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Current Research

Updated October 2022

The AGA currently oversees or is a financial contributing partner to the following research efforts:

The AGA provides technical support or acts as a technical liaison/oversight committee member to the following research efforts:

HDG of High Strength/A490 Bolts

Allowing HDG A490s within ASTM F3125 standard would give designers an additional, cost-effective and maintenance-free solution to producing a higher strength bolted connection infrastructure instead of the current proprietary liquid coating solutions applied to A490 bolts.

Historically, HDG has not been allowed for A490 bolts in North American standards due to (a) concerns of hydrogen embrittlement susceptibility, (b) minor concerns over HDG fastener lubrication/testing, and (c) the high loads associated when using A490s. Research work completed in the late 70s and early 80s showed failure rates of up to 25% for hot-dip galvanized A490 bolts. Meanwhile, an equivalent standard for A490 bolts is successfully galvanized in UK, Japan, Italy, France, South Africa, and Germany. It is understood blast cleaning and/or limited pickling times are involved.

More recent independent testing has found Grade A490 bolts that have been mechanically cleaned prior to dipping are not susceptible to hydrogen embrittlement (Langill, T., Hydrogen Embrittlement Study of Hot-Dip Galvanized High-Strength Bolts, Fastener Technology International, April, 2013). This small test program evaluated hydrogen embrittlement susceptibility using ASTM F606 wedge tests held for a 48-hour period and ASTM F1624 incremental loading techniques. However, this evidence was not compelling enough to justify a change to the ASTM F3125 standard. At the suggestion of the ASTM Subcommittee on High Strength Bolts, F16.02, additional tests were conducted on bolts that have been chemically cleaned using hydrochloric acid or sulfuric acid as this is the standard practice for cleaning steel parts before hot-dip galvanizing.

The objective for this project is to test high-strength bolts meeting the chemical and
mechanical requirements of ASTM F3125 Grade A490 that have been hot-dip galvanized in accordance with IFI-144, Test Evaluation Procedures for Coating Qualification Intended for Use on High-Strength Structural Bolts. The bolts shall be cleaned before fluxing using five different cleaning methods:

  1. pickling in hydrochloric acid with an inhibitor in the acid bath
  2. pickling in hydrochloric acid without an inhibitor in the acid bath
  3. pickling in sulfuric acid with an inhibitor in the acid bath
  4. pickling in sulfuric acid without an inhibitor in the acid bath
  5. mechanical cleaning; i.e., no pickling acid

For galvanized A490 bolts to be practically specified requires

  • Adoption by ASTM F3125 specification
  • Adoption by RCSC high-strength bolt specification
  • A490 bolt producers willing to HDG their products (manufacturer approval is required)

What the HDG industry and AGA members seek to learn from this study

Can today’s F3125 Grade A490 bolts be HDG with proper bolt material and galvanizing practices (pickling or mechanical cleaning)?

Project Status

Through a variety of tests performed with support from ASTM and RCSC in 2014 and 2017, Tom Langill of AGA demonstrated today’s A490s can be HDG with proper bolt material and galvanizing practices (pickling or mechanical cleaning).

The 2013 and 2017 research efforts satisfied the ASTM committee F16 qualification requirements for Grade A490 bolt coatings. Efforts to pass a ballot item introducing the associated change to the ASTM F3125 standard are ongoing.

RCSC will publish the final report (2017 RCSC funded report) on their website sometime in 2022 with a short cover letter. A formal white paper by RCSC will be developed at a later date to describe the research history and a list of additional research requested to address tangential industry concerns.

Back to Research

Slip of Metallic Coatings

The Research Council on Structural Connections (RCSC) approved a research program on pretension loss and long-term creep properties of HDG, metallized, and HDG + metallizing connections to address industry concerns regarding the performance of HDG in these connections. AGA participates in the project oversight committee.

Between 2012 and 2014, RCSC and the American Institute of Steel Construction (AISC) co-funded research performed at the University of Texas at Austin (UT-Austin) on slip classification of hot-dip galvanized coatings. This research investigated the influence of several variables that can affect the slip coefficients of galvanized coatings, and determined the lower bound slip coefficient under short term compression loading following the RCSC Specifications for Structural Joints using High Strength Bolts (RCSC Specifications), Appendix A (RCSC Appendix A), Section A3. The research, however, did not perform the 1000 hour tension creep test as outlined in Section A4 of the Specifications, required to determine the long-term slip performance of galvanized coatings and fully define their slip coefficient. Instead, the loss of bolt preload or relaxation of bolt load, was investigated by a 1000 hour test.

