Mortar Joints Research Articles

Experimental study on the bonding behavior of post-embedded steel bar and brick masonry

Post-embedding of steel bars into mortar joints of unreinforced masonry walls is a widely used strengthening technique in historic masonry construction. Despite its widespread use, limited research is available on the effectiveness of this technique for improving the bonding performance of post-embedded steel bars. In this study, the bond-slip relationship for bars embedded in mortar joints was investigated. The masonry specimens were built using mortar with a strength of 5 MPa, and then reinforced with 5, 10, 15, and 20 MPa mortar and plain steel bars 6 and 8 mm in diameter, and a series of pull-out tests were performed on these specimens. By analyzing the bond strength between the post-embedded steel bars and brick masonry, the bond failure modes, and the bond-slip curves, a typical bond-slip curve and bonding parameters were obtained. The effects of the mortar strength, steel bar diameter, and bond length on the bond strength were discussed, and a mathematical expression for the bond strength between the post-embedded steel bars and brick masonry was presented. In addition, the relationship between the bond strength and the slip distance of the steel bars was obtained, and the interfacial bond fracture energy was analyzed.

A macro-modelling continuum approach with embedded discontinuities for the assessment of masonry arch bridges under earthquake loading

The paper presents a novel effective macro-modelling approach for masonry arches and bridges under cyclic loading, including dynamic actions induced by earthquakes. It utilises an anisotropic material model with embedded discontinuities to represent masonry nonlinearities. Realistic numerical simulations of masonry arch bridges under static and dynamic loading require accurate models representing the anisotropic nature of masonry and material nonlinearity due to opening and closure of tensile cracks and shear sliding along mortar joints. The proposed 3D modelling approach allows for masonry bond via simple calibration, and enables the representation of tensile cracking, crushing and shear damage in the brickwork. A two-scale representation is adopted, where 3D continuum elements at the structural scale are linked to embedded nonlinear interfaces representing the meso-structure of the material. The potential and accuracy of the proposed approach are shown in numerical examples and comparisons against physical experiments on masonry arches and bridges under cyclic static and dynamic loading.

Experimental assessment of in-plane behavior of traditional Tibetan three-leaf walls

Multi-leaf masonry is a technique that can be dated back many centuries, but limited reports can be found on the seismic behaviors of three-leaf walls (TLWs). This study intended to understand better the seismic capacity of traditional Tibetan three-leaf walls (TTTLWs) constructed with stones and mud mortar by an in-plane cyclic shear loading test. A series of test results, including failure modes, hysteretic behavior, ductility, stiffness degradation, energy dissipation, and in-plane and out-of-plane bulge of tested walls were analyzed and discussed. And the experimental values of the shear strength and shear modulus were compared with the predicted values given by prediction models available in literature. Three types of failure modes were found on the wall specimens in this test: sliding of mortar joints, cracking of block stone at wall toe, and separation between external and inner leaves. The TTTLWs may be concluded to have a low shear strength of 0.118–0.162 MPa and medium shear modulus of 99–147 MPa compared with the published data of multi-leaf stone masonry. The shear strength of TTTLWs is controlled by the flexural failure, and can be predicted by the formulas recommended by ASCE.

MUDis: A low computational effort multi-unit discretization procedure for modelling masonry walls with periodic arrangement

