Strengthening Of Masonry Structures Research Articles

Mesoscale Modelling of Normal Bond Behaviour between FRP and Masonry Substrate: Effect of Mortar Joint

Fibre-reinforced polymers have been widely used to strengthen masonry structures which owning to their high strength-weight ratio and good durability. The interfacial strength between masonry substrate and FRP plays an essential role in the structural bearing capacity. Plenty of experiments have revealed that interfacial failure typically occurs within a thin layer of masonry near the bond line. The mortar joint's location in the masonry substrate sample influences the bond strength and failure mode and has not been thoroughly investigated. This work focuses on the effect of mortar joints on the normal bond strength and damage process in the pull-off test. The two-dimensional mesoscale finite element model is set up, and zero thickness cohesive elements (cohesive zone model) are inserted into the inner and interface between different materials. The numerical result shows that the mortar joint in the middle of the masonry substrate sample shows the largest normal bond strength, and next to the groove is the smallest.

Seismic performance of strengthened masonry structures: actual behaviour of buildings in Norcia and Campi Alto during the 2016 Central Italy seismic sequence

The structural response of unreinforced masonry buildings designed for gravity load only or with reference to obsolete seismic provisions is widely studied in the literature in order to define proper strengthening strategies and solution to mitigate the seismic risk. However, the critical analysis of the effectiveness of past used strengthening solution is still lacking. To fill such gap, the present study deals with the evaluation of the seismic performances of buildings in Campi Alto struck by the 2016 central Italy seismic sequence. The behaviour of buildings in Campi Alto is compared with that of buildings in Norcia. A large part of the buildings in these two towns was strengthened between 1980 and 2000 during the reconstruction processes following previous earthquakes which occurred in 1979 and 1997. However, the strengthened buildings in Norcia reported limited damage while a significant and widespread level of damage was detected on several strengthened buildings in the hamlet of Campi Alto. This study focuses on the buildings in Campi Alto with the aim of investigating on the reasons of their unsatisfactory behaviour. Thus, the seismic action experienced by buildings in Norcia and Campi Alto is initially compared and the main vulnerabilities of these buildings are also evaluated. Then, 20 projects of strengthening interventions submitted to the Civil Engineering Department of the Umbria Region between 1984 and 2012 have been herein analysed and discussed in order to focus on the effectiveness of the strengthening solution adopted in the past. The analyses of such projects and of the empirical damage detected after the 2016 seismic sequence is a unique opportunity to derive useful information for future applications.

Numerical simulation of masonry walls strengthened with vegetal fabric reinforced cementitious matrix (FRCM) composites and subjected to cyclic loads

The growing concern for the preservation of old buildings and the wide use of masonry in the construction of civil works has led to a great development of specific techniques to strengthen masonry structures. FRCM (fabric reinforced cementitious matrix) of vegetal fibres arises as a strengthening option to be considered because low cost, low density, recyclability and biodegradability brought by vegetal fibres. In this study, numerical models were used to represent masonry walls unreinforced and strengthened with FRCM (hemp, cotton and glass). The numerical model presents a new way of simulation of FRCM systems and their failure pattern. This was validated by comparison to previous experimental and analytical results. Results showed that proposed model is an effective calculation tools to reproduce the maximum shear of masonry walls strengthened with FRCM and subjected to cyclic loads, with variations between 7 and 15% with respect to the experimental and analytical. The numerical model was also useful to reproduce the walls hysteresis behavior, although showed some inability to reproduce the reversible crack opening-closing that was observed in the experimental tests.

Application of Shape Memory Alloys in Retrofitting of Masonry and Heritage Structures Based on Their Vulnerability Revealed in the Bam 2003 Earthquake.

For decades, one of the most critical considerations of civil engineers has been the construction of structures that can sufficiently resist earthquakes. However, in many parts of the globe, ancient and contemporary buildings were constructed without regard for engineering; thus, there is a rising necessity to adapt existing structures to avoid accidents and preserve historical artefacts. There are various techniques for retrofitting a masonry structure, including foundation isolations, the use of Fibre-Reinforced Plastics (FRPs), shotcrete, etc. One innovative technique is the use of Shape Memory Alloys (SMAs), which improve structures by exhibiting high strength, good re-centring capabilities, self-repair, etc. One recent disastrous earthquake that happened in the city of Bam, Iran, (with a large proportion of masonry buildings) in 2003, with over 45,000 casualties, is analysed to discover the primary causes of the structural failure of buildings and its ancient citadel. It is followed by introducing the basic properties of SMAs and their applications in retrofitting masonry buildings. The outcomes of preceding implementations of SMAs in retrofitting of masonry buildings are then employed to present two comprehensive schemes as well as an implementation algorithm for strengthening masonry structures using SMA-based devices.

