Factors affecting reinforcing steel performance in concrete

April 24, 2018

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All photos courtesy American Galvanizers Association

By Laura Hanson
Concrete is used in construction projects all over the world, from roads and bridges to structural buildings. While itself a strong building material, concrete is often reinforced to increase that strength. There are various options available for this application, with the most popular being reinforcing steel bars, or rebar.

As steel tends to corrode over its life cycle, it is important to understand the factors affecting rebar performance in concrete and the methods available to protect steel from corrosion, elongating its useful life. The long lifespan of steel provides many benefits, including safety from crumbling and spalling concrete, extended durability of concrete projects and structures, and lower life cycle costs through reduced maintenance.

Influences on rebar performance
The main factors affecting reinforcing steel performance include the chemical composition of the concrete, chloride concentration, bond strength between the rebar and the concrete, environmental factors, and—most importantly—type of bar used. There are a variety of reinforcing steel options, with the most commonly used being galvanized, epoxy-coated, or stainless steel. While each type has its benefits, it is important to weigh which is better suited to the specific job. In addition to the numerous advantages each type of rebar may provide, it is also essential to understand the factors affecting their performance in concrete.

Concrete composition
Many types of concrete are used during construction, each with its own chemical composition. Variances in concrete involve the water-to-cement ratio (w/c) and aggregates such as sand, gravel, and stone. (For more, see “How Concrete is Made” at www.cement.org[2].) Use of various concrete types in construction has made the material’s chemical, physical, and mechanical properties, along with its relationship to metals, a topic of ongoing study. (Information is available via “In Concrete” at www.galvanizeit.org[3].)

As concrete is an alkaline solution, pH levels play an important role in depassivation, or breakdown of the metal reinforcing bar and the corrosion-protection coating. While black (i.e. uncoated) steel in concrete typically depassivates above a pH of 11.5, galvanized reinforcement can remain passivated up to a higher pH—usually around 12.5—thereby offering substantial protection against the effects of concrete carbonation. (Further reading can be done via Stephen R. Yeomans’ “Galvanized Steel in Concrete: An Overview,” published in Yeomans’ Galvanized Steel Reinforcement in Concrete.) To this author’s knowledge, the effect of pH level on stainless and epoxy-coated steel has not been extensively studied.

Typically, denser, less-porous concrete and adequate concrete thickness or more concrete cover create stronger corrosion protection for the rebar. The type of concrete used plays a role in the performance of the reinforcing steel and contributes to the chloride concentration within the mixture.

A30 EXPRESS HIGHWAY
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For the A30 Express Highway outside of Montreal, more than 25,000 t (27,558 ton) of galvanized steel were used.

The structures comprising the A30 Express Highway outside Montréal include a mixture of steel and traditional precast concrete construction. In total, more than 25,000 t (27,558 ton) of galvanized steel were used in the project over a four-year construction span. For this extremely intensive highway project, galvanizing was employed for everything from the steel structures, expansion joints, guard rails, bridge rails, lighting poles, signage structures, and sound barriers to the thousands of tonnes of reinforcing steel. By allowing travellers to bypass heavy traffic congestion, it is anticipated this highway will reduce greenhouse gas (GHG) emissions in the Greater Montréal Area.

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Figure 1: This graph compares black and galvanized reinforcing steel’s bond strength to concrete.

Chloride content
While total chloride content is an important aspect in the performance of reinforcing steel, the chloride content in the concrete mixture itself is only a very small fraction of the overall total. Given concrete is porous, it is susceptible to water, oxygen, chloride ions, and carbon dioxide. (This information is derived from J.P. Broomfield’s “Galvanized Steel Reinforcement in Concrete: A Consultant’s Perspective,” published in the Yeomans text mentioned in note 3.) When these elements seep into the concrete, they attack the reinforcing steel below, causing corrosion and eventually leading to failure.

