Mechanical properties and debonding strength of Fabric Reinforced Cementitious Matrix (FRCM) systems for masonry strengthening

Fabric Reinforced Cementitious Matrix (FRCM) composites are advanced cement-based materials often used for strengthening masonry or concrete structures. The system is usually composed of a dry grid of fibers embedded in a cementitious matrix enriched with short fibers.An important parameter for designing the structural reinforcement is the tensile load-bearing capacity of FRCM composites. For their heterogeneity, FRCM composites show an interesting mechanical behavior in tension, that depends on the properties of the components and of the bonding strength. These values could be estimated with mechanical models but must be validated experimentally by means of proper testing campaigns.In this work several FRCM materials made with different fiber grids were investigated. Four different types of fibers were considered: polyparaphenylene benzobisoxazole (PBO), carbon (C), glass (G) and PBO and glass (PBO-G) fibers and three different types of cementitious mortars.The behavior of FRCM under tension and the influence of the bond properties between the dry textile and the inorganic matrix are studied developing an extensive experimental program that included the characterization both of the materials components and of the composites. A series of push–pull double lap tests and pull-off tests were performed to determine the bonding properties of FRCM composites applied to masonry structures.The paper presents results and considerations that can provide background data for future recommendations for the use of FRCM systems in the rehabilitation of elements.     

High‐rise concrete and clay block masonry building in Brazil

Brazilian structural concrete and clay block masonry construction shares many common features with construction all over the world: blocks of a similar shape are bedded in mortar, vertical and horizontal reinforcement is placed in grouted cells, engineering analysis and design follows universal principles and local design codes mimic those adopted elsewhere. However, loadbearing masonry construction in Brazil has become one of the most preferred high‐rise building systems due to its cost‐effectiveness and ease of construction compared to normal reinforced concrete solutions. This paper provides an overview of loadbearing masonry building in Brazil, including case studies on notable high‐rise masonry structures, with an overview of how Brazilian materials, codes and practices differ from the rest of the world. Finally, the paper explains how the use of high‐strength units assists the growing demand for taller and taller buildings and provides insight into why owners and general contractors often prefer to use structural masonry.     

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.     

Reliability of masonry walls subjected to in‐plane loading: A slip failure along head joints / Zuverlässigkeit von Mauerwerkswänden bei Belastung in der Ebene: Versagen entlang der Stoßfugenflucht

Masonry structures are a sustainable, economical and traditionally widely used type of construction. However, current masonry design codes are rather conservative, so there is a growing need for revision i.e. calibration of safety factors to improve the allocation of material resources. In this paper, we investigate the probability of occurrence of slip failure along head joints (perpends) in masonry subjected to in‐plane loading. An appropriate limit state function is established and the masonry material properties and loads are defined as random variables in order to simulate likelihood of occurrence of a slip failure regime along the head joints. Furthermore, an example of masonry wall with probabilistic analysis outcomes using Monte Carlo simulation is presented and recommendations for further work are provided.     

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.     

A non‐linear interface element for 3D mesoscale analysis of brick‐masonry structures

This paper presents a novel interface element for the geometric and material non‐linear analysis of unreinforced brick‐masonry structures. In the proposed modelling approach, the blocks are modelled using 3D continuum solid elements, whereas the mortar and brick–mortar interfaces are modelled by means of the 2D non‐linear interface element. This enables the representation of any 3D arrangement for brick‐masonry, accounting for the in‐plane stacking mode and the through‐thickness geometry, and importantly it allows the investigation of both the in‐plane and the out‐of‐plane responses of unreinforced masonry panels. A co‐rotational approach is employed for the interface element, which shifts the treatment of geometric non‐linearity to the level of discrete entities, and enables the consideration of material non‐linearity within a simplified local framework employing first‐order kinematics. In this respect, the internal interface forces are modelled by means of elasto‐plastic material laws based on work‐softening plasticity and employing multi‐surface plasticity concepts. Following the presentation of the interface element formulation details, several experimental–numerical comparisons are provided for the in‐plane and out‐of‐plane static behaviours of brick‐masonry panels. The favourable results achieved demonstrate the accuracy and the significant potential of using the developed interface element for the non‐linear analysis of brick‐masonry structures under extreme loading conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Modeling and numerical analysis of the bond behavior of masonry elements strengthened with SRP/SRG

