SA-based monitoring of concrete seismic shear stress within the elastic regime: influence of normal stress on sensitivity

Seismic shear force monitoring is critical for assessing the structural response of concrete buildings. PZT-based smart aggregates (SA) enable internal shear stress measurement within concrete. However, existing calibration studies typically treat normal stress as a negligible factor. Consequently, under practical compression-shear states, a fundamental knowledge gap remains regarding how normal-stress-driven interfacial stiffness nonlinearity and friction limits affect SA sensitivity. To bridge this gap, this study combines experimental and numerical approaches to investigate the nonlinear interfacial sensing mechanism of SA. Cyclic push-pull loading tests were conducted on a reinforced concrete short column embedded with 16 SAs under varying axial compression ratios (0 - 0.5). Subsequently, numerical simulations and parameter inversions were performed to elucidate the stress transfer mechanism. Experimental results show that SA sensitivity increases with the axial compression ratio but plateaus beyond 0.3, achieving a maximum growth rate of 29.1% relative to the baseline value (9.84×10³ pC/MPa). Numerical analysis suggests the underlying mechanism: initial normal stress induces compaction of the equivalent interfacial layer, enhancing shear transfer efficiency. However, at higher pressures, stiffness saturation and friction limits restrict further growth. Parameter inversion identified the optimal interfacial combination: a bilinear elastic modulus (increasing to 1.9% of the mortar modulus at a 0.1 axial compression ratio) and a 0.20 MPa friction limit, yielding a minimum root-mean-square error of 0.037×10⁴ pC/MPa. This study elucidates the nonlinear contact behavior of SAs under combined compression-shear states, establishing a quantitative baseline for practical structural monitoring strictly within the elastic, uncracked regime.