This paper presents a study of the electrical and mechanical behavior of several continuous carbon fibers epoxy composites for both strengthening and monitoring of structures. In these composites, the arrangement of fibers was deliberately diversified to test and understand the ability of the composites for self-sensing low strains. Composites with different arrangements of fibers and textile weaves, mainly unidirectional continuous carbon reinforced composites, were tested at the dynamometer. A two-probe method was considered to measure the relative electrical resistance of these composites during loading. The measured relative electrical resistance includes volume and contact electrical resistances. For all tested specimens, it increases with an increase in tensile strain, at low strain values. This is explained by the improved alignment of fibers and resulting reduction of the number of possible contacts between fibers during loading, increasing as a consequence the contact electrical resistance of the composite. Laboratory tests on strengthening of structural elements were also performed, making hand-made composites by the “wet process”, which is commonly used in civil engineering for the strengthening of all types of structures in-situ. Results show that the woven epoxy composite, used for strengthening of concrete elements is also able to sense low deformations, below 1%. Moreover, results clearly show that this textile sensor also improves the mechanical work of the strengthened structural elements, increasing their bearing capacity. Finally, the set of obtained results supports the concept of a textile fabric capable of both structural upgrade and self-monitoring of structures, especially large structures of difficult access and needing constant, sometimes very expensive, health monitoring.

Health and safety are relevant in transportation infrastructures. Most health evaluations are performed by spot measurements using conventional transducers. This study aims to explore a self-sensing composite that can be integrated into a structural layer of a pavement or rail track to continuously evaluate the full layer performance in critical zones and anticipate damage occurrence. For this purpose, a 10 % cement-stabilized standard sand used in conventional structural layers was transformed into a self-sensing composite by synergistic incorporation of 3 % and 4 % multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs). In addition, Pluronic F-127 and tributyl phosphate 97 % (TBP) were used for better dispersion of carbon nanomaterials in the mixture, and these materials were compacted at maximum density and optimum moisture conditions. Through a detailed laboratory investigation, including piezoresistivity measurements, digital image correlation (DIC), and microstructure analysis through scanning electron microscopy (SEM), it was possible to propose a novel method to detect the propagation of damage under monotonic loading based on changing the slope of normalized electrical resistance against time. This was supported by a theoretical background able to explain that the emergence and propagation of damage result in increasing electrical resistance and hence fractional changes in resistance (FCR).

A self-sensing cementitious geocomposite was developed based on laboratory data, and it demonstrated good physical and mechanical properties, durability, and piezoresistivity performance. It consisted of stabilized cemented sand containing multiwalled carbon nanotubes (MWCNTs) with graphene nanoplatelets (GNPs) as conductive fillers. This geocomposite could be used to detect damage based on the relationship between electrical impedance and mechanical performance, making it suitable for use as structural layers in railway lines. In this study, the effects of MWCNTs and GNPs, as well as of degree of saturation, were evaluated on the compaction, secant modulus, resilient modulus, electrical resistance, and piezoresistivity of the geocomposite. Scanning electron microscopy (SEM) and microscopic imaging were used to analyse microstructural alterations induced by varying concentrations of MWCNTs and GNPs. This innovative geocomposite, intended for installation in Portuguese railway lines as part of the EU project IN2TRACK3, is aimed at capturing performance data, identifying structural damage levels, and estimating load intensity, axle numbers, and train speed. The feasibility of its use is discussed based on literature studies and research conducted under the IN2TRACK2 and IN2TRACK3 projects, and its potential advantages over traditional methods for monitoring rail track health are highlighted.

In the present research, we have investigated the possibility of using core reinforced braided composite rods (BCRs) for self monitoring of deformation and damage. The BCRs were produced by braiding polyester fibres around a resin (unsaturated polyester) impregnated core, composed of glass and carbon fibre mixture, using a simple, single-step and low cost braiding process. The sensing behaviour was characterized by monitoring the change in electrical resistance with deformation using two terminal dc method. According to the experimental results, the composition with less amount of carbon fibre in the core showed the best strain sensing behaviour.

Monitoring of transport infrastructures, in terms of early damage detection, can prevent the loss of life and economic damage associated with sudden infrastructure collapse and inform timely intervention, such as repair, to increase the sustainability and service life of infrastructures. Self-sensing cementitious geocomposites with the ability to detect stress, strain, and damage based on a piezoresistive mechanism enable the development of more integrated and viable geomaterial monitoring solutions than existing monitoring technologies. Self-sensing cementitious geocomposites are composed of conductive phases embedded in cementitious geomaterials that exhibit both sensing ability and superior mechanical properties. The states of stress, strain, displacement, and damage in infrastructures can be investigated by analysing the change in their electrical resistance. In this review, different types of self-sensing composites, their preparation, influential parameters, and associated theoretical investigations are discussed in detail to inform future advances in the development of self-sensing geocomposites. The challenges of this technology have also been summarised. This review is expected to stimulate and inform research that explores the development and application prospects of self-sensing cementitious geocomposites.

The sensing ability induced by carbon nanomaterials has attracted considerable attention in the scientific community for use in cement and concrete. However, the application of carbon nanomaterials in stabilized soils has been scarce. Therefore, in this study, the use of multiwalled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) in cement-stabilized sand was investigated. MWCNTs and GNPs not only improve the mechanical characteristics of cement-stabilized sand but also can change conventional cement-stabilized sand into an intrinsic self-sensing material, which is important for structural health monitoring (SHM). The proposed composite was submitted from small to large stresses under cyclic loading to detect strain, cracks, and damage. To identify different levels of damage, various macro- and microscale analyses were carried out to explain the composite self-sensing ability. The key results used in the analysis were from the fractional changes in resistivity (FCR) and digital image correlation (DIC) and scanning electron microscope (SEM) measurements. These results demonstrate the capability of the proposed MWCNT/GNP cement-stabilized sand for detecting the strain, stress, and damage of materials through different self-sensing trends in the elastic and plastic regions. It is very important to monitor the serviceability of a composite in a real structure through different self-sensing trends in the elastic and plastic regions.

