Degradation and restoration of masonry walls of historical buildings

Masonry walls have been tentatively classified according to all the possible combinations of the available original materials of historical buildings: lime, gypsum, hydraulic lime, stone, brick, etc. Fifty potential masonry walls of historical buildings have been assumed on this basis. The possible causes of chemical deterioration of masonries due to interactions among the original materials have been analysed: the potential reasons for chemical degradation are basically conductive to the alkali-amorphous silica reaction (in stones) and, above all, to sulphate salt reactions causing the formation of ettringite and/or thaumasite. In both cases, water plays a basic role in the course of the above-mentioned reactions: only in the presence of moisture-even occurring occasionally-have historical building masonries been subjected to significant deterioration. The recovery of historical buildings is a very delicate operation, since the materials employed in restoration work can interact negatively with some of the compounds that might be present in masonries. Owing to these reactions, the restoration operation can worsen the state of masonries. These reactions cause swelling, pop-out and falling of jointing and rendering mortars applied during the restoration work, as well as of cement grout injections for interior consolidation of structures. Again, ettringite and/or thaumasite may be found as deterioration products, provided that moisture is present in building masonries.     

An experimental analysis about the effects of mortar joints on the efficiency of anchored CFRP-to-masonry reinforcements

Fiber reinforced composite materials are widely used for structural rehabilitation and retrofitting of existing buildings; recent studies, devoted to Carbon Fiber Reinforced Polymers (CFRP) reinforcements of concrete structural elements [1–5], demonstrated that spike anchors are able to effectively increase the load carrying capacity and the ductility of CFRP bonded joints. However, application to masonry structures is disregarded by research since few experimental results are available. One of these, described in Refs. [6,7], compares the efficiency of a CFRP strengthening system provided with one or more CFRP spike anchors, also in dependence of some geometrical parameters; reinforcement sheets and spike anchors were applied only on the brick surface in order to evaluate the effects due to anchors only. In this paper the authors investigate the influence of mortar joints on the efficiency of anchored CFRP reinforcements on brick masonry. For this reason, an experimental campaign was planned on masonry pillars built with the same materials employed in Refs. [6,7], subjected to Near End Supported Single Shear Tests. Masonry pillars were built according to two different patterns, in order to detect the influence of both bed and perpend joints. The results are compared with results obtained from previous experimental campaign.     

Current practice and open issues in strengthening historical buildings with composites

Modern techniques and innovative materials are often quite rapidly proposed and allowed in current practice, even for restoration of historical constructions, in which essential preservation criteria must be taken into account. The considerable variability and complexity of masonry structures and types means that choosing the most appropriate structural models and interventions is particularly difficult, since they must be based on suitable knowledge of both existing and new materials, and on their interactions in environmental and loading conditions. This paper discusses the potentials and limitations of externally bonded composite materials in masonry structures and components, in the light of knowledge acquired from research in the field, together with the requirements and recommendations of codes and restoration documents. The analysis of some case studies is presented, to highlight the advantages and constraints in the use of composites for strengthening historical buildings.     

Strengthening masonry vaults with organic and inorganic composites: An experimental approach

Polymer-reinforced fibers are now commonly applied to buildings for structural retrofitting purposes. These materials add greater tensile strength to structures, at the expense of a slight increase in weight. However, they also have other disadvantages such as brittle behavior and lack of water vapor permeability, which are not desired in the conservation of heritage buildings.Alternative composite materials embedded in an inorganic matrix are presented, which solve some of the drawbacks associated with organic matrices. Long steel fibers and basalt textiles are applied to the resistant core of the inorganic matrix to produce a steel-basalt reinforced mortar-based composite. Firstly, a mechanical characterization of the individual components and the resulting material was performed. Secondly, non-strengthened and strengthened real-scale (2.98 m span, 1.46 m high and 0.77 m deep) brick masonry vaults were tested up to failure, in order to demonstrate the mechanical effectiveness of these composite materials. Finally, a comparison between two mortar composite materials (steel-strips/basalt-textiles embedded in a polymer matrix) was performed, with the same real-scale brick-vault failure tests.The experimental campaign demonstrates that the steel/basalt composite mortar is a feasible alternative, which is physically compatible with masonry structures, easy to apply, and effective for the reinforcement of brick vaults.     

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.     

