R. Esposito
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1
During the Dutch Golden Age in the 17th and 18th centuries, the city of Amsterdam underwent rapid urban expansion, driven by trade and migration. This led to the construction of numerous earth-retaining walls and bridges, forming part of the Canal District (Grachtengordel). This historic infrastructure serves both functional and a monumental role as part of Amsterdam’s UNESCO World Heritage Site. The oldest constructions in the city centre are predominantly composed of thick, multi-wythe clay brick masonry supported on timber pile foundations. The timber piles forming the foundations of these retaining walls and bridges have exhibited deterioration in recent years. However, the masonry superstructure has shown a capacity to redistribute stresses. Accordingly, mechanical characterisation of these structures is essential to evaluate its current condition and to design appropriate interventions. The multi-wythe walls of these structures are typically 600-1400 mm thick, fully-bonded and constructed using clay brick masonry with a lime-trass based mortar. Despite their critical role in the city's transport infrastructure, the mechanical properties of this historic masonry remain poorly documented. In particular, interface shear and bond properties, which can be critical in out-of-plane failure of the masonry superstructure, are lacking. To this end, this extended abstract describes the challenges encountered so far in the characterisation of interface properties and a possible strategy together with preliminary results for a quay wall structure in the city of Amsterdam.
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During the Dutch Golden Age in the 17th and 18th centuries, the city of Amsterdam underwent rapid urban expansion, driven by trade and migration. This led to the construction of numerous earth-retaining walls and bridges, forming part of the Canal District (Grachtengordel). This historic infrastructure serves both functional and a monumental role as part of Amsterdam’s UNESCO World Heritage Site. The oldest constructions in the city centre are predominantly composed of thick, multi-wythe clay brick masonry supported on timber pile foundations. The timber piles forming the foundations of these retaining walls and bridges have exhibited deterioration in recent years. However, the masonry superstructure has shown a capacity to redistribute stresses. Accordingly, mechanical characterisation of these structures is essential to evaluate its current condition and to design appropriate interventions. The multi-wythe walls of these structures are typically 600-1400 mm thick, fully-bonded and constructed using clay brick masonry with a lime-trass based mortar. Despite their critical role in the city's transport infrastructure, the mechanical properties of this historic masonry remain poorly documented. In particular, interface shear and bond properties, which can be critical in out-of-plane failure of the masonry superstructure, are lacking. To this end, this extended abstract describes the challenges encountered so far in the characterisation of interface properties and a possible strategy together with preliminary results for a quay wall structure in the city of Amsterdam.
The mechanical behaviour of masonry is strongly influenced by the brick-mortar interface. To isolate and benchmark the pure frictional response of this interface in mortared masonry, this study uses a reciprocating sliding tribometer test on cracked brick-mortar specimens. Tests were conducted under varying pre-compression levels and two reciprocating sliding frequencies, using unreinforced clay brick masonry with lime-cement mortar. Frictional behaviour was analysed through hysteresis curves relating the measured friction force to the sliding displacement. From these, the mean sliding friction coefficient and tangential stiffness were extracted for each cycle. Both parameters showed considerable variability across tests. The mean kinetic friction coefficients, typically ranging between 0.4 and 0.6, aligned with values from indirect tests methods (e.g., triplet or couplet shear tests), and their variability appeared independent of the applied normal load. In contrast, tangential stiffness showed a qualitative correlation with the normal load, despite a high scatter. To interpret the observed variability, statistical analyses including ANOVA and Principal Component Analysis were performed. Although further testing is required, these initial results offer valuable insights into the frictional mechanics of cracked masonry interfaces and suggest new testing avenues for more accurately characterising brick-mortar interaction in structural assessments.
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The mechanical behaviour of masonry is strongly influenced by the brick-mortar interface. To isolate and benchmark the pure frictional response of this interface in mortared masonry, this study uses a reciprocating sliding tribometer test on cracked brick-mortar specimens. Tests were conducted under varying pre-compression levels and two reciprocating sliding frequencies, using unreinforced clay brick masonry with lime-cement mortar. Frictional behaviour was analysed through hysteresis curves relating the measured friction force to the sliding displacement. From these, the mean sliding friction coefficient and tangential stiffness were extracted for each cycle. Both parameters showed considerable variability across tests. The mean kinetic friction coefficients, typically ranging between 0.4 and 0.6, aligned with values from indirect tests methods (e.g., triplet or couplet shear tests), and their variability appeared independent of the applied normal load. In contrast, tangential stiffness showed a qualitative correlation with the normal load, despite a high scatter. To interpret the observed variability, statistical analyses including ANOVA and Principal Component Analysis were performed. Although further testing is required, these initial results offer valuable insights into the frictional mechanics of cracked masonry interfaces and suggest new testing avenues for more accurately characterising brick-mortar interaction in structural assessments.
Micromechanical models offer a physics-based alternative to phenomenological approaches to simulate the behavior of quasi-brittle materials. By combining mean-field homogenization techniques with fracture mechanics principles, these models aim to capture anisotropic damage evolution, unilateral effects, and multiphysics coupling with a small number of inputs. However, despite their theoretical appeal, their practical application is often hindered by critical limitations. This paper presents a comprehensive and critical examination of micromechanical formulations, focusing on the influence of homogenization schemes, damage evolution criteria, and loading type in model response. It highlights key issues such as the limited accuracy of homogenization estimates at high crack densities, the instability of post-peak responses, spurious damage localization, and the challenges of modeling tensile-compressive asymmetry and non-homothetic crack growth. Through analytical derivations and numerical examples, the study demonstrates that many micromechanical models rely on assumptions that break down under more general conditions, leading to non-physical predictions outside the scope of model conception. The findings suggest that while micromechanical models are valuable in specific contexts, their broader applicability requires careful scrutiny and further development.
