Time-lapse monitoring of internal alteration of a concrete structure using ground penetrating radar

Early recognition of concrete deterioration is an important engineering task with many associated monitoring challenges. Ground penetrating radar (GPR) is an important non-destructive testing tool commonly used by engineers to locate internal reinforcement and voids within concrete. However, the general standard of interpreting subsurface conditions from one GPR survey is insufficient for determining the rate and extent of concrete deterioration. Differences in the amplitude response of reflected radar waves within a subsurface region, as measured by repeat (4D) GPR surveys over calendar time, provide a more robust indication of changes in the condition of concrete material. Variations in the 4D amplitude response may be associated with alteration (moisture content or pore-space) in material properties (dielectric permittivity, electrical conductivity). We present a 4D GPR monitoring study of the internal deterioration of a purpose-built reinforced concrete wall subjected to increasing mechanical load until reaching structural failure (107.4 kN). At 10 kN load increments, we scanned the wall surface with a 1.6 GHz GPR system and computed difference data sets that highlight changes in the 4D amplitude response. Significant 4D anomalies were observed between the 60 kN and 100 kN data sets, including spatially distributed alteration zones co-located with the wall failure location. We interpret these zones as experiencing stress-induced porosity reduction and/or an increase in micro- or macro-fracturing. Overall, this work demonstrates that 4D GPR is useful for highlighting regions of 4D strain variations, and is thus an important monitoring tool for early recognition of concrete deterioration.