WIXLIB

HindawiShock and VibrationVolume 2017, Article ID 3648403, 11 pageshttps://doi.org/10.1155/2017/3648403Research ArticleStatic and Dynamic Strain Monitoring ofReinforced Concrete Components through Embedded CarbonNanotube Cement-Based SensorsAntonella D’Alessandro,1 Filippo Ubertini,1Enrique García-Macías,2 Rafael Castro-Triguero,3 Austin Downey,4Simon Laflamme,4 Andrea Meoni,1 and Annibale Luigi Materazzi11Department of Civil and Environmental Engineering, University of Perugia, Via G. Duranti 93, 06125 Perugia, ItalyDepartment of Continuum Mechanics and Structural Analysis, School of Engineering, University of Seville,Camino de los Descubrimientos s/n, 41092 Seville, Spain3Department of Mechanics, University of Cordoba, Campus de Rabanales, 14071 Cordoba, Spain4Department of Civil, Construction, and Environmental Engineering, Iowa State University, 394 Town Engineering, Ames,IA 50011, USA2Correspondence should be addressed to Antonella D’Alessandro; antonella.dalessandro@unipg.itReceived 28 February 2017; Accepted 28 June 2017; Published 7 August 2017Academic Editor: Marcello VanaliCopyright 2017 Antonella D’Alessandro et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.The paper presents a study on the use of cement-based sensors doped with carbon nanotubes as embedded smart sensors for staticand dynamic strain monitoring of reinforced concrete (RC) elements. Such novel sensors can be used for the monitoring of civilinfrastructures. Because they are fabricated from a structural material and are easy to utilize, these sensors can be integrated intostructural elements for monitoring of different types of constructions during their service life. Despite the scientific attention thatsuch sensors have received in recent years, further research is needed to understand (i) the repeatability and accuracy of sensors’behavior over a meaningful number of sensors, (ii) testing configurations and calibration methods, and (iii) the sensors’ ability toprovide static and dynamic strain measurements when actually embedded in RC elements. To address these research needs, thispaper presents a preliminary characterization of the self-sensing capabilities and the dynamic properties of a meaningful numberof cement-based sensors and studies their application as embedded sensors in a full-scale RC beam. Results from electrical andelectromechanical tests conducted on small and full-scale specimens using different electrical measurement methods confirm thatsmart cement-based sensors show promise for both static and vibration-based structural health monitoring applications of concreteelements but that calibration of each sensor seems to be necessary.1. IntroductionStructural health monitoring (SHM) is a topic of growing scientific interest in various fields of engineering, with potentialto increase engineering systems’ safety and lead to the optimization of repair, maintenance, and restoration activities[1, 2]. Strategically designed monitoring systems can detectdamage or variations of structural behavior during the servicelife of a structure [3–5]. SHM is also useful for fast screeningof structural conditions after severe events, such as blasts andearthquakes. The choice of the monitoring sensors and oftheir proper placement is critical for accurately analyzing thestructural performance and for the measurement of damagesensitive features and their statistical investigation. A spatiallydistributed dense sensing system increases the probability offinding and characterizing a given amount of damage. Anideal solution is to transform the entire structure into a selfsensing system, analogous to the biological nervous system.Recent developments in the field of nanotechnologyhave led to cement-based sensors doped with piezoresistive

2nanofillers, which can be used as a potential solution fordistributed sensing in reinforced concrete (RC) structures[6–8]. Nanomodified cement-based sensors can be easilyfabricated and embedded in a structure at critical locations.In addition, they possess approximately the same durability asthe materials to be monitored and exhibit lower maintenancecosts with respect to traditional sensors. Promising conductive nanoinclusions for cement-based sensors are carbonbased particles [9–12]. Among them, carbon nanotubes areparticularly suitable owing to their particular aspect ratio.They consist of concentric cylindrical graphene sheets ofnanometric diameters and lengths up to some micrometers [13–15]. Their dispersion into a cementitious matrixenhances the electrical properties of the original materials,providing them with self-sensing capabilities [16–18]. Theself-monitoring ability is achieved through the correlationof strains or stresses of the material to electrical features,such as electrical resistance or impedance [19–22]. Differentelectrical effects contribute to the strain-sensing mechanism: the piezoresistivity of the conductive nanofillers, thecontact resistance