This paper presents an innovative modelling approach for assessing the in-plane behaviour of masonry walls characterized by periodic arrangements of units and mortar joints. The proposed model is a typical d-FEM (Discontinuum Finite Element Model) consisting of deformable macro-blocks covering more than a single unit of masonry and separated by a reduced number of interface elements, which are properly arranged along pre-established potential crack/slip surfaces. To account for the modelling simplifications deriving from the proposed multi-unit discretization (MUDis), three of the mechanical parameters describing the adopted interface model, i.e. the Composite Interface (CI) model formulated by Lourenço & Rots, are optimised through an Evolutionary Polynomial Regression (EPR) technique relying on a Multi-Objective Genetic Algorithm (MOGA).In the first part of the paper, benchmark experimental tests taken from literature are reproduced to calibrate conventional CI-based FEM models and to generate therefrom a wide stock of masonry panels. Afterwards, the novel MUDis approach is applied to the generated wall population in order to evaluate its capability of replicating the in-plane response of a considerable number of masonry walls regardless of their geometrical and mechanical features. Extensive details on how deriving the optimal parameters of the CI model to allow for the reduction of interface elements inherent to the MUDis approach are provided, followed by a sensitivity analysis to evaluate the mesh dependency of the results.Finally, the proposed MUDis procedure is cross validated against the experimental results of the initial benchmark tests, allowing to demonstrate its computationally efficiency with respect to conventional micro-modelling procedures.

Construction Forms and Seismic Performance of the Ancient Chinese Buildings Joined by Tenon–Mortise Joints

Zijincheng, also known as the Forbidden City, is the largest and best-preserved ancient palace-type wooden building in China, built without using a single nail. Since it was built in 1420, it has served as a palace where the emperor lives and works. Almost 600 years old, it has gone through two dynasties (i.e., the Ming and Qing dynasties (AD 1368–AD 1912)) and 24 emperors. It has survived more than 200 devastating earthquakes throughout its history, and it is still standing strong. In this paper, the authors introduce the construction technology of the ancient Chinese wooden structure as well as the greatest secret of the well seismic performance of the Forbidden City. The study found that the secret to the Forbidden City’s ability to withstand many powerful earthquakes lies in the flexibility of its timber structure, which is mainly reflected in the application of the tenon-mortise joints, energy-dissipation capacity of Dougong brackets and shallow-buried columns. The seismic responses of a 1:5 reduced-scale model of Shoukang Palace of the Forbidden City under different earthquake magnitudes are studied through a series of shaking table tests.

Experimental Study on Tenon and Mortise Joints of Wood-Structure Houses Reinforced by Innovative Metal Dampers

To improve the seismic performance of Chinese traditional wood-structure houses, this paper proposes to strengthen their mortise and tenon joints by applying an innovative metal damper. According to the dimensions of the “Yikeyin” wood-structure houses in the Tonghai area of Yunnan Province, two groups of six samples of three types of mortise and tenon joints were manufactured, in which one group was mounted with dampers made of Q235 steels. Subsequently, a low-cycle repeated loading test was conducted to examine the overall behavior of these joints. Various characteristics of seismic performance indexes, such as the moment–rotation hysteresis curve, skeleton curve, stiffness degradation, energy dissipation capacity, residual amount of tenon and the removal before and after reinforcements of straight, penetrated and dovetail tenon joints were analyzed. The test results show that these tenons exhibit good deformation capacity, their hysteresis curves became fuller and their “pinch” effects were significantly reduced, all after their joints became strengthened, indicating that their joint slips were reduced during the loading processes and their residual amounts of tenon removals were under effective control. Compared with the blank group, the joint stiffness was substantially improved, and the increase in the reverse stiffness turned greater than that of the positive stiffness at each stage of loading, while the degradation curve of the whole joint stiffness became steeper. After mounting the dampers, the bearing capacity and energy dissipation of the joints were significantly improved, the equivalent viscous damping coefficients of the straight and penetrated tenon joints were increased, but that of the dovetail joint was slightly reduced. These study results can provide a reference for the reinforcement and protection of traditional wood-structure houses.

