Introducing Textured Epoxy Coating (TEC) for Rebar

Concrete, a crucial component in a wide range of infrastructure, faces durability challenges due to the corrosion of steel reinforcements embedded inside. Known as rebar, that reinforcing steel increases the concrete’s tensile strength to help it resist cracking and breaking. However, rebar can also weaken concrete when it corrodes. 

In many instances, concrete’s susceptibility to cracking therefore necessitates corrosion protection for rebar. While most rebar is uncoated – leaving it vulnerable to corrosion – some receives an epoxy coating that has well-documented benefits and drawbacks. However, an innovative secondary coating for rebar has been proven to improve on the qualities of both uncoated and epoxy-coated rebar (ECR), offering the concrete and construction industries a superior option that delivers long-term benefits.

Known as textured epoxy coating (TEC), this innovative technology is officially recognized under the newly approved ASTM A1124/A1124M-23: Standard Specification for Textured Epoxy-Coated Steel Reinforcing Bars guidelines. TEC offers enhanced protection for ECR, which is commonly known as green bar and is the industry’s most-used method of corrosion protection.

In bond strength, damage tolerance and corrosion resistance, rebar coated with a TEC (TEC rebar) offers significant improvements over both ECR and uncoated rebar, known as black bar. As such, TEC presents a compelling case for adoption – promising cost savings, extended asset life and a more sustainable approach to infrastructure development. 

In early 2024, Sherwin-Williams Protective & Marine launched the first damage-tolerant textured epoxy coating for rebar, known as Sher-Bar TEC™. The coating – which is applied on top of the smooth ECR surface – serves to improve rebar’s interface with concrete. Through a proprietary resin technology, the coating augments bonding capacity by adding texture to the rebar surface. This reproduces the effect from the deformations that exist on the uncoated rebar surface but would be smoothed out during the initial green bar epoxy application, thereby establishing a more distinct anchor profile on the rebar.

While various factors influence an asset’s lifespan, TEC rebar has the potential to significantly extend the life of horizontal and vertical projects. The cost-effective use of TEC rebar can potentially benefit taxpayer-funded infrastructure projects by delaying construction or maintenance cycles, resulting in substantial cost savings and reduced associated environmental impacts.

Most rebar used in roads and bridges goes without a coating, mainly because of the costs and limitations linked to ECR. Most vertical structures also contain uncoated black bar. However, recent research indicates that TEC rebar provides a chance to offer the corrosion-prevention advantages of a rebar coating while improving construction project outcomes. When applied to ECR-coated rebar, TEC introduces an additional level of protection and durability to assets (Figure 1).

Balancing Corrosion Protection and Practicality 

Contemporary concrete formulations are alkaline, with a pH above 7. This alkalinity induces the development of a passivation layer on the surface of rebar embedded in the concrete, effectively mitigating corrosion. This phenomenon begs the question: should rebar be coated at all? The answer is an easy “yes,” due to a fundamental reality: concrete is prone to cracking.

Even miniscule fractures in concrete act as gateways for deicing salts, acid rain and road contaminants to infiltrate and reach the steel rebar within. Concurrently, concrete’s absorption of moisture and carbon dioxide from the air triggers a gradual decline in its pH level. This progression eventually creates cells of corrosion. Rust can then expand the rebar, exerting escalating pressure on the concrete and gradually weakening its adhesion – culminating in the phenomenon known as spalling.

Since the 1970s, ECR has been used as long-term corrosion protection for concrete, establishing an effective barrier against detrimental elements like oxygen and electrolytes to prevent the formation of galvanic cells. However, coating rebar with epoxy also results in the formation of a smooth, hard surface that consequently decreases rebar’s bond strength with concrete by 15% – and diminishes the pullout strength of ECR compared to black bar.

To counter this effect, engineers must use more ECR for the same volume of concrete, incurring a splice length penalty – a factor that adds both substantial costs and weight to projects on a larger scale.

The cost-effectiveness of using ECR varies, depending on the climate. In many instances, expenses associated with ECR outweigh its corrosion resistance benefits. Still, in regions with frequent freeze-thaw cycles and substantial road salt usage, ECR has offered consistent protection and significantly prolonged the lifespan of infrastructure.

Compared with ECR, black bar has superior interaction and bond strength with concrete. Still, as uncoated steel, it’s highly susceptible to rust. TEC rebar has superior performance in these aspects – forging a lasting connection with concrete and exhibiting bond strength comparable to black bar (Figure 2), which allows for using similar rebar splice lengths.

