Historical masonry structures with a greater thickness can often be made as so called multi-leaf masonry with outer leaves lined with the application of some of the masonry bonding methods and with the inner leave of the so-called cast core formed by fragments of stones of different sizes that are bonded with more mortar. The load capacity, stiffness and the failure mechanism of multi-leaf masonry are influenced by the interaction of individual leaves with different deformation and physico-mechanical properties and the nature of the mutual connection of individual leaves of masonry. The performed analyses showed a significant influence of especially the cast core masonry tensile strength and the contact joint strength between the core masonry and the outer leaves

Brožovský, J.

Kubát, J.

Witzany, J.

Zigler, R.

Čejka, T.

Scipedia

12th International Conference on Structural Analysis of Historical Constructions SAHC 2020 P. Roca, L. Pelà and C. Molins (Eds.)    STABILIZATION AND CONSOLIDATION OF HISTORICAL  MULTI-LEAF MASONRY JIŘÍ WITZANY1, JIŘÍ BROŽOVSKÝ2, TOMÁŠ ČEJKA1,  JAN KUBÁT1 AND RADEK ZIGLER1* 1 Faculty of Civil Engineering (FCE) Czech Technical University in Prague (CTU in Prague)  Thákurova 7, 166 29 Prague, Czech Republic e-mail: {witzany, cejka, jan.kubat.2, zigler}@fsv.cvut.cz, kps.fsv.cvut.cz (*corresponding author)  2 Faculty of Civil Engineering (FAST) VSB - Technical University of Ostrava (VSB-TUO) Ludvíka Podéště 1875/17, 708 00 Ostrava-Poruba, Czech Republic email: jiri.brozovsky@vsb.cz, www.fast.vsb.cz  Keywords: Multi-leaf Masonry, Historical Structures, Failure Mechanism, Stabilization  Abstract. Historical masonry structures with a greater thickness can often be made as so-called multi-leaf masonry with outer leaves lined with the application of some of the masonry bonding methods and with the inner leave of the so-called cast core formed by fragments of stones of different sizes that are bonded with more mortar. The load capacity, stiffness and the failure mechanism of multi-leaf masonry are influenced by the interaction of individual leaves with different deformation and physico-mechanical properties and the nature of the mutual connection of individual leaves of masonry. The performed analyses showed a significant influence of especially the cast core masonry tensile strength and the contact joint strength between the core masonry and the outer leaves.    1 INTRODUCTION Failure mechanism of multi-leaf masonry with outer layers (facing leaves) of coursed masonry of regular (dressed) stone blocks, or of roughly coursed masonry with irregular, partially treated (roughly dressed) quarry stones and with an inner layer, the so-called cast core, consisting of fragments of stones differing in sizes and shapes, which are laid in a larger amount of mortar, is different from the failure mechanism of coursed stone masonry or roughly coursed irregular masonry [1,2]. The determination of the residual compressive loadability of multi-leaf masonry is a demanding task at which the calculation procedures used for single-leaf masonry cannot be fully applied. Due to the nature of the inner leaf containing a larger amount of binder (e.g. 40-60% of the generally more flexible binder) than the outer masonry leaves (e.g., 15-30% binder), different deformation properties, a different load capacity and failure mechanism of this type of masonry can be expected [3]. The “bonding” of the outer leaves with the inner leaf contributes to the increase in the coherence and integrity of multi-leaf masonry. Also, the natural irregularity of Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  2the inner surface of the outer (facing) leaves made of undressed stones improves the bonding of the outer and inner leaves and increases the resistance of the multi-leaf masonry structure to “splitting”, which usually precedes complete failure of masonry. The process of multi-leaf masonry failure under concentric compressive loads is characterized by the detachment of individual masonry leaves, damage to the masonry cross-section due to transverse (expanding) tensile forces usually caused by the different stiffness of the cast core and the facing leaf masonry [4,5,6]. Historical masonry is characterized by considerable heterogeneity and different properties of its individual components [7,8,9,10]. The study of the influence of different material properties of individual parts of multi-leaf masonry on its failure mechanism is therefore a basic prerequisite for its reliable stabilization and consolidation design. 2 THEORETICAL AND EXPERIMENTAL RESEARCH INTO THE BEHAVIOR OF MULTI-LEAF MASONRY UNDER COMPRESSIVE LAODING The research projects (NAKI DF12P01OVV037 and NAKI DG16P02M055) involved the theoretical and experimental analysis of the failure mechanism of multi-leaf masonry under concentric compressive load exerted by force or by forced deformation was performed. The theoretical and experimental analysis was carried out on a model of multi-leaf masonry (Fig. 1) with outer (facing) leaves approx. 