Characterization of the strain-sensing behavior of smart bricks: A new theoretical model and its application for monitoring of masonry structural elements

https://doi.org/10.1016/j.conbuildmat.2020.118907Get rights and content

Highlights

  • Smart bricks are a new technology for structural health monitoring of masonry.

  • A new model describing the strain-sensing behavior of smart bricks is proposed.

  • Experiments demonstrate the accuracy of the model at low and high compressive states.

  • Steel microfibers boost electrical and piezoresistive properties of smart bricks.

  • The use of the model for strain estimation is exemplified in a masonry wall specimen.

Abstract

The paper proposes a new theoretical model describing the strain-sensing response of smart bricks, a novel class of piezoresistive smart sensors for structural health monitoring of masonry structures. The proposed model is experimentally validated by carrying out electromechanical tests on single smart bricks. An illustrative application is also presented to exemplify the use of the model when smart bricks are embedded within a masonry wall. Results demonstrate that the novel theoretical model is able to accurately reproduce the bricks’ response when subjected to compressive loads and is also effective for strain estimation and damage detection in a typical structural setting.

Introduction

The European historical, cultural and residential building heritage is largely composed of masonry structures which are often prone to structural pathologies, typically caused by differential foundation settlements, excessive dead loads, aging of materials and natural hazards, such as seismic events. In all these cases, inadequate maintenance and poor conservation can lead masonry structures to premature brittle collapses, which can be extremely dangerous for human lives and for cultural heritage preservation [1], [2], [3]. The rapid assessment of the health state of masonry constructions and the identification of incipient or developing damages, therefore represent an urgent priority that demands for new methodological developments. In this regards, Structural Health Monitoring (SHM) systems, capable to evaluate structural performance in a real time style and to allow an optimization of retrofitting and restoration activities, deserve a special attention [4].

Many recent literature works proposed to apply long-term vibration-based monitoring to slender historic structures, where dynamic response parameters, such as natural frequencies of vibration, are taken into account for the detection of anomalies in the overall structural behavior caused by a damage. Indeed, the development of crack patterns on masonry structural elements, as well as materials’ degradation, produce local modifications in structural stiffness, which imply a decay in natural frequencies of vibration that are usually estimated by employing a limited number of sensors deployed on the building and automated operational modal analysis tools [5], [6], [7]. It is worth noting, however, that similar approaches may fail when damage severity is not sufficient to affect the global dynamic behavior of the structure, which is often the case when dealing with local damages. In these cases, the detection of cracks’ propagation and the assessment of their evolution can be performed by adopting monitoring approaches that are based on the evaluation of locally meaningful static response parameters. Among the various available alternatives, widely adopted sensors for static SHM of masonry structures are resistive strain gauges (RSG)s for strain measurements, linear variable differential transformers (LVDT)s for crack amplitudes or relative displacements and inclinometers for tilts [8], [9]. A major limitation of these sensors is that they need to be deployed in the critical portion of the structure or to be directly installed on cracks to be effective. Crack monitoring can be also performed with contactless sensing techniques, such as tomographic imaging, infrared thermography [10], [11] and ground penetrating radar (GPR) [12], while engineering properties of masonry and its state of stress can be estimated by employing non-destructive testing methods using flatjacks, by in situ shear tests, or through non-invasive techniques, such as rebound hardness and acoustic emission approach [13], [14], [15], [16].

Nowadays, practical drawbacks in off-the-shelf sensing technologies limit their applications in large-scale deployments of continuous SHM systems to masonry structures. Traditional sensors are often expensive and characterized by physical and mechanical properties that are quite different to those of the monitored building. Oftentimes they have to be externally attached to the structure, hence, being exposed to atmospheric agents with consequent degradation of the bonding or of the sensor itself, while at other times they can also cause aesthetic issues, representing an especially limiting factor in the case of historical buildings. Last but not least, these sensing technologies require frequent maintenance and visual inspections during their service life, which could be difficult and expensive depending on the number of installed sensors and on their locations.

