Strengthening Of Masonry Structures Research Articles

Design of masonry strengthening with one-sided shotcrete applications

Load-bearing masonry is quite widespread, including in earthquake-prone areas and zones in which military operations were carried out. At the same time, the existing treasure models, as well as reinforcement technologies, are mainly focused either on strengthening in order to increase the load-bearing capacity, or on restoring structures damaged during operation, actually static loads. Complex retrofitting methods, as well as models of reinforced masonry that have been damaged as a result of seismic or mine-explosive impacts, for the most part are not focused on mass use and imply mainly the re-laying of destroyed areas. At the same time, there is a need to adapt existing high-tech methods of strengthening masonry structures for mass use, taking into account the latest developments in the field of materials science. The most optimal variant of reinforcement, according to the authors, was the use of single-sided shotcrete applications. For this method, a model of reinforced masonry was defined and described in order to carry out a calculation justification and a comprehensive calculation and design methodology was developed, including for load-bearing multi-storey buildings structures. On the basis of laboratory studies, a constructive solution for fixing applications was justified only due to the forces of adhesion to the surface of the base masonry. The specified complex technique was adapted on real objects of multi-storey buildings with load-bearing masonry walls.

Enhancing fire resistance of masonry structures: The potential of ultra high performance concrete (UHPC)

The potential of ultra high performance concrete (UHPC) for strengthening masonry structures is evaluated in this work, both under normal service conditions and after exposure to high temperatures, as a significant gap has been identified on this topic at the present time. The mechanical properties of UHPC exposed up to 800 °C are analyzed by means of destructive and non-destructive tests, and the UHPC-to-masonry bond capacity is tested, depending on temperature and substrate preparation. The strengthening of masonry columns by confinement is studied, proving that UHPC jackets can double the compressive strength and provide ductility, even after exposure to high temperatures. Strengthening of masonry walls to in-plane loads is also evaluated, finding that UHPC layers can increase the shear strength and prevent brittle failures. Experimental results are compared with the predictions of theoretical models from some design guides, slightly adapted to the context of this research, obtaining a good degree of accuracy.

Bond Analysis of Titanium Rods Embedded in Masonry.

Among the techniques utilized for strengthening masonry structures with advanced materials, the adoption of near-surface mounted (NSM) titanium rods stands out as a promising method for increasing the flexural and shear strength of masonry structures. This method is also known as Bed Joint Reinforcement. Ensuring an effective performance of this technique hinges on establishing a strong bond between the NSM reinforcement and the substrate masonry material. The primary objective of this project was to study the mechanics of this bond using NSM threaded and smooth titanium rods while scrutinizing the impact of key parameters on bond performance. Variables under investigation encompassed the rod type (smooth and threaded), bond length, and the material used to fill the groove (type of mortars). It was found that threaded rods outperformed all other types investigated, and pull-out strengths can be significantly improved through careful selection and optimization of the mortar type and bond length.

Preliminary study on double-sided direct shear test of microbial mortar

A new method of strengthening masonry structures of ancient architecture has been proposed, which involves the use of microbial grouting to generate mineralized mortar. To study the shear mechanical properties of microbial mortar, a comparative experiment was designed with 9 groups of specimens, including acid washing method to test the amount of calcium carbonate, penetration method to test compressive strength of mortar, double-sided direct shear method to test shear strength of mortar and get shear stress-strain curve. According to the experimental results, when the brick joint is thick, more bubbles remain at the contact between the brick and the mortar, reducing the saturation of the grouting liquid and affecting the amount of calcium carbonate generated, thereby affecting the shear strength of the mortar. When the brick is placed vertically, the contact area between the brick and the mortar is on the side, and it is not easy for bubbles to remain, so the amount of calcium carbonate generated is high, and the corresponding shear strength of the mortar is high. Reducing the concentration of calcium solution and implementing bidirectional perfusion can slow down the deposition of calcium carbonate and blockage of perfusion pathways, thereby improving the homogeneity of the carbonate level. A well-graded filling of sand particles is an effective measure to improve the binding efficiency of calcium carbonate.These measures not only increase the production of calcium carbonate, but also result in higher compressive and shear strength.The shear stress-displacement curve of microbial mortar specimens with damagedmortar layer exhibits strain-softening behavior, while the specimens with inter facial damage show strain-hardening behavior.The shear strength of specimens with interface damage is lower than that of specimens with mortar layer damage, so measures should be taken to avoid interface damage.

