Rethinking Rebar: reinforcement goes green

by arslan_ahmed | October 18, 2022 7:00 am

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By Chris Bennett, iSCS, CDT, and Aurora Jensen

Reinforcing bar, commonly known as “rebar,” comes in the form of mesh, wires, or bars, and is designed to strengthen concrete under tension. Describing it in a more humanistic manner[2], one could consider rebar as a part of the muscular skeletal system of a building. Together, rebar and concrete work to resist tensile forces and increase the utility of a structure.¹ Without rebar, people would not be able to build high into the sky or safely cross large expanses of water.

However, the embodied carbon generated while creating and transporting traditional rebar is substantial. Luckily, new alternatives to traditional reinforcement are becoming more present in the market and the steel industry is making strides to build more sustainably.

Steel rebar

Historically, reinforcement has come in the form of steel. Common on all manner of job sites, the basic “black” bar is carbon steel rebar. It has high tensile strength, but corrodes easily, making it a poor choice in matrices with high likelihood of moisture exposure. The CSA Group (CSA) G30.18-09, Carbon Steel Bars for Concrete Reinforcement and CSA A23.1:19, Concrete Materials and Methods of Concrete Construction, covers parameters for lengths and coils of various reinforcement. ASTM A615M-09, Standard Specification for Deformed and Plain-Carbon Steel Bars for Concrete Reinforcement, provides additional guidance when needed.

Epoxy-coated rebar is basically standard steel rebar with a thin coat of epoxy. It offers improved resistance to corrosion; however, the coating itself is delicate and chips easily during transport and installation. This leaves affected areas susceptible to corrosion. Guidelines for rebar with protective epoxy coatings are covered in ASTM A775/A775M-19, Standard Specification for Epoxy-Coated Steel Reinforcing Bars.

Galvanized rebar fills a similar role to epoxy-coated rebar, by way of the galvanizing process. Compared to epoxy-coated rebar, some galvanized varieties are not as resistant to corrosion, but they are more difficult to chip and damage. The coating material for galvanized rebar is zinc, which is applied in various thicknesses through a process called “dipping” or “hot dipping.” Bolts, ties, dowel bars, and anchors can all be galvanized. CSA A23.1 refers to ASTM A153/A153M, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware for galvanized steel reinforcement.

Stainless steel rebar has significant resistance to corrosion and does not require mitigating chipped areas like epoxy-coated rebar. This makes it an optimal choice for ease of installation and durability of the reinforced concrete. However, stainless steel reinforcement [3]is traditionally very expensive and typically only found in projects which require long-term defense against corrosion, such as bridges or in geographies with higher seismic activity.  It is also more embodied-carbon-intensive[4] than standard rebar, by a margin of more than 10 per cent. In North America, the main standard for stainless steel rebar is ASTM A955/A955M, Standard Specification for Deformed and Plain Stainless-Steel Bars for Concrete Reinforcement. It includes dimensional bar profiles and other information to assist project teams.

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While steel provides excellent strength, its Achilles’ heel is its embodied carbon footprint. The process to produce a ton of steel, in total, creates about a ton of greenhouse gas (GHG) emissions, approximately equivalent to the amount of carbon dioxide (CO₂) burned in 40 home barbecue propane tanks. Steel also has a higher embodied carbon footprint (by weight) compared to concrete. According to information from the World Steel Association, steel production equates to six to seven per cent of global GHG emissions[6].

The main driver of steel emissions is its production process. There are two primary types of steel factories: the basic oxygen furnace (BOF) and the electric arc furnace (EAF). Steel is an alloy of iron and carbon. Legacy BOF furnaces burn fossil fuels to heat iron ore, coke, and limestone to temperatures exceeding 2000 C (3632 F). This is then mixed with 25 to 40 per cent iron and steel scraps to make new steel. Modern EAF furnaces melt iron and steel scraps to create new steel, achieving much higher recycled content rates of 90 to 100 per cent. While EAFs have become the predominant method of steel production in North America, BOFs are still the most common furnaces used around the world.²

Given the high recycled content, steel from EAF production types tends to be about half as carbon intensive as BOF steel. However, reducing steel’s carbon footprint is not always as easy as specifying steel from an EAF factory, as production type is tied to steel shape. Hot rolled steel shapes, for example, are typically produced in EAF factories in North America. However, hollow structural section (HSS) shapes are more likely to come from BOF factories. Rebar can be produced using either BOF or EAF processes, thus sourcing rebar from EAF furnaces should be preferred.

