Intrinsic self-sensing piezoresistive behaviors of ultra-high strength alkali-activated concrete

Abstract

<p>In this work, the self-sensing piezoresistive behaviors of carbon fiber (CF)-reinforced ultra-high strength concrete (UHSC) are investigated using alternating current impedance spectroscopy (ACIS) technique. The effects of different fiber contents, fiber lengths, and curing ages are considered. The electrical behaviors of alkali-activated slag (AAS)-based UHSC (AAS-UHSC) and ordinary Portland cement (OPC)-based UHSC (OPC-UHSC) at similar strength from 80 to 110 MPa are compared. The findings reveal that the intrinsic electrical resistivity of AAS-UHSC is significantly lower than that of OPC-UHSC, approximately 1/30 of the latter. The addition of CF results in a substantial reduction in electrical resistivity for both UHSC types, with reductions of up to 96 %, and shows better compatibility with the AAS-UHSC matrix. The piezoresistive properties of CF-reinforced UHSC are investigated using ACIS at a fixed frequency and further evaluated by parameters including sensitivity, linearity, repeatability, and hysteresis. The results indicate that plain OPC-UHSC does not exhibit piezoresistivity, whereas the addition of CF enhances both the sensitivity and linearity of piezoresistivity. In contrast, AAS-UHSC demonstrates inherent piezoresistive behaviors even without CF. The introduction of 0.5 wt% CF improves the sensitivity (by up to 50 %) and linearity (R2 increased from 0.76 to above 0.90) of piezoresistivity in AAS-UHSC. However, increasing the CF content to 1.0 wt% diminishes the sensitivity (by up to 62.5 %) due to decreased fiber dispersion uniformity. Moreover, the addition of 1.0 wt% 8-mm CF reduces the hysteresis of UHSC regardless of the binder type, with the maximum reduction reaching 62.2 %.</p>