No abstract

Flexible sensors with a high sensitivity and wide-frequency response are essential for structural health monitoring (SHM) while they are attached. Here, carbon nanotube (CNT) films doped with various PVA fractions (CNT/PVA) and ZnO nanowires (nano-ZnO) on zinc sheets were first fabricated by functionalized self-assembly and hydrothermal synthesis processes. A CNT/PVA/nano-ZnO flexible composite (CNT/PVA/ZnO) sandwiched with a zinc wafer was then prepared by the spin-coating method. The piezoresistive and/or piezoelectric capabilities of the CNT/PVA/ZnO composite were comprehensively investigated under cyclic bending and impact loading after it was firmly adhered to a substrate (polypropylene sheet or mortar plate). The results show that the piezoresistive sensitivity and linear stability of the CNT/PVA films doped with 20%, 50%, and 100% PVA during bending are 5.47%/mm, 11.082%/mm, and 11.95%/mm and 2.3%, 3.42%, and 4.78%, respectively. The piezoelectric sensitivity, linear stability, and response accuracy of the CNT/PVA/ZnO composite under impulse loading are 4.87 mV/lbf, 3.42%, and 1.496 ms, respectively. These merits support the use of CNT/PVA/ ZnO as a piezoresistive and/or piezoelectric compound sensor to monitor the static/dynamic loads on concrete structures while it is attached. Structural health monitoring (SHM) of the structural integrity of concrete structures under external static/ dynamic loading has become an important tool to ensure the safety of various types of infrastructure. SHM aims to design, verify and optimize structures and also provide real-time structural damage detection and assessment by comprehensively utilizing signal processing, sensing, communications, and other cross-subject technologies. Sensors with accurate and fast-response sensing capabilities are fundamental and essential to SHM, and sensor development has attracted considerable worldwide attention 1. Over the last several decades, fiber Bragg grating (FBG) sensors have increasingly been developed for SHM owing to their advantages of high linearity, accurate sensing, corrosion resistance, small size, and long-distance distribution 2. However, FBG has some disadvantages that restrict its wider application in SHM, such as high brittleness if there is no packaging process, incompatibility when embedded in concrete structures, and sophisticated synthesis, substrate selection and packaging 3,4 processes.; in addition, the FBG needs to be equipped with a signal demodulator 5. Recently, Chung et al. 6 , Sun et al. 7 , Li et al. 8 , Han et al. 9 , Luo et al. 10 , Dong et al. 11 , and Huang et al. 12 introduced conductive fillers, such as steel fibers, carbon fibers, carbon black, nickel powder, carbon nanotubes, carbon nanofibers, and lead zirconium titanate powder, into cement/mortar to develop intrinsic piezoresistive or piezoelectric sensors for the SHM of concrete structures and achieved good compatibility, a relatively high sensitivity, and high linearity. However, most of the

There is a constant drive to improve the properties of recycled concrete owing to its inferior strength and fracture toughness compared to normal concrete and recent progress in graphene oxide (GO) nanomaterials impelling nanosized reinforcements to recycled concrete. Here, GO-modified natural sand (NS)- or recycled sand (RS)-based mortars (GONMs or GORMs) with six GO fractions (wGOs) were fabricated to explore their 28 d mechanical strengths (f28t, f28c), fracture toughness (KIC, δc), and microhardness (Hv), as well as their crystal phases (using X-ray powder diffraction) and microstructures (using scanning electronic microscopy). Results reveal, greater enhancements in mechanical strengths (4.50% and 10.61% in f28t, 4.76% and 13.87% in f28c), fracture toughness (16.49% and 38.17% in KIC, 160.14% and 286.59% in δc), and microhardness (21.02% and 52.70% in Hv) of GORM with just 0.025 wt‰ and 0.05 wt‰ GO, respectively, with respect to the control are achieved when comparing with those of GONM with the same wGO. More zigzag surfaces, more irregular weak interface slips, and the relatively lower strengths of RS bring the superiority of the template and reshaping effects of GO into full play in GORM rather than in GONM. These outcomes benefit a wide range of applications of recycled concrete products.