Integrated Ground-Penetrating Radar and Electrical Resistivity Tomography for Concrete and Masonry Assessment: A Critical Review and Research Agenda

Abstract

Ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) provide complementary, but non-unique, observations of concrete and masonry conditions. GPR is primarily sensitive to dielectric contrasts, interfaces, reinforcement geometry and electromagnetic attenuation, whereas electrical measurements respond to ionic conduction, moisture state, material connectivity and electrode configuration. This paper presents a structured critical review updated to 21 July 2026. It distinguishes surface or bulk resistivity measurements from electrical resistance tomography around specimens and from multi-electrode geophysical ERT, an important terminological separation that is frequently blurred in the literature. This review’s contribution is a terminological separation of electrical measurement classes, an evidence-coding scheme that distinguishes corroboration from independent validation, and a staged, conditional research agenda built from that scheme; it is not a claim that GPR–ERT integration is routinely sufficient on its own. Verified evidence is synthesised across reinforced concrete, masonry, coastal infrastructure, laboratory calibration, field validation and forward modelling. The review shows that GPR is mature for reinforcement mapping and conditional detection of interfaces and delamination, while resistivity methods are well established for durability-related screening. True ERT can image spatial conductivity changes associated with moisture ingress and cracks, but inversion regularisation, electrode contact, reinforcement and three-dimensional effects limit resolution and quantitative recovery. Integrated GPR–ERT studies now include controlled masonry experiments, heritage structures, a field heritage pier and laboratory calibration on reinforced concrete; therefore, the principal remaining gap is not the absence of integration. It is the shortage of independent destructive field verification, scale-aware transfer rules and uncertainty-calibrated decision thresholds across structural types. A revised evidence matrix and detectability taxonomy show that neither method directly identifies active corrosion or chloride concentration. The paper concludes with a staged research agenda based on co-registration, physics-informed feature extraction, forward-modelled resolution assessment and targeted ground truth.