Marco Rallini

An Experimental Study on Static and Dynamic Strain Sensitivity of Embeddable Smart Concrete Sensors Doped with Carbon Nanotubes for SHM of Large Structures

The availability of new self-sensing cement-based strain sensors allows the development of dense sensor networks for Structural Health Monitoring (SHM) of reinforced concrete structures. These sensors are fabricated by doping cement-matrix mterials with conductive fillers, such as Multi Walled Carbon Nanotubes (MWCNTs), and can be embedded into structural elements made of reinforced concrete prior to casting. The strain sensing principle is based on the multifunctional composites outputting a measurable change in their electrical properties when subjected to a deformation. Previous work by the authors was devoted to material fabrication, modeling and applications in SHM. In this paper, we investigate the behavior of several sensors fabricated with and without aggregates and with different MWCNT contents. The strain sensitivity of the sensors, in terms of fractional change in electrical resistivity for unit strain, as well as their linearity are investigated through experimental testing under both quasi-static and sine-sweep dynamic uni-axial compressive loadings. Moreover, the responses of the sensors when subjected to destructive compressive tests are evaluated. Overall, the presented results contribute to improving the scientific knowledge on the behavior of smart concrete sensors and to furthering their understanding for SHM applications.

Strain sensitivity of carbon nanotube cement-based composites for structural health monitoring

Cement-based smart sensors appear particularly suitable for monitoring applications, due to their self-sensing abilities, their ease of use, and their numerous possible field applications. The addition of conductive carbon nanofillers into a cementitious matrix provides the material with piezoresistive characteristics and enhanced sensitivity to mechanical alterations. The strain-sensing ability is achieved by correlating the variation of external loads or deformations with the variation of specific electrical parameters, such as the electrical resistance. Among conductive nanofillers, carbon nanotubes (CNTs) have shown promise for the fabrication of self-monitoring composites. However, some issues related to the filler dispersion and the mix design of cementitious nanoadded materials need to be further investigated. For instance, a small difference in the added quantity of a specific nanofiller in a cement-matrix composite can substantially change the quality of the dispersion and the strain sensitivity of the resulting material. The present research focuses on the strain sensitivity of concrete, mortar and cement paste sensors fabricated with different amounts of carbon nanotube inclusions. The aim of the work is to investigate the quality of dispersion of the CNTs in the aqueous solutions, the physical properties of the fresh mixtures, the electromechanical properties of the hardened materials, and the sensing properties of the obtained transducers. Results show that cement-based sensors with CNT inclusions, if properly implemented, can be favorably applied to structural health monitoring.

STRAIN-SENSING CARBON NANOTUBE CEMENT-BASED COMPOSITES FOR APPLICATIONS IN STRUCTURAL HEALTH MONITORING: PREPARATION AND MODELLING ISSUES

The authors have recently explored the use of electrically conductive cement-based composites doped with carbon nanotubes for dynamic monitoring of strain in concrete structures. While the technology appears to be very promising for cost-effective structural health monitoring, some challenges still limit its applicability to full-scale constructions. The dispersion of the nanoparticles, typically based on sonic treatment and on other special procedures, is not compatible with distributed full-scale deployments and essentially limits the applications of the technology to the fabrication of embeddable sensors. Also, the electromechanical behaviour of the composites is complex and a proper analytical model linking electrical output to accurate strain measurements is yet to be established. This work discusses these open issues in fabrication and modelling of carbon nanotube composite concrete. A fabrication procedure with potential applicability to large casting volumes is presented and experimental results highlighting its effectiveness are discussed. Results cover analysis of nanoparticles dispersion, electrical percolation, strain sensitivity and polarization.

A comparative study between carbon nanotubes and carbon nanofibers as nanoinclusions in self-sensing concrete

Structural Health Monitoring is the automated process of damage detection and performance assessment aiming at providing reliable information regarding the integrity and the safety of a structure. Conventional measurement devices usually employed in structural health monitoring systems are affected by known drawbacks that limit their performance for health assessment, such as reduced durability against environmental actions, difficulty of access, high costs and low scalability. New nanomodified self-sensing materials may enable dense network sensing applications. Such materials are fabricated by integrating conductive nanoparticles into a building material, for instance a cementitious matrix. The self-sensing function is obtained by correlating the state of strain with appropriate material parameters, such as electrical resistance. This paper investigates and compares the smart properties of cement-based nanocomposite materials fabricated using carbon nanotubes and carbon nanofibers as conductive nanoinclusions. The work discusses the preparation procedures to obtain a homogeneous distribution of nanoparticles in the cementitious materials and the strain sensitivity of different nanocomposite specimens.