Experimental investigation on BFRCM confinement of masonry cylinders and comparison with BFRP system

Fabric reinforced cementitious mortar (FRCM) materials have started to be employed during the last years with the aim of overcoming the drawbacks related to the use of fibre reinforced polymer (FRP) composites, proving to be potentially suitable for strengthening masonry structures. Moreover, the will to develop materials able to guarantee a certain degree of sustainability without renouncing to adequate mechanical properties has drawn the attention to the use of basalt fibres, which appear to be a valid alternative to carbon or glass fibres. This work presents an experimental investigation on a basalt FRCM (BFRCM) system to confine circular masonry columns, aimed at evaluating the effectiveness of this system in comparison with data obtained by basalt FRP (BFRP) jacketing. A total of eighteen clay brick masonry cylinders were prepared by using two different assembling schemes and subjected to uniaxial compression. Six cylinders were tested as control specimens, while the rest were reinforced by using either one or two layers of basalt textile. Traditional measuring instruments were integrated with the digital image correlation (DIC) technique. The experimental results are presented in terms of stress–strain curves, and strength and strain enhancements of confined cylinders compared to control specimens. The failure modes are also discussed. All outcomes are compared to those obtained by the authors in a similar study performed on BFRP-confined cylinders realized with the same manufacturing in order to have an effective comparison.

Statistical analysis of the bonding properties of the fabric-reinforced cementitious matrix for strengthening masonry structures

The fabric-reinforced cementitious matrix (FRCM) has been extensively studied and applied for the strengthening of masonry structures. Special attention needs to be given to the bonding properties between the FRCM and masonry substrate for strengthening applications. This paper presents a statistical analysis of the bonding properties based on the available literature. First, the collected test results were discussed in terms of the interfacial failure mode. Second, the factors influencing the ultimate bond load were analysed based on the different failure modes, and a corresponding prediction formula was further determined via regression analysis for interfacial debonding and slippage failures. Then, the characteristic values of the ultimate bond load were determined via a probabilistic method. Finally, the fracture energy for the slippage failure at the fabric-matrix interface was analysed, and a corresponding prediction formula was obtained via regression analysis. Additionally, some of the collected test results present higher dispersion due to the large variability of the FRCM material properties and the differences in the testing procedures used by different institutions. More studies are needed to improve the reliability of the proposed procedure.

Cyclic behavior of autoclaved aerated concrete block infill walls strengthened by basalt and glass fiber composites

In this study, the effects of 10 mm bilaterally applied basalt and glass fiber reinforced (BRC and GRC) cementitious plasters with different fiber content (1.0%, 2.0% and 3.0%) on the behavior of the autoclaved aerated concrete (AAC) block infill walls were investigated. For this purpose, 8 infill walls with dimensions of 150 × 150 × 20 cm were produced to examine the behavior of the infill walls under reversed cyclic loading. The load carrying capacities, stiffness degradation and energy dissipation capacities of the infill walls placed in a steel frame with hinges on all four corners were determined by using hysteretic load–displacement curves to evaluate effects of fiber reinforced cementitious plaster. The test results show that BRC and GRC plaster applications considerably increase the load carrying and energy dissipation capacities of the infill walls. However, the experimental results illustrated that the usage of BRC plasters in strengthening of the AAC block infill walls needs more attention. Having similar results for different fiber ratios in the use of GRC reveals that it may be more rational to use 1.0% fiber content for the most economical solution for strengthening. Although the results obtained in this study are valid for infill walls, the experimental results show that GRC plasters can also be used in strengthening of masonry walls. It is recommended that this method can be used quickly and effectively in strengthening of masonry structures, which occupy an important place in the existing building stock.

