All state highway agencies use galvanized guardrail manufactured to the AASHTO M‑180 Specification as part of roadway safety elements. A number of agencies permit both hot dip batch and continuous galvanization of W-beam guardrail elements as both of these fabrication processes conform to the AASHTO M-180 specification. In the hot dip batch process, W-beam guardrail elements are first fabricated (shaped, cut, holes punched) and then hot dip galvanized. Guardrail fabricated by the hot dip batch process typically conforms to the AASHTO Type II classification, which has a minimum zinc coating of 3.60 oz/ft2 (minimum single spot). By contrast, in the continuous process, rolled steel is continuously galvanized and then fabricated (cut, holes punched, shaped) to manufacture the guardrail elements, resulting in bare steel exposed to weathering at the transverse cut edges and punched holes. Guardrail fabricated by the continuous process is typically made to the AASHTO Type I classification, with a minimum zinc coating thickness of 1.80 oz/ft2; however, AASHTO Type II classification is also available.
These coating types and coating thicknesses have been compared in the past using the ASTM B-117 Salt Fog test; however, this test does not directly correlate to corrosion seen in real world environments. For instance, the ASTM B-117 test corrodes zinc at a much higher rate, as compared to the steel. Because of these drawbacks, a number of accelerated corrosion tests that directly correlate to weathering seen in real world environments have been devised by GM, SAE, and others.
Since laboratory corrosion studies to date do not directly correlate to real world corrosion, a direct comparison of the corrosion resistance and life cycle of these two galvanization processes through either laboratory or field testing is not available in the literature. This lack of information provides the potential for lower life cycle/higher costs to agencies if one process provides less corrosion resistance than the other. Accelerated laboratory testing will yield preliminary guidance on the corrosion resistance and life cycle of guardrail elements prepared by the two galvanization processes, which guidance may be verified in the future by field testing.
The objective of this research is to determine and compare the corrosion resistance of guardrail materials prepared by continuous and hot dip galvanization using accelerated corrosion testing.
Accomplishment of the project objective will require the following tasks.
(1). Prepare a work plan for a laboratory experiment to determine and compare the corrosion resistance of guardrail materials prepared by continuous and hot dip galvanization. The plan shall be approved by NCHRP before proceeding to Task 2.
In the plan, identify:
(a) The accelerated corrosion test proposed to determine the corrosion rate of guardrail materials that best replicates the real world application of the guardrail and field environment such as wet and dry cycles, end coatings on guardrail, etc. from available ASTM, AASHTO, GM, and SAE standard tests.
(b) Any changes to the proposed standard test method(s) and an explanation of why the changes are necessary.
(c) The estimated time to conduct at least one accelerated test and the total testing program.
(d) The sample size (with necessary replication) and configuration of continuous and hot dip galvanized specimens to yield statistically meaningful results.
(e) The method by which initial and subsequent total galvanize coating thickness will be determined.
(f) The protocol proposed to track corrosion rates through measurements of the penetrations of each zinc or zinc-iron alloy phase and associated charts and visual comparisons.
(g) Proposed source(s) for procurement of test specimens.
(2). Conduct the accelerated corrosion testing in accordance with the approved plan to determine the corrosion rate and thickness of the two galvanization coatings over time. Analyze the results of multiple tests to compare corrosion rate and coating thickness between continuous and hot dip galvanized guardrail materials and determine the statistical significance of observed differences. Compare accelerated corrosion test results with those of specimens that have been conditioned (e.g., in salt and fresh water for 1 month) and clearly state the conditioning method used and the basis for using it.(3). Prepare a final report that documents results, summarizes findings, draws conclusions, and presents proposed guidance to state highway agencies on the corrosion resistance and life cycle of guardrail elements prepared by the two galvanization processes. An appendix to the report shall include electronic files of all test data and results on appropriate media.
STATUS: Proposals have been received in response to the RFP. The panel will select a contractor shortly to perform the work requested.