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Micromechanical models offer a physics-based alternative to phenomenological approaches to simulate the behavior of quasi-brittle materials. By combining mean-field homogenization techniques with fracture mechanics principles, these models aim to capture anisotropic damage evolution, unilateral effects, and multiphysics coupling with a small number of inputs. However, despite their theoretical appeal, their practical application is often hindered by critical limitations. This paper presents a comprehensive and critical examination of micromechanical formulations, focusing on the influence of homogenization schemes, damage evolution criteria, and loading type in model response. It highlights key issues such as the limited accuracy of homogenization estimates at high crack densities, the instability of post-peak responses, spurious damage localization, and the challenges of modeling tensile-compressive asymmetry and non-homothetic crack growth. Through analytical derivations and numerical examples, the study demonstrates that many micromechanical models rely on assumptions that break down under more general conditions, leading to non-physical predictions outside the scope of model conception. The findings suggest that while micromechanical models are valuable in specific contexts, their broader applicability requires careful scrutiny and further development.
Extracting cores with diameters of 100 to 150 mm from masonry structures has emerged as a novel, less destructive method for assessing the mechanical properties of masonry units, particularly their compressive strength. Unlike traditional methods, such as using larger wallets, this approach requires less material and causes minimal damage to the original structure, which is critical when dealing with historical buildings. However, to obtain consistent and reliable results, certain parameters, specifically the dimensions of the core cap, must be carefully defined, as they significantly influence the overall behaviour of the samples. The study employs a detailed block-based modelling approach, incorporating zero-thickness cohesive elements at the brick-mortar interfaces. Additionally, tangential and normal contact interactions were defined between the cap and core components. The concrete damage plasticity (CDP) model, implemented in ABAQUS, has been adopted as the constitutive model to account for the nonlinear behaviour of brick, mortar, and cap. The results indicate that the length of the cap has a more pronounced effect on the sample’s mechanical behaviour than its height. Additionally, the study investigates the mechanical properties of the interface between the cap and the core, identifying friction and normal stiffness as critical factors. These findings provide valuable insights for optimizing the core capping process and improving the reliability of masonry mechanical property assessments, particularly in the preservation of historical structures.
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Extracting cores with diameters of 100 to 150 mm from masonry structures has emerged as a novel, less destructive method for assessing the mechanical properties of masonry units, particularly their compressive strength. Unlike traditional methods, such as using larger wallets, this approach requires less material and causes minimal damage to the original structure, which is critical when dealing with historical buildings. However, to obtain consistent and reliable results, certain parameters, specifically the dimensions of the core cap, must be carefully defined, as they significantly influence the overall behaviour of the samples. The study employs a detailed block-based modelling approach, incorporating zero-thickness cohesive elements at the brick-mortar interfaces. Additionally, tangential and normal contact interactions were defined between the cap and core components. The concrete damage plasticity (CDP) model, implemented in ABAQUS, has been adopted as the constitutive model to account for the nonlinear behaviour of brick, mortar, and cap. The results indicate that the length of the cap has a more pronounced effect on the sample’s mechanical behaviour than its height. Additionally, the study investigates the mechanical properties of the interface between the cap and the core, identifying friction and normal stiffness as critical factors. These findings provide valuable insights for optimizing the core capping process and improving the reliability of masonry mechanical property assessments, particularly in the preservation of historical structures.
Bacteria-based self-healing agent for masonry crack repair
Assessing watertightness restoration
The preservation and long-term durability of masonry structures, particularly in the context of built heritage, are strongly affected by water ingress. Cracking in mortar joints or at the brick–mortar interface compromises watertightness, accelerates decay mechanisms such as salt crystallisation and freeze–thaw damage, and increases maintenance demands. Traditional repair strategies, such as repointing, are effective but require repeated interventions and monitoring.
In this context, autonomous self-healing mortars are currently gaining increasing attention as a complementary approach to conventional repair. Inspired by developments in self-healing concrete, bacteria-based healing agents have recently been explored for masonry applications. Previous pilot studies demonstrated that a polylactic acid (PLA)-based bacterial agent can promote crack filling in masonry couplets, contributing to aesthetic recovery and partial restoration of bond strength. However, the ability of such systems to restore watertightness, a performance aspect directly linked to durability, has not yet been systematically investigated.
Assessing watertightness recovery in masonry presents methodological challenges. Existing standards typically focus on large-scale wall assemblies or surface water penetration tests, which are not well suited for early-stage material evaluation. Consequently, there is a need for adapted, small-scale methods capable of quantifying changes in permeability associated with cracking and healing. This study addresses this gap by introducing an adapted permeability test derived from research on self-healing concrete and applying it to evaluate watertightness recovery in masonry containing the above-mentioned agent. ...
In this context, autonomous self-healing mortars are currently gaining increasing attention as a complementary approach to conventional repair. Inspired by developments in self-healing concrete, bacteria-based healing agents have recently been explored for masonry applications. Previous pilot studies demonstrated that a polylactic acid (PLA)-based bacterial agent can promote crack filling in masonry couplets, contributing to aesthetic recovery and partial restoration of bond strength. However, the ability of such systems to restore watertightness, a performance aspect directly linked to durability, has not yet been systematically investigated.
