Reactive porcelain enamel coatings have been developed for a wide range of steel products used in reinforced concrete structures and applications.
The Porcelain Enamel Institute (PEI) and several of its member companies are involved in the improvement and commercialization of a new technology that was developed by the U. S. Army Corps of Engineers. The goal is to increase the bond between concrete and reinforcing steel while also protecting the metal from corrosion in typical environments where steel is used to reinforce concrete structures. Steel-reinforced concrete is the most common construction material in our global infrastructure-it is the material of choice for the construction of large, durable structures throughout the world. Although modern deformed steel bars date from the 1800s, engineers have been placing metal into concrete or mortar to transfer forces and make connections since the early Roman times.
Deformed steel bars used in concrete construction have been standardized since the 1940s. Modern construction technology typically employs concrete with steel reinforcement to form a composite material that combines the high tensile strength of the steel with the good compression strength of the concrete. This combination of durability and strength has made reinforced concrete particularly attractive in environments with extreme exposure to the elements, such as roadways, bridges and port facilities.
PEI's efforts are directed toward the development of reactive porcelain enamel coatings for a wide range of steel products used in reinforced concrete structures and applications. In addition, the coating of various steel and stainless steel alloys is being explored. Our aim is to make reinforced concrete structures stronger and more durable by coating the reinforcing steel with porcelain enamel.
Figure 1. Significant and pervasive steel reinforcement corrosion-with serious consequences-has been observed in locations with high chloride concentrations and wet, oxygenated environments.
The first fundamental problem related to reinforced concrete construction is the interface between the steel and the surrounding concrete. No strong chemical bond exists between the metal and the surrounding hydrated calcium silicate paste. As reinforced concrete cures and gains strength, the water in the concrete is distributed unevenly and the boundary between an impervious surface (steel) and cement paste becomes a water-rich layer that results in a weak, pervious zone adjacent to the reinforcing steel surface.
The second problem is corrosion of the reinforcing steel. While concrete can temporarily protect embedded steel from corrosion, it is only temporary. Initial corrosion is inhibited through two mechanisms: the restriction of contact between corrosive materials and the steel, and the formation of a protective layer of high pH around the embedded steel that causes a film of stable ferrous oxides to form around the steel bars. This layer stays relatively inert until the pH in the surrounding layers is lowered by carbonic reactions or until sufficient chloride ions concentrate near the bar surface. Conditions for many structures allow these corrosion reactions to begin in timeframes often measured in only months. Corrosion is beginning far too soon, as departments of transportation are looking for structure design lives of 50 or more years.
Even when covered with a relatively thick layer of concrete, embedded metals are subjected to an environment where corrosion can occur. In the presence of chloride ions, water and oxygen, the passivity of the steel is overcome and electrolytic reactions occur. These reactions cause some areas of the steel to act as anodes, reducing the effective section, and other areas to act as cathodes where ferrous and ferric oxides are formed. As these oxides are formed, the volume of the original steel grows by a factor of 3-6 times and produces significant internal stresses/cracks in the concrete. The corrosion of the steel reinforcement not only weakens the composite section through the reduction of the steel cross-section, but also through the spalling of the concrete. Concrete is relatively weak in tension and these internal stresses produce large cracks and delamination, leading sections of the concrete to crack away (spall). These actions also accelerate the corrosion since they allow more water, air and chloride ions to contact the reinforcing steel.
The length of time that the concrete can provide corrosion protection to the metal within depends on the corrosion potential of the metal, how much cracking occurs in the concrete, the concentration of chlorides and other corrosive anions at the metal surface, and the availability of oxygen and moisture. This is particularly evident in many marine and highway structures where a high chloride environment is prevalent. Significant and pervasive steel reinforcement corrosion-with serious consequences-has been observed in locations with high chloride concentrations and wet, oxygenated environments (see Figure 1).
To combat the corrosion of steel reinforcement in concrete, a number of different methods have been investigated, including zinc galvanizing, active cathodic protection, stainless steel reinforcing bars and organic coatings. While each of these technologies can be effective, most have significant drawbacks such as cost, variability of protection, construction issues, and the reduction of the bond strength between the steel and the concrete.
Figure 2. The new porcelain enameling procedure incorporates cement particles into a vitreous porcelain enamel coating matrix.
