Periodic Reporting for period 1 - S-RePaIR (Smart Restoration with Particle Infused Repointing)
Periodo di rendicontazione: 2021-09-01 al 2023-08-31
The S-RePaIR action explores the use of smart materials based on traditional building techniques as multifunctional intervention agents in historic masonry structures.
Masonry structures of all typologies are commonly encountered worldwide and constitute a large percentage of the architectural heritage inventory, particularly in urban environments. Due to the inherent brittleness of the materials used in traditional masonry architecture, and following the effects of aging, historic masonry suffers from accumulation of damage in the form of cracks and loss of durability. Modern urban development, which includes tunnelling and an increase in ambient vibration due to traffic, is a continuous source of strain on historic masonry buildings.
Preventive maintenance through structural intervention can help protect architectural heritage structures and extend the service life of existing infrastructure. The use of traditional materials is preferred from a conservation engineering standpoint as it maximises aesthetic, chemical and mechanical compatibility between the in-situ and intervention materials. Especially in the case of historic masonry, this translates to the use of lime-based mortars for repointing, rendering or reconstruction. However, lime-based mortars are characterised by low strength and an extended hardening period before for strength development.
Complementing preventive maintenance, structural health monitoring (SHM) is a valuable tool for detecting damage and extraordinary structural deformation in existing structures. Early warning on these effects can help optimise the timing of preventive maintenance, leading to reduced overall intervention costs and reduction of the detrimental effects of damage. However, off-the-shelf sensors for SHM are costly, often suffer from lack of durability and are only able to provide an indirect measurement of damage onset and propagation.
The practical and technological requirements of both preventive maintenance and SHM for protecting historic masonry structures can be satisfied through the use of smart multifunctional materials, namely intervention materials applied for the structural repair and maintenance of structures while also possessing self-sensing capabilities for deformation and damage. Smartness in cementitious materials, such as mortars, can be achieved through enhancement of their piezoresistive properties by means of dispersion of conductive nano- and micro-scale fillers in the material. These fillers include carbon nanotubes, graphite and carbon microfibres, all of which are characterised by different properties and different effects in the materials they are dispersed in.
The primary objective of the project is the development of a smart intervention mortar based on natural hydraulic lime to be used as a repointing agent in historic masonry structures. This is accomplished through the extensive comparison of the mechanical, physical, durability and electromechanical properties of lime-based mortars doped with different types of conductive fillers at different concentrations.
Masonry structures of all typologies are commonly encountered worldwide and constitute a large percentage of the architectural heritage inventory, particularly in urban environments. Due to the inherent brittleness of the materials used in traditional masonry architecture, and following the effects of aging, historic masonry suffers from accumulation of damage in the form of cracks and loss of durability. Modern urban development, which includes tunnelling and an increase in ambient vibration due to traffic, is a continuous source of strain on historic masonry buildings.
Preventive maintenance through structural intervention can help protect architectural heritage structures and extend the service life of existing infrastructure. The use of traditional materials is preferred from a conservation engineering standpoint as it maximises aesthetic, chemical and mechanical compatibility between the in-situ and intervention materials. Especially in the case of historic masonry, this translates to the use of lime-based mortars for repointing, rendering or reconstruction. However, lime-based mortars are characterised by low strength and an extended hardening period before for strength development.
Complementing preventive maintenance, structural health monitoring (SHM) is a valuable tool for detecting damage and extraordinary structural deformation in existing structures. Early warning on these effects can help optimise the timing of preventive maintenance, leading to reduced overall intervention costs and reduction of the detrimental effects of damage. However, off-the-shelf sensors for SHM are costly, often suffer from lack of durability and are only able to provide an indirect measurement of damage onset and propagation.
The practical and technological requirements of both preventive maintenance and SHM for protecting historic masonry structures can be satisfied through the use of smart multifunctional materials, namely intervention materials applied for the structural repair and maintenance of structures while also possessing self-sensing capabilities for deformation and damage. Smartness in cementitious materials, such as mortars, can be achieved through enhancement of their piezoresistive properties by means of dispersion of conductive nano- and micro-scale fillers in the material. These fillers include carbon nanotubes, graphite and carbon microfibres, all of which are characterised by different properties and different effects in the materials they are dispersed in.
The primary objective of the project is the development of a smart intervention mortar based on natural hydraulic lime to be used as a repointing agent in historic masonry structures. This is accomplished through the extensive comparison of the mechanical, physical, durability and electromechanical properties of lime-based mortars doped with different types of conductive fillers at different concentrations.
Three different conductive fillers were used for doping natural hydraulic lime mortar: graphite, carbon nanotubes and carbon microfibers. These fillers were introduced at different doping levels, high enough to have a positive impact on the properties of the modified mortar but low enough to avoid overpercolation, loss of workability or discolouration.