Improved Bond Stress-Slip Relationships for Carbon Fibre-Reinforced Polymer-Strengthened Masonry Triplets

Carbon fibre-reinforced polymer (CFRP) emerges as a viable solution for reinforcing unreinforced masonry (URM) walls subjected to shear loads. While masonry structures are straightforward to construct, the complexity of the construction materials, especially in terms of their mechanical properties, poses challenges for numerical studies of their structural behaviour. Walls, being fundamental components in masonry construction, play a crucial role in transferring both horizontal and vertical lateral forces. This study investigates the enhancement of masonry wall behaviour through the reinforcement of CFRP. CFRP reinforcement increases ductility and strength, reducing the risk of failure under shear conditions. Additionally, CFRP composites present a practical solution to strengthening masonry structures compared to traditional reinforcement. However, brick, mortar, and CFRP have not been thoroughly investigated. Experimental tests on the bond behaviour of different configurations of CFRP-retrofitted masonry triplets have not been performed before and are therefore presented in this paper. Triplet specimens, comprising three bricks and two mortar joints, both with and without CFRP strengthening, were subjected to bond testing. The study affirms that masonry triplets strengthened with CFRP under shear loads exhibit strength levels at least four to six times greater than those without CFRP. The experimental work was carried out with eight different CFRP configurations on triplet masonry, and each test was repeated four times. Further, the bond stress-slip relationship in the case of masonry triplets with and without CFRP was predicted with new mathematical equations based on the conducted test results. These equations were included in the commercial finite element software ANSYS and used to conduct simulations of CFRP-reinforced masonry triplets. The numerical results indicate good agreement between the finite element model and the test results. The outcome of this research improves the current knowledge on the use of CFRP to reinforce masonry walls with brick and mortar, which will contribute to the understanding of the effect of CFRP on masonry structures.

ВОССТАНОВЛЕНИЕ КАМЕННОЙ КЛАДКИ ПОСЛЕ ДИНАМИЧЕСКИХ ВОЗДЕЙСТВИЙ. МЕТОДЫ РАСЧЕТА И ПРОЕКТИРОВАНИЯ

Civil engineers are encouraged to apply novel techniques, to improve and to adapt well-known methods amid the reality of modern life. An impressive number of restoration and strengthening techniques are developed for unreinforced masonry (URM) material well-known for centuries. Masonry may need restoration and strengthening due to errors made in the course of design, construction or long-term operation of buildings and structures. Besides, masonry needs strengthening in seismic areas. Strengthening of masonry structures, subjected to dynamic impacts during military operations, was initiated after the Second World War. Construction technologies advanced considerably over the last seventy years, and today shotcrete, a widely known strengthening technique, can be applied for a good reason. This article addresses shotcrete as a method for restoring masonry damaged by explosion impacts. Results of the laboratory testing of materials and parts of structures are provided together with improved methods of analysis. The methodology for computer-aided analysis of buildings is also presented, taking into account the staged nature of work and the ability of external shotcrete to support loads. Practical restoration of buildings is addressed in the conclusions section, and conclusions are drawn there.

An Experimental Study of Strength Increase in Masonry Wall Reinforced by One-sided Khorasan Mortar with Steel Mesh

The strengthening of masonry structures is of paramount importance due to their inherent lack of tensile elements, rendering them susceptible to tensile stresses induced by horizontal loads and the resultant substantial damage. Consequently, this study aims to develop a reinforcement system for the strengthening of historical masonry structures by using mortars devoid of cement and modern pozzolan. Experiments were conducted on masonry walls unreinforced and reinforced by a one-sided Khorasan mortar with steel mesh. Initially, six masonry brick walls constructed using Khorasan lime mortar were prepared. Subsequently, after a waiting period of six months, Khorasan plaster mortar, reinforced with steel mesh, was applied to three masonry walls on one side. Following an 18-month waiting period, all samples were subjected to testing in an experimental setup designed and manufactured for this purpose. The wall reinforcement resulted in a significant increase, with the average peak load by 215.73%, and the average displacement by 48.82%. The experimental shear force on unreinforced walls was found to be 41.23% lower than Eurocode-6 and 1.41% lower than TBDY-2018. In the case of one-sided reinforced walls, the experimental shear force was 9.94% lower than Eurocode-6 and 6.16% lower than TBDY-2018. This form of strengthening not only obviates the use of potentially damaging cement in historical buildings but also extends the lifespan of the reinforced structures. Doi: 10.28991/CEJ-2023-09-12-019 Full Text: PDF