Interface modeling in load transfer mechanisms of multi-leaf masonry panels

Masonry heterogeneity and manufacturing imperfections affect structural behaviour of constructions; these unpredictable aspects are amplified in multi-leaf masonry walls by interfaces which play a main role on load transfer mechanisms and on global behaviour. In this paper, the performances of multi-leaf masonry walls subjected to compressive loads are investigated using experimental and numerical approaches. 9 experimental tests were performed on three-leaf specimens constituted by external leaves made of a regular pattern of brick elements and mortar joints, and a disaggregate internal core made of brick potsherds and mortar. FE models were developed to assess the interface interaction among layers through model updating procedure based on dynamic parameters experimentally identified at incremental load conditions, keeping fixed the mechanical parameters of external layers. FE models were updated by 13 parameters: i) unidirectional stiffnesses of 12 springs located between external layers and core; ii) elastic modulus of core. Updated models were tested through non-linear static analysis and results were compared with structural performances of multi-leaf masonry panels. Interface effectiveness regulated by different stiffness hierarchies among layers and between bricks and mortar joints handles the load transfer mechanisms and failure scenario. Compressive load is transferred through out-of-plane mechanisms triggered by maximum tensile stresses by 20% of failure load. In the plane the load is transferred with maximum compressive stresses at the ends (30% of failure load) decreasing centrally. Tangential stresses among layers are maximum near to load application and are linearly distributed in the plane of panels.

An improved mean-field homogenization model for the three-dimensional elastic properties of masonry

Accurate assessment of the overall mechanical behavior of masonry, composed of bricks and mortar joints, remains challenging due to its inhomogeneous and orthotropic nature. In this study, the feasibility of various mean-field homogenization schemes for the three-dimensional orthotropic elastic properties of masonry is comprehensively investigated. Three kinds of masonry patterns are considered, including the stack bonded pattern, the running bonded pattern and the double-leaf Flemish bonded pattern that has received limited attention so far. Special attention is paid to the homogenization schemes which have not been applied to the masonry case, such as Lielens’ interpolative double inclusions (D-I) and the interaction direct derivative (IDD) schemes. After a comparison between the well-known mean-field homogenization schemes, an improved micro-mechanical model is proposed by combining the advantages of the IDD and D-I models. The validation of the proposed model is conducted through a comparison against experimental data from literature and numerical results obtained via finite element analyses (FEA). The results show that the proposed model can accurately evaluate the orthotropic elastic properties of the three masonry typologies for a wide range of stiffness ratios between brick and mortar, ranging from 1 to 1000. The proposed model also shows better performance than the classical schemes especially when the stiffness ratios between brick and mortar are higher than 10, which is of major importance for the application of mean-field homogenization based multiscale methods to the nonlinear analysis of masonry. Furthermore, the presented homogenization method can be of interest for other anisotropic materials, e.g., laminate materials.

Strength and stiffness of 3D-printed connectors compared with the wooden mortise and tenon joints for chairs

ABSTRACT In this study, 3D-printed connectors to replace the typical L-shaped joints in the construction of a chair were developed, tested and numerically analysed. Different connectors were designed and manufactured with a fused deposition modelling (FDM) 3D printer using acrylonitrile butadiene styrene (ABS) with the aim to find a simple shaped connector which could be used to build chairs and withstand standard chair loading requirements. The strength and stiffness of the joints were tested and compared with traditional beech mortise-and-tenon joints. Numerical stress and strain analyses were performed with the finite element method for an orthotropic linear-elastic model. The experimental results showed that joints with 3D-printed connectors achieved lower strength than the traditional wooden mortise-and-tenon joints with similar dimensions. The results indicate that the effect of reinforcement of the connector were not recognised due to the small thickness and inadequate geometric position and arrangement of the reinforcement ABS material. The chair assembled with 3D-printed connectors could withstand the loads for seating, but failed the backrest test according to standard EN 1728:2002. The connectors need to be optimised and reinforced to withstand standard loads.