All fillers resulted in an increase in the flexural and compressive strength of the mortar compared to its unmodified counterpart. While the maximum enhancement of mechanical properties achieved within the doping range was comparable between fillers, the maximum enhancement in the case of carbon microfibres was achieved at remarkably low levels of doping (0.01% by weight of binder). Desirable workability was not observed for any of the fillers, ensuring that the modified mortars can be applied with the same ease for repointing as their unmodified counterpart. The inclusion of fillers resulted in an increase in porosity while the bulk density was not strongly affected. While graphite and nanotubes resulted in darkening of the hardened mortar’s colour, microfibres did not have the same effect in the doping range investigated. All fillers had a positive effect on the piezoresistivity of the mortar (in terms of gauge factor and measurement linearity), with graphite and microfibres causing the strongest enhancement in the investigated doping range. Residual damage, primarily through cracks, can be measured through a residual change in the resistance of the material. The dispersion of graphite and microfibres can be achieved without substantially altering the mixing process for producing the mortar, whereas dispersing the nanotubes requires ultrasonic processing of the batch water, which can increase cost and reduce applicability in large volumes.
For deepening the mechanical, physical and durability study, the mortar doped with a low concentration of carbon microfibres was promoted to the second stage of testing. Accompanying tests on unmodified and modified paste were also conducted. Both standard mortar tests and tests on thin mortar joints were conducted for illustrating differences in the response between specimens of different shape. Mechanical testing was conducted at different stages of aging (between 4 and 16 weeks), indicating that the modified materials were able to obtain higher mechanical strength at early ages. The enhancement was more pronounced in the case of the paste. The durability of the modified mortar against salt crystallisation was slightly lower than that of the plain mortar, potentially due to higher porosity. However, the durability of the modified paste in the same test conditions was enhanced, potentially due to higher tensile strength.
The experimental results were communicated at a lecture at University of Leeds, following an invitation by the Materials and Structures Group on 30 June 2022. Further dissemination was achieved through participation at the 10th European Workshop on Structural Health Monitoring (special session: SHM of Engineering Structures Using Smart Multifunctional Materials and Systems), held in Palermo 4-7 July 2022.
All fillers resulted in an increase in the flexural and compressive strength of the mortar compared to its unmodified counterpart. While the maximum enhancement of mechanical properties achieved within the doping range was comparable between fillers, the maximum enhancement in the case of carbon microfibres was achieved at remarkably low levels of doping (0.01% by weight of binder). Desirable workability was not observed for any of the fillers, ensuring that the modified mortars can be applied with the same ease for repointing as their unmodified counterpart. The inclusion of fillers resulted in an increase in porosity while the bulk density was not strongly affected. While graphite and nanotubes resulted in darkening of the hardened mortar’s colour, microfibres did not have the same effect in the doping range investigated. All fillers had a positive effect on the piezoresistivity of the mortar (in terms of gauge factor and measurement linearity), with graphite and microfibres causing the strongest enhancement in the investigated doping range. Residual damage, primarily through cracks, can be measured through a residual change in the resistance of the material. The dispersion of graphite and microfibres can be achieved without substantially altering the mixing process for producing the mortar, whereas dispersing the nanotubes requires ultrasonic processing of the batch water, which can increase cost and reduce applicability in large volumes.
For deepening the mechanical, physical and durability study, the mortar doped with a low concentration of carbon microfibres was promoted to the second stage of testing. Accompanying tests on unmodified and modified paste were also conducted. Both standard mortar tests and tests on thin mortar joints were conducted for illustrating differences in the response between specimens of different shape. Mechanical testing was conducted at different stages of aging (between 4 and 16 weeks), indicating that the modified materials were able to obtain higher mechanical strength at early ages. The enhancement was more pronounced in the case of the paste. The durability of the modified mortar against salt crystallisation was slightly lower than that of the plain mortar, potentially due to higher porosity. However, the durability of the modified paste in the same test conditions was enhanced, potentially due to higher tensile strength.
The experimental results were communicated at a lecture at University of Leeds, following an invitation by the Materials and Structures Group on 30 June 2022. Further dissemination was achieved through participation at the 10th European Workshop on Structural Health Monitoring (special session: SHM of Engineering Structures Using Smart Multifunctional Materials and Systems), held in Palermo 4-7 July 2022.
The electromechanical enhancement of natural hydraulic lime-based mortars, both plain and modified with various conductive nano- and micro-fillers, was investigated extensively. In addition to this enhancement, improvements in mechanical strength were registered for strikingly low amounts of doping with easy to disperse fillers. The resulting modified material exhibits a high degree of compatibility with traditional lime mortar, can be produced with only a marginal increase in material cost and can be applied with the same methods as plain lime mortar. These features make the modified material suitable for wide-scale application in restoration projects involving architectural heritage built in masonry.
The piezoresistive enhancement of the modified material means that it can be used as a smart multifunctional intervention agent, replacing the use of externally mounted sensors. Thus, it is possible to carry out structural health monitoring operations on historic masonry buildings with low-cost and inconspicuous sensors, with a positive impact on the mechanical resistance of masonry and zero aesthetic impact on the repair patch.
The piezoresistive enhancement of the modified material means that it can be used as a smart multifunctional intervention agent, replacing the use of externally mounted sensors. Thus, it is possible to carry out structural health monitoring operations on historic masonry buildings with low-cost and inconspicuous sensors, with a positive impact on the mechanical resistance of masonry and zero aesthetic impact on the repair patch.