Galvanized rebar is making a comeback. 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 that easily stands up to rough handling in the field. Other than cut ends, it is extremely rare to have to perform touch up on galvanized coatings.
Is HDG rebar a new product?
Galvanized reinforcing has been used for many years and is the standard method of protecting reinforcing steel in many countries. Australia, where galvanizing is widely used, chose it 50 years ago to construct the sails of the Sydney Opera House, one of the 20th Century’s most famous buildings. These sails are still in excellent shape today while nearby construction from the same period using uncoated steel reinforcement has suffered extensive damage.
In the U.S., galvanized rebar fell into disfavor when 3M Corporation introduced its new epoxy coating in 1972. Armed with accelerated modeling data promising decades of protection, epoxy quickly became the dominant corrosion-resistant coating for reinforcing steel. With the acceptance of accelerated testing as the norm for product evaluation, galvanized reinforcing became a marginal player.
Fortunately for the galvanizing industry, there were some agencies that continued using HDG, and today there are numerous structures cast with galvanized rebar as far back as the late 1960s, including several bridges that still undergo periodic testing and are in excellent condition today. The continued success of these structures contradicts projections from the accelerated testing models and is in large part responsible for today’s renewed interest in hot dip galvanized rebar. Two large projects currently underway using HDG rebar are the re-decking of the Tappan Zee Bridge in N.Y. and Pier 5 at the Norfolk Navy Shipyard in Va. Combined, these two projects will use 12,000 tons of galvanized rebar.
What’s the cost of HDG of reinforcing bar?
Cost for galvanizing depends on many factors, including the volume of work and the nature and configuration of the material being coated. The galvanizer’s cost/pound for setting up and processing a small order of #4 bars cut and bent into a variety of shapes will be significantly higher than a large order of #9 bars in straight 50-foot lengths. Costs are typically calculated by the weight of the material being coated and the price is generally expressed as a cost/pound, which can vary greatly based on the criteria noted above. In general, the cost of protecting rebar with HDG should add a 25% to 50% premium to the cost of “black” bar.
How does the bond strength of galvanized rebar compare with other coated bars?
Testing has shown that 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 rebar coatings, such as epoxy. Design codes treat black steel and galvanized steel the same. The elimination of extra steel for the extended length requirements is one of HDG rebar’s advantages.
What HDG rebar lengths are available?
The maximum length of galvanized rebar available in any region is highly dependent on the capacity of local galvanizers and the size of the kettle and clearances in their plants. In most markets 40-foot or shorter bar is readily obtainable, but local availability of longer lengths should be determined before making design decisions.
Does galvanized rebar need to be cut and bent before or after coating?
When properly applied in the correct thickness, galvanized coatings are very ductile and easily bent. ASTM A767, the standard for galvanized rebar produced in the U.S., gives guidance for handling rebar in this manner. If you are purchasing bar that is intended for fabrication after galvanizing this should be discussed with the galvanizer and it is best to specify ASTM A767 Type 2 bars to avoid problems with excessive cracking and flaking. This bar can be handled in the shop in the same manner as black bar without the need for special equipment. Cut ends need only be touched up using zinc-rich spray paint to meet the ASTM A767 requirements. Bars with heavier coatings may also be bent, but touch up may be required in the bend area if flaking occurs.
ASTM A767 requires the use of larger pin diameters when bending black bars before galvanizing, for example, a 6d pin for grade 60 bars #3 through #6 and an 8d pin for #7 through #12 bars.
Are there special storage requirements for galvanized steel?
Galvanized material is 100% UV stable and can be stored outside with no damage from exposure to the sun or weather. It does not need to be covered. Bar should, however, be stored off the ground on wood dunnage and normal precautions should be taken during handling to avoid snagging and bending individual bars.
What’s the difference between galvanized weight and black weight?
The zinc coating on a galvanized piece of steel will add 5% to 7% to the original weight, although extremely heavy or light shapes may vary from this range. This ‘after-galvanizing’ weight is multiplied by the quoted amount/pound to determine the final invoice because the configuration of the steel that is being galvanized and its metallurgic makeup can lead to heavier coatings, which are beyond the galvanizer’s capability to control. In job-shop batch galvanizing, each lift of material is weighed as it leaves the production floor with these weights being totaled for customer billing. For highly repetitive shapes and consistent material, though, many galvanizers will quote a fixed, theoretical pickup rate to facilitate fixed per-piece billing. The after-galvanizing weight must also be considered when scheduling transportation of galvanized material to avoid over-loading of trucks.
