PSI - Issue 64

Alamgir Khan et al. / Procedia Structural Integrity 64 (2024) 539–548 Alamgir khan / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Over the last few decades, sensor/strain-sensing technologies have become vital for structural health monitoring (SHM) systems. Conventional sensors such as strain gauges, fiber optic sensors (FOS), shape memory alloys, and strain gauges are commonly used to evaluate the integrity and stability of buildings, bridges, and dams [1-3]. Among these conventional sensors, strain gauges are widely employed to assess the stress/strain state of a structure owing to their low cost and ease of installation. However, their limitations, such as the requirement for installation on a concrete surface, high dependence on the behavior of the adhesive, long-term performance, low strain sensitivity, high signal to-noise ratio, and drift due to temperature, have prompted researchers to explore new types of self-sensing cementitious composites known as smart cement composites/intrinsic sensors [4]. These smart cement composites consist of Ordinary Portland Cement (OPC) and other pozzolanic and supplementary materials mixed with conductive materials (steel fiber, carbon fiber, graphene, and graphite) for multifunctional application [5-8]. Smart cement composite sensors offer several advantages over conventional sensors, including ease of embedding, higher durability, absence of adhesives, and higher gauge factors. The function of smart cement composites is based on the concept of piezoresistivity, and they are capable of load and strain sensing. When a load is applied, the change in the conductive fiber/filler causes an increase or decrease in electrical resistance [9, 10]. The synergetic utilization of MWCNT and NCB carbon nanomaterials offers great potential for the development of new intrinsic sensors. Carbon nanomaterial-based smart intrinsic sensors are primarily composed of OPC-based materials (concrete, mortar, and cement paste) and offer multifunctional applications [12, 13]. However, the mechanical properties, electrical resistivity, and piezoresistivity of smart OPC-based sensors are greatly affected by exposure to elevated temperatures [14]. Recently, there has been growing interest among researchers in fire-resistant materials for use in structures owing to their elevated temperature resistance and rapid strength development [15]. Calcium aluminate cement (CAC) can be used as a binder to address the issue of elevated temperatures in mortars and concrete. CAC is known for its superior spalling performance at elevated temperatures compared with OPC-based concrete because of the presence of alumina phases, which increase the concrete porosity and release pore pressure caused by water evaporation [16]. This versatile specialty cement has a wide range of applications including fast repair mortars, furnaces, sealants, and fireplaces [17, 18]. CAC rapid strength development, good sulfate resistance, increased abrasion resistance, and endurance to repeated heating to elevated temperatures are among its key unique qualities. Within 24 h of hydration, CAC attains approximately 80% of its maximum strength, making it ideal for multifunctional applications. The primary feature of CAC is its ability to withstand elevated temperatures of up to 1600-2000 ºC and prevent spalling at these temperatures [18]. Therefore, this study focused on the mechanical properties, electrical conductivity, and piezoresistive properties of a CAC-based MWCNT/NCB sensor at room and elevated temperatures (200 °C and 400 °C). Previous studies have focused on the mechanical properties, electrical conductivity, and piezoresistivity of OPC cementitious composites with conductive fibers and fillers. The potential impact of conductive cementitious composites at elevated temperatures on their mechanical properties, electrical conductivity, piezoresistivity, and stress/load sensitivity has not been adequately considered. Therefore, the objective of this study was to develop a CAC-based MWCNT/NCB conductive composite to optimize a mix design that can withstand different elevated temperatures and prevent spalling to stabilize the compressive strength, electrical conductivity, and piezoresistivity. 2. Experimental program 2.1. Raw materials, mix design, and specimen casting. In this study, multi-walled carbon nanotube (MWCNT)/carbon black (NCB) composite fillers, calcium aluminate cement (CAC), silica fume, silica sand, quartz powder, water, and a polycarboxylate superplasticizer (SP) were incorporated into cement-based conductive composites to investigate their mechanical and electrical conductivities. The physical and electrical properties of the MWCNT/NCB composite fillers are listed in Tables 1 and 2, respectively. Table 3 lists the chemical composition of calcium aluminate cement (CA50), which was employed as