1. Introduction
Structural health monitoring (SHM) is a vital research area in civil engineering, aimed at intelligently and non-destructively ensuring the safety and reliability of everyday structures. Traditional SHM methods, which depend heavily on external sensors and monitoring devices, often incur high installation and maintenance costs, and can sometimes cause structural damage [
1,
2]. However, recent advancements in nanotechnology have given rise to self-sensing materials, a revolutionary approach to SHM that has garnered significant research interest. Among these, self-sensing cementitious composites stand out due to their unique properties and potential applications. Their distinguishing characteristic is their ability to monitor structural states non-intrusively [
3,
4]. Compared to traditional sensor installation methods, the application of these materials reduces the maintenance costs of monitoring systems, and improves the long-term reliability of the monitoring setup [
5].
The core principle of self-sensing cementitious materials lies in the direct integration of sensing capabilities. This allows the sensors to autonomously monitor structural forces in real-time by observing changes in their own electrical resistance [
6]. Currently, carbon and metal fillers are the two main types of conductive fillers in use. Sevim, O et al. noted an improved piezoresistive response in the sensor when 7.5% M graphene nanoparticles (GNPs) were added to the self-sensing cementitious material [
7]. Suchorzewski, J et al. discovered that adding 0.05% MWCNT to concrete enabled damage detection at a tensile force of 85% Fmax, as evidenced by wedge splitting tests [
8]. Roshan et al. developed self-sensing cementitious materials by integrating two types of conductive fillers, namely MWCNTs and GNPs. They uncovered a direct correlation between FCR changes and crack propagation within a structure [
9]. In their study, Han et al. observed a direct link between the stability of piezoresistivity variations in self-sensing cementitious sensors and the applied external stresses [
10]. Feng Xu’s experiments demonstrated that reactive powder cement concrete (RPC) with 1.0% nano-stainless steel powder (NSP) showed optimal strain-sensing sensitivity. Furthermore, the research depicted a cubic relationship, suggesting a decrease in the electrical resistance and drying shrinkage rate of RPC with the volume ratio of NSP [
11].
Cyclic loading testing is a prevalent method for assessing the sensing capabilities of self-sensing cementitious materials [
12,
13]. By simulating the actual stress conditions experienced by sensors through repeated vertical loading and unloading, it can evaluate the sensing ability of self-sensing cementitious materials. This is significantly beneficial for assessing the stability and durability of sensors [
14]. Saptarshi Sasmal and his colleagues found in their cyclic loading tests that the content of conductive fillers had a significant impact on the piezoresistive response. An ideal result was not achieved when the filler concentration was too high. Instead, the sensitivity of resistance changes under loading decreased. They had also discovered that the microstructure within the specimen changed during the loading process [
15]. In the cyclic loading tests conducted by Yoo et al., they discovered that the self-sensing capacity of cement composites, inclusive of CF and GNF, under compression was not directly influenced by their conductivity. Furthermore, they identified that composites containing CNTs exhibited superior self-sensing capacity under cyclic compressive force at both 0.5 and 1 vol% [
16].
In the study of self-sensing cementitious materials, fibers and powders emerge as the two primary categories of conductive fillers [
17]. The hydrophobic nature of most fibrous fillers complicates their integration into these materials. In contrast, conductive powders, typically non-hydrophobic, disperse well in water [
18,
19,
20]. Nanosilver, with its unique physical and chemical properties, has attracted significant interest in pressure-sensitive research. Unlike fibrous fillers, nanosilver’s small particle size, typically 10–50 nm, considerably reduces the composite system’s percolation threshold. Furthermore, nanosilver’s outstanding electrical properties offer substantial potential for advancements in the pressure sensitivity field [
21,
22]. Li et al. enhanced the surface conductivity of AgNW-PI films significantly by blending silver nanowires (AgNW) with polyimide (PI) polymers and employing a wet etching method for surface treatment. They further fabricated this material into a flexible pressure sensor. Experimental results revealed that this sensor demonstrated exceptional sensitivity, approximately 1.3294 kPa
−1, under a pressure of about 600 Pa [
23]. Alessandro Paghi et al. utilized control over the piezoresistive properties of the AgNP electrical network formed on PDMS foams to manufacture flexible and wearable pressure sensors. These sensors exhibit high deformation (GF) and pressure (S) sensitivities, allowing the detection of small displacements of up to 4 μm and low stresses of up to 25 Pa [
24].
In the present study, a novel conductive filler, AgNP, was selected with the objective of addressing the challenges of high dosage, limited stability, and reversibility that are associated with traditional fillers. The stability and stress sensitivity of AgNP self-sensing cementitious materials were evaluated under two distinct loading regimes: long-cycle fixed amplitude intervals and long-cycle variable amplitudes.
4. Summary and Conclusions
In this study, five groups of self-sensing cementitious bases were prepared, each containing AgNPs at various doses: 0.0022 wt%, 0.0044 wt%, 0.0066 wt%, 0.0088 wt%, and 0.011 wt%. The dispersion, percolation threshold, polarization under varying water contents, pressure-sensitive stability, and stress sensitivity of AgNPs were investigated. The following conclusions were derived from the experiments:
At equivalent concentrations, AgNPs exhibited 1.15 to 9 times greater dispersibility in aqueous systems compared to conventional conductive fillers.
The percolation threshold of AgNPs in the cement matrix was determined to be 0.0066 wt% through polarization testing on five sets of specimens.
Long-term cyclic loading tests, both of equal and variable amplitude, revealed that self-sensing cementitious materials with 0.0066 wt% AgNPs exhibited optimal pressure-sensitive stability. The change in fractional change resistance (ΔFCR) reached up to 41.92%, and the maximum value of the stress sensitivity (SS) was as high as 11.736.
The utilization of AgNPs leads to a significant reduction in the quantity of conductive filler by approximately 90% or more. Their minute size and extremely high numbers contribute to a substantial increase in the number of conductive pathways within cementitious composite systems, thereby ensuring the stability of the pressure-sensitive effect.