An aggregated non-destructive testing (NDT) framework for the assessment of mechanical properties of unreinforced masonry Italian medieval churches

Medieval churches constructed of unreinforced masonry (URM) represent critical assets of Italian architectural heritage. In order to preserve these churches against earthquakes, obtaining robust information regarding their material mechanical characteristics is necessary as part of a reliable structural analysis and strengthening intervention program. Given the drawbacks of semi-destructive or destructive testing of heritage material, non-destructive testing (NDT) is the most viable approach to obtain data regarding the mechanical characteristics of the material composing the structure of the churches. However, there are several uncertainties inherent within NDT techniques based on the current state of the art. Thus, two different NDT techniques (i.e., rebound hammer testing, and pulse velocity testing) and two expert judgment-based investigation techniques (i.e., masonry quality index, and mechanical properties ranges based on the Commentary to the Italian building code) were applied to 170 specimens belonging to the walls of 72 URM Italian medieval churches to assess the quality of the URM and its components. The surveyed churches walls, although highly variable in geometry, materials, and conditions, can be classified into four URM types: a) irregular stone masonry with pebbles, erratic and irregular stone units; b) roughly cut stone with good bond; c) ashlar masonry with regular squared blocks and mortar joints; and d) solid fired clay bricks with lime mortar. Subsequently, using the SonReb technique, predictive equations that aggregate the two NDT techniques and the correlation coefficient specific for each URM type were developed to define some of the critical mechanical properties of the URM (i.e., compressive strength, Young’s modulus, and shear modulus). The mechanical properties determined via predictive equations were then plotted and compared with the predictions of the two well-established expert judgment-based investigation techniques to evaluate the accuracy of the approach. Finally, a partial validation based on NDT and destructive testing techniques of six URM prisms was performed to evaluate the accuracy of the proposed predictive equations. Ultimately, three equations to determine the compressive strength, the Young’s modulus, and the shear modulus were developed. The developed equations offer to engineering practitioners a rapid NDT technique to assess URM properties that do not solely rely on the judgment and expertise of the practitioner.

Parametric Analysis of Failure Loads of Masonry Textures by Means of Discontinuity Layout Optimization (DLO).

Several masonry structures of cultural and historical interest are made with a non-periodic masonry material. In the case of periodic textures, several methods are available to estimate the strength of the masonry; however, in the case of non-periodic masonry, few methods are available, and they are frequently difficult to use. In the present paper we propose using discontinuity layout optimization (DLO) to estimate the failure load and mechanism of a masonry wall made with non-periodic texture. We developed a parametric analysis to account for the main features involved in the estimation of failure: in particular we considered three different textures (periodic, quasi-periodic, and chaotic), variable height-to-width ratio of the wall (from 0 to 3) and of the blocks (from 0.25 to 1), different mechanical properties of mortar joints and blocks, and possible presence of a load on the top. The results highlight the importance of the parameters considered in the analysis, both on the values of the failure load and on the failure mechanism. Therefore, it is found that DLO can be an useful and affordable method in order to assess the mechanical strength of masonry wall made with non-periodic textures.

Nonlinear truss modeling method for masonry-infilled reinforced concrete frames

This paper presents a new modeling approach for masonry-infilled, reinforced concrete (RC) frames, based on the nonlinear truss analogy. The unreinforced masonry infill walls are modeled with truss cells containing horizontal, vertical and inclined elements. Zero-length macroelements are also used to account for the localized damage on mortar joints. The frame members are modeled with a recently proposed scheme that can account for inelastic response and can capture the effect of shear damage and failure of RC columns. The modeling approach is validated through analysis of experimentally tested masonry-infilled frames, and its capability in capturing the strength and damage patterns is evaluated. A sensitivity study is also provided, to elucidate the impact of several model parameters on the obtained analytical results.