of the electrodes, the intrinsic resistanceof the different materials, and the tunneling and the fieldemission effects due to the nanosize of the fillers [16, 23].Although several studies were recently devoted to investigating challenges related to the dispersion of the conductivenanoparticles in the cement matrix [24, 25], the fabricationof nanocomposites with different amounts of fillers [26, 27],and electromechanical testing [28–30], the repeatability andaccuracy of the electrical behavior upon dynamic sensing stillrequire in-depth investigations. The authors have researcheda new cement-based sensor doped with multiwalled carbonnanotubes, termed carbon nanotube cement-based sensor(CNTCS) [15, 24, 31, 32]. The CNTCSs were fabricated withdifferent types of cement matrices (pastes, mortars, andconcretes) and various amounts of carbon nanoinclusions.Building on previous work, the objective of this paperis to investigate the use of CNTCSs as smart embeddedsensors for dynamic strain monitoring of RC elements. Ameaningful number of samples are first characterized andthe application to a full-scale RC beam is studied. The restof the paper is organized as follows. Section 2 presentsmaterials and preparation procedures of the cementitioussensors with multiwalled carbon nanotubes (MWCNTs). Section 3 describes the experimental methodologies. Section 4discusses results of electromechanical tests under applicationof slowly varying cyclical loads and sinusoidal dynamic loadswith increasing frequencies. The sensitivity of the samplesand their frequency response functions are analyzed and theelectrical and electromechanical properties of the sensors areinvestigated through both two- and four-probe measurementmethods. This section also includes test results from theembedment of CNTCSs in a full-scale RC beam. Section 5concludes the paper.Shock and Vibrationof cement (Figure 1). Before solidification, four stainless steelmeshes were embedded symmetrically through the sample.The inner meshes were placed at a mutual distance of 2 cm,while the outer ones were at a distance of 4 cm. Figure 1 showsthe geometry of the samples and of the electrodes.The carbon nanotubes were type Arkema GraphistrengthC100. Their dispersion in the cementitious matrix wasachieved using a physical dispersant, preliminary mechanicalmixing, and sonication (Figures 2(i)–2(iii)). A stable watersuspension was obtained and then mixed with cement and0.5% of a plasticizer with respect to the weight of cement(Figure 2(iv)). The cement was type 42.5, pozzolanic. Thewater/cement ratio was 0.45. The smooth mixture was pouredinto oiled molds and the steel nets were embedded (Figure 2(v)). After solidification, the samples were unmoldedfor curing in laboratory conditions for the next 28 days (Figure 2(vi)). Each sample was then instrumented with two 2 cmlong electrical strain gauges, with a gauge factor of 2.1, placedonto the center of opposite lateral sides, as shown in Figure 1.3. Methodology3.1. Electrical Tests. Electrical tests were performed on the tensensors using a data acquisition system consisting of an NIPXIe-1073 chassis with dedicated modules. Both two-probeand four-probe measurement configurations in DC currentwere adopted. The chassis was equipped with two modules:the electric power was supplied through an NI PXI-4130,capable of providing a four-quadrant 20 V and 2 A outputin a single isolated channel, while electrical measurementswere conducted through a high speed digital multimeter,model NI PXI-4071. This last module acquired voltage inthe two-probe measurement setup and current in the fourprobe measurement setup. The temperature of the sensorswas controlled before and during the tests using a climaticchamber to avoid temperature-induced drifts.The electrical resistance of each sensor was evaluatedafter 6000 s of polarization in order to mitigate the signaldrift due to the dielectric nature of the cementitious matrix.In the two-probe method, the sensors were subjected to anelectrical voltage of 5 V with a current measurement rangeof 1.0 mA. The inner electrodes with a mutual distance of20 mm were used as the active electrodes. In the four-probeconfiguration, the sensors were subjected to a current levelequal to 15 mA with a voltage measurement range of 10 V. Inthis case, the current was applied to the external electrodesplaced at a mutual distance of 40 mm, while the voltage dropwas measured across the internal electrodes, as in the twoprobe method. The electrical resistance was calculated, inboth cases, based on the first Ohm’s law. Equations (1) and (2)refer to the computation of the electrical resistance for twoand four-probe configurations, respectively:2. Materials and Preparation of Samples 𝑅2𝑃 (𝑡) 𝑡 𝑡𝑝 𝑉,𝐼 (𝑡) 𝑡 𝑡𝑝Ten cube samples of 5 cm side were fabricated for the experimental campaign. The cementitious matrix was a cementpaste doped with 1% of MWCNTs with respect to the weight 𝑅4𝑃 (𝑡) 𝑡 𝑡𝑝 𝑉 (𝑡) 𝑡 𝑡𝑝𝐼(1),(2)