Design and evaluation of hydraulic lime grouts for the strengthening of stone masonry historic structures

The present paper discusses an experimental procedure realized in order to design hydraulic lime based grouts adequate for the strengthening of stone masonry historic structures. With the aim to minimize incompatibility problems between the original materials and the grouts, several natural hydraulic lime based grouts, as well as a ternary (lime–pozzolan–cement) grout with reduced cement content, have been studied. The selection of the most suitable grouts was performed based on a set of criteria, namely injectability, mechanical and durability characteristics. The selected grouts were subsequently injected into cylindrical specimens that simulate the infill of three-leaf stone masonry. The experimental results obtained from mechanical tests carried out on the injected cylinders demonstrated that all grout mixes studied within this work were efficient in strengthening the infill material; they exhibited, however, differences in terms of durability properties. Finally, an empirical formula was developed to predict the compressive strength of the injected infill, as a function of the mechanical properties of grouts.     

Mechanical behaviour of FRP-confined masonry by testing of full-scale columns

The vulnerability of masonry constructions under seismic forces, or more generally under the mechanical actions during the centuries, has been highlighted in the last years by several events that caused the loss of significant heritage buildings. Faced with this difficulty, the use of composite materials, fiber reinforced polymers (FRP) may be a solution for mitigating the vulnerability of masonry buildings. This solution has been tested in the laboratory by researchers in the last decade. In particular, studies regarding elements such as walls, arches and vaults, strengthened with FRP materials are available. A few numbers of studies are known for columns, which have been tested only as small or middle scale samples. The current state of the art does not report studies on FRP-confined masonry columns tested in real scale. The research presents the results of an experimental program performed on full-scale masonry columns strengthened with different composite systems. The same kind of study had been previously performed by the authors on medium scale masonry columns, using the same materials for both the masonry core and for the FRP system. Prismatic columns with a square cross section were subjected to compression tests according to the following test schemes: two control unconfined columns; column with continuous wrapping by using unidirectional glass FRP (GFRP) sheets; column with discontinuous wrapping by using GFRP unidirectional sheets; column with continuous GFRP wrapping and internal carbon FRP bars bonded in the transverse directions; column wrapped with continuous alkali resistant GFRP grid and steel spikes bonded together in lime based matrix. The experimental results are presented and discussed in the paper along with the comparison with the results obtained from the experimental tests on medium scale specimens. The comparison between experimental data and theoretical predictions provided by the analytical model found in the guidelines of the CNR technical document is also illustrated.     

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.     

Seismic retrofitting solution of an adobe masonry wall

Adobe constructions represent a high percentage of the national patrimony, with high historical, cultural and architectonic value. Well-preserved adobe structures can exhibit a particular architecture with very attractive geometric characteristics while also incorporating natural materials. However, the behavior of these structures is deficient under horizontal loads, such as those induced by an earthquake, which endangers their structural integrity and human lives. To develop a seismic retrofit solution, a real-scale wall was characterized and tested by considering permanent vertical actions with cyclic horizontal forces of increasing amplitude. To retrofit the wall, repair and seismic reinforcement solutions were developed and combined to evaluate their efficiencies. To repair the damages, hydraulic lime gum was injected by pressure into the cracks. The reinforcement solution included the use of a synthetic mesh in the wall. The retrofitted wall was then tested, and the results indicated that the retrofit solutions significantly improved the seismic performance of the wall. This study contributes to the characterization of walls constructed with adobe masonry and their behavior under horizontal actions. Furthermore, an economic, sustainable and efficient solution is presented for the retrofitting of adobe walls, with significant performance improvements obtained.     

Microstructural investigation of a silica fume–cement–lime mortar

Several additions, minerals and organic, are used in mortars, such as pozzolanic materials, cementicious materials and polymers. Literature about the use of additions in masonry mortars (cement/lime/sand mixes) is scarce; usually, studies are about concrete mortars. The purpose of this work is to study the microstructural effects of the substitution of 10% of Portland cement by silica fume in a 1:1:6 (cement/lime/sand mix proportion by volume) masonry mortar. Scanning electron microscopy with energy dispersive X-rays analysis (SEM/EDX) shows that, with silica fume, the C–S–H formed is type III at early ages and that type III and type I coexist at later ages. Silica fume lowers the total porosity and increases compressive strength only at later age and, as expected, the pore structure of mortar with silica fume is found to be finer than of non-silica fume mortar.     