150 mm thick made up of roughly dressed sandstone blocks (approx. size of 170 x 150 x 120 mm, compressive strength approx. 40 MPa) and a core masonry leaf approx. 300 mm in thickness made up of sandstone fragments with a lime mortar binder (mortar compressive strength of facing leaves of 1.15 MPa in the case of bonded leaves and 1.08 MPa in the case of non-bonded leaves, mortar compressive strength of cast (inner leaf) masonry of 0.92 MPa in the case of bonded leaves and  0.97 MPa in the case of non-bonded leaves).   Figure 1: Multi-leaf masonry specimens Numerical analysis (2D linear finite element analysis) included a total of 7 different arrangements of segments (bodies) of multi-leaf masonry loaded at the upper free end by a uniform vertical compressive load and 7 segments (bodies) loaded at the upper free end by Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  3forced vertical deformation (Fig. 2, Tab. 1). The numerical analysis covered different alternatives of computational models (single-leaf, multi-leaf, bonded and non-bonded leaves) as well as computational load models (force, forced deformation).  Figure 2: a) Multi-leaf masonry with full bond between leaves, b) Multi-leaf masonry with no shear bond between leaves (*horizontal stresses are transferred), c) Single-leaf masonry Table 1: Overwiev of theoretically studied cases of multi-leaf masonry  Geometry Load type Em,f [GPa] Em,c [GPa] 1  Force 3.8 1.6 2 3.8 1.2 3  3.8 3.8 4 1.6 1.6 5 1.2 1.2 6  Deformation 3.8 1.6 7 3.8 1.2 8  3.8 3.8 9 1.6 1.6 10 1.2 1.2 11  Force 3.8 1.6 12 3.8 1.2 13 Deformation 3.8 1.6 14 3.8 1.2  The experimental research was carried out on a total of 6 test specimens of the same dimensions as in the theoretical analysis (width x depth x height - approx. 500 x 600 x 720 mm) loaded by a monotonically increasing compressive force exerted by a hydraulic press with a digital pressure sensing center (manufacturer MFL Germany, load range 0 - 10000 kN, Fig. 3). Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  4Based on the theoretical and experimental analysis of multi-leaf masonry segments, two different failure mechanisms of multi-leaf masonry under concentric compressive load were formulated.   Figure 3: Test specimens of multi-leaf masonry under compressive load 3 MULTI-LEAF MASONRY FAILURE MECHANISM The failure mechanism of multi-leaf stone masonry of the "coursed facing masonry leaf - cast masonry leaf – coursed facing masonry leaf" composition is characterized by the development of transverse (horizontal) tensile forces with the maximum value in the middle of the masonry and shear forces with the maximum in the joint between the facing masonry leaves and the cast core leaf (Fig. 4).    Figure 4: Results of numerical analysis – a) Horizontal stresses, b) Vertical stress, c) Shear stresses Based on the results of the numerical analysis, deflection of stress trajectories in multi-leaf masonry loaded by concentric compression due to the higher stiffness of facing masonry leaves Em,f (lower compressibility) compared to the stiffness of the core masonry leaf Em,c can be observed. This deflection of the stress trajectories (change from the vertical trajectory in case Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  5of single-leaf masonry or masonry where Em,f ≈ Em,c to more complex or inclined stress pattern in case of Em,f ≠ Em,c) is accompanied by the formation of horizontal stresses which increase in size with the increasing difference in the stiffness of the facing and core masonry leaves (Em,f > Em,c).  In the case of Em,c/Em,f  ration equal to 0.42, the horizontal stresses +σx reached 646% and 861% in case of facing leaf or core leaf, respectively, compared to a single leaf masonry. In the case of  Em,c/Em,f  ration equal to 0.32 these stresses reach 769% and 1028% in case of facing leaf or core leaf, respectively, compared to a single leaf masonry (Fig. 5). These additional horizontal tensile stresses, combined with no uniform distribution of vertical stresses +σz (Fig. 6) can lead to significant crack formation and masonry failure. This mechanism is further enhanced by the increase in shear stresses +τxz reaching up to 775%, depending on the Em,c/Em,f  ration (Fig. 7).  