he degree of corrosion of metal reinforcing is a major concern with bridge decks and similar concrete structures. Since the presence of saline water and corrosion by-products causes strong attenuation of GPR signals, measuring and mapping the variations in GPR signal attenuation has become an accepted practice to assess the condition of concrete structures.

Challenges

A local municipality needed to know if one of their aging bridges was in need of bridge deck maintenance. The municipality had a limited budget and were looking for an assessment in order to plan maintenance and repair work. A GPR survey was performed to help decide where to collect core samples.

Solution

GPR data were acquired on the 6 lanes of a 90-meter long bridge using a Noggin® 1000 GPR in a SmartChariot vehicle-towed configuration. Three lines were collected in each lane: in the left wheel path, right wheel path and the center of the lane. This provided 18 lines with a total of approximately 1620 meters of GPR data on the bridge. A multi-ball hitch system on the tow vehicle allowed the SmartChariot to be towed in the center of the lane or the left or right wheel paths; this meant the tow vehicle was always driving safely in the middle of the lane.

Figure 1

A multi-ball hitch system makes data acquisition in any part of the lane safe

GPR Data Collection

Figure 2

Bridge Survey Lines. Data acquisition of about 54,000 sample points on the bridge took less than an hour – in fact, more time was spent driving to the next exit and turning the tow vehicle around to collect another pass of the data than was spent actually acquiring the data.

After the data collection, the data were transferred from the data logger to a PC and the EKKO_Project™ software was used to process the bridge deck data. Specifically, the Interpretation Module was used to pick the rebar responses; a total of 5480 interpretations (or picks) were made.

Figure 3

A cross-section of a portion of one GPR line across the bridge is shown.

The data show that the rebar spacing on the bridge deck is 25 centimeters (approximately 10 inches). Using the EKKO_Project Interpretation module, the hyperbolic responses from the rebar were picked – the Smart Point feature was used to pick the best position of each rebar (blue dots).

The Bridge Deck Condition Report Module

The Bridge Deck Condition Report module processed the picked rebar amplitude values and created signal attenuation map images. The Bridge Deck Condition report software generates two types of response amplitude outputs:

1. raw amplitude GPR signal strength in millivolts (mV)
2. a normalized display presented in decibels (dB)

The normalized data shown in decibels (db) is often referred to as a Deterioration Index map and uses an enhanced version of the processing outlined in ASTM standard 6087 for using GPR to evaluate asphalt-covered bridge decks.

In the map below, red areas have low amplitude indicating possible corrosion from salt infiltration.

Figure 4

Rebar Amplitude map. Red areas have low amplitude indicating possible corrosion from salt infiltration.

The picked rebar data also provided useful statistics for the bridge engineers, including the minimum, maximum and average amplitude, rebar depth and rebar spacing.

All the information was presented as a Bridge Deck Condition Report in PDF format.

Figure 5

Bridge Deck Condition Report

Results

Based on the findings, the municipality planned to core one of the areas of the bridge with the highest attenuation of the GPR signals; indicating that the concrete in this area was in the worst condition. By correlating the coring results to the GPR results, they could assess the actual condition of the asphalt, concrete, and rebar and decide if bridge deck maintenance was required.

Using GPR for bridge deck assessment meant less time and fewer cores were needed to analyze the bridge condition. In addition, GPR allowed for continuous coverage of the scanned area, whereas solely coring doesn’t show anomalous areas in between cores. This allowed the municipality to save time and money while prioritizing repair and maintenance efforts.

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