Assessing watertightness recovery in masonry presents methodological challenges. Existing standards typically focus on large-scale wall assemblies or surface water penetration tests, which are not well suited for early-stage material evaluation. Consequently, there is a need for adapted, small-scale methods capable of quantifying changes in permeability associated with cracking and healing. This study addresses this gap by introducing an adapted permeability test derived from research on self-healing concrete and applying it to evaluate watertightness recovery in masonry containing the above-mentioned agent. ...
The preservation and long-term durability of masonry structures, particularly in the context of built heritage, are strongly affected by water ingress. Cracking in mortar joints or at the brick–mortar interface compromises watertightness, accelerates decay mechanisms such as salt crystallisation and freeze–thaw damage, and increases maintenance demands. Traditional repair strategies, such as repointing, are effective but require repeated interventions and monitoring.
In this context, autonomous self-healing mortars are currently gaining increasing attention as a complementary approach to conventional repair. Inspired by developments in self-healing concrete, bacteria-based healing agents have recently been explored for masonry applications. Previous pilot studies demonstrated that a polylactic acid (PLA)-based bacterial agent can promote crack filling in masonry couplets, contributing to aesthetic recovery and partial restoration of bond strength. However, the ability of such systems to restore watertightness, a performance aspect directly linked to durability, has not yet been systematically investigated.
Assessing watertightness recovery in masonry presents methodological challenges. Existing standards typically focus on large-scale wall assemblies or surface water penetration tests, which are not well suited for early-stage material evaluation. Consequently, there is a need for adapted, small-scale methods capable of quantifying changes in permeability associated with cracking and healing. This study addresses this gap by introducing an adapted permeability test derived from research on self-healing concrete and applying it to evaluate watertightness recovery in masonry containing the above-mentioned agent.
In this context, autonomous self-healing mortars are currently gaining increasing attention as a complementary approach to conventional repair. Inspired by developments in self-healing concrete, bacteria-based healing agents have recently been explored for masonry applications. Previous pilot studies demonstrated that a polylactic acid (PLA)-based bacterial agent can promote crack filling in masonry couplets, contributing to aesthetic recovery and partial restoration of bond strength. However, the ability of such systems to restore watertightness, a performance aspect directly linked to durability, has not yet been systematically investigated.
Assessing watertightness recovery in masonry presents methodological challenges. Existing standards typically focus on large-scale wall assemblies or surface water penetration tests, which are not well suited for early-stage material evaluation. Consequently, there is a need for adapted, small-scale methods capable of quantifying changes in permeability associated with cracking and healing. This study addresses this gap by introducing an adapted permeability test derived from research on self-healing concrete and applying it to evaluate watertightness recovery in masonry containing the above-mentioned agent.
Historical masonry quay walls in the inner city of Amsterdam need maintenance due to aging, and deterioration of foundations. These structures are not only critical to the transport infrastructure of the city, but also hold great heritage and monumental value being UNESCO World Heritage site. However, there is a notable lack of knowledge and documentation regarding the mechanical properties of these multi-wythe, unreinforced brick masonry walls, which poses challenges to the assessment of their structural safety. This research investigates the mechanical properties of masonry for typical quay walls presenting the results of experiments for four different quay wall locations in Amsterdam. Masonry properties were determined via core testing due to its minimal invasiveness to the structure and its ability to adopt standard extraction and testing facilities. Companion tests on brick and mortar were performed to evaluate their compressive properties. Samples were obtained from both above and below the waterline to assess the effect of submersion on the mechanical properties. This study offers an insight into the mechanical properties of quay walls in Amsterdam, enhancing understanding of multi-wythe masonry and supporting the assessment and preservation of urban infrastructure in Dutch cities.
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Historical masonry quay walls in the inner city of Amsterdam need maintenance due to aging, and deterioration of foundations. These structures are not only critical to the transport infrastructure of the city, but also hold great heritage and monumental value being UNESCO World Heritage site. However, there is a notable lack of knowledge and documentation regarding the mechanical properties of these multi-wythe, unreinforced brick masonry walls, which poses challenges to the assessment of their structural safety. This research investigates the mechanical properties of masonry for typical quay walls presenting the results of experiments for four different quay wall locations in Amsterdam. Masonry properties were determined via core testing due to its minimal invasiveness to the structure and its ability to adopt standard extraction and testing facilities. Companion tests on brick and mortar were performed to evaluate their compressive properties. Samples were obtained from both above and below the waterline to assess the effect of submersion on the mechanical properties. This study offers an insight into the mechanical properties of quay walls in Amsterdam, enhancing understanding of multi-wythe masonry and supporting the assessment and preservation of urban infrastructure in Dutch cities.
Salt crystallisation and weathering in masonry retaining walls
A multiphase modelling approach
Journal article
(2025)
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Satyadhrik Sharma, Rita Esposito, Antonio Maria D'Altri, Giovanni Castellazzi
This study presents a numerical investigation into the effects of salt crystallisation-induced weathering on masonry earth-retaining walls, with a specific focus on historic quay walls in Amsterdam. A multiphase modelling strategy is adopted to simulate moisture and salt transport, capturing the impact of environmental exposure on these ageing structures. The numerical model is first applied with masonry assumed as a homogeneous continuum and is subsequently refined to incorporate masonry texture. The influence of boundary conditions, multiple weathering cycles, and long-term humidity variations is examined to assess salt accumulation patterns. Results indicate that evaporation pathways significantly influence crystallisation depth, while explicitly modelling masonry texture leads to greater salt accumulation. Furthermore, an analytical estimation of the effective Young's modulus suggests that salt deposition within pores may contribute to through-thickness stiffness variations observed in experimental studies on samples collected from a multi-wythe masonry bridge pillar, with masonry type and exposure conditions comparable to those of Amsterdam's quay walls. These findings provide new insights into the deterioration mechanisms of historic quay walls and highlight the importance of considering environmental effects in their structural assessment.
