**1. Introduction**

Concrete is the most widely used construction material and usually placed in the plastic form. Fluidity, homogeneity, consistency and workability are the key elements that greatly depend on viscosity of concrete and need to be considered during the mixing and placing of concrete. Any shortcoming in these parameters will lead to bleeding, segregation, laitance and cracking of the concrete [1]. Therefore, the rheological properties of concrete are of utmost significance to achieve homogeneity and good workability and have a strong influence on the overall fresh properties of the concrete [2]. With the advancement in nanotechnology, researchers are emphasizing more on the effect of nanomaterials on cement composite [3]. Numerous researchers studied the effect of various engineered nanomaterials on flow characteristics of the cement paste [4–6] and most of the research was focused on the rolled sheets of graphene and its derivatives, i.e., CNTs (carbon nanotubes) and graphene oxide [6–8]. However, the rheological properties of graphene cement paste remained unexplored and rarely reported. Therefore, an in-depth knowledge of the connection of graphene and

rheology of cement paste is required. The flow properties of the cement paste are usually acquired by shear stress and shear rate. Later from the flow curves, viscosity and other flow parameters are calculated by using mathematical models. These rheological models statistically determine the yield stress (shear stress at zero shear rate), plastic viscosity (generalized viscosity for a range of shear rate) and predict the specific trend of the flow. As, these mathematical models possess statistical errors; therefore, one model cannot predict accurately the deformation of cement paste [9]. Therefore, in this research, four rheological models were used to investigate the flow properties of the graphene cement based composite.

Graphene possesses some amazing and extraordinary properties such as huge specific surface area (2630 <sup>m</sup>2·g<sup>−</sup>1), high intrinsic strength (130 GPa), firm Young's module (~1.0 TPa) and high electrical transport properties [10]. Therefore, incorporation of graphene in cement composite will not only alter the rheological characteristics but also affect the electrical properties of the composite. The electrical properties of the GNP–cement composite are important and can be used to monitor the damage in a concrete structure for the purpose of maintaining safe, reliable and sustainable civil infrastructure. It is known that non-destructive test offers skills for speedily and effectively monitoring these structures [11,12]. However, Self-sensing concrete, which can monitor its own strain, is the need of this era. The cement based composite reinforced with conducting fillers can observe its own strain by monitoring the changes in the electrical resistivity values [12]. Self-sensing ability is related to the breaking of conducting fibers when cracks are initiating in the cement based composite consequently enhancing the resistivity of the overall sample. If cracks are opening up due to tensile or fracture loading then resistivity values will be positive while it will be negative when subjected to compressive loading. Newly developed engineering nanocarbons are capable of fabricating new kind of high-performance tailored multifunctional cement-based composite that is capable of self-sensing the real-time damage [13–16]. Carbon nanotubes (CNTs) are composed of sp<sup>2</sup> hybridized carbon atoms, sheets of single layered graphene sheet rolled up in a cylindrical tube [17]. If these rolled sheets of CNTs are opened up in one plane, then it forms the two-dimensional sheet-like structure. These two-dimensional sheets, i.e., graphene nanoplatelets (GNPs) have even greater surface area and aspect ratio. Sixuan [18] investigated the effect of crack depth of the GNP reinforced mortar on the change in the electrical resistance. It was revealed that the specimens (cube and prism shapes) responded to an increase in electrical resistance as the depth of crack became larger. For the same relative crack depth, the change in electrical resistance for the cube was more significant as compared to that of the prism. However, as per authors' knowledge, the response of the GNP–cement composite to various damage levels has not been investigated.

Therefore, in this study, the flow properties of the GNP cement paste were investigated by using Bingham, Modified Bingham, Herschel–Bulkley and Casson models. Variation in flow curves of cement paste with different percentages of graphene nanoplatelets was determined. Rheological properties of graphene cement paste with various resting time (time between sample preparations to casting) and shear rate cycles were also evaluated. Later, the self-sensing properties of the GNP-based cementitious material were determined. The four-probe method was used for electrical resistance measurement purpose. Strain-sensing and fractional change in resistance were observed and used for determining self-sensing characteristics. Finally, application of GNP–cement composite specimen was evaluated on the reinforced beam.