Accordingly there is a need to verify the findings of the UT-Austin study by performing the slip tests in full compliance with the RCSC Appendix A. The UT-Austin study identified the steel chemistry, particularly the steel reactivity or silicon content as the most significant factor affecting the slip performance of the coating, including slip coefficient and the loss of clamping force. The roughening of the galvanized surface did not have any significant effect on the slip coefficient. The study, however, was inconclusive regarding the effect of coating thickness on the loss of bolt pretension, particularly with respect to the variation of coating composition and reactivity of steel on creep deformation of the coating.

Moreover, there has been a growing trend of using thermal spray coatings (TSC) or metalizing in lieu of galvanizing; however, there has not been a modern comprehensive study on their slip performance, particularly for the thicknesses commonly employed and the application requirements set forth in the Society for Protective Coatings (SSPC) specifications.

Additional testing is needed for reliable slip classification of metallic coatings (both galvanized and metalized) and for strengthening the RCSC Specifications.

Fundamental Project Goals and Objectives

  1. Determine if the findings of UT-Austin short term slip tests of galvanized coatings (without the roughening effect) remain valid when evaluated with the long term 1000 hour creep test specified in RCSC Appendix A
  2. Determine the slip classification of thermal sprayed coatings of zinc, aluminum and 85% zinc+15% aluminum coating compositions considering both short- and long-term testing specified in RSCS Appendix A
  3. Determine the relationship between metallic coating (both galvanizing and metallizing) thickness and relaxations of bolt pretension.

Additional objectives of this research are:

  • Differentiate creep performance based on variations in thickness and composition of galvanized coatings, as occurs in response to silicon content or reactivity of steel
  • Determine the effect of multiple galvanized plies on the creep.
  • Determine the effect of application-dependent porosity on the slip classification of thermal sprayed zinc, aluminum and 85/15 zinc-aluminum coatings.
  • Determine the role of sealers in modifying the slip classification of thermal spray coatings
  • Determine the slip classification of mixed faying surfaces comprised of galvanized and thermal spray coatings considering both short and long term testing specified in RCSC Appendix A

Design Variables tested for the HDG coating include:

  • Steel Chemistry: reactive vs. non-reactive steels
  • Coating Thickness: 7 vs. 13 mils. Existing research presents conflicting results whether coating thickness has a significant effect on the slip performance or bolt relaxation.
  • Multiple Plies and Bolt Grip Length: Existing research shows longer bolt grip reduces relaxation of bolt preload. Typical splices involving multiple plies consist of 4 to 6 in. material.
  • Aging of Galvanized Coating: Galvanized coatings tend to harden with age beneficially influencing slip and creep performance. It is, however, not uncommon in the industry to put components in service soon after galvanizing, sometimes within a few hours that adversely affects the coating performance.
  • Bolt Type: Existing research concluded the use of galvanized fasteners may contribute to the creep of galvanized connections and relaxation of bolt preload.
  • External Loading: Existing research noted a significant drop in bolt preload during application of external shear loading that was attributed to Poisson effect in addition to that by coating creep.

What the HDG industry and AGA members seek to learn from this study are

  • Adoption of an established slip coefficient for metallized-to-galvanized connections.
  • Clarification or removal/adjustment of 20% reduction in allowable clamping force (to be published in AASHTO LRFD Bridge Design Specifications 10th edition) developed to address concerns related to coating thickness in galvanized connections (HDG coating thickness on faying surface and/or fastener coating thickness).

Project Status

Alana Fossa is a member of the project oversight committee. Test setup and is fully complete and they are available to begin slip & tension creep testing using the final samples. A draft report is scheduled for late 2023.

Back to Research

Blasting Before HDG Impact on Coating Thickness

Blast cleaning before galvanizing is often specified to roughen the steel surface and promote thicker galvanized coatings for low-silicon or Aluminum-killed steels, whereas blast cleaning before galvanizing is also a method used to limit coating thickness for reactive steel chemistries. The difference in outcome is due to the initial steel chemistry. The reason for this contradiction and the amount of reduction/increase possible is not fully determined, but galvanizers and specifiers continue to seek information that can help them determine whether blasting before galvanizing is a worthwhile additional cost for certain projects.