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This study presents a numerical investigation into the effects of salt crystallisation-induced weathering on masonry earth-retaining walls, with a specific focus on historic quay walls in Amsterdam. A multiphase modelling strategy is adopted to simulate moisture and salt transport, capturing the impact of environmental exposure on these ageing structures. The numerical model is first applied with masonry assumed as a homogeneous continuum and is subsequently refined to incorporate masonry texture. The influence of boundary conditions, multiple weathering cycles, and long-term humidity variations is examined to assess salt accumulation patterns. Results indicate that evaporation pathways significantly influence crystallisation depth, while explicitly modelling masonry texture leads to greater salt accumulation. Furthermore, an analytical estimation of the effective Young's modulus suggests that salt deposition within pores may contribute to through-thickness stiffness variations observed in experimental studies on samples collected from a multi-wythe masonry bridge pillar, with masonry type and exposure conditions comparable to those of Amsterdam's quay walls. These findings provide new insights into the deterioration mechanisms of historic quay walls and highlight the importance of considering environmental effects in their structural assessment.
Masonry quay walls are vital infrastructure in many historic cities, serving both functional and historical purposes. Originally designed as gravity retaining walls, they now face increased vehicle loads and widespread material degradation, particularly in timber foundations. Traditional assessment methods are often overly conservative, lacking standard procedures for multi-wythe masonry characterisation.With over 200 km of quay walls in Amsterdamrequiring renovation, there is an urgent need for practical, reliable assessment methods. This paper provides an overview of recent research conducted at TU Delft with focus on the response of masonry superstructure, presenting and discussing key advancements in the development of high-fidelity static and dynamic finite element models and minor-destructive testing for masonry mechanical property characterisation.
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Masonry quay walls are vital infrastructure in many historic cities, serving both functional and historical purposes. Originally designed as gravity retaining walls, they now face increased vehicle loads and widespread material degradation, particularly in timber foundations. Traditional assessment methods are often overly conservative, lacking standard procedures for multi-wythe masonry characterisation.With over 200 km of quay walls in Amsterdamrequiring renovation, there is an urgent need for practical, reliable assessment methods. This paper provides an overview of recent research conducted at TU Delft with focus on the response of masonry superstructure, presenting and discussing key advancements in the development of high-fidelity static and dynamic finite element models and minor-destructive testing for masonry mechanical property characterisation.
Estimating the mechanical properties of existing masonry structures still represents a challenge since there is the need to find a balance between the invasiveness of the testing method and the accuracy and representativeness of the test results. Minor destructive tests are usually preferred, especially when dealing with historical constructions. In the last decades, the use of masonry cores, tested to obtain either compression or shear properties, has progressively become a valid and promising alternative to other testing methods. However, the application of core testing is often limited to the academic research, mainly due to the lack of specific standards and testing guidelines. The objective of the present work is to analyze procedures adopted to test masonry cores for the characteriza-tion of compressive properties of unreinforced masonry with regular units. Through the collection of a database, an inventory of available experimental data on various masonry is provided, and the main features of test setups and protocols are examined and compared. Factors influencing the results, such as boundary conditions, geometry of the samples, bond pattern, etc., are identified. Collection of data related to standard experimental tests is also performed to support the definition of specific correlations between the results of core testing and of standard procedures, and, in general, to evaluate the reliability of the different testing methodologies. The present research represents the basis for a possible standardization of core testing methods.
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Estimating the mechanical properties of existing masonry structures still represents a challenge since there is the need to find a balance between the invasiveness of the testing method and the accuracy and representativeness of the test results. Minor destructive tests are usually preferred, especially when dealing with historical constructions. In the last decades, the use of masonry cores, tested to obtain either compression or shear properties, has progressively become a valid and promising alternative to other testing methods. However, the application of core testing is often limited to the academic research, mainly due to the lack of specific standards and testing guidelines. The objective of the present work is to analyze procedures adopted to test masonry cores for the characteriza-tion of compressive properties of unreinforced masonry with regular units. Through the collection of a database, an inventory of available experimental data on various masonry is provided, and the main features of test setups and protocols are examined and compared. Factors influencing the results, such as boundary conditions, geometry of the samples, bond pattern, etc., are identified. Collection of data related to standard experimental tests is also performed to support the definition of specific correlations between the results of core testing and of standard procedures, and, in general, to evaluate the reliability of the different testing methodologies. The present research represents the basis for a possible standardization of core testing methods.
The integration of bacteria-based self-healing mortars has emerged as a promising solution to address repair due to recurring cracks and preserving masonry durability. Building upon a recent pilot study demonstrating the efficacy of a self-healing agent in the repair of masonry made with cement-based mortar, this follow-up study explores the potential of integrating the added-in healing agent in a pre-bagged cement-lime mortar - more commonly used in masonry applications. Through bond wrench tests and a 30-day healing period involving wet-dry cycles, the study evaluates aesthetic and flexural bond strength recovery of couplets built with solid clay bricks. Results showed that the addition of the agent altered the initial flexural bond strength, with bacteria-based masonry couplets four times stronger than the plain reference ones - without containing the agent. The mortar’s colorwas also affected. Additionally, bacteria-based specimens demonstrated automatic repair, restoring up to 33% of the original flexural bond strength, while referencemasonry couplets showed no evidence of autonomous healing. However, instances of leaching, possibly attributed to the agent’s substrate, prompted a revision of the strategy employed for the healing environment. Further research will specifically target the observed leaching issue by exploring the effects of multiple healing environments.