Mechanical behaviour of ancient masonry

The aim of this research was to build a behaviour law for ancient masonry made during the nineteenth century with bricks and lime mortar bonds. This work should be of interest to researchers involved in the study of ancient masonry structures like arch bridges built in this period. To assess the masonry capacity vaults to support service loads and to determine their collapse loads, engineers need mechanical behaviour laws for their component parts. This experimental research was performed to explore the behaviour of the bricks, of the lime mortar, and of a wall until their failure in compression. In parallel the bricks / mortar interface criterion failure under shear and tensile load is characterised. After laboratory tests, numerical simulations were carried out using a finite element method (FEM) to define an homogenised behaviour law for a macro element including bricks and lime mortar bonds. In this goal, a behaviour law was firstly found for each component and then for the masonry as a whole by a FEM homogenisation process, including the non-linear behaviour domain up to the compression failure. The tension failure being reported into an interface element for which the failure criterion was adjusted on specific tests.     

Stress rate sensitivity of stone masonry units bound with fibre reinforced hydraulic lime mortar

It is well established that most construction materials behave differently under static and dynamic loading. However, the literature on the time-dependent response of masonry joints is scarce, particularly with regard to the bond behaviour in historical stone masonry. This paper describes the dynamic response of sandstone masonry units bound with hydraulic lime mortars (HLMs). A drop weight impact machine was used to generate stress rates up to 10⁷ kPa/s. The dynamic impact factor and stress rate sensitivity were evaluated for the flexural strength of the sandstone, mortar and for the bond strength of the unit and further, the pattern of failure was noted in the units for each mortar mix and loading rate. Based on a related study on the fracture toughness of HLM, polypropylene micro-fibres were incorporated at 0, 0.25 and 0.5% volume fraction into the mortar. Results show that the flexural bond strength was more sensitive to stress rate than the flexural strength of the mortar, at similar rates of loading. Further, the stress rate sensitivity of the bond strength decreased with an increase in the fibre content. Also, whereas the mode of failure in the masonry units under quasi-static loading was through fracture at the mortar-block interface, the failure plane transferred to within the mortar under dynamic loading, particularly in the presence of fibre reinforcement.     

Investigation of horizontal floor acceleration in historic masonry buildings

Earthquakes with comparatively low magnitudes can lead to serious damage to non‐structural components of historical masonry buildings, such as architectural facade elements. In order to assess the vulnerability of non‐structural components, the horizontal floor acceleration is used. This depends on the dynamic characteristics of the building, the ground acceleration and dissipative effects. In the present article comprehensive probabilistic FE time history analyses with different hazard levels have been carried out for selected masonry structures. In order to take induced non‐linearities into account, a macroscopic material model for historic masonry was calibrated and applied. It was shown that the chosen methodology enables the determination of the distribution of floor acceleration over the building height for historic masonry structures. In addition, due to the detailed scope, a robust comparison with the simplified design methods is possible. Finally, the applicability of the simplified design approach according to EN 1998‐1 [1] is discussed for the investigated case.     

On the vulnerability of historical masonry structures: analysis and mitigation

This work presents a methodological approach to the study of historical structures in particularly hazardous conditions. Preliminary knowledge, diagnostic methods and assessment procedures are selected and proposed for specific structural problems, in order to define proper improvement techniques to increase safety levels according to preservation and restoration criteria. Both stone and brick masonry structures, and their performance under severe horizontal (seismic) and vertical (high-compression) actions, respectively, are examined. The use of traditional, modern and innovative materials and techniques is also discussed, in the light of experimental validation, in order to calibrate analytical and numerical models for reliable analyses and simulations. Dr Valluzzi presented a lecture of this paper at the 2005 Annual Meeting in Moscow, as she was awarded the 2005 Robert L’Hermite Medal in recognition of her work. Dr. Valluzzi’s work on masonry is closely linked to the work of RILEM Technical Committee RHM ‘Repair mortars for historic masonry’ and to the extensive work on masonry carried out by the previous TC 177-MDT ‘Masonry durability and on-site testing’. Her work is exceptional in that she is able to integrate a deep understanding of masonry structures with sophisticated analytic techniques in order to develop practical engineering solutions for their repairs.This combination of skills is, unfortunately, all too rare. Her work should lead to the more effective restoration and repair of the many historic masonry structures throughout Europe.     

