Another aspect of hot-dip galvanizing is proven durability. Hot-dip galvanized steel has been specified extensively for reinforcing steel because of its unmatched durability in harsh environments. Hot-dip galvanizing remains durable thanks to its abrasion resistance, uniform protection, complete coverage, and bond strength.
A strong metallurgical bond between the steel rebar and zinc coating is created during the galvanizing process. Galvanizing is metallurgically bonded to the steel, meaning when rebar is dropped, kicked, stepped on, or rubbed against existing concrete or other rebar pieces on the job-site, the protective coating will remain tightly adhered. This durable zinc coating will withstand the effects of UV light, temperature extremes, and exposure to rain or snow, thus protecting the rebar both as it waits to be used and while in use. From installation through the use phase, galvanized rebar is abrasion resistant and durable.
A unique characteristic of the hot-dip galvanized coating is the development of metallurgically bonded (~3,600 psi), abrasion resistant intermetallic layers. The photomicrograph below is a cross-section of a galvanized steel coating showing the three intermetallic layers (Gamma, Delta, and Zeta) and top layer of pure zinc (Eta). During the galvanizing process, these layers develop naturally during a metallurgical reaction between the iron in the steel and zinc in the kettle. As the photomicrograph also shows the hardness of each of the layers as a Diamond Pyramid Number (DPN), you can see the three intermetallic layers are harder than the base steel, while the eta layer has ductility which makes damaging the HDG coating very difficult. Hot-dip galvanizings abrasion resistance provides unmatched protection against damage caused by rough handling during transport and erection, as well as in service. Other coatings with lower bond strengths (300-600 psi) can be easily damaged during shipment and construction, weakening their effectiveness, as barrier protection is dependent upon the integrity of the coating.
Another aspect of HDGs durability is its uniform protection. During the metallurgical diffusion reaction in the galvanizing kettle, the galvanized coating grows perpendicular to all surfaces. Therefore, the coating is naturally as thick on corners and edges as flat surfaces. Since coating damage commonly occurs at edges, added protection at these junctures is important. Brush- or spray-applied coatings have a natural tendency to thin at corners and edges leaving the part prone to attack. The uniform protection of hot-dip galvanized steel leaves no weak points for accelerated corrosion.
Hot-dip galvanizing is a total immersion process, meaning the steel is fully submerged into cleaning solutions and the molten zinc coating all interior and exterior surfaces. This complete coverage ensures the entire bar and all design details are coated. Fully coating bars is equally important as they are utilized at connection points which are critical to structural integrity.
Good bonding between reinforcing steel and concrete is essential for the reliable performance of reinforced concrete structures. When protective coatings on steel are used, it is essential to ensure they do not reduce bond strength. Studies on the bonding of galvanized and black steel bars to Portland Cement concrete have been investigated.
The results of these studies indicate:
- Development of the bond between black or galvanized steel and concrete depends upon cure time and environmental factors.
- In some cases, the full bond for galvanized rebar may take longer to form than for uncoated steel, depending on the zincate/cement reaction.
- As reported by Stephen Yeomans in Galvanized Steel Reinforcement in Concrete, there are a number of studies that have concluded the fully developed bond strength of galvanized rebar has no significant difference from black rebar bond strength.
- A study by C. Andrade in Spain monitored bond strength of galvanized rebar samples over 10 years immersed in seawater and found no deleterious effects on bond strength over that time.
Durability Case Study
Jesup Bridge- Jesup County, IA, 2013
The Jesup Bridge in Buchanan County, Iowa, has been a vital market link for farmers, growers and the community as a whole since it was constructed in the late 1940s. According to the Iowa Department of Transportation, an average of 4,360 vehicles cross this bridge every single day between rural Jesup, Iowa, and Highway 20, which leads to the city. This might seem like small potatoes (pun intended) to a big city municipality, but for this rural county bridge, this is heavy traffic. The loss of this bridge could effectively isolate the residents of this community and cause severe economic duress, and thats exactly what happened. The 22-foot-wide concrete bridge became functionally obsolete, too narrow to handle two-way traffic.
Like any municipal project, funding a new bridge is a major undertaking. Estimated at nearly $250,000, finding the money would require some creativity. With a sufficiency rating below 50, something needed to be done quickly and federal funding would take too long. Fortunately for Buchanan County, Keierleber is South Central Vice President of the National Association of County Engineers (NACE) and sits on several other industry committees, which exposes him to the latest construction ideas and product developments. It was here that he saw an opportunity to design and help fund an innovative Jesup Bridge. Through his involvement in NACE, Keierleber learned about and joined the Short Span Steel Bridge Alliance (SSSBA). The SSSBA worked with two college professors to develop a new software program, eSPAN140 (www.espan140.com), a free, web-based tool utilizing standard designs and details for short-span steel bridges and/or buried soil structures up to 140 feet long.
Galvanized steel beams and rebar are key components in this design. It all goes back to economics, Keierleber says. It will dictate everything we do. It goes back to cost comparisons and being worried about cost savings in the long run instead of the short term. Beyond research, this project is an enhancement to the community because it removes a barrier to commerce and mobility for county residents. Typically contractors are hired for this job, but Keierleber wanted his crews to be intimately familiar with the construction of this bridge, so it also provided jobs and income for local county crews.