The efficacy of specifying higher recycled content as a method to reduce the embodied carbon footprint of steel has also recently been challenged. Since the amount of scrap steel available is limited, some experts suggest specifying higher recycled content in metal products only arbitrarily changes the manufacturer’s allocation of accounting for scrap shifting the recycled percentage from one buyer to another, rather than creating any substantive change in supply chain practices. Until specifying higher recycled content starts to make steel deconstruction and recovery practices more desirable, and effectively adds more scrap to the system, it is challenging to create change through specifying higher recycled content.

Composite reinforcement

Composite rebar is not necessarily new, but it may not be as widely utilized as it should be. Unlike its steel cousin, composite rebar generally does not have any natural weakness to corrosion, which reduces the risk to the concrete around it. Composite reinforcement is also non-conductive, and in many instances, it has greater tensile strength compared to many types of steel rebar. Additionally, since composite reinforcement is lighter than steel, there are more benefits to the economic and carbon costs of its transportation.

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There are two common types of composite rebar: glass fibre reinforced polymer (GFRP) rebar and basalt fibre reinforced polymer (BFRP) rebar. Fibreglass rebar is produced from glass fibres in a polymeric matrix. The polymer makes load transfer to the glass fibres possible, while the fibres themselves carry the load.³  ASTM D7957, Standard Specification for Solid Round Glass Fiber Reinforced Polymer Bars for Concrete Reinforcement, provides guidance for GFRP in structural applications.⁴ CSA also provides specifics on building with composite rebar in CAN/CSA-S806-12 (R2017), Design and Construction of Building Components with Fibre-Reinforced Polymers.⁵ GFRP composite rebar is generally considered to be more carbon intensive than BFRP, 100 per cent EAF recycled rebar, but less carbon intensive than stainless steel rebar, galvanized rebar, or standard BOF rebar[8]. Basalt fibre reinforced polymer, sometimes known as “basalt rock rebar,” has higher tensile strength compared to steel and, like fibreglass, is non-corrosive. Basalt rebar also shares concrete’s same thermal coefficient of expansion and can be up to 89 per cent lighter than steel, making its related carbon expenditures in transportation the lightest yet.

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Research suggests the emissions from manufacturing BFRP rebar[10] are 74 per cent less than typical steel and 22 per cent less than 100 per cent recycled content EAF steel. Not only are composites lighter, and in some ways stronger than steel, but they also make sense in scenarios where water exposure is a risk to the long-term health of the concrete system.

One example would be using composite reinforcement in a slab which will also have a hydronic heating system. Hydronic systems embed tubes into concrete, which will carry heated water to warm the slab. However, if there is a leak and the hydroponic tubing is tied to metal reinforcement, the risk of corrosion is certain. Basalt and fibreglass reinforcements do not corrode. While this does not prevent a leak from happening, it will reduce the possible damage the leak will have on the entire slab. Basalt and glass fibre rebar installation standards for composite reinforcement of concrete are described in ACI 440.6-08, Specification for Carbon and Glass Fiber-Reinforced Polymer Bar Materials for Concrete Reinforcement.⁶

As previously noted, steel and composite rebar do not have identical mechanical characteristics. While composite rebar may have greater tensile strength, it can also have a lower modulus of elasticity than steel rebar. This can limit its application in certain scenarios. It is therefore important to check with the project team’s engineers to ascertain best uses, as well as serviceability limit state (SLS) and any provincial design codes related to sustained loads.

European rebar, produced with large amounts of manganese, has tremendous flexibility and is easy to work with. However, this same workability also makes it unsuitable for bearing heavier loads, extreme thermal change, or the effects of seismic shock.⁷ However, it can be less expensive than traditional rebar and have a smaller carbon footprint, making it potentially useful on smaller projects in areas with only subtle temperature swings.

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Green steel

Composite rebar is certainly leading the pack when it comes to lowering embodied carbon in reinforcement, but this does not mean the steel industry is not pursuing other changes in the face of climate change. As previously described, newer electric arc furnaces are operating with far fewer emissions than older basic oxygen furnaces.⁸ Along with modernization of many of the world’s electrical grids, the steel industry’s carbon footprint has been reduced by 36 per cent since 1990, according to the Steel Market Development Institute (SMDI).⁹

In the present market, specifying suppliers must have product-specific Type III Environmental Product Declarations (EPDs), or specifying rebar producers who are already disclosing their impacts through EPDs are some of the best ways to support data transparency and better data quality, and ultimately drive decarbonization in the steel industry. Sourcing rebar supplied from EAF mills is also advisable, as it will help support the transition to a greater share of EAF production over BOF-milled steel.