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The integration of bacteria-based self-healing mortars has emerged as a promising solution to address repair due to recurring cracks and preserving masonry durability. Building upon a recent pilot study demonstrating the efficacy of a self-healing agent in the repair of masonry made with cement-based mortar, this follow-up study explores the potential of integrating the added-in healing agent in a pre-bagged cement-lime mortar - more commonly used in masonry applications. Through bond wrench tests and a 30-day healing period involving wet-dry cycles, the study evaluates aesthetic and flexural bond strength recovery of couplets built with solid clay bricks. Results showed that the addition of the agent altered the initial flexural bond strength, with bacteria-based masonry couplets four times stronger than the plain reference ones - without containing the agent. The mortar’s colorwas also affected. Additionally, bacteria-based specimens demonstrated automatic repair, restoring up to 33% of the original flexural bond strength, while referencemasonry couplets showed no evidence of autonomous healing. However, instances of leaching, possibly attributed to the agent’s substrate, prompted a revision of the strategy employed for the healing environment. Further research will specifically target the observed leaching issue by exploring the effects of multiple healing environments.
The preservation of historic masonry structures requires durable repair techniques that minimize intervention while maintaining the original appearance. In this context, bio-based self-healing mortars present a promising solution, yet their application in masonry remains underexplored. Previous research demonstrated the potential of a poly-lactic acid (PLA)-based agent to heal cracks in masonry with cement-based mortars, restoring both bond strength and aesthetics. However, when extended to more porous cement–lime mortars and clay bricks, new challenges emerged. Leaching, likely resulting from interactions between the mortar’s porosity and the healing environment, underscored the need for further investigation. To this purpose, the study investigates clay brick couplets built with two mortar compositions of increasing lime content, exposed to two different healing environments. Specifically, a PLA-based healing agent was incorporated into cement–lime mortars with lime/cement ratios of 1.0 and 3.33 by volume. Each mortar was used both plain and with 10% of the healing agent (by binder volume) to build the specimens. After pre-cracking through bond wrench testing, the couplets were left to heal for 60 days under either high humidity or wet–dry cycle conditions. The results highlight the critical influence of the healing environment on overall performance. While wet–dry cycles proved ineffective in this case, humid conditions enabled partial recovery of flexural bond strength, with values reaching up to 15% in samples containing the healing agent and higher lime content. Visual inspection confirmed complete crack closure, supporting the aesthetic—and potentially watertightness—restoration. Additionally, reference samples with higher lime content also exhibited minor strength recovery, likely due to the migration of free lime and its reaction within the crack. These advancements underscore the potential of bio-based self-healing mortars for masonry repair and establish a foundation for further research.
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The preservation of historic masonry structures requires durable repair techniques that minimize intervention while maintaining the original appearance. In this context, bio-based self-healing mortars present a promising solution, yet their application in masonry remains underexplored. Previous research demonstrated the potential of a poly-lactic acid (PLA)-based agent to heal cracks in masonry with cement-based mortars, restoring both bond strength and aesthetics. However, when extended to more porous cement–lime mortars and clay bricks, new challenges emerged. Leaching, likely resulting from interactions between the mortar’s porosity and the healing environment, underscored the need for further investigation. To this purpose, the study investigates clay brick couplets built with two mortar compositions of increasing lime content, exposed to two different healing environments. Specifically, a PLA-based healing agent was incorporated into cement–lime mortars with lime/cement ratios of 1.0 and 3.33 by volume. Each mortar was used both plain and with 10% of the healing agent (by binder volume) to build the specimens. After pre-cracking through bond wrench testing, the couplets were left to heal for 60 days under either high humidity or wet–dry cycle conditions. The results highlight the critical influence of the healing environment on overall performance. While wet–dry cycles proved ineffective in this case, humid conditions enabled partial recovery of flexural bond strength, with values reaching up to 15% in samples containing the healing agent and higher lime content. Visual inspection confirmed complete crack closure, supporting the aesthetic—and potentially watertightness—restoration. Additionally, reference samples with higher lime content also exhibited minor strength recovery, likely due to the migration of free lime and its reaction within the crack. These advancements underscore the potential of bio-based self-healing mortars for masonry repair and establish a foundation for further research.
Friction significantly affects masonry, especially in determining lateral resistance. This study evaluates the contribution of friction in cracked masonry by performing reciprocating sliding tests on cracked brick-mortar interfaces, by means of a tribometer, whose application to masonry materials remains unexplored. Enabling the evaluation of the sole contribution of the friction mechanism at the cracked brick-mortar interface is rather novel for the field of mortared masonry, in which friction properties are typically calculated through an inverse analysis of experiments involving both de-cohesion and friction mechanism. A solid clay brick masonry with cement-lime mortar is considered as case study. The influence of various parameters is studied, such as the normal force and the sliding frequency. The preliminary study shows a large variability in terms of the kinetic coefficient of friction and tangential stiffness. This may suggest that the roughness and waviness of the cracked interface - and consequently, wear and debris formation - highly influence the overall friction response.