In construction projects involving concrete, rebar constitutes a significant cost factor. TEC rebar helps promote cost savings by eliminating the need for additional “penalty” rebar splice lengths associated with ECR projects. This efficiency also alleviates on-site grid congestion, enabling faster settling of concrete upon pouring, ensuring smoother installations and less idle time (Figure 3). 

Testing TEC Technology

The recently established ASTM A1124/A1124M-23 standard for TEC rebar provides a comprehensive framework for evaluating the performance of different coatings. Since 2019, research universities and independent labs have conducted rigorous testing that has validated the efficacy of TEC applied to rebar by evaluating corrosion resistance, bond strength and damage tolerance. 

The new ASTM specification will allow for this technology to be codified in three performance criteria, which are supported by specific test methodologies: 

• Corrosion resistance (ASTM A775/A775M-19): Chemical resistance (ASTM G20), cathodic disbondment (ASTM G8), salt spray resistance (ASTM B117) and chloride permeability (ASTM A775 A1.3.4)
• Relative bond strength: Beam end test (ASTM A944) and lap splice test
• Damage tolerance: Impact resistance (ASTM G14), chip resistance (ASTM D3170), and flexibility (ASTM A775 A1.3.5)

Proving Grounds

Although testing will persist until 2027, the current findings offer verified insights into TEC rebar.

A study conducted at the University of Minnesota subjected concrete beams containing both coated (ECR and TEC) and uncoated rebar to weight and force. The concrete cleanly separated from the ECR, indicating weaker adhesion. Rebar coated with the Sherwin-Williams TEC (Sher-Bar TEC) exhibited superior adhesion, even compared to black bar, with researchers needing to chisel off concrete from the rebar specimen to be able to inspect it.

In beam-end experiments at the University of Kansas, rebar coated with the Sherwin-Williams TEC demonstrated significantly better bond strength than ECR. The splice strength of the TEC rebar averaged 1.05 times that of uncoated rebar, suggesting comparable, if not better, bonding capabilities. These results indicated that the development length for TEC-coated bars can be used without modification factors, while ECR incurs a penalty, as previously mentioned.

A study by the Wisconsin Department of Transportation revealed that the use of a TEC could reduce rebar overlap by up to 60% compared to black bar and ECR. This reduction may lead to decreased bridge deck weights, potentially enabling the use of higher grades of concrete and resulting in lower raw material and logistical costs.

The University of Illinois conducted microcracking tests (ASTM A944-10) with the Sherwin-Williams TEC that resulted in cracks approximately half as wide as those that occurred with ECR. The total crack area was 33% less. 

The textured rebar engaged more effectively in the structure, reducing stress levels in the concrete. In fact, in flexural tests, TEC rebar exhibited a higher initial slip resistance compared to uncoated rebar or ECR – up to 74%, researchers noted. 

In drying shrinkage tests, TEC rebar demonstrated an appropriate level of interaction with concrete, indicating that the reinforcement from TEC specimens actively resisted the shrinkage stresses induced by the concrete. Following these tests, the Illinois Department of Transportation adopted TEC rebar as an “essential innovation” for bridge construction projects in 2020.

Clemson University conducted studies comparing the flexural cracking of concrete using four different rebar systems: black bar (as the baseline control), ECR, TEC and galvanized rebar. The findings indicated that the ECR showed larger and less frequent cracks, yet it performed about 12% worse than the average in the study. In contrast, the Sherwin-Williams TEC produced smaller and finer cracks, reducing the overall area of cracking. This suggests that less road salt and water will be able to infiltrate the concrete and reach the rebar, reducing its corrosion potential and related problems.

Clemson tests also demonstrated that TEC rebar enhances the point load dissipation of concrete specimens, mitigating the development of large cracks in favor of multiple smaller ones. With increased bond strength at numerous small focal points between the textured coating and concrete, energy entering the concrete dissipated more easily, resulting in more microcracks. Yet, because the overall energy per crack was lower, fewer cracks propagated to the surface, showcasing the added durability and increased damage resistance in concrete featuring TEC rebar.

A Better Bonding Experience

The connection between rebar and concrete is pivotal. Although ECR provides a protective barrier that shields uncoated rebar from moisture and corrosion potential, its smoother surface introduces a challenge by compromising the bond strength between concrete and rebar, potentially leading to separation over time.

In contrast, the textured surface of TEC rebar reintroduces the surface area available for bonding with concrete – and can even increase that area compared to what’s available on the original uncoated rebar surface. This reestablished surface profile of peaks and valleys provides the bonding sites concrete needs to hold fast to the rebar under stress compared to when smoother ECR is used.