Experimental investigation on a FRCM bio-composite for sustainable retrofitting of masonry buildings

Strengthening masonry structures by using green and natural materials has received increasing attention. Natural fibers can be obtained from vegetables, animals or mineral sources and can be used to meet modern buildings safety standards and to reduce carbon dioxide emissions. This paper presents a novel method to strengthen masonry buildings through the application of a mortar coating reinforced with natural fibers made of a jute or a basalt grid. This class of materials is known as Fiber Reinforced Cementitious Matrix (FRCM). FRCM composites can produce an increment in the mechanical properties of masonry members and contribute to the thermal-acoustic insultation of the building envelope. The mortar coating is made of a hydraulic lime: to increase thermal properties this was admixed with short spelt fibers. Both matrix (lime mortar) and fibrous material were initially mechanically and thermally characterized. Laboratory tests have been carried out to study the bond characteristics between the FRCM composite and brickwork masonry. It was found the low mechanical properties of jute fibers are aggravated by a low FRCM-to-masonry bonding, making difficult to use this material for structural retrofit. On opposite, basalt fibers exhibit high mechanical properties and their use in conservation is surely promising and interesting.

Bond behavior between Fe-SMA strips and mortar in masonry joint after resistive heating

In recent years, iron-based shape memory alloys (Fe-SMA) have attracted much interest as an innovative prestressing material with the ability to eliminate the need for on-site tensioning and complex anchoring. This unique property offers application potential for using Fe-SMA to effectively strengthen masonry structures, especially in narrow space joint strengthening applications. Here, based on the premise that the Fe-SMA needs heating to generate prestress, the bond characteristics between the Fe-SMA strips and mortar in the masonry joint after resistive heating were investigated. The effects of various factors, such as heating time, strips surface condition, and bond length, on the bond behaviors were analyzed. The results show that under reasonable resistance heating conditions, Fe-SMA did not significantly damage the bonding interface during masonry joint strengthening. After resistive heating at a heating temperature of 200 °C and a heating time of 30 s, the bond strength of the specimens remained high, and it only decreased by 9.38 %∼10.36 % of the bond strength of the unheated specimen. Following the rough surface treatment of Fe-SMA strips, the bonding properties of the Fe-SMA strips were obviously improved before and after resistive heating, and bond energy dissipation was the clearest performance increase to over 5 times the original value. In addition, a prediction model for the bond strength between the Fe-SMA strip and mortar in masonry joint strengthening after resistive heating was proposed and could provide references for subsequent studies.

Behavior of grout injected solid stone masonry walls under in-plane loading

It is a shared duty of all humanity to ensure the preservation of historical structures for future generations. The discontinuities, cracks, and gaps in the masonry walls, especially in historical structures, prevent the building from working monolithically. For this reason, historical buildings need repair and strengthening efforts in time. Grouting is an effective technique for consolidating and strengthening masonry structures. This study aims to establish a general evaluation of the injection method used to repair damaged stone walls. For this purpose, solid stone masonry walls were constructed and tested under in-plane loading to simulate the actual conditions. The solid stone masonry walls are made of saw-cut travertine stones and plastered with English bond, a pattern formed by laying alternate courses of stretchers and headers. The mechanical behavior of the solid stone masonry walls, both in non-damaged condition and with grout injected into the cracks caused by the preloading, was monitored and demonstrated in terms of the load-displacement relationship. As a result of the wall tests, it was observed that there was an increase of up to 28% in the maximum lateral load that the walls can withstand without any crack formation after grout injection.

Compressive behavior of brick masonry columns confined with composites embedded in the horizontal mortar joint

