Electrical resistivity probe (a machined jig with either wooden dowels or foam contacts wetted with surfactant solution), configured to permit sufficient vertical movement of contact points to conform to irregular surfaces; some instruments may provide units of resistivity directly based on preset probe spacing.

2.1.4

Digital camera.

2.2

Personnel: PRE-PL-LO-005, Personnel Qualifications.

3. Methodology

3.1

Use the global rectangular grid (FLD-OP-SC-001, Data Collection Grid and Coordinate System for Bridge Decks) to locate test points on the deck.

3.2

Test preparation:

3.2.1

Determine if there are any electrical conduits in the structure producing an electromagnetic field that may affect the measurement stability.

3.2.2

Mark the position of the cable in the resistivity map as a sign of possible interference in its proximity.

3.2.3

Prewet the bridge deck (FLD-OP-SP-001, Site Preparation) until it is in a saturated-surface-dry (SSD) condition. The electrodes require ionic coupling to the concrete to measure resistivity, and uniformly prewetting the deck provides this water evenly, thereby removing variability induced by irregular weather and rainfall patterns. Supplemental wetting is necessary immediately before measuring, particularly when the weather is hot, windy, and/or the relative humidity is low. For both prewetting and supplemental wetting, it is important that no surface water or visible film is present during measurements.

3.3

Measurements:

3.3.1

Place the four wetted probe points of the Wenner array in contact with the concrete surface, and apply current (if required by the particular instrument) between the two outer electrodes.
To ensure a minimal effect of the reinforcing steel, the probe spacing of the Wenner probe must be less than the depth of the concrete cover over the reinforcing steel. Where practical, avoid placing probe points on individual, exposed aggregate particles that may inordinately influence the reading.

3.3.2

Apply consistent and even pressure to the probe points, because variable contact pressure will influence the readings.

3.3.3

Read and record the indicated resistance or resistivity.

3.3.4

Monitor each point for at least 3 seconds before recording to ensure the reading is stable (not increasing). If readings are not stable, it may indicate inadequate moisture or interference by an external electrical source. To remedy this situation, remove the probe from the test point, rewet the probe, and then retake the measurement. If that does not work, pour more water on the deck surface, and repeat the test at the test point when it reaches an saturated-surface-dry (SSD) state.

3.3.5

For comparison with other complementary nondestructive evaluation (NDE) test data at selected point locations, take additional detailed measurements. To ensure repeatability of the localized measurements, repeat the readings five times at each location. For each measurement, remove the probe from the test point, rewet it, and place it again on the test point. Take extra care to ensure the proper contact between probes and deck surface.

3.4

Traffic in the lanes outside of the work zone is permissible during data collection and does not affect data quality.

3.5

Storing data, documents, and images:

3.5.1

FLD-DS-LS-001, Data, Document, and Image Storage—Local, for local storage.

3.5.2

FLD-DS-RS-001, Data, Document, and Image Storage—Remote, for remote storage.

3.6

Reporting: Transfer all metadata, data, documents, and images to Federal Highway Administration (FHWA), and/or upload all metadata, data, documents, and images into the Long-Term Bridge Performance (LTBP) Bridge Portal.

4. Data Collection Table

4.1

Table:

#	Field Name	Data Type	Accuracy	Unit	Field Description	Row Color
1	State	Text			State Code; e.g., Virginia = VA	Green
2	NBI structure number	Text			Item 8, structure number; from NBI Coding Guide	Green
3	Structure name	Text			Descriptive name for the bridge; e.g., Route 15 SB over I–66	Green
4	Protocol name	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			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	Range: -50 to 150	Green
9	Deck surface temperature	Number	1	ºF	Range: -50 to 150	Green
10	Equipment name	Text				Green
11	Equipment manufacturer	Text				Green
12	Equipment model name and number	Text			If available	Green
13	Comments (equipment)	Text	Unlimited			Orange
14	Test site	Text			Describe the location of data collection on the bridge (e.g., spannumber, lane number, right or left shoulder, substructure unit, etc.)	Blue
15	Location of test site (x‑coordinate)	Number	1	ft	Longitudinal distance from the local grid origin	Blue
16	Location of test site (y‑coordinate)	Number	1	ft	Transverse distance from the local grid origin	Blue
17	Electrical resistivity reading	Number	1	kΩ/m³	Range: 0 to 200	Yellow
18	Comments	Text				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 from the same concrete member using other NDE methods, including acoustic methods, chemical/potential methods, and electromagnetic methods.

5.2

If half-cell potential results are available, compare results from the current test to the half-cell potential results.

6. Commentary/Background

6.1

The purpose of this protocol is to provide a standard procedure for determining electrical resistivity of the concrete in a reinforced concrete member. ER is one measure of a concrete member’s ability to protect reinforcing steel from corrosion. Low ER is indicative of an environment that will support corrosion but does not indicate that corrosion is ongoing.

6.2

Applied on a broad scale, ER provides spatial variability of resistivity for the tested concrete member; applied at finite locations, it can be compared to corrosion measures, such as half-cell, linear polarization, and chloride profiles.

6.3

Electrical resistivity surveys are used to map corrosion activity in tandem with another corrosion technique, like half-cell potential (FLD-DC-NDE-003, Half-Cell Potential Testing). Concrete with a high ER has a greater resistance to the corrosion current passing between anodic and cathodic areas of the reinforcing steel. In contrast, damaged and cracked areas with increased porosity create preferential paths for fluid and ion flow and have low resistivity. There is a relationship between electrical resistivity and corrosion rate for steel in concrete. Resistivity of less than 1.97kΩ/inch supports very rapid corrosion of steel, whereas resistivity greater than 7.88 kΩ/inch supports very low corrosion rate.

6.4

A common device to measure electrical resistivity is the Wenner array. The Wenner array uses four probes arranged linearly with equal spacing. Current is applied between the outer electrodes and the resulting potential is measured between the two inner electrodes. The resistivity is calculated as ρ=2πaV/I, where “a” is the probe spacing, “V” is the voltage, and “I” is the current.

6.5

Any steel located in the concrete being measured can affect electrical resistivity. To decrease the effect, measure the resistivity on a set grid over the whole deck so that any influence from steel is statistically removed due to sampling.

6.6

Overlays can affect the ability to take ER measurements. If the overlay is highly resistive, like epoxy or asphalt, ER measurements will be extremely high and will only be an indication of the overlay’s resistivity. These measurements will give no indication of the concrete member’s environment regarding corrosion. If the overlay is a concrete overlay that has similar electrical properties to the concrete substrate, resistivity measurements can be conducted without any issue. However, during interpretation of the results, it is important to know that the resistivity values measured will be a combination of the overlay and the concrete below.

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-DC-NDE-003, Half-Cell Potential Testing.

7.1.5

FLD-DC-PH-002, Photographing for Documentation Purposes.

7.1.6

FLD-DS-LS-001, Data, Document, and Image Storage—Local.

7.1.7

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

Nondestructive Evaluation Protocols (NDE)

Electrical Resistivity TestingLTBP Protocol #: FLD-DC-NDE-001

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

Electrical Resistivity Testing
LTBP Protocol #: FLD-DC-NDE-001