Once chlorides from de-icing salts or marine environments begin to attack the steel, the corrosion threshold for chloride metal attack factors into the rebar’s performance. The corrosion protection afforded by galvanized rebar in concrete is due to a combination of beneficial effects, including the sacrificial zinc coating. Of primary importance is the substantially higher chloride threshold for zinc coatings to start corroding (two to four times that of uncoated steel). (See the International Zinc Association’s “Performance of Galvanized Rebar in Concrete” at www.galvanizedrebar.com[6].)

Bond strength
Bond strength between the rebar and the concrete significantly affects the performance of reinforcing steel. The structure’s overall strength highly depends on this bond strength, so it is important to use a bar or coating that does not reduce it. Affected by the bar’s physical structure, depth of concrete cover, and any potential coatings on the steel, bond strength can be improved with bars that are ribbed or deformed, as this provides more bearing surface in contact with the concrete. Coatings on bars produce varying bond strength results, with galvanized bars typically performing better than epoxy or uncoated bars. (For more, consult O. Kayali’s “Bond of Steel in Concrete and the Effect of Galvanizing,” published in Yeomans’ text.) This can be partially attributed to the chemical bond formed between the zinc layer on the galvanized steel bar and the concrete.

Concrete cure time and environmental factors also affect bond strength; in some cases, the bond between the type of bar and the cement may take varying amounts of time to form. Three studies compared the bond strength of hot-dip galvanized rebar compared to black rebar: “Zinc-coated Reinforcement for Concrete,” “The Influence of Steel Galvanization on Rebar’s Behavior in Concrete,” and “Bond of Ribbed Galvanized Reinforcing Steel in Concrete.” (All three of these studies were referenced in O. Kayali and S.R. Yeomans’ “Bond of Ribbed Galvanized Reinforcing Steel in Concrete,” published in the Journal of Cement and Concrete Composites in 2000.) In each of these studies, hot-dip galvanized rebar had higher bond strength to concrete compared to black rebar. Figure 1 shows the various bond strengths observed for hot-dip galvanized rebar and black rebar in different studies.

Further, studies from the American Society of Civil Engineers (ASCE), the International Journal of Cement Composites and Lightweight Concrete, and the Proceedings of the Institution of Civil Engineers–Structures and Buildings found epoxy-coated rebar had a reduction of 20 to 50 per cent in bond strength to concrete when compared to black rebar. (These studies are R.G. Mathev and J.R. Clifton’s 1976 “Bond of Coated Bars in Concrete,” K. Kobayashi and K. Takewaka’s 1984 “Experimental Studies on Epoxy-coated Reinforcing Steel for Concrete Protection,” published in International Journal of Cement Composites and Lightweight Concrete, and J. Cairns’ 1996 “Performance of Epoxy-coated Reinforcement at the Serviceability Limit State.”)

Type of rebar
Environmental factors ultimately decide the overall performance of a project. As concrete is porous, the steel within is still subject to anything permeating it, despite the fact the steel is not directly exposed to the atmosphere. In areas where road salts are used during harsh, icy winters, the reinforcing steel is subject to opportunities for corrosion due to the chlorides from the road salts. Additionally, in tropical marine areas such as Bermuda, reinforcing bars are highly susceptible to chlorides, which—as mentioned—can be detrimental to their performance.

Considering these factors, it is important to protect steel embedded in concrete from corrosion. With the increasing importance of sustainable design partnered with economic efficiency, specifying projects with 100-year lifespans is essential. Studies have shown while epoxy-coated reinforcing steel may be an economical choice, it does not last as long as other options due to many factors. Stainless steel comes at a high initial cost, yet can withstand corrosion for 100 years according to accelerated testing by the Michigan Department of Transportation. (Visit www.michigan.gov/documents/mdot/MDOT_Research_Report_RC-1560_407022_7.pdf[7].) In addition, epoxy-coated rebar requires special handling at the jobsite to ensure the coating is not damaged. Other bar types, including galvanized, stainless, and uncoated, do not require special handling and are resistant to abrasion incurred at the jobsite.

Epoxy-coated rebar comes at a low cost and performs well in roads and structures not subject to extreme weather and de-icing salts, or those located in dry, non-marine environments. While epoxy is the most widely used form of corrosion protection for reinforcing steel, it has a shorter lifespan than its alternatives.