Effect of fiber-to-matrix bond on the performance of inorganic matrix composites

The strengthening of masonry structures is nowadays performed by means of high-strength fibers embedded in inorganic matrix (FRCM) where lime or cement-based matrix is used instead of epoxy adhesive to reduce debonding issues between substrate and matrix. However, some sliding phenomena and cohesive failures between fibers and the matrix mortar can occur. The paper examines the effect on the FRCM efficiency of the mechanical properties of fiber and matrix and potential geometrical defects, which are possible in real field applications or in qualification tests. The model application to simulate bond tests on typical PBO-FRCM and Glass-FRCM allowed to analyse slips as well as normal and shear stresses both in the bundle and in the matrix constituting the FRCM, for different defects due to application issues. The result of numerical simulations seems to interpret well the results of the qualification tests with a multi-bundle effect that justifies their scatter.The approach can be applied by varying main mechanical properties of the materials (e.g. elastic modulus, fiber cross section, bond properties) to consider their intrinsic variability in the assessment of the performance of the FRCM system or by changing the type of materials (i.e. mortar and fibre) to optimize the FRCM system.

In-plane behaviour of masonry brick walls strengthened with mortar from two sides

The most destructive effects of earthquakes in Turkey are seen in rural areas and slums. Since most of the masonry structures in those areas are not built in accordance with regulations and standards, they are not able to withstand earthquake effects and, eventually collapse. This situation demonstrates the importance of developing effective strenghening techniques for masonry structures. In Turkey, however, scientific studies usually focus on reinforced concrete, steel, and prefabricated structures. Studies on the strengthening of masonry structures, which constitute the majority of the buildings in Turkey, are more limited. Therefore, the shear behaviour of masonry bearing walls must be analyzed to develop appropriate strengthening techniques against potential earthquakes. In this study, walls made of masonry bricks were strengthened by applying lean and steel fibered repair mortar with different thicknesses on both sides. The mortar was applied by plastering the wall surfaces with a trowel. Seven wall specimens were tested diagonally; one was the reference specimen, and six were the reinforced specimens. The results obtained were compared in terms of strength, ductility and energy consumption values.

Some Key Aspects in the Mechanics of Stress Transfer Between SRG and Masonry

The use of composite materials to strengthen masonry structures has become common practice within the civil engineering community. Steel-reinforced grout (SRG), which comprises high-strength steel fibers embedded in a mortar matrix, is part of the family of the fiber-reinforced cementitious matrix (FRCM) composites that represent a suitable alternative to fiber-reinforced polymer (FRP) composites for strengthening existing structures. Although studies on FRCMs have already reached a certain level of maturity, some key issues remain open, such as the role of matrix type and layout, substrate properties, and test rate. This paper focuses on some of these issues. The results of single-lap direct shear tests on masonry blocks strengthened with SRGs are presented to analyze the bond behavior between the composite material and the substrate. Four aspects are considered: (1) the change in the width of the SRG mortar matrix while keeping the width of the fiber sheet fixed; (2) the type of mortar used for the SRG; (3) the influence of the test rate, and (4) the type of substrate (i.e., concrete vs. masonry). The results obtained indicate the active role of the matrix layout and the importance of the test rate, encouraging further investigations to clarify these aspects.

Impact of FRP and FRCM on the ductility of strengthened masonry members

Strengthening strategies have become extremely efficient as demonstrated in recent scientific works and real field applications. The recent calamitous events focused the interest on existing masonry structures. In the common practice the strengthening strategies are performed improving the load capacity of existing structural elements. However, especially for slender masonry elements like as arches and barrel vaults, the increase of load capacity may not be the only and optimal approach. The ductility capacity represents an important aspect that should be taken into account in a strengthening strategy. A great number of applications is performed without relying on and assessing the ductility of the strengthened elements. This approach could promote deleterious effects on the structural behavior due to the brittle behavior provided by excessive amount of strengthening. Furthermore, a strengthening strategy must respect the compatibility with the masonry substrate especially for heritage applications.The present paper focuses on the out-of-plane behavior of slender masonry elements strengthened with FRP or FRCM systems. A parametrical analysis was performed and results, in terms of bending moment–curvature diagrams and ultimate curvatures were analyzed in dimensionless form. The behavior of strengthened masonry is independent on the type of stress–strain constitutive relationship of composite, depending on mechanical fiber reinforcement ratio, ω. It is very important for practical applications when an overestimation of the effective amount of reinforcement is designed. In this sense the type of composite can be optimized, especially for poor masonry, where higher ω could promote brittle behaviors. At low ω, the type of the stress–strain constitutive relationship becomes a key aspect (e.g. linear or bi-linear strongly correlated to the type of composite: FRP or FRCM). Furthermore, the impact of the axial load both on the ductility capacity and on the load capacity becomes negligible at high values of ω. The results, provided in a dimensionless form, constitute the basis for a valid support to the design of interventions using composites on masonry structures.This work represents a preliminary study to highlight some issues often overlooked in the strengthening of masonry structures, based on direct application of available design guidelines. It is the basis for future development of normalized approaches to perform targeted experimental validations and optimize the design of strengthening systems.

