and live) loads but also under earthquake loads. Loads transverse to the plane (out‐of‐plane) often have a decisive influence on the load‐bearing capacity. In practical applications, simplified methods from codes, guidelines and literature are often used to analyse and evaluate the out‐of‐plane capacity of load‐bearing and non‐load‐bearing URM walls. The results of these simplified methods can be significantly conservative and inaccurate since essential influencing effects are neglected. For many existing buildings, the simplified methods underestimate the capacity, which leads to cost‐intensive retrofitting and strengthening measures or complete replacement by other wall systems. In order to realistically estimate the out‐of‐plane capacity, parameters such as wall geometry, boundary conditions, vertical loads and especially dynamic effects (e.g. inertia forces) have to be taken into account. In this paper, non‐linear time history simulations are presented to investigate the influence of these effects. The numerically determined maximum acceptable earthquake acceleration is compared with results from simplified analysis models. The comparison shows that the out‐of‐plane capacity is significantly higher than the values predicted by simplified models. Finally, several initial experimental seismic tests conducted on the shaking table of the TU Kaiserslautern are presented, together with the planned extensive experimental test program on the out‐of‐plane capacity of masonry walls.     

高延性混凝土加固砖柱轴压性能试验研究

高延性混凝土（HDC）是一种具有高强度、高韧性和高耐损伤能力的新型结构材料。该文提出采用HDC面层加固砖柱,对27个砖柱试件进行了轴压性能试验研究。结果表明:1）HDC作为砌筑砂浆,可对砌体形成一定的约束作用,使砖柱的轴压承载力和变形能力均有所提高;2）HDC面层发挥了较强的套箍作用,使砖柱处于三向受压状态,承载力和变形能力均得到较大幅度提高,且改善了砖柱的脆性破坏特征;3）HDC面层与砖柱具有良好的协调工作能力,对提高砖柱的整体性能具有重要作用。考虑HDC面层对砖柱的约束作用,提出了HDC面层加固砖柱的轴压承载力计算方法,计算结果与试验结果吻合较好。该文研究结果为砌体结构加固提供了一种新方法,具有良好的推广应用前景。     

Determining the masonry compressive strength / Bestimmung der Druckfestigkeit von Mauerwerk

Masonry is a building material primarily suited for building structures under compressive load and therefore it is mainly used for vertical load transfer. The decisive characteristic to assess the load bearing capacity of such building members is their compressive strength. It can either be derived experimentally from compression tests on wall specimens or by calculation. In this paper, firstly the current procedure in the derivation of compressive strength values based on experimental compression tests is described. Furthermore, empirical approaches and theoretical models for determining the masonry compressive strength, which have been developed in the past, are presented.     

Compressive behaviour of a new reinforced masonry system

This paper presents the behaviour in compression of an innovative reinforced masonry system. The system is made of horizontally perforated units, having common steel bars or prefabricated trusses as horizontal reinforcement. At the wall edges or crossings, confining columns for vertical reinforcement are built with vertically perforated units. Experimental tests, aimed at obtaining basic mechanical characterisation of the construction system, were performed on single constitutive elements i.e., confining columns and masonry panels made of horizontally perforated units, and on completed reinforced masonry walls. Non-linear numerical models, interpreting stress and strain distributions, were developed on the basis of the results. In particular, this paper presents: (a) results of compression tests on columns, masonry panels, and complete reinforced masonry system; (b) comparison of walls built with two types of horizontal reinforcement; (c) outcome of numerical models; and (d) effectiveness of various design equations to evaluate the compressive strength of the system.     

A students' challenge for the estimation of the maximum compressive load of masonry prisms / Studentenwettbewerb zur Ermittlung der maximalen Druckfestigkeit von Mauerwerksprismen

The paper presents the “IMC Students' Challenge” competition held in 2014, during the 9th International Masonry Conference, in Guimarães. The objective of the competition was to predict the maximum compressive load of two masonry prisms built of solid bricks, and of hollow blocks, with mortar joints. To increase the complexity of the problem, all prisms were tested under eccentric load. The students, who enthusiastically participated in the final laboratory tests, presented different approaches to estimate the maximum eccentric compressive force on masonry prisms. The challenge was a great experience, not only for students and conference participants, but also for sponsors and organizers.