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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-16-007    Date:  January 2016
Publication Number: FHWA-HRT-16-007
Date: January 2016

 

Long-Term Bridge Performance (LTBP) Program Protocols, Version 1

Long-Term Bridge Performance Program Logo

Ground Penetrating Radar Testing for Bridge Decks
LTBP Protocol #: FLD-DC-NDE-002


1.

Data Collected

 
1.1 Characterization of broad scale concrete deterioration based on ground penetrating radar (GPR) signal attenuation.  
1.2 Estimate of broad scale concrete cover (rebar depth) or deck thickness, overlay thickness, etc.  
1.3 Localized mapping or locating of rebars, posttensioning, conduits, etc.; verification of reinforcement layout shown on drawings and plans.  
1.4 Detection and localization of voids, rock pockets, or honeycombs.  

2.

Onsite Equipment and Personnel Requirements

 
2.1 Equipment:  
2.1.1 PRE-PL-LO-004, Personal Health and Safety Plan.  
2.1.2 Ground-coupled GPR instruments, consisting of portable, single channel digital data acquisition systems with a dynamic range of 60 dB or greater and 16-bit data acquisition or greater. GPR scanning (1.0 to 2.5GHz) must occur with a minimum spatial sampling of 60 scans per foot using a distance measuring instrument (DMI). Systems must maintain a stable, steady-state signal and must collect at least 512 samples per scan at scan rates of 120 scans per second or greater. GPR Ground penetrating radar systems must be able to store at least 2 GB of data on a hard drive or flash drive and be capable of rapidly transferring data via Universal Serial Bus (USB), ethernet, or other backup media to a computer for storage, processing, and interpretation.

NOTE—Though multichannel GPR arrays, dual-polarization antennas (sensors), air-coupled (horn) antenna GPR systems, etc., can be used for bridge deck condition assessment, most systems in use are single-channel, ground-coupled GPR instruments. This protocol applies only to ground-coupled systems used in the Long-Term Bridge Performance (LTBP) Program because of accuracy, portability, and ease of use.
 
2.1.3 Vehicle or cart; speed is dependent on the system used, the scan rate and signal quality, and the spatial sampling along the GPR profile line (path). Normally, walking speed with a cart is sufficient for high-resolution data using 1.0 to 2.5 GHz ground-coupled antennas. For other systems, refer to the manufacturer’s operations manual.  
2.1.4 Digital camera.  
2.2 Personnel: PRE-PL-LO-005, Personnel Qualifications.  

3.

Methodology

 
3.1 Use the GPR equipment’s manufacturer procedures with the following protocol steps to ensure proper equipment settings and data collection procedures.  
3.2 Use the grid described in FLD-OP-SC-001, Data Collection Grid and Coordinate System for Bridge Decks, to provide lines along which to collect GPR data.  
3.3 Collect GPR data along lines perpendicular to the orientation of the upper rebar mat.  
3.4 Record the GPR signal (A-scan), including the entire surface reflection (combination of surface reflection and internal, direct coupling) and the “flat-line” (no amplitude/noise only) data immediately above the surface reflection. This ensures that all data from the deck are captured.  
3.5 Conduct a preliminary scan of at least three 50-ft lines on the bridge deck surface with the A-scan visible at all times in order to set up the GPR signal properly. Doing this ensures that the GPR signal does not “clip” (saturate measurement window width) during data collection and storage. The preliminary scan will find locations of strong signal reflection from the top rebar mat. Then, set the gain for the GPR signal waveform at the location with the strongest rebar reflection so that the resulting signal does not exceed two-thirds to three-quarters of the measurement window width.  
3.6 Keep the gain constant while scanning the entire bridge deck. Avoid automatic gain settings except during initial GPR calibration and for fine tuning after the system is first turned on.  
3.7 Most bridge decks can be scanned using the same settings, except for signal gain and position, which are often site specific. When returning to any bridge for successive, periodic tests, use the same system settings, such as gains, filters, samples/scan, scan rate, spatial scan density. The following minimum settings for GPR data collection are applicable for most reinforced concrete bridge decks and other reinforced concrete structures:  
3.7.1 512 samples per scan.  
3.7.2 12 ns signal duration.  
3.7.3 16-bit data.  
3.7.4 Appropriate vertical filter settings for the antenna frequency used.  
3.8 Correct position of the signal guarantees appropriate sampling of the entire deck without cutting out the upper surface or amplifying noise and misinterpreting it as data. To set the signal gain and position, the following procedures must be performed:  
3.8.1 Since the GPR records waveforms of 10 to 20 ns, representing the entire bridge deck thickness, the signal must be positioned after the waveform is recorded on oscilloscopes.  
3.8.2 Locate the first arrival (transmit–receive pulse).  
3.8.3 Place the antenna on the ground, and identify the surface reflection. The surface reflection is a merger of the transmit–receive reflection generated internally within the antenna and the reflection of the GPR waveform from the surface of the deck.  
3.8.4 Move surface reflection at the beginning of the sample window of interest.