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Friction significantly affects masonry, especially in determining lateral resistance. This study evaluates the contribution of friction in cracked masonry by performing reciprocating sliding tests on cracked brick-mortar interfaces, by means of a tribometer, whose application to masonry materials remains unexplored. Enabling the evaluation of the sole contribution of the friction mechanism at the cracked brick-mortar interface is rather novel for the field of mortared masonry, in which friction properties are typically calculated through an inverse analysis of experiments involving both de-cohesion and friction mechanism. A solid clay brick masonry with cement-lime mortar is considered as case study. The influence of various parameters is studied, such as the normal force and the sliding frequency. The preliminary study shows a large variability in terms of the kinetic coefficient of friction and tangential stiffness. This may suggest that the roughness and waviness of the cracked interface - and consequently, wear and debris formation - highly influence the overall friction response.
Anatolia has been home to various long-standing civilizations, many of which have left historical monuments for future generations. The Cappadocia area, which covers over 5000 km2 in Central Anatolia, Turkey, is home to several rock-cut constructions and masonry buildings built of tuff stone. Preserving these monuments, listed as UNESCO World Heritage sites, is critical for the region. This study focuses on the in-plane behavior of walls constructed of tuff stone and alkali-activated waste earth mortar, which is typical in the Cappadocia region. First, an experimental investigation is described. Second, a simplified block-based modeling technique is used to perform 3D nonlinear finite element analysis, which replicates the experimental results. The findings of the numerical modeling approach were then compared to experimental data, emphasizing the lateral load-displacement response of masonry tuff stone walls. There was good agreement between the experimental and numerical results.
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Anatolia has been home to various long-standing civilizations, many of which have left historical monuments for future generations. The Cappadocia area, which covers over 5000 km2 in Central Anatolia, Turkey, is home to several rock-cut constructions and masonry buildings built of tuff stone. Preserving these monuments, listed as UNESCO World Heritage sites, is critical for the region. This study focuses on the in-plane behavior of walls constructed of tuff stone and alkali-activated waste earth mortar, which is typical in the Cappadocia region. First, an experimental investigation is described. Second, a simplified block-based modeling technique is used to perform 3D nonlinear finite element analysis, which replicates the experimental results. The findings of the numerical modeling approach were then compared to experimental data, emphasizing the lateral load-displacement response of masonry tuff stone walls. There was good agreement between the experimental and numerical results.
Innovative Application of Self-Healing Technology to Masonry
A Proof of Concept
Cracks represent a prevalent form of damage in masonry structures, posing not only aesthetic concerns but also compromising structural durability; therefore, they are undesirable and need to be repaired. The repointing technique is traditionally implemented in this context, especially in historical masonry. However, this method fails to provide a long-term solution, leaving structures vulnerable to future damage. The paper investigates the applicability of a bio-based self-healing mortar to enable autonomous repair of masonry. This innovative mortar, developed to repair concrete structures, was implemented to explore the capacity of couplets to recover their original bond capacity and aesthetic aspect after multiple damaging events. Specimens built with calciumsilicate and clay bricks were subjected to subsequent cracking cycles using a crack-mouth-opening -displacement controlled bond-wrench test. Experimental results showed that self-repair, in terms of bond restoration and aesthetic filling of cracks, occurs even after multiple cracking cycles when the bio-based mortar is used with both types of bricks, optimizing the autogenous healing (intrinsic) of cement-based mortars. The effectiveness varied also according to the types of brick and healing environment used, e.g. under humid conditions (RH ~ 95%), 50% vs 80% of the original capacity was regained in fully separated couplets made respectively with clay and calcium-silicate bricks.
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Cracks represent a prevalent form of damage in masonry structures, posing not only aesthetic concerns but also compromising structural durability; therefore, they are undesirable and need to be repaired. The repointing technique is traditionally implemented in this context, especially in historical masonry. However, this method fails to provide a long-term solution, leaving structures vulnerable to future damage. The paper investigates the applicability of a bio-based self-healing mortar to enable autonomous repair of masonry. This innovative mortar, developed to repair concrete structures, was implemented to explore the capacity of couplets to recover their original bond capacity and aesthetic aspect after multiple damaging events. Specimens built with calciumsilicate and clay bricks were subjected to subsequent cracking cycles using a crack-mouth-opening -displacement controlled bond-wrench test. Experimental results showed that self-repair, in terms of bond restoration and aesthetic filling of cracks, occurs even after multiple cracking cycles when the bio-based mortar is used with both types of bricks, optimizing the autogenous healing (intrinsic) of cement-based mortars. The effectiveness varied also according to the types of brick and healing environment used, e.g. under humid conditions (RH ~ 95%), 50% vs 80% of the original capacity was regained in fully separated couplets made respectively with clay and calcium-silicate bricks.
Environmental factors, projected to intensify due to climate change predictions, can expedite the degradation and aging of historic building materials like masonry. Among the primary degradation risks, salt crystallization stands out. Historical masonry quay walls, a vital component of the infrastructure of numerous European cities, notably in the Netherlands, present a unique case study in this aspect. This uniqueness arises from their continuous and long-term exposure, not only to environmental influences but also to salts in the canal water. To investigate this, a coupled multiphase modeling strategy for the hygrothermal analysis of masonry structures is used to simulate the impact of salt crystallization on multi-wythe masonry quay walls in the city of Amsterdam. This modeling strategy is governed by four highly nonlinear and fully coupled differential equations addressing moisture mass conservation, salt mass conservation, energy balance, and salt crystallization/dissolution kinetics. The model has been previously validated against laboratory experiments, but it is here applied for the first time to a real case study. A parametric study adopting a 2D sectional numerical model of the quay wall was performed. Parameters investigated include the effects of boundary conditions at different faces of the quay wall, masonry bond pattern, salt concentration in the water as well as time variance of environmental relative humidity. The findings of this paper can be used to identify critical environmental conditions for quay walls as well as provide the basis for explaining the through-thickness variation of mechanical properties found in previous research.
