about 3 years ago

Self-sensing concrete materials, also known as smart concretes, are emerging as a promising technological development for the construction industry, where novel materials with the capability of providing information about the structural integrity while operating as a structural material are required. Despite progress in the field, there are issues related to the integration of these composites in full-scale structural members that need to be addressed before broad practical implementations. This article reports the manufacturing and multipurpose experimental characterization of a cement-based matrix (CBM) composite with carbon nanotube (CNT) inclusions and its integration inside a representative structural member. Methodologies based on current–voltage (I–V) curves, direct current (DC), and biphasic direct current (BDC) were used to study and characterize the electric resistance of the CNT/CBM composite. Their self-sensing behavior was studied using a compression test, while electric resistance measures were taken. To evaluate the damage detection capability, a CNT/CBM parallelepiped was embedded into a reinforced-concrete beam (RC beam) and tested under three-point bending. Principal finding includes the validation of the material’s piezoresistivity behavior and its suitability to be used as strain sensor. Also, test results showed that manufactured composites exhibit an Ohmic response. The embedded CNT/CBM material exhibited a dominant linear proportionality between electrical resistance values, load magnitude, and strain changes into the RC beam. Finally, a change in the global stiffness (associated with a damage occurrence on the beam) was successfully self-sensed using the manufactured sensor by means of the variation in the electrical resistance. These results demonstrate the potential of CNT/CBM composites to be used in real-world structural health monitoring (SHM) applications for damage detection by identifying changes in stiffness of the monitored structural member.

Introduction It is a fact that materials and structures degrade with time and usage. Within the context of civil structures (e.g., buildings, bridges, tunnels, dams, among others), natural and human-made hazards (e.g., earthquakes, typhoons, hurricanes, fire, or collisions) represent also threats to the structural integrity that may cause catastrophic failures involving economic and human losses Xu and He (2017). This requires the implementation of inspection procedures to guarantee serviceability and reliability of structures in their long lifetimes. These inspections are often carried out at a fixed time intervals using different methods comprised within the field of nondestructive testing (NDT), where the quality or integrity of a component is determined nondestructively by interrogating one or several physical variables that are damage-sensitive Shull (2002).

Some NDT methods include visual testing (both direct and remote) Agnisarman et al. (2019), ultrasonics Zhao et al. (2018), acoustic emissions Meo (2014), infrared thermography Yamazaki et al. (2018), ground penetrating radar techniques, and electrical resistivity tomography Salin et al. (2018). These methods commonly require human intervention that increases uncertainty and implies costly equipment and personnel. SHM overcomes these factors using approaches where sensors are permanently installed into the structure, so that NDT is performed continuously or online Xu and He (2017). In this regard, SHM can reduce costs by providing valuable information for maintenance management, improve reliability by the possibility of finding damages at incipient stages, and improve future designs by making available valuable information about the performance of the current ones Ogai and Bhattacharya (2018).

Recent advances in SHM for civil structures include the use of piezoelectric sensors Liao and Chiu (2019), fiber-optic sensors (FOS) Glisic et al. (2013), Barrias et al. (2019), Xu et al. (2019), electrochemical sensors (e.g., potentiometric, amperometric, and conductometric) Hu et al. (2011), Qiao et al. (2012), and self-sensing composite materials Tian et al. (2019). A deep review of the sensing technologies applied to civil structures was recently reported by Taheri (2019). Considering the available sensing technologies, one concern when developing effective SHM systems is the selection of the type of sensor to be used. The resistance and durability of the sensor operating in harsh environments and its integrability to large structures are paramount. That is why, self-sensing composites, materials that not only bear loads but provide measures as a response to an external stimulus, have raised the interest of the research community (D’Alessandro et al. 2016a; Han et al. 2015a; Rana et al. 2016; Yang et al. 2020).

As cement-based materials or cementitious composites (e.g., paste, mortar, and concrete) are the most popular building materials Xu and He (2017), much of the scientific research around self-sensing composites is related to such materials. Self-sensing cementitious composites, also known as smart concretes, are fabricated by mixing functional fillers (e.g., carbon fibers Baeza et al. 2013; Teomete 2015; Sarwary et al. 2019, carbon nanotubes Elkashef 2015, steel fibers Kang et al. 2018; Ding et al. 2019, graphite powder Šimonová et al. 2018, nickel powder Wang et al. 2015 , among others Tian et al. 2019). Particularly, CNTs have raised the interest in the last years due to their high specific mechanical and transport properties (Schumacher and Thostenson 2014; Ubertini et al. 2016).

The incorporation of CNTs in cementitious materials can endow these materials with a piezoresistive behavior. The CNTs are materials that, when subjected to strain, their electrical properties change up to two orders of magnitude, exhibiting a proportional and reversible piezoresistive response to the external stimulus (Garcia-Macias et al. 2017; García-Macías et al. 2017; Minot et al. 2003). This electromechanical behavior is explained first by various authors as a change in the conductivity of the CNT due to the change in the energy band induced by the strain applied on its volume (Minot et al. 2003; PHAM 2008; Han et al. 2015b; Njuguna 2012; Tjong 2009; Xinxin Sun 2009).