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Ground Penetrating Radar Testing for Bridge Decks - Ltbp Protocol - Fld-Dc-Nde-002

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

	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.

Page Owner: Office of Research, Development, and Technology, Office of Corporate Research, Technology, and Innovation Management

Topics: research, infrastructure, structures
Keywords: research, structures, LTBP Program, bridge performance, bridge testing, bridge inspection, Bridge, bridge inspection, Long-Term Bridge Performance Program, LTBP, testing protocols, data collection
TRT Terms: research, infrastructure, Facilities, Structures
Scheduled Update: Archive - No Update needed

This page last modified on 03/24/2017

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

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

1.

Data Collected

2.

Onsite Equipment and Personnel Requirements

3.

Methodology

4.

Data Collection Table

5.

Criteria for Data Validation

6.

Commentary/Background

7.

References

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