The sample window (10 to 20 ns, two-way travel time) is used to measure GPR reflection energy throughout the GPR survey. Reflections from internal, embedded elements, such as rebars, conduits, cables, deck bottom, overlays, etc., are captured, stored, then ultimately analyzed and interpreted prior to mapping results.
 
3.9 Traffic in the lanes outside of the work zone is permissible during GPR data collection and does not affect data quality. Take extra safety precautions and coordinate with the traffic control crew when GPR lines are oriented across traffic lanes, even if only the closed lane(s) are accessed by the GPR operator. In such a field operation, the front end of the equipment/cart must temporarily penetrate beyond the lane closure (while no traffic is oncoming). This ensures that a complete dataset is obtained when the partial GPR lines are “stitched together” during processing.  
3.10 Storing data, documents, and images:  
3.10.1 FLD-DS-LS-001, Data, Document, and Image Storage—Local, for local storage.  
3.10.2 FLD-DS-RS-001, Data, Document, and Image Storage—Remote, for remote storage.  
3.11 Reporting: Transfer all metadata, data, documents, and images to Federal Highway Administration (FHWA), and/or upload all metadata, data, documents, and images into the LTBP Bridge Portal.  

4.

Data Collection Table

 
4.1 Table:  
# Field Name Data Type Accuracy Unit Field Description Row Color
1 State
Text
Text
 
State Code, e.g., Virginia = VA
Green
2 NBI structure number
Text
Text
 
Item 8, Structure Number from NBI Coding Guide
Green
3 Structure name
Text
Text
 
Descriptive name for the bridge, e.g., Route 15 SB over I–66
Green
4 Protocol name
Text
Text
 
Title of the protocol
Green
5 Protocol version
Text
Month and year
 
Month and year the protocol version was published; e.g., May 2015
Green
6 Personnel performing data collection activities
Text
Text
 
First name(s) Last name(s)
Green
7 Date data was collected
Text
Exact date
 
mm/dd/yyyy
Green
8 Ambient air temperature
Number
1
ºF
Numeric means negative and positive integers, range: -50 to 150
Green
9 Deck surface temperature
Number
1
ºF
Numeric means negative and positive integers, range: -50 to 150
Green
10 Equipment name
Text
Text
 
 
Green
11 Equipment manufacturer
Text
Text
 
 
Green
12 Equipment model name and number
Text
Text
 
If available
Green
13 Data acquisition system model
Text
 
 
 
Green
14 Comments (equipment)
Text
Unlimited
 
 
Orange
15 Testing site
Text
 
 
Location and date of the test on the bridge (e.g., shoulder and lane 1)
Blue
16 Pulse length
Number
 
ns
 
Blue
17 Center frequency
Number
 
MHz
 
Blue
18 Bandwidth
Number
 
MHz
 
Blue
19 Stress S1
Number
 
 
 
Blue
20 Stress S2
Number
 
 
 
Blue
21 Longitudinal strain, Ɛ2
Number
 
 
 
Blue
22 Transverse strain, Ɛt1
Number
 
 
 
Blue
23 Transverse strain, Ɛt2
Number
 
 
 
