Determining a Field Inspection for Guardrail Beam Integrity (405160-29)

DETERMINING A FIELD INSPECTION TECHNIQUE FOR GUARDRAIL BEAM INTEGRITY (2011 WA/25)

PROBLEM STATEMENT

Many states use weathering steel guardrail (Cor-Ten steel) along their roadways.  The Federal Highway Administration (FHWA) recently posted a Frequently Asked Questions (FAQ) list on their roadway departure safety webpage, which states that the use of the weathering steel guardrail should be limited, but may be used if the owner agency adopts a frequent periodic inspection and replacement schedule.  Rail deterioration appears to vary from state to state, with severe deterioration reported in some locations and no noticeable deterioration in other locations.  An inspection procedure needs to be developed to comply with the direction in the FHWA’s FAQ.

BACKGROUND

 Several states across the nation use the weathering steel guardrail for aesthetic purposes.  Instead of the zinc galvanization used to prevent corrosion of the standard steel guardrail, the outer surface of the weathering steel guardrail corrodes a certain thickness and maintains a specified core metal thickness.  This outer corrosion layer gives a rustic look to the rail, which is considered more aesthetically pleasing compared to the metallic look of the regular steel guardrail with galvanized zinc coating.

It was believed that once the outer surface of weathering steel has corroded a certain thickness, the corrosion process stops and the specified metallic core thickness is maintained without the need of further surface treatment.  However recent observations and in-field evaluations have shown that while the weathering steel is resistant to further corrosion of the rail, it does not completely prevent corrosion under certain environmental circumstances [1].  More specifically, areas of the rail that overlap, such as in locations of rail splices, or near posts, are more prone to increased corrosion due to water retention or other factors.   Corrosion increase deteriorates the rail by reducing its tensile capacity and can ultimately result in loss of the rail’s cross section.

Due to such observations, FHWA issued a response on their Frequently Asked Questions website limiting the use of weathering steel guardrails unless a frequent and periodic field inspection program was adopted by the user agency [2].

Currently there are no established techniques for conducting field inspection of the weather steel guardrails.  Since it is desirable to perform these inspections in a non-destructive manner and without having to dismantle the rail, Non-Destructive Testing (NDT) methods seem to hold a good promise for this application.

OBJECTIVE

Develop inspection technique(s) for determining the integrity of guardrail beams.  The inspection should focus on lap splices in the rail elements and any other areas where large areas of metal to metal surface contact are present.  In addition to the focus on lap splices, the rail elements should be spot checked elsewhere to assess the ribbon strength of the rail.  A field inspection manual and inspection form are to be produced in this project.  An FHWA representative should be part of the development effort to ensure the final procedures are acceptable to FHWA and consequently can move forward into an implementable program.

This project should start with an investigation and outreach effort to determine if similar efforts are underway elsewhere.  The technique(s) developed should result in measureable, pass/fail assessment criteria.  The assessment technique(s) are to be implemented without disassembling the guardrail

BENEFITS

A successful project will allow states to continue using the weathering steel through a documented inspection program when the functional integrity of the rail has not been compromised due to exposure to the elements.  Once implemented, an inspection program is also expected to result in identification and subsequent replacement of rail that has been compromised.  This approach is expected to result in sustainable levels of safety while avoiding the expense of replacing systems that are still fully functional.

PRODUCTS

TTI will provide a final report suitable for submittal to Federal Highway Administration (FHWA) documenting the research.  Discussion needed to request FHWA’s acceptance of the field inspection techniques and procedures will be provided.

A copy of all deliverables will be provided for each participating member state.

IMPLEMENTATION

As stated above, TTI will provide all the supporting information and written discussion for submitting a request to FHWA for acceptance of the field inspection techniques and procedures.

WORK PLAN

The work plan for this research will comprise four tasks as described below.

Task 1: Survey of States

In this task, the researchers will conduct a survey of states to determine their experience with the use of weathering steel W-beam guardrail.  The survey will aim at determining the extent and location of rail damage observed, methods or procedures employed to inspect and determine the rail failure, and special equipment used for the inspection. The survey will also ask the participating states if they will be willing to donate corroded guardrail for use in Task 3.

Task 2: Search and Review of NDT Products

In this task, the researchers will search for existing nondestructive testing (NDT) products and review product information to determine suitability of use for this project. NDT devices will be used to determine the cross-section thickness of the non-corroded metal core in the weathering steel W-beam.  If it is found that the thickness has reduced to less than the minimum thickness specified in the AASHTO M180 specification for the 12-guage W-beam guardrail, the section will be considered damaged. 

Products will be searched using Internet search engines and by contacting various NDT labs for information they may have on NDT equipment and their manufacturers.  At the end of this task, the researchers will shortlist several products for a detailed evaluation in Task 3.

Task 3: NDT Product Demonstration and Selection

In this task, the researchers will contact manufacturers of the NDT products shortlisted in Task 2 for demonstrating the use of their products.  Weathering steel guardrail with different levels of corrosion will be installed at TTI’s Proving Ground facility for use in the demonstration.   The guardrail is expected to be donated by one or more of the states who participate in the Task 1 survey.  TTI will arrange for shipping the guardrail to its facility using funds from this project. 

A request will be made to manufacturers of NDT products to perform a demonstration at TTI’s facility.  If a manufacturer cannot be present, the researchers will request they loan the equipment to TTI for use in the demonstration.  The researchers will evaluate each product for accuracy of the measured data, ease of use, robustness and quality, maintenance procedures, and cost.

After the demonstrations, the researchers will disassemble the damaged weathering steel guardrail.  The rust from the surface of the guardrail will be removed using muriatic acid solutions and the true thickness of the rails will be measured.  These values will be compared to those determined by the candidate NDT devices to determine the accuracy of the devices.

To involve FHWA officials in the demonstration and selection process, the researchers will attempt to hold most product demonstrations on a single day and invite an FHWA representative to the demonstration.  Demonstrations that cannot take place on the same day will be held on alternate days. All demonstrations will be video recorded for future reference and documentation.

Task 4: Development of Field Inspection Methodology

In this task, the researchers will develop a procedure for inspecting the weathering steel guardrail in a consistent manner.  The procedure will involve inspecting some of the areas of the guardrail that are known to result in greater damage due to the weather (e.g. slices and at posts), as identified from the survey in Task 1.  Additionally, the procedure will consist of two levels of screening.  In the first level, the guardrails will be inspected for any visual signs of damage.  If none are found, the procedure will call for inspecting a certain number of locations on the guardrail for a specified segment length of the system using the NDT device(s) selected in Task 3.  If no damaged areas are detected in the initial screening, the maintenance personal will move on to the next segment of the guardrail.  If however some spots are found damaged in the initial screening, or if visual signs of damage are observed, a secondary screening of the segment length will be conducted.  This second inspection will be more thorough and will identify segments that need to be replaced or repaired.  

Using the two-level screening approach, the researchers intend to speed up the process of periodic in-field inspection of weathering steel guardrail.

At the end of this Task, the researchers will prepare a final report documenting all research findings and recommendations for the NDT technology to use, and recommended procedures for inspecting weathering steel guardrail.

TTI will provide the test facility, video recording of the demonstrations, still photographs, and a final report suitable for submittal to Federal Highway Administration (FHWA).  The report will include details of the NDT products selected and recommended procedures for inspecting the guardrails.

 

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[1]   New Hampshire Department of Transportation Research, “A588 Weathering Steel Guardrail.” Research poster, 2007.
[2]   Federal Highway Administration Website. Retrieved May 2, 2011.
http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/qa_bttabr.cfm.