New Hampshire Office: 9 Tyler Drive, Goffstown, NH 03045 (603) 235-9108 fromero@geovision.com Main Office: 1151 Pomona Road, Corona, CA 92882 (951) 549-1234 FAX: (951) 549-1236

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GPR Prediction

Deterioration

Deterioration

HIGH-SPEED, DUAL-POLARIZATION METHOD OF GROUND PENETRATING RADAR BRIDGE DECK EVALUATION

Ground penetrating radar (GPR) is a high-speed NDT method that WaveTech—GEOVision commonly applies to a number of engineering problems associated with both new and aging concrete structures, including identification of concrete deterioration in bridge decks—caused primarily by intrusion of chlorides into the concrete matrix and corrosion of the reinforcing steel. Traditional GPR Surveys. High-speed GPR surveys have been performed for many years to map the condition of aging bridge deck structures. Most often, one or more 1.0 GHz horn antennas are driven along unique survey paths spaced anywhere from 3 to 4ft apart—and 2 to 3 GPR scans are collected along every linear foot of the deck surface that is being sampled. Surface dielectric properties, signal attenuation through the deck thickness, and reinforcement reflection amplitude, as well as a number of other variables, are correlated to deck condition. Although this traditional GPR method is moderately successful in terms of its ability to estimate the percentage of deck area that is deteriorated, it has produced very mixed results in terms of its ability to accurately locate deterioration quantities on a deck and define their boundaries. Particularly, when deterioration quantities (determined by other methods such as sounding) fall under 10% of total deck area, or above 45% of deck area, these traditional, high-speed GPR surveys often under-predict or over-predict deterioration quantities by a factor of two or more. High-Resolution GPR Surveys. New bridge deck evaluation techniques have been developed over the past five years which have markedly improved the ability of GPR to locate and define the boundaries of deterioration significant enough to require removal. Along with the big improvement in pinpointing deteriorated zones on a deck has been added accuracy in GPR’s ability to estimate total deterioration quantity (historically expressed as % of total deck area). Top plan-view map shows areas on a deck that where half-cell and corrosion potential data indicated active corrosion (dark and light orange) and regions (greens/white) where corrosion was approaching threshold or insignificant. Bottom plan-view map shows results from high-resolution ground-coupled, 1.5 GHz survey illustrating predicted zones of deterioration (all colors but blue and black,with reds being most deteriorated) on a plan view (overlay present during GPR survey).

Initially, these high-resolution surveys were performed exclusively with high-frequency, 1.5 GHz ground-coupled antennas, still used in situations where their deployment is simpler and more effective. The ground-coupled antenna’s size and higher resolution gives it the unique ability to accurately measure top rebar reflection amplitude without interference from other signals returning from within the deck—particularly interference from longitudinal bars in the upper mat of rebar. This—the top mat transverse rebar amplitude—is the one measurement in the GPR signal which consistently has the single, greatest influence on accurately determining relative deck deterioration quantities.

New Hampshire Office: 9 Tyler Drive, Goffstown, NH 03045 (603) 235-9108 fromero@geovision.com Main Office: 1151 Pomona Road, Corona, CA 92882 (951) 549-1234 FAX: (951) 549-1236

Any single, air-coupled antenna cannot match the ability of a 1.5 GHz ground-coupled sensor to cleanly measure rebar reflection amplitude. Also, other interface reflections—concrete surface, lower rebar, deck bottom, total travel time—do not yield consistent or reliable results from one survey to the next. And as the results of traditional, high-speed 1.0 GHz surveys have shown…using these variables to predict deterioration often results in as many “false-positives” and “false-negatives” as accurate identifications of deteriorated locations on a given deck. To date, nothing has proven as accurate, reliable or more simple than a 1.5 GHz ground-coupled GPR analysis when estimating deterioration quantity and delineating boundaries between sound and damaged concrete are both critical—and measurement of the top transverse rebar signal is critical to its success.

However, ground-coupled antenna surveys have their limitations, which are significant: slow speed of data collection; lane-closure requirements (exposure of survey crew to traffic and economic impact of lane closures on businesses); and the absolute necessity to run GPR survey lines perpendicular to the MOST shallow reinforcement in the deck—if longitudinal steel is tied on top of transverse steel (as is the case in continuously-reinforced decks), a high-quality GPR survey with a single antenna (ground-coupled or air-coupled) MUST be performed with GPR lines oriented ACROSS the deck’s width. This is totally impractical, very uneconomical, and extremely time-consuming on a large deck or on a series of decks surveyed together.

STATE-OF-THE-ART GPR SURVEYS (HIGH-RESOLUTION, HIGH-SPEED)

Dual-Polarization (1.0 GHz) Horn Antenna Surveys: State-of-the-Art & State-of-the-Practice. Continued investigation into the use of air-coupled horn antennas (1.0 GHz) resulted in the development of a novel technique enabling two independently polarized antennas to effectively simulate the higher-resolution ground-coupled antenna. When two air-coupled antennas, mounted in-line and deployed so that one is sensitive to transverse steel in the deck and the other is rotated 90° so that it is most sensitive to longitudinal steel, the signals from both antennas are sampled from exactly the same survey path, allowing them to be measured and evaluated in a unique manner. The signal from one antenna is subtracted from the other, taking the offset between the two antenna’s positions into account, and the resulting signal yields a top rebar reflection which is similar to the one measured by a high-frequency (1.5 GHz) antenna. Removed from this signal are unwanted interference from either a thin overlay (less than 2 inches), shallow concrete cover (less than 2.5 inches), or both. Additionally, the negative influence of longitudinal steel—which is randomly measured along every GPR profile running the deck’s length because the antennas are sometimes directly over a bar, but more often are not—is removed from the top transverse bar

