Project 2b: Coated Steel Rebar for Enhanced Concrete-steel Bond Strength and Corrosion Resistance
Matching Research Agency
Missouri Department of Transportation (MoDOT) and Missouri University of Science & Technology (Missouri S&T)
|Three graduate students and one undergraduate student.|
The conventional approach to improve the corrosion resistance of steel rebar is to apply a coating, like epoxy, that provides a physical barrier to the corroding environment. These coatings do not improve the bond strengths between the steel and the surrounding concrete environment. The chemically bonded reactive enamel coatings proposed in this study increase that bond strength while providing corrosion protection. The overall goal of the proposed study is to optimize this technology for the construction of RC structures for bridges, with the objective of reducing construction costs and improving structure performance. The scope of work or main research tasks include:
1) Characterize and quantify the bond strength between coated rebar and concrete;
2) Characterize the corrosion resistant properties of coated rebar in alkaline environments, including reinforced concrete. The corrosion mechanism of silicate-based enamels in alkaline environments will be studied, as will the role of the steel interface chemistry in the oxidation protection of the steel rebar, and the formation of chemical bonds between the coatings and the concrete matrix;
3) Develop a new design equation for the tension development lengths of coated steel rebar in flexural members based on the enhanced bond strengths between the steel and concrete. Study the behavior of beam-column concrete structures reinforced with enamel-coated rebar, develop and validate new design equations for the tension development lengths of coated steel rebar in RC members;
4) Develop a design specification for RC members using coated rebar and provide design training for MoDOT engineers as well as compare costs with conventional RC members.
Approximately 90% of the bridges in the MoDOT inventory are reinforced concrete (RC) structures and the construction and maintenance of RC bridges constitute a major portion of the up-front costs in MoDOT bridge budget. Therefore, the development and utilization of new technologies that reduce the amount of steel need for safe RC structures, and that prolong the useful lifetimes of steel RC members, beam-column joints, and column-footing connections could have a significant impact on MoDOT operations, especially as the steel price continues to increase. More importantly, using less reinforcement in joint and connection areas will alleviate the congestion of reinforcing bars, and reduce the time-consuming construction process from rebar caging to concrete casting, and will ultimately improve the quality and thus structural safety of a constructed RC bridge. In order to reduce the RC bridge construction cost by 10%, innovative yet cost-effective reinforcing bars need to be developed that can significantly increase concrete-steel bond strength and offer long-term corrosion resistance compared to conventional steel rebar.
About $10 Billion is spent annually to directly remediate corrosion problems with our nation’s bridges, and indirect costs push that annual expenditure up by a factor of ten. Epoxy coated rebar has been the most widely used method for corrosion protection. The epoxy coating is intended to isolate the steel from the corrosive alkaline environment associated with the concrete. However, under certain conditions, the epoxy coating can actually accelerate the corrosion of the steel. The relatively weak physical bond between the epoxy and steel is not a sufficient barrier to attack by moisture. Furthermore, the epoxy coating is detrimental to the concrete-steel bond strength, thus requiring longer development lengths for epoxy coated rebar.
Enamel coatings significantly increase the concrete-steel bond strength, reduce the development length of rebar in RC constructions, and improve the construction quality at congested areas such as beam-column joints. Missouri S&T faculty members are currently leading an on-going effort with the Army Corps of Engineers and Army Research Laboratory to demonstrate the bond strength and corrosion characteristics of enamel-coated bars for blast impact resistance applications. This technology is ready for further investigation in bridge applications with the intent of significantly reducing bridge construction costs and improving long-term corrosion-protection.
Many different types of rebar (zbar, stainless steel, stainless steel clad, fiberglass) have been evaluated for reinforcement performance in RC bridge structures, but they often are not cost effective or produce limited benefits in most conditions and do not warrant the additional cost. In addition, most alternative reinforcing systems do not form chemical bonds with the surrounding concrete matrix, and so offer no opportunity to enhance the overall strength of the RC structure. The proposed project is an opportunity for Missouri S&T to lead an effort to develop a new materials technology for cost competitive and corrosion resistant RC structures that could have a national impact.
Relationship to other Research/Projects
A project under the Missouri S&T-UMC-UMKC MoDOT Structures Program Initiative
Technology Transfer Activities
|Project meetings with MoDOT engineers are anticipated throughout the period of performance.|
Transportation Research Board Keywords
Coated rebar, corrosion, bond strength, deterioration