Galvanized reinforcing steel is an extremely durable option with a middle price range, and has great performance records in a variety of concrete projects. In both galvanized and stainless steel reinforcement options, the upfront price ends up being the price of the life of the project. When working on public projects, selecting the more economical option is usually easier to justify.

 STONEHAM ARCH BRIDGE HIGHWAY 73
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The unique Stoneham Arch Bridge on Quebec’s Highway 73 used galvanizing for the durability and low cost in confers.

This suspended arch bridge in Stoneham, Qué., is an esthetically pleasing solution to a unique site requirement. The arched bridge design, unusual for a highway overpass, was chosen primarily to overcome two problems: uneven, rocky field conditions and the obtuse angle at which the road passes over the highway (a configuration that does not fit well with typical overpass construction).

This type of bridge, combining steel and concrete, is not often used, and its unique design is very interesting both architecturally and structurally. Galvanizing was employed in all aspects of the bridge, including the structural steel, reinforcing steel in the concrete, and the supporting cables. The choice of hot-dip galvanizing was preferred by the engineer because of its durability and cost compared to weathering steel in this highly corrosive environment. The resulting bridge is a beautiful and cost-effective marriage between structural steel and reinforced concrete.

Conclusion
This article discusses aspects affecting performance of rebar in concrete in broad terms, but the wide variety of factors combine to make it hard to specifically predict which application is ideal for certain types of rebar. The conditions to which the reinforcement is subjected will affect its performance. Each type of rebar performs differently depending on the environment and the corrosion protection benefits each type provides. (With respect to how each type of bar resists corrosion, this author recently wrote a piece for the August 2017 issue of The Construction Specifier, entitled “Embedded in Concrete: Reinforcing Steel Corrosion Protection[9].” Additional background information can be found in the June 2016 Structure article, “Steel Rebar Coatings for Concrete Structures[10],” by Fujian Tang, PhD.)

Protecting reinforcing steel from corrosion is important for many reasons. From avoiding unsafe roads and bridges where concrete has begun to fail to practising sustainable and economic building practices, it is easy to see how important it is to choose the right material to stand up to corrosion. While the factors affecting corrosion vary depending on the type of rebar used and the environment in which the project is located, the choice to use a high-performing, economic bar is essential.

Laura Hanson is the digital marketing manager for the American Galvanizers Association (AGA), where she has worked for more than seven years. She leads the Rebar Focus Group for AGA members while managing the association’s various digital marketing initiatives. Hanson holds degrees in design from the University of Nebraska Lincoln and in communications from the University of Denver. She can be reached via e-mail at lhanson@galvanizeit.org[11].

Endnotes:
  1. [Image]: https://www.constructioncanada.net/wp-content/uploads/2018/04/corrosion_Stoneham_Arch_12.jpg
  2. www.cement.org: http://www.cement.org
  3. www.galvanizeit.org: http://www.galvanizeit.org
  4. [Image]: https://www.constructioncanada.net/wp-content/uploads/2018/04/corrosion_A30_Express_Highway_16.jpg
  5. [Image]: https://www.constructioncanada.net/wp-content/uploads/2018/04/Bond-Strength-Graph.jpg
  6. www.galvanizedrebar.com: http://www.galvanizedrebar.com
  7. www.michigan.gov/documents/mdot/MDOT_Research_Report_RC-1560_407022_7.pdf: http://www.michigan.gov/documents/mdot/MDOT_Research_Report_RC-1560_407022_7.pdf
  8. [Image]: https://www.constructioncanada.net/wp-content/uploads/2018/04/corrosion_Stoneham_Arch_13.jpg
  9. Embedded in Concrete: Reinforcing Steel Corrosion Protection: http://www.constructionspecifier.com/embedded-in-concrete-reinforcing-steel-corrosion-protection
  10. Steel Rebar Coatings for Concrete Structures: http://www.structuremag.org/?p=10047
  11. lhanson@galvanizeit.org: mailto:lhanson@galvanizeit.org

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