NOT ALL GALVANIZED REBAR IS THE SAME. Hot-dip galvanizing (HDG) produces a galvanized coating typically three times (or more) as thick as Continuously Galvanized Rebar (CGR), a sole-sourced product developed in China in 2011, and brought to the American market in 2018. The thicker the coating, the longer it provides corrosion protection.
To-scale micrographs showing comparative thickness of galvanized coatings. Source: Polytechnique Montreal and images courtesy of Corbec
Recent independent scientific testing by LPI, Inc., consulting engineers, and the Centre for Characterization and Microscopy at Polytechnique Montréal documented the dramatic difference in galvanized coating thickness between HDG and CGR.
Marketing claims vs. reality about CGR
LIFESPAN
MARKETING CLAIM: CGR’s lifespan is the same as hot-dip galvanized rebar.
REALITY: CGR is produced using a continuous galvanizing process that yields a minimum coating thickness of approximately 2 mils (50 microns). By contrast, hot-dip galvanized offers a minimum coating thickness of 5.9 mils (150 microns), and typical thicknesses of 8 to 9 mils. According the American Galvanizing Association, the thicker the coating, the longer it provides corrosion protection, with a relationship between thickness and service life that is approximately linear.[1] Thus, once the zinc surface begins to dissolve from chloride exposure, the service life of hot-dip galvanized rebar can be expected to be three times or more that of CGR.
[1] American Galvanizers Association, “Hot-Dip Galvanized Reinforcing Steel; A Specifier’s Guide;” International Zinc Association, “Galvanized Rebar: It Works”
HISTORY and USE
MARKETING CLAIM: CGR is “widely used” in the US and Canada.
REALITY: First placed into service in China in 2011 and brought to the American market in 2018,[I] CGR has virtually no extended track record in the field, certainly not in any high chloride environment for any meaningful period of time. Any reference to longevity is piggybacked onto the proven history of HDG as it has no history of its own. By contrast, hot-dip galvanized rebar has been in successful use for some seven decades. Hot-dip galvanized bar’s performance has been repeatedly studied decades after installation and shown to be superior to black and epoxy coated rebar.[ii]
[i]AZZ, Inc., Securities & Exchange Commission Form 8-K filing for the period ending 7/3/2018,
[ii]American Galvanizers Association, “Hot-Dip Galvanized Reinforcing Steel; A Specifier’s Guide;” International Zinc Association, “Galvanized Rebar: It Works”
VALUE
MARKETING CLAIM: CGR is the most economical choice.
REALITY: Hot-dip galvanized rebar’s documented superior performance to, and minimal price difference from, epoxy coated rebar establish it as the proven choice for best long-term value. CGR offers only negligible savings on upfront purchase cost versus hot-dip galvanized rebar. While some studies have estimated CGR's longevity by extrapolating accelerated tests, its long-term performance, in-situ, is unproven. However, its thin coating (one third the thickness of ASTM A767, Class1), would indicate CGR's longevity to be much less.
Photo credit: New York State Thruway Authority