Blue
24 Antenna model
Text
 
 
 
Blue
25 Gain
Number
 
 
 
Blue
26 Range
Number
 
 
 
Blue
27 Longitudinal rebar
Yes/No
 
 
 
Blue
28 Pulse repetition rate
Number
 
 
 
Blue
29 Samples/scan
Number
 
 
 
Blue
30 Scans/second
Number
 
 
 
Blue
31 Scans/unit
Number
 
 
 
Blue
32 Vertical filters
Text
 
 
 
Blue
33 Horizontal filters
Text
 
 
 
Blue
34 Line location
Number
1
ft
Transversal distance from origin
Range: 0 to 300
Yellow
35 Data collection direction
Predefined list
 
 
Transversal
Longitudinal
Yellow
36 Data
BLOB
 
 
 
Yellow
37 ASCII file
CLOB
 
 
DZT or DT file
Yellow
38 Comments
Text
Unlimited
 
 
Orange
4.2 Table Key:  
Column Descriptions
#
Sequential number of data item
Field Name
Data field name
Data Type
Type of data, such as text, number, predefined list, binary large object (BLOB), or PDF file
Accuracy
Accuracy to which the data are recorded
Unit
Unit in which a measurement is taken and recorded
Field Description
Commentary on the data or list of items in a predefined list
Row Color Key
Green
Data items only entered once for each protocol for each day the protocol is applied
Pink
Logical breakdown of data by elements or defect types (not always used)
Blue
Data identifying the element being evaluated or the type of defect being identified
Yellow
LTBP data reported individually for each element or defect identified
Orange
Comments on the data collection or data entered

5.

Criteria for Data Validation

 
5.1 Verification and comparison should be made with results obtained using other NDE methods, including acoustic methods and chemical/potential methods, as well as with ground truth data.  

6.

Commentary/Background

 
6.1 The purpose of this protocol is to provide a standard procedure for using GPR to detect and characterize deterioration in bridge decks. GPR can also serve additional purposes, including the following:  
6.1.1 Characterizing presence, pattern, depth, and density (layout) of structural steel reinforcement in the deck.  
6.1.2 Estimating deck thickness.  
6.1.3 Identifying anomalies or locating construction elements such as posttensioning conduits.  
6.2 The principle of GPR is the reflection of electromagnetic waves from interfaces of two materials that differ in relative dielectric permittivity. Different reflectors within or on the boundary of a bridge deck, such as overlays, upper and lower reinforcement mats, deck bottom, etc., are identified in the time domain by viewing what is commonly referred to as a B-scan (line scan). The image is a recognizable representation of the cross-section of the deck proved by the moving GPR antenna. The vertical scale (two-way travel time) can be associated with a physical depth, and the horizontal scale (distance) is controlled by a distance measuring instrument (DMI) that fixes the scan density (scans per meter or scans per ft). Typical bridge deck surveys use antennae ranging in frequency between 1.0 GHz to 2.5 GHz, with those in the 1.5 GHz range most common. Testing equipment consists of a digital data acquisition unit that controls signal stability, filtering and scan rate; cabling; a DMI; and ground-coupled sensors in the frequency range specified. The equipment usually includes a deployment system (pushed or hand-towed cart, vehicle, etc.).  
6.3 For the LTBP Program, GPR testing is done using ground-coupled, high-frequency instruments to ensure the most accurate results. GPR scan density for the LTBP Program is 60 scans per foot. This high spatial density aids not only during processing and interpretation but also in activities where raw data must be interpreted and used in the field; for example, to validate and update core locations.  

7.

References

 
7.1 LTBP Protocols:  
7.1.1 PRE-PL-LO-004, Personal Health and Safety Plan.  
7.1.2 PRE-PL-LO-005, Personnel Qualifications.  
7.1.3 FLD-OP-SC-001, Data Collection Grid and Coordinate System for Bridge Decks.  
7.1.4 FLD-DS-LS-001, Data, Document, and Image Storage—Local.  
7.1.5 FLD-DS-RS-001, Data, Document, and Image Storage—Remote.  
7.2 External: None.  

 

 

 

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