Parametric Analyses of the Response of Masonry Walls with Reinforced Plaster

This paper presents the results of a parametric study on the response of unreinforced and retrofitted masonry specimens. The adopted strengthening technique is the steel-reinforced plaster, which is very commonly used but it is not supported by a proper theoretical and experimental characterization in the scientific literature. The aim was to investigate the main parameters that affect the structural performances of the walls. Several numerical models were implemented using the finite element method to analyze the influence of the bricks’ arrangements, the mechanical properties of the mortar joints, the number of connectors, and the mechanical properties and thickness of the plaster coating. A concrete damage plasticity model was adopted to describe the bricks, the mortar joints, and the plaster behaviors. For the unreinforced specimens, the outcomes confirmed that the mortar strength had a significant influence on the performance of the wall, together with the presence of potential weaknesses in the bricks, while the bond effect was negligible. For reinforced walls, the connectors do not have a significant influence on retrofitted wall capacity but may prevent instability if a proper number is considered. Furthermore, the strength of the plaster coating does not affect the collapse load significantly, while increasing the fracture energy, which can be produced, for instance, by using fiber-reinforced concrete, increases the capacity of retrofitted walls, with more limited damage. Finally, an increase in the plaster thickness may be beneficial in terms of collapse load, even though greater thickness may increase the seismic masses significantly.

Development and Application of Non-Destructive Testing Instrument for Wall Impermeability Based on a Water Drenching Method

This article introduces a new type of on-site non-destructive testing instrument for the impermeability measurement of walls. The research and development of this instrument is based on the water drenching method. The influence of symmetry is mainly considered in the design process, and it has on-site testing functions such as water pressure adjustment and wind pressure simulation. The water seepage of the four types of masonry walls under the combined working conditions of three levels of spray strength and wind pressure strength was tested by using instruments. The results show that the weak location regarding the impermeability of the masonry wall is the junction of mortar joints. Parameters such as wall leakage time, seepage area and water seepage amount are significantly affected by wind pressure. Furthermore, during on-site inspection, the influence of wind pressure on water seepage conditions must be considered when simulating the real wind-driven rain conditions. The testing instrument will be promoted and used according to the compiled industry regulations.

Geogrid reinforced brick buildings for earthquake disaster mitigations

In a severe earthquake, the walls of brick buildings experience in-plane shear and out-of-plane bending, leading to diagonal crack and corner failure, respectively. In this study, an experimental investigation was carried out to observe the above damages on brick masonry buildings reinforced with geogrid embedded in bed joint mortar of the walls. It was observed that the geogrid reinforced brick panels showed better shear strength, lateral strength, ductility, etc. A qualitative comparison was made using a sinusoidal shake table test on a one-fourth single-room building model consisting of two sets of corner walls with and without geogrid reinforcement. It was observed that the corner wall without reinforcement showed crack initiation at 0.45 g and complete collapse with over toppling of the transverse wall at 0.90 g. In contrast, no sign of damages for the corner walls strengthened with geogrid reinforcement for any level of shaking.

Experimental Study of Masonry Wall Strengthened by Polypropylene Fiber Mortar

Based on previous studies, the average strength of Indonesia's masonry wall shows a weak compressive strength that increases the vulnerability of buildings with masonry walls towards the seismic load. This study presents an experimental investigation of the masonry wall's flexural capacity strengthened with Polypropylene Fiber (PP Fiber). In general, the experiments were divided into two groups: the masonry wall with PP Fiber in a joint mortar and the masonry wall with PP Fiber in a plastering. The investigation was carried out on twelve specimens. The specimens consisted of three standard masonry wall (DBK) samples as the controlled specimens, which are without plastering and PP Fiber, three masonry wall samples with PP Fiber (DBP) in a joint mortar, three masonry wall samples with normal plastering (DBKP), and three masonry wall samples with PP Fiber in a joint mortar and plastering (DBPP). The experimental investigation proved that the addition of PP Fiber to the mortar mixture at joint masonry mortar could increase the masonry wall's flexural capacity. The results showed that the mortar with 8% PP Fiber improves the compressive strength by 58.46%. The flexural testing showed that 8% PP Fiber to the mortar could increase the flexural capacity to 35.8%. The maximum deflection also increases as much as 38.58% for masonry walls with PP Fiber on mortar and plastering, compared to the masonry wall without PP Fiber. In addition, the presence of Polypropylene Fiber contributes to give a higher flexural capacity.