Allocating different capabilities to structural elements simultaneously is still challenging. In this study, a field-applicable multifunctional planar braided composite with the abilities of reinforcing, self-sensing and self-heating was developed for the first time. In this route, three commercial fabrics were used, including cotton, cotton/polyamide, and polyester. The fabrics were first chemically treated and then coated with a carbon nanomaterial-based polymeric conductive paste using screen printing with different concentrations and layers. The samples were then covered and sealed with a thermoplastic polyurethane-based polymer to avoid environmental factors effects. Smart planar composites (SPC) were also used as reinforcement for cementitious specimens. The electrical conductivity and joule heating capability of the samples were also evaluated. The microstructure of the SPCs was investigated using various tests. The mechanical and self-sensing performances of the cementitious composite reinforced with different SPCs were assessed using different load patterns. The results showed a heating rate of 0.44 ˚C/s, a joule heating power of 0.7 W/˚C, and a maximum temperature of 44 ˚C which proved the proper heating capability of the cementitious composites reinforced with SPCs. The great correlation between electrical resistivity changes and strain values indicated the high potential of the composite in strain sensing for different applications. The SPCs also improved the post-crack behaviour of the specimen and its flexural strength and failure strain by approximately 50% and 118%, respectively. The outcomes of this study draw a bright horizon in multifunctional braided composite development with different applications in civil infrastructures, which is a crucial step for intelligent cities' advances.

O objetivo do presente trabalho consistiu no desenvolvimento de uma nova classe de polímeros reforçados por fibras, nomeadamente Varões Compósitos Entrançados (BCR´s). Assim, o objetivo traçado para a presente tese centrou-se no desenvolvimento de BCR´s com pseudoductilidade com vista ao melhoramento do desempenho mecânico e BCR´s com monitorização que permitam monitorizar deformações experimentadas pelo reforço quando solicitado por uma determinada ação mecânica. Assim, numa primeira parte do trabalho foram desenvolvidas técnicas que permitissem a produção de BCR´S com a incorporação das tecnologias mencionadas anteriormente. Para além disso, neste âmbito, foram igualmente adaptados métodos de ensaio que permitissem avaliar o desempenho associado à pseudoductilidade e à capacidade de monitorização dos varões. Para o desenvolvimento de BCR´s com Pseudoductilidade foram exploradas duas abordagens, nomeadamente: (a) pseudoductilidade a partir da incorporação de materiais fibrosos com diferentes propriedades mecânicas (alongamento na rotura); (b) pseudoductilidade a partir da utilização de materiais dúcteis (aço) em combinação com fibras de Vidro e Basalto. Desta forma, foi possível concluir que a partir de fibras com diferentes alongamentos na rotura é possível induzir um comportamento pseudo-ductil em BCR´s. No que diz respeito à monitorização, foram estudadas as seguintes situações: (a) introdução de filamentos intrinsecamente condutores; (b) funcionalização de filamentos de fibra de vidro com materiais condutores, nomeadamente nanotubos de carbono. No âmbito do presente estudo foi possível desenvolver novas funcionalizações no que diz respeito aos BCR no sentido de as capacitar de propriedades piezoresistivas em ensaios de tração cíclicos.

In this study, the effects of multiscale conductive carbon fillers, including carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and carbon fibres (CFs), on the mechanical and microstructural properties, durability, and piezoresistivity of cementitious stabilised sand (CSS) were investigated. In this route, the surface of the CFs was modified via an oxidation process to improve their interfacial performance and dispersion. An optimum amount of hybrid CNT/GNP with different concentrations of CFs was incorporated into the CSS, and specimens were fabricated using the standard compaction method at the optimum water content. The interfacial properties of the CFs were studied by performing pullout tests and several chemical analyses. The variations in the mechanical and microstructural, durability, and piezoresistivity of the CSS, were investigated by various tests. In addition, the status of the specimens in terms of residual strain and damages was identified by the digital image correlation technique. The results showed a considerable improvement in the interfacial properties of the modified CFs in terms of physical and chemical bonding with the cement matrix. In addition, a combination of 0.17% CNT/GNP (1:1, by weight of dry sand) with 0.75% CF can improve the maximum dry density and mechanical properties, as well as the ductility and durability of the CSS. In addition, using multiscale conductive fillers resulted in a considerable enhancement in the electrical conductivity and piezoresistivity of the CSS. The outcomes indicate the immense potential of CNT/GNP/CF for the development of a sustainable self-sensing cementitious geocomposite.

The development of innovative sensing technologies is essential for implementing smart road pavement monitoring systems. The design of asphalt-based materials with intrinsic self-sensing capabilities represents a promising solution in that regard. Despite this, this technology is still not mature and further efforts should be made for its development. With this aim, the present paper evaluates the self-sensing response of asphalt mixtures incorporating graphene nanoplatelets (GNPs), and proposes the use of a digital signal processing technique, based on wavelet transform, for the analysis of the electrical signals generated by the mixture. The results showed that the mixtures exhibited both stress and damage sensing functions, albeit some issues related to the dispersion of GNPs should be further investigated. In addition, wavelet transform analysis seems to be able to capture insightful information about the electrical response of the mixture, as well as its structural condition, useful for traffic and pavement health monitoring purposes.