Experimental and analytical investigation of bond behavior in glass fiber-reinforced composites based on gypsum and cement matrices

In this paper, an innovative composite, formed by a glass fiber net embedded in a gypsum-based matrix and aimed at strengthening masonry structures, is proposed. Experimental and analytical investigations on composite systems with gypsum- and cement-based matrixes, both coupled with a glass fiber net, are illustrated. Bond tests between brick and composite systems were carried out on different bond lengths. The gypsum-based composite (FRGM) showed higher peak loads and a more brittle behavior compared to the response of the standard FRCM system. In the framework of mode-II fracture mechanics, analytical modeling of the tested behavior was undertaken based on local cohesive laws derived from experimental evidence. Both short- and long-bond lengths were modeled, and the results well agree with the experimental behavior of the cement-based system.

Experimental Study on Seismic Behavior of Masonry Walls Strengthened by Reinforced Mortar Cross Strips

Due to the poor seismic performance, strengthening of masonry structures is always a significant problem worthy to study. It has been proven that the bearing capacity of existing masonry buildings can be enhanced greatly with efficient strengthening measures. An experimental program was conducted to investigate seismic performance of un-reinforced masonry (URM) walls strengthened by reinforced mortar (RM) cross strips. Eleven walls were tested under horizontal low-cyclic load, simultaneously with a vertical constant load on the top face. Three URM walls were tested as reference. The other eight walls were externally strengthened with 40 and 60 mm thick of RM cross strips on one or both faces. Test results showed that externally strengthening with RM cross strips was an efficient way to enhance the seismic performance of URM walls. The failure modes were divided into shear failure and shear-compression failure. All the tested walls did not collapse until the test ended, while many diagonal cracks and few vertical cracks appeared on mortar strips. After strengthening, the shear capacity of the strengthened walls increased by at least 38.2%, and the reinforcement ratio was noted to be the key factor to influence the shear capacity with positive correlation. Besides, RM cross strips did improve deformation capacity greatly.

Strengthening of Masonry Structures: Current National and International Approaches for Qualification and Design

The strong seismic events that have occurred in Italy during the past few years have shown the vulnerability of unreinforced masonry structures all over the country. The growing interest in the use of composite materials as a fast and effective way for structural strengthening pushed Italian public authorities to define common rules for qualification and design. Different guidelines are being developed at the national, but also at the international level, considering the different composite systems currently present on the market, generally constituted by continuous fibres in the form of fabrics or meshes embedded in a polymeric or inorganic matrix. The paper will summarize and compare the various assessment and design approaches of the composite system, highlighting the different aspects which characterize national and international procedures of certification.

Seismic strengthening of masonry walls using bamboo components

Bamboo is a sustainable green material and has been gradually applied in the construction industry; however, little research on strengthening masonry structures with bamboo has been carried out. In this article, strengthening methods using bamboo were developed including bamboo grid reinforced cement mortar layer, externally bonded bamboo mats, additional confining horizontal bamboo reinforced concrete band beams, and bamboo strips placed in mortar joints. Ten masonry walls were designed including two reference walls. Experimental results showed that all the strengthening methods can improve certain aspects of the seismic performance of masonry walls. The shear strength, deformability, and energy dissipation capacity of masonry walls strengthened with bamboo grid reinforced cement mortar and externally bonded bamboo mats were the most improved. The limit states of tested walls were discussed. Strengthened masonry structures with bamboo components are promising methods and can be used especially in remote areas.

