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    Effects of graphene nanoplatelets type on self-sensing properties of cement mortar composites
    (Elsevier Sci Ltd, 2022) Sevim, Ozer; Jiang, Zhangfan; Ozbulut, Osman E.
    Graphene nanoplatelets (GNPs) that possess high electrical conductivity and relatively low cost have been considered to obtain self-sensing capability in cementitious composites. However, there is limited understanding on the effects of physical properties of GNPs such as particle size and surface area on the self-sensing charac-teristics of the cement composites. In this study, nine types of GNPs that have different surface areas, particle sizes, and thicknesses are considered in the development of self-sensing mortar composites. For each type of GNPs, specimens with GNP concentrations of 2.5%, 5%, and 7.5% by weight of cement were prepared. The bulk electrical resistivity of the developed mortar composites was measured at different curing ages. The compressive strength of the specimens was also evaluated. The piezoresistive behaviors of the GNP-reinforced mortar com-posites were studied through cyclic compressive loading tests at different load levels. During piezoresistivity tests, the measurements were conducted through both direct current (DC) and alternating current (AC) and the results obtained from each method were evaluated. Results reveal that GNPs with very small particle sizes and large surface areas cannot disperse effectively in the cement matrix and do not provide piezoresistive charac-teristics. For GNPs with relatively smaller surface areas, the GNPs with higher particle sizes form effective conductive paths and exhibit better piezoresistive characteristics.
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    Graphene-reinforced cement composites for smart infrastructure systems
    (Elsevier, 2022) Schulte, Justine; Jiang, Zhangfan; Sevim, Ozer; Ozbulut, Osman E.
    In traditional strategies for structural health monitoring of concrete structures, distributed sensors are commonly utilized to monitor and evaluate the state of the structure. This requires deployment of a large number of sensors to obtain sufficient information that can help the owners and engineers to timely detect anomalies in the structural performance. Considering the challenges, such as high deployment cost, low durability, and weak compatibility, for the use of conventional sensors in the long-term monitoring of concrete structures, self-sensing cement composites with intrinsic strain- and damage-sensing capabilities can be a more practical and sustainable approach for condition monitoring. Although various nano- and microfillers have been used to develop such smart cement composites, graphene and its derivatives have gained significant attention in developing functional cement-based composites over the past decade. Graphene sheets are two-dimensional nanomaterials and offer various advantages such as excellent mechanical properties, ultrahigh specific surface area, and relatively low cost. This chapter provides a comprehensive review on the use of graphene-based nanomaterials in the development of self-sensing cementitious composites. First, different forms of graphene-based nanomaterials are described and their use for self-sensing applications is assessed. Then various techniques for the dispersion of graphene nanomaterials into a cement matrix are discussed, followed by a discussion on the experimental techniques that can be used to assess the quality of dispersion. Also, different methods used for electrical and self-sensing characterization tests of cement composites are described. A summary of earlier studies on self-sensing graphene-reinforced cement composites is provided. Next, the piezoresistive behavior of graphene-reinforced cement composites under applied loads is presented for a mortar composite filled with different types of graphene materials. Finally, opportunities for the use of these self-sensing composites in smart infrastructure applications are discussed and potential challenges are noted. © 2022 Elsevier Inc. All rights reserved.
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    Mechanical properties of graphene nanoplatelets-reinforced concrete prepared with different dispersion techniques
    (Elsevier Sci Ltd, 2021) Jiang, Zhangfan; Sevim, Ozer; Ozbulut, Osman E.
    In this paper, an experimental investigation is carried out to study the effects of graphene nanoplatelets (GNPs) on the mechanical properties of cementitious composites with coarse aggregates. The concrete mixtures with GNPs concentrations ranging from 0.025% to 0.10% by weight of the cement are prepared, where a wet dispersion technique that employs high shear mixing and polycarboxylate-based superplasticizer to disperse GNPs in water. The effects of various dispersion parameters including different high shear mixing durations as well as the use of ultrasonication together with high shear mixing on the dispersion of GNPs are studied. The dispersion quality of GNPs is assessed through optical microscopy, Raman spectroscopy, and Scanning Electron Microscopy tests. Compressive strength and flexural strength tests are conducted to assess the effects of GNPs on mechanical properties of the fabricated GNP-reinforced concrete specimens. Results show that all the dispersion procedures considered in this study can disperse GNPs in water without causing any basal or vacancy defects in graphene sheets and reduce the size of graphene flakes. The use of ultrasonication together with high shear mixing leads to the smallest size graphene sheets. When the GNPs are added to the concrete mixture at a dosage of 0.025%, a maximum increase of 17% in compressive strength is observed, while no significant effect of GNPs on the flexural strength is noticed.