Another bond critical to the successful outcomes that TEC rebar enables is the covalent bond between the textured coating and the base epoxy layer. Both formulations are powdered fusion-bonded epoxy (FBE) coatings. During coating applications, the separate powders transform into a monolithic coating that extends seamlessly from the steel to the air interface (Figure 4). This approach ensures prolonged protection of rebar and concrete structures by combining the corrosion resistance of the ECR with the inherent bond strength of the TEC.

The application of a TEC to rebar is a two-step, nearly simultaneous process. First, the rebar is blasted to eliminate surface contamination. Then, it undergoes heating before passing through the ECR application booth. Powdered FBE is sprayed onto the heated rebar, rapidly transforming into a liquid coating that uniformly covers all surfaces to produce green bar.

Immediately after, the heated green bar advances through the TEC application booth. At this stage, a textured FBE is sprayed onto the surface of the rebar. As this textured powder melts and flows over the rebar, it establishes a covalent bond with the ECR layer.

Durability Advantages

Due to the strength of the covalent bond between the ECR and TEC FBEs, TEC rebar exhibits enhanced durability and chip resistance, minimizing the instances of areas of the steel rebar being exposed before concrete is poured and therefore being vulnerable to corrosion (Figure 5) – as observed in damage tolerance assessments conducted by Sherwin-Williams. Simulating poor handling on a construction site, the rebar was spiked on a gravel parking area. ECR demonstrated a greater susceptibility to incur damage and chips compared to rebar coated with Sher-Bar TEC. The TEC material’s matrix – with occluded areas acting as rubbery springs – efficiently dissipated energy and showed greater flexibility than ECR, which tended to develop cracks and chips in similar conditions.

Sustainable Foundations

The TEC formulation is made from repurposed materials. Coupled with the recycled steel in rebar, the solution contributes to reducing the environmental impact of concrete production and aligns with sustainability goals. 

The new coatings also open doors to using alternative cementitious technologies, expanding ingredient options beyond the limitations imposed by the need to protect uncoated rebar from corrosion. This development is particularly promising considering concrete’s significant contribution (around 8%) to global anthropogenic carbon dioxide emissions.

With the potential to substantially extend asset life, TECs offer a promising avenue to postpone or eliminate the necessity for new infrastructure projects, thereby reducing associated emissions. An optimistic Environmental Product Declaration (EPD) is expected for these coatings compared to existing technologies.

Vertical Integration

TEC rebar also offers various advantages for vertical construction, which typically does not use coated rebar. The coating improves barrier protection, which may allow for the use of native sand – without desalination – as an ingredient in the formulation of concrete. Sand with high levels of salinity can hasten corrosion of uncoated rebar. The use of TEC rebar not only facilitates the utilization of local sand but also reduces raw material and logistical costs, promoting sustainability.

TEC rebar could also allow for reduced concrete deck thickness and overlap, potentially making structures lighter and enabling greater height. By enhancing the interaction between concrete and rebar, coated rebar can lead to weight savings on each floor of a structure. This weight reduction can be crucial for building taller structures.

Interestingly, TEC rebar could also play a critical role in seismic-resistant construction, given its enhanced bond strength and higher pullout strength. This could prove pivotal, as the danger in earthquakes is from both the collapse of building floors and detached building parts. Vertical rebar, when detached from concrete, poses a significant injury risk to individuals outside the building during seismic events.

Different specifications apply to horizontal (ASTM) and vertical (American Concrete Institute, or ACI) rebar applications. ACI specifications are only periodically published, with the next update expected in 2030. In the interim, opinion specifications will likely bridge the gap.

The market for coated rebar in vertical structures is in the early stages of development and will require collaboration with engineers and designers to establish its value proposition. The gradual acceptance of coated rebar in vertical applications is expected to increase, as awareness and understanding of its benefits grow.

Pioneering Progress

The new Sherwin-Williams headquarters in downtown Cleveland, currently under construction, stands as its own testament to the new rebar coating technology, as the first-ever commercial building employing TEC rebar (Figure 6).

A ripple effect is anticipated, driven by TEC adoption by various organizations. State departments of transportation will likely call upon TEC technology for bridges and other critical infrastructure. Rigorous independent evaluations – including assessments by the International Code Council – will contribute to the possible endorsement of TEC rebar by the American Association of State Highway and Transportation Officials (AASHTO). Subsequently, this testing will pave the way for code adoption by the ACI under ACI 318, “Building Code Requirements for Structural Concrete and Commentary.”

With such authoritative backing, the concrete industry is positioned and poised to adopt TEC technology, unlocking the enhanced bond strength, damage tolerance and corrosion resistance that the coating offers – alongside cost savings and prolonged asset lifespans – when compared to existing options.