Several producers in the industry are also seeking to tackle the challenge of finding electric alternatives to the high-heat processes, which make it difficult to reduce the carbon footprint of steel production.

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This includes exploring substitutes for traditional fossil fuels for driving high-heat processes, such as biomass-based fuels, or hydrogen-based production.¹⁰ The Canadian steel sector has an aspirational ambition to produce net zero emissions steel by 2050 and has already voluntarily invested to reduce energy consumption and GHG emissions, achieving a 31.5 per cent reduction in absolute emissions in 2016.¹¹ No doubt, more innovation in these markets and production technologies is a critical part of the decarbonization pathway.

A future with lower carbon steel starts with specifying submittals of EPDs for reinforcement products. Project teams should prioritize lower carbon products, like composite reinforcement or EAF rebar, in the earlier stages of planning and communicate with supply team partners to stay informed on best practices.

Notes

¹ Refer to Concrete: Microstructure, Properties, and Materials, 2nd ed., by P. Mehta and P. Monteiro, published in 2006 by McGraw-Hill.

²See “Steel,” published by Carbon Smart Materials Palette. For more information, visit materialspalette.org/steel[13].

³ Read “GFRP, Glass Fiber Reinforced Polymer,” from An Introduction to Composite Materials by Derek Hull, published in 1992 by Cambridge University Press. For more information, visit princeton.edu/~maelabs/mae324/glos324/gfrp.htm[14].

⁴ Refer to ASTM D7957, Standard Specification for Solid Round Glass Fiber Reinforced Polymer Bars for Concrete Reinforcement.

⁵ Consult to CAN/CSA-S806-12 (R2017), Design and Construction of Building Components with Fibre-Reinforced Polymers. For more information, visit ictturkey.com/assets/images/can.csa.s806-02.pdf[15].

⁶ Refer to ACI 440.6-08(17)(22), Specification for Carbon and Glass Fiber-Reinforced Polymer Bar Materials for Concrete Reinforcement.

⁷ Consult “What is Rebar? Types and Grades of Steel Reinforcement,” published by The Constructor. For more information, visit theconstructor.org/practical-guide/steel-reinforcement-types-grades/24730[16].

⁸ See “Basic Oxygen Furnaces vs. Electric Arc Furnaces,” by Chris Deziel, published by Hunker. For more information, visit hunker.com/13401389/basic-oxygen-furnaces-vs-electric-arc-furnaces[17].

⁹ Read “The Urgency of Embodied Carbon and What You Can Do About It,” by Ed Whalen, published on February 27, 2020, by CSSBI. For more information, visit cssbi.ca/blog/article-the-urgency-of-embodied-carbon-and-what-you-can-do-about-it[18].

¹⁰ Refer to “Decarbonization Challenge for Steel,” by Christian Hoffmann, Michel Van Hoey, and Benedikt Zeumer, published on June 3, 2020, by McKinsey & Company. For more information, visit mckinsey.com/industries/metals-and-mining/our-insights/decarbonization-challenge-for-steel[19].

¹¹ Consult Canadian Steel Industry Energy & Greenhouse Gas Emissions Intensity, Technology and Carbon Reduction Roadmap, published by Canadian Steel Producers Association, canadiansteel.ca/files/resources/Golder-Report-CSPA-NRCan.pdf[20].

Authors

[21]Chris Bennett, iSCS, CDT, is CEO of a North American concrete consultancy that provides owner and designer representation, helping oversee documents and installation in the development of sustainable concrete solutions, as well as risk and schedule reductions. Bennett has provided input to the National Research Council Canada for National Master Specification (NMS) content. His firm is currently working on a number of commercial projects across North America in the logistics, mixed-use, and residential markets.

[22]Aurora Jensen is a project manager and materials specialist in Brightworks’ office in New York, where she specializes in embodied carbon measurement and reduction strategies. She also has experience in operational carbon modelling and evaluating passive strategies, and helps clients link operational and embodied carbon considerations and evaluate trade-offs. Jensen currently sits on the steering committee for the Carbon Leadership Forum in New York City and teaches environmental design as a part-time faculty member at Parsons at The New School. Jensen received her masters in design studies in energy and environment with distinction from the Graduate School of Design at Harvard University. She can be reached at aurora.jensen@brightworks.net.

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