Infrasense, Inc., a national leader in infrastructure nondestructive evaluations, is carrying out subsurface investigations for 15 bridge decks throughout Minnesota’s Metro region. The completion of this project will bring the total bridge decks evaluated in Minnesota to 137 (over 2.5 million square feet) since 2009. Ground Penetrating Radar (GPR) tests were performed on each bridge deck, accompanied by underside visual inspections. These tests provide a condition assessment of the reinforced concrete bridge decks without requiring any cores or exposed rebar, and with minimal disruption to traffic flow.
Ground penetrating radar (GPR) data is collected at highway speeds to estimate rebar depth, corrosion conditions and deteriorated concrete. The GPR data is collected in a series of lines spaced 3 feet transversely across the width of the deck, with each line representing a cross sectional slice of the deck at a particular offset. Decks in good condition consist of strong and uniform radar reflections from the rebar. GPR data with weak and inconsistent reflections indicate rebar-level deterioration in the bridge deck.
Recently a study evaluating the accuracy of Infrasense's results provided to the Minnesota DOT was carried out by an independent consultant for MnDOT. The study compared the predicted quantities for 12 decks surveyed by Infrasense against subsequent construction repair quantities. The study found that, on average, Infrasense’s predicted deterioration quantities were within 3.5 % of the documented construction quantities.
Ground penetrating radar surveys provide transportation agencies with accurate and comprehensive bridge deck condition information, enabling effective preservation, rehabilitation, and replacement decisions. With large bridge deck inventories, highway agencies have primarily relied on visual inspection at the network level. Since the mechanisms of deterioration occur below the surface, their manifestations are not readily seen in the visual inspections, often leading to subsequent project-level focus.
Traditionally, highway agencies have employed sounding (chain or hammer) to identify delaminated areas for project-level rehab. Although sounding has proven reliable, the labor and closures required for a sounding survey makes it prohibitive for obtaining data of a large number of decks. Also, sounding is not effective when there is an asphalt overlay. In response to these limitations, a number of highway agencies, including MnDOT, have utilized alternative methods, particularly ground penetrating radar.