signal. After all, what we really want to be measuring—without the influence from longitudinal bars—is transverse rebar signal strength (amplitude). The result is a survey which nearly matches the accuracy of a ground-coupled inspection, but can be performed at speeds between 30 and 40 mph. Furthermore, it makes no difference whether longitudinal or transverse steel is on top because the influence of the longitudinal steel—even in a continuously reinforced structure (with longitudinally-oriented steel on top)—is minimized during this subtraction process, allowing top transverse rebar reflection to be

measured and compared throughout the deck area. The resulting contour plots are virtually the same from a dual-polarization (1.0 GHz) horn antenna survey as those obtained from a 1.5 GHz ground-coupled analysis on most decks—a far better correlation than any other GPR method with what is commonly called “ground truth”, which really consists of “verification and validation” of GPR predictions using sounding, half-cell corrosion potential, or chloride tests…even cores (bottom map on previous page shows delaminated areas detected by hammer-sounding after overlay was removed).

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(c) Deteriorated areas of bridge deck mapped using hammer-sounding

(a) Deteriorated areas of bridge deck assessed using 1.5 GHz ground-coupled antenna (GSSI Model 5100)

(b) Deteriorated areas of bridge deck assessed using dual-polarized horn antennas (GSSI Model 4208)

New Hampshire Office: 9 Tyler Drive, Goffstown, NH 03045 (603) 235-9108 fromero@geovision.com Main Office: 1151 Pomona Road, Corona, CA 92882 (951) 549-1234 FAX: (951) 549-1236

What else contributes to accurate, ground-coupled (1.5 GHz) and dual-polarization (1.0 GHz horn antenna surveys—and sets the dual-polarization survey apart from the rest?

• Precise survey positioning (using either a camera or laser to maintain GPR lines at precise 2ft spacing, and surveying as close as 2ft to curbs and gutters—as shown in the adjacent figure), and extremely dense signal sampling along each GPR line.

• Add a method for removing compromising influences of

shallow cover, overlays, and the variable presence of longitudinal steel—and an ability to use the highest-performance GPR equipment capable of performing these surveys at high speed, and you have what make this high-speed method more accurate, repeatable, reliable and versatile than the traditional single-polarization surveys that have been performed over the last decade—and more convenient and practical than the high-resolution (1.5 GHz) ground-coupled surveys.

Regardless of methodology, however, it is always important to

calibrate GPR (any survey) using selective NDT or physical/petrographic samples to achieve the best results. Not doing so will severely limit the potential effectiveness of any GPR survey.

Case Study: I-89 Bridge, Connecticut River (1988)—Using GPR Survey to Select Patch Locations

Project Maintenance Summary: Project costs were overrun due to lack of GPR information prior to budget decisions. As it turned out, no one knew how reliable the GPR survey would be able to predict verifiable deterioration—or accurately map its location—once it was calibrated with some underside and chloride sample data.

Item 520.5519 - Quick Set Concrete (Night)

Item Unit Unit Bid Price Estimated Quantity Phase I

Actual Quantity Phases I & II

511.02 Yd2 $340.00 400.00 698.10 520.5519 Yd3 $1,245.00 50.00 77.50

1) Total cost of project = $923,266.41 The original project budget, including contingency costs = $703,778.90. Approximately $106,000.00 of the project increase was attributed to traffic control.

New Hampshire Office: 9 Tyler Drive, Goffstown, NH 03045 (603) 235-9108 fromero@geovision.com Main Office: 1151 Pomona Road, Corona, CA 92882 (951) 549-1234 FAX: (951) 549-1236

Phase I and Phase II Maintenance (Typical of I-89 Project)

White Border: Phase I Maintenance ZonesGray (Pink, in CD-ROM version of paper) Border: Phase II Maintenance Zones

Use of GPR, based on quality of predictions, would have provided better defined regions for repair resulting in an improved capability to estimate project costs and budget appropriately. More than likely, a different maintenance strategy might have been selected (hydro-demolition and replacement of concrete, for example) if the goal was to increase service life beyond ten years. Because the structure was considered to be one that would become functionally obsolete within ten years (due to increased traffic demand)…the decision was more one of repair or replace; meaning the DOT didn’t want to repair more (or incur more expense) than absolutely necessary. More previous experience with high-resolution GPR methods (precision positioning/dense sampling) would have also reduced the amount of time spent on Phase II maintenance, the most expensive part of the work (due primarily to increased cost of traffic control). Because of the unknowns surrounding the capabilities of this method, both NHDOT and GSSI (who provided the data for this project) agreed that a more conservative approach toward identification of “patch locations” could be used, even though GSSI felt that its predicted deterioration quantities and locations (more extensive than what NHDOT used as an action level) were essentially correct. A detailed, published paper on this project (or other work) is available upon request.

SUMMARY OF BENEFITS

• Accurate, specific location information for better planning • Rapid data acquisition – less inconvenience and liability for owner • Elimination of lane closures reduces cost and improves worker safety and reduces injury liability • Cost benefit due to better estimation of repair project costs, lower cost of planning, speed and safety • Better quantification of repair zones will

o extend bridge deck service life by minimizing future repairs and frequency of maintenance o improve ability to plan extent of daily maintenance activities and estimate material quantities o reduce costs associated to unplanned traffic control

Data for much of this informational brochure provided courtesy of Geophysical Survey Systems, Inc.