Figure 5: Horizontal stresses +σx in multi-leaf masonry – a) Ratio of Em,c/Em,f = 0.32, b) Ratio of Em,c/Em,f = 0.42, c) Ratio of Em,c/Em,f = 1, d) Change of horizontal stress due to the Em,c/Em,f ratio  Figure 6: Vertical stresses +σz in multi-leaf masonry – a) Ratio of Em,c/Em,f = 0.32, b) Ratio of Em,c/Em,f = 0.42, c) Ratio of Em,c/Em,f = 1, d) Change of vertical stress due to the Em,c/Em,f ratio  Figure 7: Shear stresses +τxz in multi-leaf masonry – a) Ratio of Em,c/Em,f = 0.32, b) Ratio of Em,c/Em,f = 0.42,  c) Ratio of Em,c/Em,f = 1, d) Change of shear stress due to the different Em,c/Em,f ratio Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  6Based on the experimental results (Fig. 8 and Fig. 9, Tab. 1 and Tab. 2), it can be stated that the characteristic failure of multi-leaf masonry depends on the tensile strength of the cast core masonry and the strength of the contact joint between the facing and the core masonry leaves.    Figure 8: a) Scheme of experimental specimens, b) Ultimate compressive stress, c) Transverse and longitudinal horizontal strains for un-bonded masonry, d) Transverse and longitudinal horizontal strains for bonded masonry, Note*: Due to test setup, longitudinal strains were not measured up to ultimate load  Figure 9: a) The change of tensile stresses due to the Em,f/Em,c and tensile strength of the core leaf (rubble) masonry,  b) The change of shear stresses due to the Em,f/Em,c ratio and shear strength of the core/facing leaf contact  In terms of the response and the type of failure of multi-leaf masonry loaded by concentric compressive force, which precedes reaching the limit load, the loss of stability and bearing function of multi-leaf masonry, two different failure mechanisms of multilayer masonry can be distinguished: Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  7a) Failure mechanism of multi-leaf masonry due to reaching the ultimate strength of the core (cast) masonry. This failure mechanism of multi-leaf masonry can be observed in cases where the ultimate tensile strength of the cast core masonry is exceeded due to vertical tensile cracks, which gradually extend from the middle third of the height of the compressed masonry (walls, columns) towards the heel and head. The formation of vertical tensile cracks in the masonry core is subsequently accompanied by an uneven distribution of normal compressive stresses across the cross-section of multi-leaf masonry, which precedes reaching the limit load. The reduction of the stability - buckling strength - of the masonry parts (columns) delimited by individual vertical tensile cracks contributes to the progressiveness of the multi-leaf masonry failure mechanism. b) Failure mechanism of multi-leaf masonry due to reaching the ultimate shear strength in the contact joint between the facing masonry leaves and the core masonry leaf. This failure mechanism of multi-leaf masonry can be observed in cases where the ultimate shear strength in the “facing leaf – core leaf – facing leaf” contact joint is exceeded and individual masonry leaves become detached. The ultimate compressive strength of multi-leaf masonry at this stage of failure depends primarily on the intensity of the uneven distribution of normal stresses and the buckling strength of the facing leaves.  Table 2: Overview of experimental multi-leaf and single-leaf masonry specimens and their material properties Masonry type Masonry units comp. strenght [MPa] Mortar comp. strenght [Mpa] Dimensions [mm] Width Depth Height Multi-leaf,  un-bonded 46.4 0.95-1.10 510 580 710 Multi-leaf, bonded 46.4 0.93-1.13 510 590 740 Single-leaf, coursed 46.4 1.01 485 595 730 Single-leaf,  rubble* 19.51 1.68 540 540 1710 Note: *Test specimen from another test series (full height specimens), included for comparison Table 3: Results of experimental testing of multi-leaf and single leaf masonry specimens Masonry type Ultimate vert.  stress σz [MPa] Ultimate strains ε [-] Vertical (εz) Transverse horizontal (εx) Longitudinal horizontal (εy) Multi-leaf,  un-bonded 2.37 0.0117 0.0302 0.0073* Multi-leaf,  bonded 2.61 0.0164 0.0143 0.0057* Single-leaf,  coursed 2.89 0.0109 0.0169 x Single-leaf,  rubble 1.99 0.0201 0.0342 x Note: Strains at approx. 75% of ultimate load level (*), strains not measured (x) Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  84 STABILIZATION AND CONSOLIDATION OF MULTI-LEAF HISTORICAL MASONRY Stabilization and consolidation measures should be based on the failure mechanisms characterized by masonry splitting accompanied by reduced stability and compressive load capacity. In terms of prevention of the appearance of premature tensile cracks, which result in multi-leaf masonry splitting, the most effective methods that can be recommended, based on the research conducted and authors’ experience of in-situ applications, are: • In the case of insufficient contact joint strength or damage   - Reinforcement of multi-leaf masonry by transverse anchors (connectors) designed on the principle of the so-called “nail” grouting. After the insertion of steel bars (e.g. with periodic surfaces) into transversally drilled holes (slightly sloping inwards – approx. 5°, reaching to approx. 