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Environmental factors, projected to intensify due to climate change predictions, can expedite the degradation and aging of historic building materials like masonry. Among the primary degradation risks, salt crystallization stands out. Historical masonry quay walls, a vital component of the infrastructure of numerous European cities, notably in the Netherlands, present a unique case study in this aspect. This uniqueness arises from their continuous and long-term exposure, not only to environmental influences but also to salts in the canal water. To investigate this, a coupled multiphase modeling strategy for the hygrothermal analysis of masonry structures is used to simulate the impact of salt crystallization on multi-wythe masonry quay walls in the city of Amsterdam. This modeling strategy is governed by four highly nonlinear and fully coupled differential equations addressing moisture mass conservation, salt mass conservation, energy balance, and salt crystallization/dissolution kinetics. The model has been previously validated against laboratory experiments, but it is here applied for the first time to a real case study. A parametric study adopting a 2D sectional numerical model of the quay wall was performed. Parameters investigated include the effects of boundary conditions at different faces of the quay wall, masonry bond pattern, salt concentration in the water as well as time variance of environmental relative humidity. The findings of this paper can be used to identify critical environmental conditions for quay walls as well as provide the basis for explaining the through-thickness variation of mechanical properties found in previous research.
Bacteria-based self-healing agent for masonry repair
Applicability to cement-lime mortars
In the pursuit of introducing bacteria-based self-healing mortar for masonry repair, this study examined the potential of incorporating a poly-lactic acid (PLA) agent—already established in concrete repair—into cement-lime mortars, typical of historical constructions. Testing prisms constructed with varying lime/cement ratios revealed decreased flexural and compressive strength in high-cement-concentration mortars upon the addition of the agent; for mortars with high lime concentration, however, the agent led to an increase in both strengths. Furthermore, the agent's potential to self-repair was confirmed by allowing the remaining portions of tested samples to heal under humid conditions. Irrespective of mortar composition, cracks were resealed thus confirming the aesthetic, and potentially watertightness, restoration.
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In the pursuit of introducing bacteria-based self-healing mortar for masonry repair, this study examined the potential of incorporating a poly-lactic acid (PLA) agent—already established in concrete repair—into cement-lime mortars, typical of historical constructions. Testing prisms constructed with varying lime/cement ratios revealed decreased flexural and compressive strength in high-cement-concentration mortars upon the addition of the agent; for mortars with high lime concentration, however, the agent led to an increase in both strengths. Furthermore, the agent's potential to self-repair was confirmed by allowing the remaining portions of tested samples to heal under humid conditions. Irrespective of mortar composition, cracks were resealed thus confirming the aesthetic, and potentially watertightness, restoration.
Testing Methods for Masonry Cores
A way forward to increase reliability of mechanical properties evaluation
The assessment of unreinforced brick masonry structures and infrastructure is a worldwide challenge for the development of resilient urban areas and preservation of historical assets. Among other factors, the estimation of mechanical performance of masonry in existing construction is of importance. However, the characterisation effort does not always satisfy the requirements from structural analyses point of view, i.e. need of elastic, strength and toughness properties, and/or from technical point of view, i.e. use of conventional technical expertise and limited invasiveness. In this respect, the new RILEM Technical Committee CTM aims at promoting the use of tests on masonry cores for the evaluation of compression and shear properties of unreinforced masonry with regular units. Upon a state-of-the-art review, a database of previous experimental test series will be created to identify influencing factors (e.g., core’s geometry, boundary conditions). Selected testing procedures will be compared at various international institutes for a variety of masonry types typically used in existing structures and infrastructure. By comparing results with standardise tests, correction factors will be identified. Eventually, testing guidelines to characterise masonry with core specimens will be defined and shared within the research and engineering community.
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The assessment of unreinforced brick masonry structures and infrastructure is a worldwide challenge for the development of resilient urban areas and preservation of historical assets. Among other factors, the estimation of mechanical performance of masonry in existing construction is of importance. However, the characterisation effort does not always satisfy the requirements from structural analyses point of view, i.e. need of elastic, strength and toughness properties, and/or from technical point of view, i.e. use of conventional technical expertise and limited invasiveness. In this respect, the new RILEM Technical Committee CTM aims at promoting the use of tests on masonry cores for the evaluation of compression and shear properties of unreinforced masonry with regular units. Upon a state-of-the-art review, a database of previous experimental test series will be created to identify influencing factors (e.g., core’s geometry, boundary conditions). Selected testing procedures will be compared at various international institutes for a variety of masonry types typically used in existing structures and infrastructure. By comparing results with standardise tests, correction factors will be identified. Eventually, testing guidelines to characterise masonry with core specimens will be defined and shared within the research and engineering community.
This paper presents the results of an experimental campaign carried out to characterise the mechanical properties of multi-wythe masonry infrastructure in the city of Amsterdam. Samples were extracted from a 1.2 m thick bridge’s pillar constructed in 1882. For the characterisation of shear and compressive properties of masonry, tests on cores with a 100 mm diameter were performed at the Stevinlaboratorium of Delft University of Technology. Samples were extracted along different locations in the wall thickness to evaluate the effect of exposure to environment conditions. Overall, the study provides a first insight on the mechanical properties of multi-wythe masonry city infrastructure and knowledge regarding the sampling and testing strategy for these structures. In turn, this will increase the knowledge on multi-wythe masonry, which is limited in literature, and will support the assessment of many infrastructures in typical Dutch canal cities.