Experimental tests for the characterization of sisal fiber reinforced cementitious matrix for strengthening masonry structures

The experimental characterization of a Natural Fiber Reinforced Cementitious Matrix (NFRCM), made of sisal fibers yarns impregnated with a water-based resin and embedded in an inorganic matrix based on natural lime is presented in this paper. The Sisal-NFRCM performances are investigated through tensile and single-lap shear tests, adopting the dispositions of the Round Robin Tests developed for the FRCM characterization by the RILEM Technical Committee 250-CSM.The tensile stress-strain curve evidenced the tension stiffening effect of the mortar between cracks, which tended progressively to reduce. In shear tests, the failure and the progressive slippage of the sisal yarns was observed.

Numerical Modeling of Traditional Masonry Walls Strengthened with Grout Injection

ABSTRACT The main purpose of this article is to experimentally and numerically study the improvements introduced by the grout injection strengthening technique for the structural rehabilitation of traditional unreinforced masonry (URM) walls. In this context, four damaged URM walls with four different types of lime mortars under shear-compression are strengthened using grout injection method. Injection materials (Grout) which have same contents with existing mortar types are applied to all cracks in damaged walls. Besides, for the numerical simulations, a simplified micro modeling strategy is preferred combining the mortar and the mortar-unit interface behavior and then an elasto-plastic damage (EPD) approach is adopted calibrating the parameters of Oliver’s damage model and the modified von-Mises yield criterion for the masonry constituents. Results show that the proposed approach can be useful to describe the behavior of mortar and mortar-unit interface in strengthened masonry structures with grout injection.

Assessing the performance of CFRP strengthening on masonry walls using experimental modal analysis

Strengthening masonry structures using FRP laminates has been widely studied from a resistance point of view. However, there is a need of a non-destructive technique to validate strengthening interventions. In this paper, experimental modal analysis is proposed as a technique to assess practitioners’ works. Fifteen brick masonry walls were built and strengthened with five different patterns of carbon-FRP laminates. Experimental modal analysis was performed before and after strengthening interventions. Different vibration modes were compared to select the more sensitive one depending on the strengthening configuration. The change in the vibration frequency was analysed and correlated with cross-section stiffness modification. The obtained results showed changes up to 30% on the vibration frequencies due to strengthening installation. On the overall, the proposed experimental methodology is supported by theoretical analytical calculations with an error under 5% in most of the cases.

Shear capacity of masonry panels repaired with composite materials: Experimental and analytical investigations

Composite materials have been widely used to strengthen masonry structures or to repair those damaged by earthquakes. While the effect of composite materials applied on undamaged masonry walls is widely investigated, their effectiveness when used to repair damaged ones still needs further investigations.This paper deals with the shear capacity of masonry panels repaired with carbon fiber reinforced polymer composites. In particular, masonry undamaged panels were tested under diagonal load up to failure, repaired with composite materials, and tested again. The repair technique proved its effectiveness, since the shear strength of the repaired specimens was similar to that of the original ones. The shear capacity of the repaired panels was computed with the approach provided by the CNR DT200 Italian guidelines. Within this framework, the cohesion of the masonry material was neglected for the evaluation of the shear capacity of the repaired panels to take into account for the pre-existing damage. This approach provides a satisfactory agreement with the experimental results.

Tensile strength of flax fabrics to be used as reinforcement in cement-based composites: experimental tests under different environmental exposures

The use of Textile-Reinforced-Matrix (TRM) systems is gaining consensus as a possible technical solution for strengthening masonry structures. In this context, the use of natural fabrics (among which those made of flax) instead of synthetic ones can have a positive impact on several sustainability-related aspects, such as renewability, recyclability, biodegradability, low price. However, both mechanical properties and durability performance of natural fibres and fabrics needs to be further investigated with the aim to make it possible their use in composites for construction. Furthermore, this paper reports the results of a fundamental study on a bidirectional flax fabric eventually intended as the reinforcement in cement-based composite systems. Specifically, it aims at determining the tensile strength of flax of fibres, threads and the fabrics, and investigating how they are influenced by various environmental exposures and aging processes. The results in the experimental tests reported herein show that fibres and fabrics suffered no significant reduction in tensile strength due to the considered environmental exposure. Despite the common belief that natural fibres may be affected by durability issues, the results demonstrate that the flax fabric under investigation can be utilised as a reinforcement in TRM systems, which is the main novelty and original contribution of this paper.