Strengthening of masonry arches with Textile-Reinforced Mortar: experimental behaviour and analytical approaches

The continuous and growing interest in the conservation of historical heritages requires easy to use and reliable strengthening systems with related calculation methods that allow evaluating the capacity of existing and strengthened masonry structures. However, the analytical models applicable to retrofitted masonry structures have not been developed at the same level as those of other modern construction materials. In particular, there is a gap between the experimental results of masonry elements strengthened with innovative systems and the predicted structural behaviour provided by analytical models. This can hinder exhaustive analysis of experimental results and potentially led to over conservative design methods for innovative strengthening solutions. The present work investigated the performance of the basalt textile reinforced mortar (BTRM) strengthening system, applied to stone masonry arches, and evaluated the applicability of three different analytical approaches for design purposes. The basic materials and the BTRM composite were tested for the definition of the main constitutive laws. Three unreinforced stone masonry arches and nine arches retrofitted according to three different layouts were tested under vertical monotonic load. The experimental and analytical results were compared for the identification of the more suitable analytical approach for design purposes.     

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.     

Strengthening of historic masonry structures with composite materials

This paper deals with the applications of unidirectional fibre-reinforced polymer tendons for the reversible strengthening of masonry monuments. The tendons, anchored to the masonry only at the ends, are circumferentially applied on the external face of the structure and posttensioned to provide horizontal confinement. The relevant properties of fibre-reinforced polymer materials and prestressing systems are summarised; in addition, the concepts for their application, including anchorage, to masonry structures are developed, and a general design procedure is presented. The effectiveness of the strengthening technique is established both analytically, for structures with simple geometries, and numerically, for a real three-dimensional structure with openings, based on the finite element method. The effects of temperature changes on the tendons and the masonry are shown to be negligible. It is concluded that the effectiveness of the proposed method in the consolidation of historic masonry structures is quite satisfactory, especially when the strengthening elements are made of carbon fibre-reinforced polymer.     

基于RVE单元的藏式古建石砌体均质化研究

在普通砖石砌体结构方面的均质化研究已经较为完善,而在构造、材料存在随机性的古建筑砌体结构方面的均质化研究相对欠缺。该文以有限尺度测试窗法为基础,提出了一种选择砌体结构代表性体积单元(RVE单元)的方法,并与试验和传统有限元模拟结果对比,验证了所提方法的可行性。在此基础上,该文进行了藏式古建石砌体结构RVE单元的选择,探讨了RVE单元的尺寸大小和所包含的组元分布对等效模量的影响,并基于所选RVE单元建立了藏式石砌体结构的均质化模型和整体式模型。结果表明:该选择方法适用于周期性和准周期性砌体结构,能选出与完整结构力学性能接近的RVE单元。随着RVE单元尺寸变大,其Voigt、Reuss等效模量会逐渐向完整结构的模量收敛,呈现先快后慢的变化趋势;组元分布的不同会改变等效模量的收敛程度,但在较大尺寸的RVE单元上,组元分布的影响将被体积造成的影响抵消。该文所建均质化模型能代替传统有限元模型进行局部结构的分析,并给出藏式古建石砌体结构的应力分布规律;所建整体模型能代替传统有限元模型,较为精确模拟结构整体的宏观变形。     

Penetrability of hydraulic grouts

Design of hydraulic grouts for strengthening of masonry historical buildings seems to follow rather empirical procedures, with all the related uncertainties, both in terms of economy and efficiency. This paper is part of a broader attempt to establish a rational methodology for the design of such grouts, based on their discrete injectability characteristics, i.e. (i) Penetrability, (ii) Fluidity and (iii) Stability. This paper deals with penetrability and constitutes the first part of this holistic methodology. The second part regarding the fluidity and the third regarding the stability are separately published. Grouting is intended to fill voids, fissures and open joints of the masonry as a system, producing a “dendrite” (a three-dimensional skeleton), directly contributing to the strength of the masonry as a whole. However, to do so, the grout should be able to pass through the “narrowest” possible width of such discontinuities, in order to reach the maximum possible internal volume of masonry and open joints, avoiding most of possible blockages. In the specific case of three-leaf masonries, the most decisive result of the grouting is expected to be the strengthening of the bond along the interfaces between the external layers and the infill; the rather small voids, as well as pre-existing fissures along these interfaces have to be penetrated. In this paper the penetrability of hydraulic grouts is discussed, and relationships between two characteristic diameters of the grains of the solid phase of the grout and the nominal minimum width of fissures and voids of the structure to be injected are proposed. Furthermore the beneficial role of replacing part of the cement or hydraulic lime with ultrafine materials in order to improve penetrability is presented, and related criteria are proposed.     

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.