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The 2013 ASCE Report Card for American Infrastructure states that one in nine U.S. bridges are considered structurally deficient, and more than $20.5 billion will be required annually to eliminate the deficiency backlog by 2028.

For many years, there’s been a nationally recognized need to improve existing techniques for assessing the condition of roads and bridges. In 2005, the U.S. Transportation Research Board tackled this issue by launching an initiative known as the Strategic Highway Research Program (SHRP2).

The research evaluates the effectiveness of applying various non-destructive testing techniques such as ground penetrating radar (GPR), impact echo, ultrasonic surface waves and electrical resistivity, among others, to capture subsurface data on corrosion, delamination, vertical cracking and concrete degradation. The report concluded that rapid, non-destructive condition assessment significantly reduces the overall resources and expenditures required for bridge renewal. It also concluded that capturing data via non-destructive methods could serve to prevent the premature and unexpected failure of bridge decks.

Of the non-destructive techniques evaluated in the SHRP2 report, GPR ranked highest in overall value for assessing bridge deck deterioration based on accuracy, precision, speed, ease of use and cost.

[caption id="attachment_5418" align="alignnone" width="640"]Ground Penetrating Radar (GPR) makes it possible to visualize varying degrees of subsurface deterioration along bridge decks. Ground Penetrating Radar (GPR) makes it possible to visualize varying degrees of subsurface deterioration along bridge decks.[/caption]

 

Getting Beneath the Surface with GPR

GPR is a non-destructive technique that works by transmitting high frequency radio waves into the ground or structure and analyzing the reflected energy to create a profile of the subsurface features. It’s leveraged to scan vast areas of infrastructure and deliver insights from deep below the surface, including asphalt and pavement thickness, subsurface cracks, voids in the concrete, rebar corrosion and deterioration.

This data is used to calibrate results from traditional core sampling, while minimizing the number of cores required. Once collected, GPR data is digitally stored and can be referenced years down the line during subsequent condition assessments to accurately monitor changes in a road's or bridge’s condition.

[caption id="attachment_5417" align="alignnone" width="640"]Cross sections of the GPR data can be viewed to assess individual layers such as pavement, asphalt and PCC Cross sections of the GPR data can be viewed to assess individual layers such as pavement, asphalt and PCC.[/caption]

Adding GPR to the Condition Assessment Process

A GPR scan is typically conducted at the outset of a condition assessment. The results are then combined with observations noted through a visual inspection to identify evidence of distress or abnormalities. Core sample locations are then selected based on this information, and data gleaned from the core sample program is calibrated with GPR data to verify accuracy.

Insights delivered through non-destructive testing are particularly useful when decisions are being made around the prioritization of rehabilitation and maintenance projects. A GPR scan collects data between widely spaced core samples to provide a more comprehensive picture of the infrastructure’s condition. This empowers decision-makers to thoroughly and accurately assess the true condition of roads and bridges so they’re able to identify infrastructure in need of immediate repair and spend limited budgets wisely.

Adding GPR to the condition assessment process serves to improve infrastructure reliability in a few key ways.

  • Identifying subsurface threats early on. Defects and wearing reduce the structural capacity of a road or bridge over time, and can lead to an eventual failure. These defects often are invisible, particularly when a surface has been repaved. GPR has the ability to detect areas that are unusually thin or thick and identify subsurface defects below asphalt overlaid roads and structures.
  • Reducing the need for destructive testing. Core sampling is costly and can create weaknesses in a road or bridge. The incorporation of GPR into a structural assessment greatly reduces the amount of drilling required to capture data and fills in information gaps between discrete points.
  • Recording digital data for future use. Each time a condition survey is completed using GPR, subsurface inspection data can be added to a digital record to be referenced years down the line for future rehabilitation projects. This makes it possible to examine patterns of deterioration overtime and evaluate the effectiveness of ongoing maintenance strategies.
  • Minimizing service disruption. With the cost of traffic congestion estimated at more than $121 billion across the United States, GPR offers the added advantage of capturing data at posted speed limits, often eliminating the need for road closures and traffic control.
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