To determine the realistic effects of blasting before hot-dip galvanizing, all blasted (experiment) samples and non-blasted (control) samples of different compositions will be galvanized on the same rack to determine a quantifiable impact on coating thickness per the following Test Matrix:

Material Type

Blasted,
5 min dip

Unblasted,
5 min dip

Blasted,
10 min dip

Unblasted,
10 min dip

Si-killed reactive plate (A36/CSA G40)

3 plates

3 plates

3 plates

3 plates

Al-killed plate (A36/CSA G40)

3 plates

3 plates

3 plates

3 plates

Reactive building beam piece (A992/A572 Gr50)

1 beam (blast half)

1 beam (blast half)

Reactive bridge beam (A709 Grade 50)

1 beam (blast half)

1 beam (blast half)

Weathering bridge beam (A709 Grade 50W)

1 beam (blast half)

1 beam (blast half)

Reactive sign structures (A499)

1 rail

1 rail

1 rail

1 rail

What the HDG industry and AGA members seek to learn from this study

  1. Perform a literature review: gather all existing research on this topic
  2. Demonstrate the outcome of blasting before HDG depends on steel chemistry and quantify the effect
  3. Define practical examples where blasting before HDG may be more cost effective up-front than dealing with potential quality issues due to thick, brittle HDG coatings (more frequently observed when HDG coatings are allowed to reach excessive coating thickness such as above 8-10 mils)

Project Status

Testing was completed at Valmont Oklahoma in September 2021 and initial results were presented at TechForum 2021 (October). A final report is under development.

Back to Research

Pickling Bath Maintenance Curves

The objective of this project is to determine how the pickling rate is influenced by the amount of zinc in the acid. From the data, the following curves will be generated: hydrochloric acid and iron (Kleingarn), sulfuric acid and iron, hydrochloric acid and zinc, sulfuric acid and zinc, hydrochloric acid with zinc and iron, sulfuric acid with zinc and iron.

The research will involve determining the pickling rate when steel with mill scale/rust and also zinc is immersed in pickling acids (hydrochloric and sulfuric). The testing will be conducted in two phases. The first phase of testing is to determine the effects of iron on the pickling rate and will verify previously published information on the amount of iron that can be added before the pickling solution is too slow to be practical. For hydrochloric acid, this should produce results similar to the Kleingarn curve. For sulfuric acid, this phase of testing will help to generate a similar curve. The results will determine how much iron can be added to the acids before the pickling is ineffective.

The second phase of testing will investigate the effects of zinc on the pickling rate. A gradual increase of zinc will be added to the acid until pickling slows or is ineffective in removing the scale from the steel. Using the information from both phases can determine how much of a factor the zinc has on the pickling rate. For repeatability, multiple samples per condition will be tested.

What the HDG industry and AGA members seek to learn from this study

  • Develop pickling curve for Sulfuric acid similar to the Kleingarn Curve (iron vs. acid concentration)
  • Investigate impact of zinc concentration on existing pickling curves to determine when maintenance needs to be performed based on the iron and zinc concentrations in the pickling acid.
  • Update to AGA's Pickling Maintenance Calculator Tools

Project Status

Testing complete and awaiting final report from KU expected 2022. An expansion of the study to investigate the impacts of temperature on the Kleingarn curve (40-90 F) and the Burnett curve is being investigated.

Back to Research

AISC Durability of Steel Bridge Corrosion Protection Systems

Part one of this study analyzed the historical performance of bridge coatings: 3-part paint systems, ZRP, weathering steel, HDG, and metallizing. HDG historically outperformed the other coatings. Part two performed an accelerated corrosion test modified to allow a patina to form on HDG and Uncoated Weathering Steel. HDG outperformed in this test as well.

A final report from U. Delaware is available. AISC will publish their own take on the report.

Project Summary From AISC:

This project, in collaboration with the National Steel Bridge Alliance (NSBA), has recently commenced. It aims at addressing the need of bridge designers and owners to have well-informed, quantifiable data on expected service lives of typical modern steel corrosion protection used for steel bridges. Ultimately, a Design Guide will be published that documents the predicted service life of the most popular steel corrosion protection systems in different environments.

This research consists of both analytical and technical components. Research tasks include:

  • Compiling data on the performance of different steel protection systems in different environments and then identifying and prioritizing corrosion protection systems of interest,
  • Developing realistic accelerated corrosion tests and then performing the corrosion tests,
  • Analyzing the test data to quantitatively compare the performance and estimate the longevity of different corrosion protection systems in different environments.

Ultimately, the end goal of this work is to help get more owners to choose steel bridge systems by showing real and meaningful data for different corrosion protection systems. This work is vitally important for steel bridges to get a larger market presence as much of the nation’s aging infrastructure will need to be replaced in the coming years.

Back to Research

NCHRP Program 12-117 Duplex Systems for Bridges

See Project Overview from NCHRP.

Project Status

The final report will head to AASHTO for approval in Oct 2022. If it passes without negatives, final publishing is expected early 2023. NSBA will likely use this study to develop a Duplex Systems specification which will involve AGA.