Steel Reinforced Polymers (SRPs) and Steel Reinforced Grout (SRG) strengthening systems have been recently introduced as an alternative solution to the traditional systems based on the use of fiber reinforced polymers materials (FRPs). Few studies on SRP/SRG are available in the current literature and all have shown the potentialities of SRP/SRG in improving structural performances of masonry and concrete elements and, at the same time, their difference with respect to FRPs particularly in terms of bond behavior. Aim of the present paper is to propose a simple approach devoted to study the bond behavior of masonry structures strengthened with SRP/SRG systems. The approach, based on experimental evidences and theoretical considerations mainly consists of deriving approximate bond stress-slip laws for the strengthening/support interface layer, able to reproduce the local bond stresses transferring mechanism. Finite Element (FE) analyses are then developed with reference to the experimental tests available in the current literature by adopting the bond stress-slip laws obtained through the proposed approach. The deduced results show the reliability of the proposed approach in simulating the bond behavior of masonry elements strengthened with SRP/SRG and the possibility to investigate further peculiarities characterizing this kind of strengthening systems.     

Fabrication of Nanofiber Reinforced Protein Structures For Tissue Engineering

Extracellular matrices and degradable nanofibers are two very promising materials in the field of tissue engineering; however both of these structures face limitations as tissue engineering scaffolds. Extracellular matrices, such as collagen, gelatin, and laminin, have excellent biocompatibility and allow cell in growth and survival, but structural weakness makes them difficult to handle and greatly limits their uses. Degradable nanofibers support cell attachment and can provide structural support and directional guidance, but individual degradable nanofibers are fragile and have a tendency to form dense fiber bundles which limit cell penetration into the spaces between the nanofibers, especially in the case of aligned nanofibers. To overcome these difficulties, degradable loose nanofibers were embedded in protein matrix in an attempt to fabricate a hybrid scaffold with improved properties, such as improved strength, guidance, spacing among nanofibers, etc. Polycaprolactone (PCL) was used as a model material for degradable nanofibers. Gelatin was employed as a model protein for matrix structure formation. Thin hybrid films (average thickness = 2.78 um) were fabricated by wetting the loose aligned undirectional nanofiber arrays or loose aligned bi-directional nanofiber grids with a gelatin aqueous solution, which also allows for live cell loading into the nanofiber-protein composite if cell are premixed with protein solution or on the surface of the films. Gelatin film alone without nanofiber reinforcement is difficult to handle due to the weakness of the thin membrane. Gelatin films with a fiber density as low as 3% v/v were structurally robust enough for handling, and manipulation into complex shapes. Mechanical testing confirmed that the addition of nanofibers enhanced the strength of gelatin films, in both dry and hydrated state. In vitro testing confirmed that nanofiber reinforced films were biocompatible and provided cells with directional guidance. Results demonstrate the promise of gelatin/PCL nanofiber composites as a tissue scaffolding material.     

Determination of tensile strength of glass fiber straps

Glass fibers straps have been used for strengthening of masonry and concrete structures in the last decade. Recently, their use has become greater. This paper describes the research of measuring tensile strength of dry glass fiber straps as well as straps that were made of glass fibers and epoxy coating. The effect of strap widths, the effect of loading speeds and the effect of epoxy coating placed on fiber straps, on tensile strength of straps have been analyzed. Differences of tension strengths of fiber straps with and without epoxy coating are shown.     

Verification of the fire resistance of clay brick masonry – hints for efficient design

In Germany, structural fire design of masonry is carried out in a simplified way using tabulated minimum wall thicknesses depending on the loading level in fire. Against this background the procedure of structural fire design is shown briefly before two approaches for a more efficient verification of the fire resistance are explained. The first possibility is to determine the reduction factor for the design value of the actions in fire more precisely and thereby reduce the loading level. Secondly, a design methodology is presented which can be applied in case of masonry walls with low vertical load but a large load eccentricity at mid‐height of the wall. Finally, the verification of the fire resistance of masonry according to national technical approval is discussed with an explanation how to obtain the same loading level in fire if the design is based on DIN EN 1996‐3/NA as when it is based on DIN EN 1996‐1‐1/NA.     

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.     

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.     

Mechanics of hollow concrete block masonry prisms under compression: Review and prospects

The aim of this work is to critically assess the mechanical properties of hollow concrete masonry using experimental results from prisms constructed with blocks of two different strengths and four types of mortar. A key conclusion is that mortar is mostly responsible for the non-linear behavior of masonry. Moreover, a strongly non-linear relationship between masonry elasticity modulus and compressive strength is found, which contradicts the simple linear relation proposed by Eurocode 6 [CEN. Eurocode 6: Design of masonry structures – Part 1 – Common rules for reinforced and unreinforced masonry structures. EN-1996-1-1; 2005.]. The porosity of mortar and the state of stress that mortar undergoes in the process of compressive loading can be responsible for changes in the mechanical properties, such as elasticity modulus and Poisson’s ratio. Finally, different types of mortars induce different failure modes in the masonry prisms and there is clear evidence that the failure of hollow concrete masonry starts after onset of mortar crushing. In order to better reproduce the observed experimental behavior, a tentative model for the mortar Poisson’s ratio variation upon loading is also presented.     

