A Forward-Modelled, Model-Informed Four-Stage Framework for Integrated GPR–ERT Preliminary Risk Screening of Reinforced Concrete

Ground penetrating radar (GPR) and electrical resistivity tomography (ERT) are increasingly combined for the condition assessment of reinforced concrete, yet published integration schemes generally rely on empirical curve-fitting or qualitative visual overlay rather than on an explicit physical model for the anomaly thresholds applied. This paper presents a forward-modelling programme used to propose provisional, model-informed GPR–ERT screening multipliers and embeds them within a four-stage preliminary risk-screening framework. The multipliers are bracketed by the modelled responses but are not derived from a closed mathematical procedure; the framework is a physics-informed preliminary screening approach, not a validated diagnostic method, and the reported sensitivity results do not constitute statistically validated diagnostic accuracy. A linearised, sensitivity-kernel-weighted (Born-type) formulation was used to compute synthetic Wenner–Schlumberger ERT pseudosections, and a semi-analytical impulse-response superposition (800 MHz Ricker source) was used to compute synthetic GPR radargrams, for four scenarios: an intact reference, a water-filled crack, an air void, and a combined crack–void–corrosion zone. The GPR model reproduces reflection polarities prescribed from electromagnetic theory (R = −0.48 for the water-filled crack mouth; R = +0.52 then −0.52 for the top and bottom interfaces of an air void), which demonstrates implementation consistency rather than independent validation. For the air void, the inverted ERT peak (250–275 Ω·m against a 180 Ω·m background) lies 39–53% above background, which is under 1% of the true resistivity contrast (about 10,000 Ω·m), illustrating the strong under-recovery of high-contrast targets by smoothed, linearised inversion. The modelled responses were used to bracket Stage 2 screening criteria—a GPR rebar-amplitude criterion (A < 0.70 × A0) and ERT conductive/resistive criteria (ρ < 0.60 × ρ0 or ρ > 1.25 × ρ0)—within a four-stage procedure comprising baseline establishment, anomaly screening, spatial cross-validation and indicative risk categorisation. The procedure was applied to previously reported classifications of four masonry field sites, for which no destructive ground truth is available and to which rebar-specific quantities do not directly apply; a preliminary one-at-a-time multiplier sweep of these classifications is under audit and supports no robustness claim. All results derive from a single reference concrete mix, electrode spacing and antenna frequency, and their transfer to other mixes, cover depths and survey geometries has not been tested.