Development and Testing
To address these limitations, a new porcelain enameling procedure for reinforcing steel products has been developed that incorporates cement particles into a vitreous (porcelain) enamel coating matrix, as shown in Figure 2. The fundamental technology requires the reinforcing steel to be prepared and coated with a porcelain enamel that is fused to the metal at temperatures of about 1500øF (815øC). Portland cement is included in the porcelain enamel and becomes bound as the glass cools. It is the hydration of this cement with the rest of the wet concrete as the desired structure is produced that completes the bond of the reinforcing steel to the concrete structure.
Figure 3. Coated sample bars for pullout testing. The top and bottom bars have cement particles dispersed throughout the porcelain enamel coating, while the middle bars have the cement primarily bound to the porcelain enamel surface.
Figure 4. Results of 40-day exposure for raw (left) and porcelain enamel-coated test rods in 3.5% NaCl solution at 20°C.
To determine structural performance, the Corps of Engineers reported over 4.2 times higher pullout strengths with the reactive porcelain enamel coating compared with uncoated bars in laboratory testing (see Figure 4). This means that the development length of reactive porcelain enamel-coated steel will be less than "raw" steel and much less than bars coated with other materials, especially organic coatings.
Finally, PEI has sponsored an ongoing research study at the University of Louisville to develop additional data on the structural engineering performance of reactive porcelain enamel-coated rebar. The data generated will enable the determination of the potential reduction in development length and bar overlap requirements for reinforced concrete structures. These results will, in turn, allow us to promote and market the technology based on the better performance and lower costs of reinforcing steel compared to other types of rebar (e.g., uncoated raw steel, epoxy coated, galvanized, metal clad, etc.).
Figure 5. Embedded portland cement grains hydrate, forming calcium silicate gel in fractures.
Supporting ancillary tests will enable the determination of the metallurgical effects of the porcelain enamel process' heat treatment (firing) of the steel in terms of any specific changes in stress/strain performance or loss of yield strength. These tests will also enable the evaluation of the performance of the coating and its adherence to the steel when the metal is strained to determine the level of strain required to cause the coating to crack and/or separate from the metal. It has been postulated that low strain levels will only produce "micro-cracking" that will be subsequently filled by the internally distributed portland cement grains as they hydrate. Scanning electron microscopy is being used to investigate these self-healing effects of the C3S-porcelain enamel composites (see Figure 5).
These test results, when positive, will be used to support proposals for demonstration projects with the Federal Highway Administration, state departments of transportation, etc. These studies will lead to the development of the data and information needed by the building code and infrastructure authorities to start the process necessary to modify their structural equations and calculations to account for the structural performance of the porcelain enamel-coated reinforcing steel and attain building code approval.
Today, many companies and industries are focused on green manufacturing technologies and sustainable products. The porcelain enamel industry is actively working to assure that the coatings/products for this new technology meet green standards through the use of recycled materials and manufacturing processes with improved efficiencies. Since reinforcing steel employing the reactive porcelain enamel coating will provide more corrosion resistance and an improved structural bond to the concrete, longer structure service lives will result. Thus, this new breed of reinforced concrete structures will consume less raw material and energy input on a structure service life basis of comparison.
PEI enamellers and members that supply frit or equipment all have the expertise to further the performance of reactive porcelain enamel-coated reinforcing steel for concrete. Work remains to be done on:
- enamel chemistries to achieve even better fired adherence to the steel
- efficient coating application processes
- new heat treatment methods
A large number of potential steel products can benefit from this reactive porcelain enamel technology, including rebar, welded wire mesh, corrugated steel decking, stay-in-place forms, wall ties, masonry fittings, steel sheet and steel fiber. PEI and many of our member companies are fully committed to doing all we can to further the development and commercialization of this technology.For additional information, contact the Porcelain Enamel Institute, Inc., P.O. Box 920220, Norcross, GA 30010; call (770) 676-9366; or visit the website at www.porcelainenamel.com.
Author's AcknowledgementsPEI would like to thank many people and companies who have been working on this technology. More than 12 papers or presentations have been given on this topic, and this article is a combination of the work and results of many of those. Specifically, the author would like to thank the following institutions or companies for their contributions and efforts: U.S. Army Corps of Engineers' ERDC laboratories in Vicksburg, Miss.; U.S. Army Corps of Engineers' CERL laboratories in Urbana, Ill.; University of Louisville Department of Civil and Environmental Engineering; A. O. Smith Corp.; EIC Group; Ferro Corp.; KMI Systems, Inc.; Pemco Corp.; Pro Perma Engineered Coatings; Roesch, Inc.; and SQM North America.