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.     

Sustainable building with masonry / Nachhaltiges Bauen mit Mauerwerk

Ecology, energy and sustainability are crucial socio‐economic keywords, especially for the construction and real estate industry. In recent years, the masonry industry faced related sustainability issues intensively in order to keep its ability of operating in a market setting which is increasingly characterized by sustainability dogmata. Scientific analyses and studies on an objectified sustainability basis – such as established certification systems – show that masonry is absolutely competitive with all other market‐relevant construction methods. Therefore, the first part of this paper deals with a sustainability assessment of buildings made of masonry as well as of other construction materials. Concerning the design and construction of masonry, further selected aspects are discussed which are important for the competitiveness of this building technique. Outlining the simple yet economic pre‐dimensioning with the help of load capacity tables is one topic which is of utmost importance for an efficient structural design. Additional aspects are the design of laterally loaded exterior walls with low vertical forces and the verification of basement walls under high earth pressure load. New and easy to use design proposals are open for discussion. Finally, the educational portal “masonry structures” of the Deutsche Gesellschaft für Mauerwerks‐ und Wohnungsbau (DGfM) (German society for masonry and residential construction), which supports the training of young engineers, is presented.     

Prototype and model masonry behaviour under different loading conditions

The aim of this paper is to systematically determine the effect of scale on masonry structural behaviour under various loading conditions. Small scale models have been employed to understand masonry structural behaviour over the years, because testing prototype masonry structures is both costly and challenging to undertake in a controlled laboratory environment. A programme of tests have been carried out at four scales namely prototype, half, fourth and sixth scale under five different loading conditions; compression, shear, diagonal tension, flexure and bond tension, with a view to understanding how each of these conditions affects masonry structural behaviour over the range of scales enumerated. The results show that scale or size effect is mostly pronounced in the masonry compressive strength.     

Hollow‐Structured Metal Oxides as Oxygen‐Related Catalysts

Metal oxide hollow structures with large surface area, low density, and high loading capacity have received great attention for energy‐related applications. Acting as oxygen‐related catalysts, hollow‐structured transition metal oxides offer low overpotential, fast reaction rate, and excellent stability. Herein, recent progress in the oxygen‐related catalysis (e.g., oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and metal–air batteries) of hollow‐structured transition metal oxides is discussed. Through a comprehensive outline of hollow‐structured spinels, perovskites, rutiles, etc., a rational design strategy is provided for an enhanced oxygen‐related catalysis performance from the viewpoint of crystal structures. Urgent challenges and further research directions are presented for hollow‐structured transition metal oxides toward excellent oxygen‐related catalysis.     

Dynamic DNA Structures

Dynamic DNA structures, a type of DNA construct built using programmable DNA self‐assembly, have the capability to reconfigure their conformations in response to environmental stimulation. A general strategy to design dynamic DNA structures is to integrate reconfigurable elements into conventional static DNA structures that may be assembled from a variety of methods including DNA origami and DNA tiles. Commonly used reconfigurable elements range from strand displacement reactions, special structural motifs, target‐binding DNA aptamers, and base stacking components, to DNA conformational change domains, etc. Morphological changes of dynamic DNA structures may be visualized by imaging techniques or may be translated to other detectable readout signals (e.g., fluorescence). Owing to their programmable capability of recognizing environmental cues with high specificity, dynamic DNA structures embody the epitome of robust and versatile systems that hold great promise in sensing and imaging biological analytes, in delivering molecular cargos, and in building programmable systems that are able to conduct sophisticated tasks.     

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.     

CJEU Judgment,Standardisation and Eurocode 6 / EuGH‐Urteil,Normung und Eurocode 6 – Aktuelle Fragen im Mauerwerksbau

This article reviews current challenges for the masonry industry from a regulatory perspective. First, the CJEU judgment and its consequences will be discussed. The judgment of 16 October 2014 will make fundamental changes in the German regulatory system necessary. How these will be implemented is not yet clear so that this article can only give an overview of the present situation (September 2015) in the discussions. What is clear is that the changes in the regulatory system will have repercussions in the field of standardisation. These repercussions will be outlined – in general and specifically for the masonry sector – in the second part of this article. Finally, the paper will look at the progress in the implementation of Eurocode 6 (Design of masonry structures).