The application of composite materials embedded in the horizontal mortar joints to strengthen masonry structures with insufficient load-bearing capacity has shown beneficial effects in terms of improving mechanical behavior and preserving the aesthetics of the building. To investigate the effectiveness of this strengthening technique, concentric axial compressive tests were performed on eight masonry columns, including two unconfined columns, five specimens confined by carbon fiber reinforced polymer (CFRP) plates, and one specimen confined by engineered cementitious composite (ECC). The failure mode, strength, deformability, and energy dissipation capacity of the specimens were investigated. The effective tensile strain of the CFRP plates was also discussed. The results showed that the CFRP-confined masonry columns exhibited brittle failure, which was characterized by the debonding failure of the CFRP caused by the sudden fracture of the epoxy resin, while the failure of the ECC-confined masonry column exhibited significant ductility, the reason for which was that the propagation of vertical cracks was delayed due to the strain-hardening characteristics of the ECC. The reinforcement technique was effective for masonry columns in obtaining better compressive performance. The peak strength and energy dissipation of the confined columns were increased by a maximum of 86.31% and 115.71%, respectively. A non-uniform distribution of the effective tensile strain of the CFRP plates along the perimeter was observed. In addition, a simple strength model for masonry columns confined by embedded material in the horizontal mortar joints was developed based on the best-fit analysis of a database assembled from the existing literature and the current study. The predictive capabilities of the proposed model and an existing model for the compressive strength of confined masonry columns were analyzed.

Ferrocement, Carbon, and Polypropylene Fibers for Strengthening Masonry Shear Walls.

This paper describes an experimental investigation into the feasibility of using ferrocement jacketing, polypropylene fibers, and carbon fiber reinforced polymer sheets (CFRP) to enhance the shear resistance of unreinforced brick masonry. The study involved testing 12 wall panels in diagonal compression, three of which were strengthened using each of the above-mentioned techniques. The results showed that all three strengthening techniques led to a significant improvement in the shear resistance and deformation capacity of the unreinforced walls. Furthermore, the results showed that the strengthened walls exhibited a significant improvement in shear resistance and deformation capacity by a factor of 3.3-4.7 and 3.7-6.8, respectively. These findings suggest that ferrocement jacketing is a viable and highly effective method for strengthening masonry structures. Test results can assist in the decision-making process to identify the most suitable design and retrofitting solution, which could indicate that not only new materials, but also traditional methods and materials (ferrocement) could be interesting and effective, also considering their lower initial cost.

Estimation of the Ultimate Strength of FRP Strips-to-Masonry Substrates Bond

Fiber-Reinforced Polymers (FRP) were developed as a new method over the past decades due to their many beneficial mechanical properties, and they are commonly applied to strengthen masonry structures. In this paper, the Artificial Neural Network (ANN), K-fold Cross-Validation (KFCV) technique, Multivariate Adaptive Regression Spline (MARS) method, and M5 Model Tree (M5MT) method were utilized to predict the ultimate strength of FRP strips applied on masonry substrates. The results obtained via ANN, KFCV, MARS, and M5MT were compared with the existing models. The results clearly indicate that the considered approaches have better efficiency and higher precision compared to the models available in the literature. The correlation coefficient values for the considered models (i.e., ANN, KFCV, MARS, and M5MT) are promising results, with up to 99% reliability.

Closed-form model for curved brittle substrates reinforced with FRP strips

Externally bonded Fiber Reinforced Polymer (FRP) strips have been proved to be effective for strengthening masonry structures. The role played by the bond between composite and substrate is crucial for transferring tensile stresses from masonry to the reinforcing system, thus has become the object of extensive investigation. When the reinforced components are curved, such as arches and vaults, which appear quite commonly in masonry buildings, the influence of the substrate curvature on the bonding cannot be ignored. The debonding mechanism can be studied with a concise model in which the substrate is rigid and the FRP plate is elastic, and the interface where the two are bonded is assumed to lump all the nonlinearities. In this paper, a fully analytical approach for FRP strengthened curved masonry prisms is proposed based on this model, providing closed-form solutions for different geometrical configurations. The interface cohesive law is assumed to be piecewise linear with elastic, softening and residual strength stages. The influence of the normal stress appearing at the interface due to the substrate curvature is considered in the cohesive law for both extrados and intrados strengthening cases. An extensive validation carried out considering both existing experimental results and alternative numerical approaches indicates that the procedure proposed is able to accurately predict the bond strength as well as to reproduce the global bond behavior, requiring few parameters and little calculation effort.

Effect of early curing and substrate preparation conditions on the physical and mechanical properties of TRM-masonry composites

• The effect of different curing conditions on TRM composites performance is investigated. • The influence of moisture content bricks on TRM-to-substrate bond behavior is investigated. • The development of the fiber-to-mortar bond behavior is influenced by the curing conditions. • The substrate initial water content affects the TRM-to-substrate bond behavior. Textile reinforced mortars (TRMs) are innovative sustainable composite materials that recently received extensive attention for strengthening of masonry structures. Understanding the mechanical performance of these composites has therefore been the subject of many recent studies. However, most of these investigations have focused on the mechanical properties of these composites under controlled laboratory conditions, and their performance under field conditions is still unknown. The critical role of substrate preparation and curing conditions on the performance of these repair systems have never been addressed before. To address this gap, this paper presents an experimental investigation on the role of these factors on the performance of two different TRM systems (a steel-based TRM and a glass-based TRM) commonly used for strengthening masonry structures. The results show the importance of these factors and the need for the identification of standard procedures for the application of these systems aiming the optimization of their performance in the field.