Recent developments in the field of materials’ science are offering engineers a wide range of multifunctional construction materials with enhanced physical, chemical and electromechanical properties [17], [18], [19]. Particularly promising are smart materials for SHM of concrete structures enabling the automated self-monitoring of global structural conditions and the detection and localization of damages [20], [21], [22]. To achieve this goal, common cement-based materials can be doped with carbon nanoinclusions, such as carbon nanofibers and carbon nanotubes, providing the material with enhanced piezoresistive capabilities and, therefore, with strain-sensing properties [23], [24], [25]. Such rising sensing technologies have low costs, high durability and can be directly embedded within the structure being monitored, inasmuch they are made with a similar material. These attractive features have been introduced in the field of SHM of masonry structures with the work by Downey et al. [26] that for the first time proposed special bricks doped with titanium dioxide, called “smart bricks”, and demonstrated their piezoresistive behavior, while also illustrating an example of their application for monitoring the health state of a small-scale wall subjected to eccentric compression loads. An improvement in smart brick technology has been achieved with the work of D’Alessandro et al. [27] which promoted the use of stainless steel micro fibers as conductive fillers and the use of external copper plate electrodes, rather that inner ones in stainless steel, for electrical measurements. García-Macías and Ubertini proposed a theoretical investigation concerning the effectiveness of the smart brick technology to reconstruct strain field maps when embedded into either new or pre-existing constructions, by post-processing smart bricks’ outputs with suitable spatial interpolation methods [28]. More recently in Meoni et al. [29], the Authors presented the first full-scale experimental application of smart bricks for monitoring an unreinforced masonry building subjected to shaking table tests. That paper demonstrated that smart bricks can be used to allow damage detection and localization after an earthquake, even at an early stage, through the definition of global and local damage indexes based on their electrical outputs. In that work, the need of performing a more detailed characterization of the smart bricks’ strain-sensing behavior was anticipated, given that the simple linear strain gauge equation fails at accurately modeling the smart bricks’ strain-sensing behavior at relatively low compressive states.

The present paper addresses the characterization of the strain-sensing behavior of smart bricks through the definition of an improved electromechanical model consisting of a series of resistors and linking bricks’ electrical outputs to their state of strain, considering both internal piezoresistivity and sensing at the electrodes’ contact interface. After presenting the new theoretical model, the paper proposes its experimental validation and characterization through a campaign of electromechanical tests carried out on single smart bricks containing different amounts of steel micro fibers, selected by performing an investigation on the smart bricks’ internal electrical properties. Furthermore, in order to exemplify the application of the series resistors model in strain estimation in typical structural settings, smart bricks are employed for monitoring strain within a small-scale wall specimen subjected to eccentric axial compression load.

The rest of the paper is organized as follows. Section 2 briefly reviews the smart brick technology and its application to SHM of masonry buildings. The production process of the doped bricks is described in the same section, together with the method applied to perform electrical measurements during laboratory tests. Section 3 introduces the novel series resistors model adopted to properly describe the smart bricks’ strain-sensing behavior. In addition, the methodology of laboratory tests performed to investigate internal electrical properties of the smart bricks and to experimentally validate the proposed model is illustrated. Afterwards, construction details of the small-scale wall specimen are provided and the eccentric axial compression test is fully described. Section 4 reports results obtained from electrical and electromechanical tests on single smart bricks as well as those achieved by testing the wall specimen. Finally, Section 5 concludes the work with comments and remarks.

Section snippets

Smart brick

This section introduces the smart brick technology, presenting the production process of the sensors and the methodology adopted to perform electrical measurements.

Analytical model and experimental methodology

This section illustrates the novel theoretical model describing the smart bricks’ strain-sensing behavior. Afterwards, the methodology adopted for its experimental validation is presented. In particular, two types of experiments are carried out. The first experiments deal with the study of electrical percolation, while, the second set of experiments validates the proposed electromechanical model. An example application of smart bricks inserted within a masonry wall specimen is also introduced

Results and discussion

This section summarizes the results obtained from electrical and electromechanical tests carried out on single smart bricks following the methodology described in the previous section. Furthermore, results obtained from the experimental application of the smart brick technology for monitoring strain within the wall specimen are discussed.

Conclusions

The paper has proposed a novel theoretical model for describing the smart bricks’ strain-sensing behavior under compression loads. This rising technology for structural health monitoring of masonry structures consists in clay bricks doped with stainless steel micro fibers, a conductive filler used to improve their piezoresistive capability, and equipped with two external copper plate electrodes for electrical measurements. When subjected to compression loads, variations in smart brick’s total

CRediT authorship contribution statement

A. Meoni: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Funding acquisition. A. D’Alessandro: Conceptualization, Methodology, Validation, Investigation, Writing - review & editing. F. Ubertini: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Italian Ministry of Education, University and Research (MIUR) through the funded project of Relevant National Interest “SMART-BRICK: Novel strain-sensing nanocomposite clay brick enabling self-monitoring masonry structures” (Protocol No. 2015MS5L27). The authors also wish to acknowledge the contribution of “Fornaci Briziarelli Marsciano Spa”, the brick manufacturing company that contributed to this research by providing technical support for preparing the smart

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