1/2 of the core thickness), pressure grouting (with lime, cement or polymer based binders, resins etc.) is performed. The optimal spacing of individual grouted nail anchors (connectors), which can be arranged in one or several rows one above the other, is less than or equal to the thickness of the facing masonry leaf (Fig. 10a). - Pressure grouting aimed at reinforcing the contact joint with grouting mixtures based on polymer or epoxy resins or grouting mixtures based on lime or cement binders. The grouting hole should reach into the core leaf to a maximum depth equal to the thickness of the facing masonry leaf (Fig. 10b). • In the case of insufficient strength or quality of the core leaf masonry - Grouting of the cast core masonry with a suitable grouting mixture, depending on the pore system, voids or cavities. In order to increase the efficiency and reliability of grouting, it is advisable to carry out two-stage grouting aimed at increasing the stiffness and strength of the core masonry. Grouting holes should reach to 2/3 - 4/5 of the thickness of the core masonry leaf (Fig. 10c).   Figure 10: a) Stabilization of multi-leaf masonry by “nail” grouting, b) Stabilization of multi-leaf masonry by contact joint grouting, c) Stabilization of multi-leaf masonry by core grouting 5 CONCLUSIONS The determination of the residual compressive load capacity of multi-leaf masonry and the design of remediation and reinforcement methods of damaged multi- leaf masonry requires a Jiří Witzany, Jiří Brožovský, Tomáš Čejka, Jan Kubát and Radek Zigler  9specific approach based on the results of detailed research, diagnostic investigations and theoretical analysis. Massive multi-leaf masonry usually shows sufficient reserves in the load capacity due to the magnitude of real loads. The tests and analyses performed, in contrast, pointed out the risk factors of multi-leaf masonry that may be the cause of its sudden failure (collapse).  Acknowledgements. The article was written with support from the NAKI DG16P02M055 project “Development and Research into Materials, Procedures and Technologies for Restoration, Conservation and Strengthening of Historic Masonry Structures and Surfaces and Systems of Preventive Care of Historic and Heritage Buildings Threatened by Anthropogenic and Natural Risks”.  REFERENCES [1] Stavroulaki, M., Papalou, A. Parametric analysis of old multi-leaf masonry walls International Journal of Conservation Science (2014) 5(4):435-446. [2] Demir, C., Ilki, A., Characterization of the materials used in the multi-leaf masonry walls of monumental structures in Istanbul, Turkey. Construction and Building Materials (2014), 64:398-413. [3] Aldreghetti, I., Baraldi, D., Boscato, G., Cecchi, A., Massaria, L., Pavlovic, M., Reccia, E., Tofani, I. Multi-leaf masonry walls with full, damaged and consolidated infill: Experimental and numerical analyses.  Key Engineering Materials (2017) 747:488-495 [4] M. R. Valluzzi, F. da Porto, C. Modena, Behaviour of multi-leaf stone masonry walls strengthened by different intervention techniques, In: P.B. Lourenço, P. Roca (Eds.) Historical Constructions (2001)  [5] Candela, M., Borri, A., Corradi, M., Righetti, L. Effect of transversal steel connectors on the behavior of rubble stone-masonry walls: Two case studies in Italy. Brick and Block Masonry: Trends, Innovations and Challenges - Proceedings of the 16th International Brick and Block Masonry Conference, IBMAC 2016 (2016), pp. 2029-2038. [6] Corradi, M., Borri, A., Poverello, E., Castori, G. The use of transverse connectors as reinforcement of multi-leaf walls.  Materials and Structures/Materiaux et Constructions (2017) 50(2), art. no. 114 [7] Witzany, J., Čejka, T., Kroftová, K., Šmidtová, M., 2016. The effect of degradation processes on the serviceability of building materials of historic buildings, The Civil Engineering Journal 3(10), art. no. 11 [8] Witzany, J., Zigler, R., Čejka, T., Pospíšil, P., Holický, M., Kubát, J., Maroušková, A., Kroftová, K., Physical and Mechanical Characteristics of Building Materials of Historic Buildings  The Civil Engineering Journal (2017) 4(12):343-360 [9] Witzany, J., Zigler, R., Kroftová, K., Čejka, T., Kubát, J., Holický, M., Karas, J., The Effect of Pore Distribution in Historic Masonry on the Grouting Method and Grouting Mix Selection, The Civil Engineering Journal (2018) 3(10):307-329 [10] Witzany, J., Zigler, R., Čejka, T., Libecajtová, A., Polák, A., Kubát, J., 2019. Research into the effect of grouting on physical-mechanical properties of historic masonry, In: A. Zingoni (Ed.) Advances in Engineering Materials Structures and Systems: Innovations, Mechanics and Applications (2019). London: Taylor & Francis, p. 617-618. 

Stabilization and Consolidation of Historical Multi-Leaf Masonry

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Stabilization and Consolidation of Historical Multi-Leaf Masonry

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