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.

Numerical Analysis of Masonry Structures through a Modified Composite Interface (MCI) Model

This paper presents an innovative modelling approach for evaluating the structural response of new and existing masonry buildings characterized by a periodic arrangement of units and mortar joints. The aim is to provide a calculation tool that allows to model the non-linear behaviour of masonry structures under lateral and vertical loads with a reduced numerical effort, without compromising the accuracy of the results. The proposed model is a typical D-FEM (Discontinuum - Finite Element Model) consisting of deformable blocks, which incorporates several masonry units. The blocks are separated by interface elements placed along predetermined surfaces on which cracks, sliding or crushing planes can develop. The surface layout is conceived to replicate all the fundamental failure mechanisms that can occur in real masonry structures. To adequately describe the non-linear behaviour of the interface elements, a Modified Composite Interface (MCI) model is formulated by modifying the "Combined Cracking-Shearing-Crushing" model originally proposed by Lourenço to simulate the cracking mechanisms along the interface elements of simplified micro-models in FEM analysis. The proposed D-FE model has been already calibrated and validated by the Authors on different masonry panels and, through an Evolutionary Polynomial Regression (EPR), expressions in closed form have been defined to calculate the mechanical parameters of the new MCI model starting from the original CI. Particularly, for each mechanical parameter, six formulas of increasing complexity have been identified through non-linear regression, progressively including a greater number of input variables to increase the precision of results. The aim of this study is to determine the accuracy of these formulas. Numerical analyses are carried out on a population of sixty-four masonry walls already used in the previous calibration phase, modeling each panel with all available equations. This allows to evaluate the average accuracy of each formula and to understand their efficiency in terms of calculation effort and correctness of the results.

An Updated Discrete Element Model for the In-Plane Behaviour of NFRCM Strengthened Masonry Walls

Abstract. Masonry strengthened with natural fabric-reinforced cementitious matrix (NFRCM-strengthened masonry) is investigated by updating an existing discrete element model. Masonry walls are modelled by rigid blocks and elastoplastic interfaces that are able to account for mortar joints and block cracking. The reinforcement is modelled in a simplified manner considering perfect adhesion between wall and reinforcement and by adopting further spring elements connecting block centres. The model is validated by comparing it with an existing FEM based on a multi-step homogenization, where reinforced masonry is considered as a whole. Both approaches are used for performing nonlinear pushover tests with an increasing shear action applied to unreinforced and reinforced panels. The updated discrete model turns out to be able to represent the strength increment given by the reinforcement, but it is less able to represent the corresponding ductility increment.

Experimental and Numerical Mechanical Characterization of Unreinforced and Reinforced Masonry Elements with Weak Air Lime Mortar Joints

This paper deals with the results of an experimental and numerical campaign aimed at characterizing the mechanical response of masonry components and panels made of limestone units kept together by weak air lime mortar joints. The selected air lime mortar, typical of ancient masonry buildings but difficult to be built-up artificially, was specifically prepared for the experimental analyses, with the aim of obtaining a laboratory compression strength of 0.25–0.50 MPa. In the first part of the paper, the performed tests concerning the strength of the units (mean compression strength of 80 MPa) and of the mortar (mean compression strength after 28 days of 0.30 MPa), are described for different curing periods. Moreover, tests of masonry triplets in shear (shear strength of 0.11 MPa for null axial forces) are shown and used in order to establish the main parameters of the Mohr–Coulomb failure criterium. Then, the calibration of a continuous numerical micro-model implemented in Kratos Multiphysics is presented. The model is used for reproducing the behavior of an unreinforced panel in shear made of the studied masonry and to appraise the effectiveness of a FRCM- (Fiber Reinforced Cementitious Matrix) based reinforcement intervention applied. The obtained results proved that FRCM allows to increase the strength of the considered masonry type by about eight times and the ductility by about thirteen times.