Visual assessments of roadways and bridges typically do not provide an accurate assessment of their condition and the need for future repair or replacement. Conversely, subsurface investigations offer specifics that highway agencies require for future planning.

Infrasense Inc., www.infrasense.com, Arlington, Mass., a provider of rapid subsurface scanning and inspection, recently conducted subsurface investigations for nine bridge decks, representing nearly 700,000 sq.ft., at the Waterbury Stacks interchange in Waterbury, Conn.

The investigations were performed using vehicle-mounted high-speed GPR (ground penetrating radar) to scan selected bridges within the complex interchange. Unlike traditional inspection methods, these tests provided a condition assessment of the reinforced concrete bridge decks without requiring lane closures or disruption to traffic flow.

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 three 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. Infrasense uses its proprietary software to analyze and map this data to provide comprehensive results to its clients.

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. As a result, visual deck assessments are often inaccurate, and do not provide a sound basis for planning repair and rehabilitation.

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