What do I need to know about installing HDG reinforcing?
Galvanized rebar can be installed in the same manner as uncoated reinforcing. And there are no real limits on how far in advance of a pour it can be put in place nor is there a need for elaborate placing schemes to minimize damage to already installed bars. Any touch-up is minimal and usually caught before a bar is installed.
Galvanized tie wire or plastic clips should be used when assembling or installing galvanized bar, and bar supports also should be galvanized steel, plastic, or some other inert material such as masonry. If mechanical couplers are being used, they should be galvanized as well.
Can unrepaired areas of damage cause accelerated corrosion?
Zinc is naturally cathodic in the presence of steel, and the zinc surrounding any exposed steel will automatically sacrifice itself to prevent the corrosion of base metal steel and quickly act to re-passivate the exposed area under attack. For small voids of limited size, the coating essentially re-seals itself. Even for larger voids, the surrounding zinc continues to significantly impede corrosion of the base steel and may also lead to re-passivation of the area. Because the galvanized coating is metallurgically bonded to the steel, under no circumstances can moisture travel under the coating to create an accelerated corrosion cell.
How do I touch up cut ends and any damaged areas of the galvanized coating?
ASTM A780 outlines several methods to repair galvanized coatings. The most widely used is zinc-rich paint, sold in either brush-on or spray form. Paints should have dry-film zinc content greater than 92%; paints meeting this criterion are widely available at industrial supply and paint centers.
Can I tie galvanized bar and black bar together?
When black and galvanized reinforcement are used together in concrete, a bimetallic couple may occur between the zinc and the black steel in much the same manner as the zinc will couple with the exposed steel surface of a coating void on the galvanized bar (although the larger anodic area of a black bar mat would consume zinc at a much higher rate than a simple void). In concrete, corrosive reactions would not be expected to occur between black and galvanized bars so long as the two metals remain passive. To ensure this is the case, the concrete cover over non-galvanized steel should not be less than the cover required to protect black steel alone under similar conditions.
If there is a need to prevent the zinc from being consumed in defense of other metals, polyethylene and dielectric tape can be used to provide insulation between the two metals.
Can HDG reinforcing be welded?
Galvanized bars may be welded in the field with the A/E’s authorization if the base steel meets ASTM A706. Welding of galvanized reinforcing steel must conform to American Welding Society AWC D1.9 and AWS ZWC, which require welds be made on steel that is free of zinc to prevent strength reduction from zinc inclusion in the weld. The zinc coating needs to be ground off at least one inch from either side of the intended weld zone and on all sides of the steel part. Once the weld is completed, the coating in the area of the weld can be repaired using zinc-rich paint, metalizing, or hot-melt zinc.
Fumes from welding galvanized steel can contain noxious substances, so proper ventilation that minimizes worker exposure to fume is essential. Specific precautions are found in ANSI Z-49.1.
What are the different standards for HDG reinforcing?
The various standards that may apply to HDG rebar are shown in the box on page 64, HDG Rebar Standards, although ASTM A767 is the most common material specification in the U.S. Material galvanized to any of these standards might well be indistinguishable even to a trained observer, and the coatings might even be identical in metallurgical structure, but there are significant differences in the coating thickness. Also, ASTM A767 requires post treatment of the galvanized bar with a chromate solution whereas ASTM A123, along with the Canadian and ISO standards do not.
Where ASTM A767 is specified, ASTM A123, CAN/CSA G164, or ISO 1461 certified material should not be substituted without prior, written approval from the specifier.
Why does ASTM A767 specify chromate and why is it needed?
ASTM A767 requires galvanized rebar to be treated with a sodium dichromate solution to “seal” the zinc surface in order to minimize the evolution of hydrogen gas when the zinc comes in contact with high pH concrete. The necessity and desirability of chromating and its effect on bond strength have been debated for years. This debate remains unresolved, but currently chromate treatment is still mandatory under A767. Contractors, fabricators, and galvanizers should not decide to skip this step without written approval.
Carl Maki is manager for reinforcing products with South Atlantic Reinforcing, Wilmington, N.C.