Hot-dip galvanizing (HDG) is a process for protecting steel from corrosion by completely cleaning the steel member and then immersing it in molten zinc. The molten zinc reacts with the steel surface to form stratified zinc-iron alloy layers which are metallurgically bonded to the steel.
A properly applied HDG coating is highly impact and abrasion resistant with alloy layers harder than the underlying steel and easily stands up to rough handling in the field.
Cost for galvanizing depends on many factors, including volume of work as well as the nature and configuration of the material being coated.
The galvanizer’s cost in setting up and processing a small order of #4 bars which have been cut and bent into a variety of shapes will be significantly higher than an order for multiple truckloads of large diameter strait bars in mill cut lengths, and the price for each type of order will vary accordingly.
Galvanizing costs are typically calculated by the weight of the material being coated and the pricing is generally expressed in a cost per pound.
As a rule of thumb, HDG for rebar should add a 25% to 50% premium to the cost of ‘black’ bar, about the same as epoxy coating. However, any touch-up beyond cut ends will be minimal and there is no need for special equipment for processing or handling at either the plant or jobsite.
In today’s global economy, concrete contractors are liable to encounter a wide spectrum of national and international standards and hot-dip galvanizing is no different.
Internationally, most standards for HDG are based on ISO 1461 with only minor variations. In the US, domestic standards have been issued. ASTM 767 Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement has been issued by the ASTM 1.05 committee on reinforcing standards and is generally viewed as the ‘proper’ domestic standard for galvanizing reinforcing bar in the US, while domestic use embeds and similar concrete accessories are properly galvanized to ASTM 123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products.
What are the significant differences between ISO based standards, ASTM 123, ASTM 767 and ASTM A-1094?
Material galvanized to ISO based standards, ASTM 123 and to ASTM 767, are often indistinguishable even to a trained observer, and the coatings can be identical in metallurgical structure.
There are, however, significant differences between the standards which should not be ignored. Foremost, ASTM 767 requires post treatment of the galvanized bar with a chromate solution.
Secondly, the minimum required coating thicknesses for each of these standards is different, ranging from 50 to 140 microns. ASTM 767 also requires the galvanizer to maintain lot identity to aid in material identification at the job site.
Where ASTM 767 is specified, ASTM 123 or ISO 1461 certified material should not be substituted without prior, written approval from the specifier. Since both ASTM 767 and ASTM 123 meet or exceed the requirements of ISO 1461 any material certifiable to either should also be certifiable under an ISO 1461 requirement.
In 2016, at the request of the International Zinc Association’s (IZA) products innovation task group, a new galvanized rebar specification, ASTM A-1094, was adopted by the ASTM A1.05 committee. This defined a hot-dip galvanized (HDG) reinforcing bar with a thinner coating weight of 1.2 oz/sq. ft. which was distributed evenly along the entire length of the bar. The intent of IZA in bringing this product to the US market was to increase overall zinc usage in this country by reducing the cost of HDG rebar to make it more competitive with epoxy bar.
Testing has shown that to fabricate pre-galvanized rebar without damage it must have a consistently uniform coating of less than 2.3 oz. /Sq. ft. Both our Airwipe Type 2 A-767 (2.0 oz sq. ft.) rebar and the ASTM A-1094 bar (1.2 oz/sg. Ft.) can successfully be fabricated after coating in much the same manner as epoxy bar without the handling damage issues often encountered in working with epoxy coatings. When seeking galvanized rebar to fabricate after coating, please note that South Atlantic’s Air wipe bar can be certified as meeting A1094, but the reverse is not true.
South Atlantic Reinforcing has been successfully providing galvanized rebar to fabricators and manufacturers across the United States for over 20 years, and our team is ready to answer all your galvanizing and rebar questions.
Galvanized reinforcing bar is post-treated with a sodium dichromate solution to ‘seal’ the zinc surface so as to minimize the potential for evolution of hydrogen gas when the zinc comes in contact with the high pH concrete ‘pour.’ The necessity and desirability of chromating and its effect on bond strength have been debated for several years.
Proponents of chromating are concerned that such gassing might adversely affect the ultimate bond strength between the reinforcing and surrounding concrete.
Advocates for removal of this requirement point to recent laboratory tests which indicate it may actually decrease ultimate bond strength, and claim it adds unnecessary environmental impact and costs.
This debate is far from resolved, and even though it is being questioned, chromate treatment is still mandatory under the standard. Contractors, fabricators and galvanizers should not decide to skip it without written approval.
Specifiers who are reluctant to specify chromating or have been asked to consider a waiver on this provision are encouraged to review the recent literature on chromating or contact the American Galvanizers Association or the International Zinc Association for more detailed information on this subject.
The bond between galvanized reinforcing and concrete exceeds that of black steel, eliminating the need for the extended development lengths and lap splices required when using other coated bars. Design codes treat black steel and galvanized steel the same.
Under ASTM 767 reinforcing bar may be ordered in two manners: as stock bar galvanized prior to fabrication or as fabricated shapes sent to a job galvanizer for coating.
Both have their advantages, and where this choice is not dictated by contract documents or specifications either is suitable. ASTM 767 also lists two coating weights options, a Class 1 (5.8 mils) or Class 2 (3.4 mils).
While there are no guidelines within the specification regarding applications for Class 1 or Class 2, the thicker Class 1 coating is typical of pre-fabricated material where coating weights tend to be heavier. A Class 2 coating is typically more ductile and may be preferable for pre-galvanized bars going to fabrication as it is likely to exhibit less cracking or flaking of the coating when bent.
When no designation is specified, a Class 2 coating is acceptable. The Class 2 coating is equivalent to international standards, which generally reference ISO1461 and specify a coating of 85 UM, or 3.35 mils.
Coatings A767 lists two coating weights. While there is no guidance within ASTM767 as to when or where each might be appropriate, it is known that coatings above 100 microns in thickness are significantly less ductile, making Type 2 bar preferable for work intended to be bent after galvanizing.
The heavier Type 1 coating may also have a cost premium as it requires almost double the amount of zinc as a normal galvanized coating. We will be happy to review your RFQ or other documents if you are unsure as to which type is required for your job.
We realize you may still have some questions about coatings and specifications. Don’t worry—we’re dedicated to creating a wonderful customer service experience. Just contact us and we’ll be glad to supply the information you need. It’s one more way that we’ve got you covered.