Wireless surface acoustic wave sensors for displacement and crack monitoring in concrete structures

In this work, we demonstrate that wireless surface acoustic wave devices can be used to monitor millimetre displacements in crack opening during the cyclic and static loading of reinforced concrete structures. Sensors were packaged to extend their gauge length and to protect them against brittle fracture, before being surface-mounted onto the tensioned surface of a concrete beam. The accuracy of measurements was verified using computational methods and optical-fibre strain sensors. After packaging, the displacement and temperature resolutions of the surface acoustic wave sensors were 10µm and 2 C respectively. With some further work, these devices could be retrofitted to existing concrete structures to facilitate wireless structural health monitoring

Hybrid graphene/geopolymeric cement as a superionic conductor for structural health monitoring applications

In this paper, we demonstrate for the first time a novel hybrid superionic long gauge sensor for structural health monitoring applications. The sensor consists of two graphene electrodes and a superionic conductor film made entirely of fly ash geopolymeric material. The sensor employs ion hopping as a conduction mechanism for high precision temperature and tensile strain sensing in structures. The design, fabrication and characterization of the sensor are presented. The temperature and strain sensing mechanisms of the sensor are also discussed. The experimental results revealed that the crystal structure of the superionic film is a 3D sodium-poly(sialate-siloxo) (Na-PSS) framework, with a room temperature ionic conductivity between 1.54 x 10-2 and 1.72 x 10-2 S/m and, activation energy of 0.156 eV, which supports the notion that ion hopping is the main conduction mechanism for the sensor. The sensor showed high sensitivity to both temperature and tensile strain. The sensor exhibited temperature sensitivity as high as 21.5 kΩ/oC and tensile strain sensitivity (i.e.,gauge factor) as high as 358. The proposed sensor is relatively inexpensive and can easily be manufactured with long gauges to measure temperature and bulk strains in structures. With some further development and characterization, the sensor can be retrofitted onto existing structures such as bridges, buildings, pipelines and wind turbines to monitor their structural integrity

Effects of lactic and citric acid on early-age engineering properties of Portland/calcium aluminate blended cements

In this study, Portland/calcium aluminate blended cement (PC/CAC) was combined with citric acid or lactic acid as additives to investigate the effects of the aforementioned carboxylic acids on the hydration reactions of PC/CAC as a potential fast hardening and low cost repair material for concrete. Mortar specimens with the carboxylic acid additives of either 0.5%, 1% or 3% by weight, prepared with a binder:sand:water ratio (by weight) of 1:3:0.5, were subjected to flexural and compressive strength tests at early ages up to 28 days. In order to understand the phase composition of the hydrates in the PC/CAC systems, XRD analyses were conducted on ground PC/CAC mortars with and without carboxylic acid at 7, 14 and 28 days. In combination with this, SEM images of selected mortar specimens were also taken at the same times for visual analyses of hydrates. Citric acid did not have any beneficial effect on enhancing the calcium silicate phase as initially assumed and instead reduced the strength of PC/CAC cement at all levels of concentration. The experiment analyses revealed that Portlandite crystals were the major hydrate phase in PC/CAC with lactic and citric acids. Lactic acid below 2% wt. improved both compressive and flexural strength gained at early ages due to improved crystallinity of the calcium hydroxide crystals. Combined with its inherent rapid setting time, PC/CAC blended cements have a potential to be developed into a suitable repair material for concrete

Novel engineered high performance sugar beetroot 2D nanoplatelet-cementitious composites

In this paper, we show for the first time that environmentally friendly nanoplatelets synthesized from sugar beetroot waste with surface area and hydroxyl functional groups similar to those of graphene oxide (GO) can be used to significantly enhance the performance of cementitious composites. A comprehensive experimental and numerical simulation study was carried out to examine the performance of the bio waste-derived 2D nanoplatelets (BNP) in cementitious composites. The experimental results revealed that the addition of BNPs decreased the workability of the cement pastes due to their high surface area and dominant hydrophilic functional groups. The experimental results also revealed that the BNP sheets altered the morphology of the hydration phases of the cementitious composites. At 0.20-wt%, the BNP sheets increased the content of the C-S-H gels. At higher concentrations (i.e., 0.40-wt% and 0.60-wt%), however, the BNP sheets increased the content of the calcium hydroxide (Ca(OH)2) products and altered their sizes and morphologies. The flexural results demonstrated that the 0.20-wt% BNPs produced the highest flexural strength and modulus elasticity and they were increased by 75% and 200%, respectively. The numerical simulations were in good agreement with the fracture test results. Both results showed that the 0.20-wt% BNPs optimal concentration significantly enhanced the fracture properties of the cementitious composite and produced mixed mode crack propagation as a failure mode compared to Mode I crack propagation for the plain cementitious composite due to combined crack bridging and crack deflection toughening mechanisms. Because of this, the fracture energy and the fracture toughness were increased by about 88% and 106%, respectively

Experimental test and analytical modeling of mechanical properties of graphene-oxide cement composites

Graphene oxide has recently been considered as an ideal candidate for enhancing the mechanical properties of the cement due to its good dispersion property and high surface area. Much of work has been done on experimentally investigating the mechanical properties of graphene oxide-cementitious composites; but there are currently no models for accurate estimation of their mechanical properties, making proper analysis and design of graphene oxide-cement-based materials a major challenge. This paper attempts to develop a novel multi-scale analytical model for predicting the elastic modulus of graphene oxide-cement taking into account the graphene oxide/cement ratio, porosity and mechanical properties of different phases. This model employs Eshelby tensor and Mori-Tanaka solution in the process of upscaling the elastic properties of graphene oxide-cement through different length scales. In-situ micro-bending tests were conducted to elucidate the behaviour of the graphene oxide-cement composites and verify the proposed model. The obtained results showed that the addition of graphene oxide can change the morphology and enhance the mechanical properties of the cement. The developed model can be used as a tool to determine the elastic properties of graphene oxide-cement through different length scales