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This paper presents the results of an experimental campaign carried out to characterise the mechanical properties of multi-wythe masonry infrastructure in the city of Amsterdam. Samples were extracted from a 1.2 m thick bridge’s pillar constructed in 1882. For the characterisation of shear and compressive properties of masonry, tests on cores with a 100 mm diameter were performed at the Stevinlaboratorium of Delft University of Technology. Samples were extracted along different locations in the wall thickness to evaluate the effect of exposure to environment conditions. Overall, the study provides a first insight on the mechanical properties of multi-wythe masonry city infrastructure and knowledge regarding the sampling and testing strategy for these structures. In turn, this will increase the knowledge on multi-wythe masonry, which is limited in literature, and will support the assessment of many infrastructures in typical Dutch canal cities.
Innovative Application of Self-healing Technology to Masonry
A Proof of Concept
Cracks are one of the most common expressions of damage in masonry structures. Aside from aesthetic issues, they can compromise the overall behaviour of the structure; therefore, they are undesirable and need to be repaired. The repointing technique is traditionally implemented in this context, especially in historical masonry. Nevertheless, future damage is not prevented and may arise again, thus requiring renewed repointing interventions. The paper describes a preliminary study conducted at Delft University of Technology to investigate the applicability of the innovative self-healing technology to enable an automatic repair of masonry cracks. A bacteria-based self-healing mortar, developed to repair existing concrete structures, was implemented to explore the capacity of couplets to recover their original strength and aesthetic aspect after multiple damaging events. Specimens built with calcium-silicate and clay bricks were subjected to subsequent cracking cycles using a crack-mouth-opening-displacement controlled bond-wrench test. Experimental results showed that self-repair, in terms of strength restoration and aesthetic filling of cracks, occurs even after multiple cracking cycles when the self-healing mortar is used with both types of bricks, optimizing the autogenous healing of cement-based mortars. In this context, the healing effectiveness tended to decrease as the crack width and the number of cycles increased. The effectiveness varied also according to the types of brick and healing environment used, e.g. under humid conditions (RH ~ 95%), 50% vs 80% of the original capacity was regained in fully separated couplets made respectively with clay and calcium-silicate bricks. This outcome provides the ground to delineate the remaining testing campaign.
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Cracks are one of the most common expressions of damage in masonry structures. Aside from aesthetic issues, they can compromise the overall behaviour of the structure; therefore, they are undesirable and need to be repaired. The repointing technique is traditionally implemented in this context, especially in historical masonry. Nevertheless, future damage is not prevented and may arise again, thus requiring renewed repointing interventions. The paper describes a preliminary study conducted at Delft University of Technology to investigate the applicability of the innovative self-healing technology to enable an automatic repair of masonry cracks. A bacteria-based self-healing mortar, developed to repair existing concrete structures, was implemented to explore the capacity of couplets to recover their original strength and aesthetic aspect after multiple damaging events. Specimens built with calcium-silicate and clay bricks were subjected to subsequent cracking cycles using a crack-mouth-opening-displacement controlled bond-wrench test. Experimental results showed that self-repair, in terms of strength restoration and aesthetic filling of cracks, occurs even after multiple cracking cycles when the self-healing mortar is used with both types of bricks, optimizing the autogenous healing of cement-based mortars. In this context, the healing effectiveness tended to decrease as the crack width and the number of cycles increased. The effectiveness varied also according to the types of brick and healing environment used, e.g. under humid conditions (RH ~ 95%), 50% vs 80% of the original capacity was regained in fully separated couplets made respectively with clay and calcium-silicate bricks. This outcome provides the ground to delineate the remaining testing campaign.
Canals delimited by masonry quay walls are integral elements of many cities in the Netherlands. Historically built to enable the efficient transportation of goods, today such infrastructure also gives the cities their historical and monumental character. In recent years, many quay walls in the Netherlands have shown substantial deformation and damage, and in few cases even collapse. Historical quay walls, which are constructed in thick multi-wythe unreinforced brick masonry and are supported on a system of timber piles, nowadays sustain traffic loads larger than the one they were designed for. Instances of collapse and severe damage has given rise to a need for research assessing the safety of these structures, which are not appropriately covered by any normative or standardised guidelines. This paper presents a novel methodology to numerically assess the performance of masonry walls in historical quays under the dynamic effect of traffic loads. Application of the proposed methodology to a case study in Amsterdam, the Netherlands, is presented.
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Canals delimited by masonry quay walls are integral elements of many cities in the Netherlands. Historically built to enable the efficient transportation of goods, today such infrastructure also gives the cities their historical and monumental character. In recent years, many quay walls in the Netherlands have shown substantial deformation and damage, and in few cases even collapse. Historical quay walls, which are constructed in thick multi-wythe unreinforced brick masonry and are supported on a system of timber piles, nowadays sustain traffic loads larger than the one they were designed for. Instances of collapse and severe damage has given rise to a need for research assessing the safety of these structures, which are not appropriately covered by any normative or standardised guidelines. This paper presents a novel methodology to numerically assess the performance of masonry walls in historical quays under the dynamic effect of traffic loads. Application of the proposed methodology to a case study in Amsterdam, the Netherlands, is presented.