A novel parallel wire steel-reinforced mortar (PW-SRM) method versus textile reinforced mortar (TRM) for out-of-plane strengthening of masonry walls

Lateral loads cause extensive damage to buildings due to the weakness of bearing out-of-plane un-reinforced masonry (URM) walls. Strengthening URM walls is consequently favored by researchers aiming at achieving building and respective occupant safety. The strengthening of masonry structures with textile reinforced mortar (TRM) or steel-reinforced mortar (SRM) has become prevalent in the last two decades. This study presents an innovative method of strengthening masonry walls using mortar reinforced with high tensile strength steel (HTSS) parallel wires. For the first time, an unprecedented investigation as to the effects of the grooving method improving the load-carrying capacity and failure mechanism of masonry walls under out-of-plane loading was performed. In addition, the parallel wires steel-reinforce mortar (PW-SRM) was compared with the textile-reinforced mortar (TRM) reinforcement method. Nine walls with dimensions of 1400 × 440 × 100 mm each were reinforced through different techniques and tested under four-point out-of-plane bending. The implications of the number of glass textiles with open mesh layers, one-sided and two-sided reinforcement, engineered cementitious composites (ECC), grooves on the masonry substrate, and, the volume of HTSS wires in increasing out-of-plane loading were compared and discussed. The findings demonstrated that the best performance is related to the specimens reinforced by the PW-SRM method. Similarly, the amount of out-of-plane load-bearing capacity and energy absorption increased more than 24 and 5300 times compared to the control specimen, respectively. It was further discovered that the grooving method transferred the tensile stress created in the TRM composite to the depth of the masonry substrate, which enhanced the failure mechanism from debonding to textile rupture and increased the strength by 88% compared to the corresponding specimen without grooves.

Tensile Behavior of Textile-Reinforced Mortar: Influence of the Number of Layers and their Arrangement

In the last decade, textile-reinforced mortar (TRM) composites have been introduced as a sustainable solution for the strengthening of masonry structures. As an externally bonded reinforcement system, it consists of textile fibers embedded in an inorganic matrix (e.g., lime or cement mortar) applied to the substrate. Even though many studies have been focused on characterizing the mechanical behavior of TRM composites in recent years, there are still some drawbacks, including their tensile performance and few studies about multilayer textiles arrangement. This work aims at clarifying the effect of adding a second textile layer to TRM composites and investigating how textile arrangement affects the tensile behavior of the composite system. For this purpose, AR-glass and steel-based TRM composites were used in single layer and multilayer with different arrangements embedded in a lime-based mortar. The results show that using two plies of textile mesh improves the tensile response of TRM composites. In addition, it is found that the arrangement of different layers in the matrix influences the TRM response in different stress stages. The addition of a thin layer of mortar between two layers seems to improve the stiffness in uncracked condition and slightly decreases the final strength of TRM. Thus, the present study makes a step toward optimizing the arrangement of textiles in multilayer TRM composites.

Experimental Investigation on the Bond Behaviour of Masonry Element Strengthened with Carbon-TRM and Steel-TRM

Textile reinforced mortar (TRM) composites are an innovative solution for strengthening existing structures. TRM composites are produced with high-strength textile embedded in inorganic mortar matrices, demonstrating compatibility with existing masonry. As TRM is externally bonded to the surface of the structure, the bond behaviour between TRM and masonry substrate is a critical issue to investigate. In this study, experimental research aims to deeply understand the bond behaviour of TRMs for strengthening masonry structures. The experiments mainly consist of a series of single-lap shear bond tests on TRMs to masonry substrates. Two types of textiles (carbon and steel textile) combined with two mortar matrices (cement and lime-based mortar) were used to construct TRM. The effect of the textile and mortar matrix is presented. The test results are discussed in terms of the full range of load-slip responses and failure modes. The result showed that TRM with steel textile and cement-based mortar matrix offers better bond behaviour, but the risk of masonry damage should also be considered.

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.