Grantee Research Project Results
Final Report: Self-healing Coatings for Steel-reinforced Infrastructure
EPA Grant Number: SU835699Title: Self-healing Coatings for Steel-reinforced Infrastructure
Investigators: Sakulich, Aaron , Peterson, Amy , Chen, Yixi , Xia, Chris , Smith, Nick
Institution: Worcester Polytechnic Institute
EPA Project Officer: Hahn, Intaek
Phase: I
Project Period: August 15, 2014 through August 14, 2015
Project Amount: $14,982
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2014) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Awards , P3 Challenge Area - Sustainable and Healthy Communities , P3 Challenge Area - Chemical Safety , Sustainable and Healthy Communities
Objective:
America is facing an unprecedented, and rapidly escalating, infrastructure maintenance crisis. Over $100 billion is spent on maintaining and improving the built environment annually, in addition to the $100 billion that was spent from 2009 to 2011 through the ARRA and TIGER programs. The majority (~70%) of this funding went towards maintenance, as opposed to new construction. However, this substantial maintenance effort is not sufficient to keep infrastructure in a state of good repair. In 2013, the American Society of Civil Engineers gave U.S. infrastructure an overall “grade” of D+, estimating that $3.6 trillion dollars of additional spending will be needed by 2020 to improve infrastructure to a satisfactory condition. This increased maintenance spending would be only a temporary solution – the Department of Transportation has acknowledged that the development of innovative material systems will be critical in improving infrastructure sustainability. It is only through the development of new materials, with enhanced durabilities, that this crisis can be addressed.
Steel-reinforced concrete is by far the most widely used infrastructure material, with some 7 billion m3 currently in place in the U.S. alone. An additional 380 million m3 is added each year. Electrochemical corrosion, which occurs when aggressive media such as chlorides from deicing salts break down the protective oxide film on reinforcing steel and accelerate the production of rust, is one of the most significant contributors to service life reduction in steel-reinforced concrete. The most common method of preventing corrosion is the use of epoxy-coated rebar (ECR). The epoxy thermoset acts as a physical barrier that can prevent, or significantly delay, the onset of corrosion. Other methods of corrosion prevention are either significantly more expensive (e.g. using stainless steel rebar) or significantly more difficult to use in the field (e.g. galvanic protection). However, ECR is only effective if the brittle epoxy coating is kept in excellent condition. Chips or cracks in the epoxy provide aggressive media access to the reinforcing steel and negate the protective properties of the system. Although improvements in the manufacture of ECR have reduced the number of imperfections, flaws are still routinely encountered. Furthermore, the epoxy coating on ECR affects the adhesion between the cementitious system and the rebar, necessitating alterations in the design of reinforced structures.
This project therefore investigated self-healing epoxy coatings for rebar for the first time. The development of such a coating system would lead to greatly increased service lives for infrastructure systems. This, in turn, would reduce maintenance costs as well as the construction waste, energy consumption, and CO2 emissions associated with infrastructure maintenance. Selfhealing rebar coatings would reduce the environmental impact of maintenance activities, improve user safety, and result in considerable economic savings by making maintenance less frequent.
Summary/Accomplishments (Outputs/Outcomes):
In addition to microscopic characterization, the experimental self-healing coatings were tested using two techniques. First, a rebar pull-out test was performed, during which a piece of coated steel was pulled out of a concrete cylinder. The results of this test indicate that a coating incorporating 20 wt.% microcapsules has no effect on the bond strength between the steel and the concrete. The results for a coating incorporating 10 wt.% microcapsules indicated a reduction in bond strength, although the reasons are not immediately clear. Bond strength is an important factor, as any significant change would require alterations in the design of concrete structures and could affect the economic feasibility of the coatings.
The second test that was performed was an accelerated corrosion test, during which samples of steel-reinforced concrete were placed in a saltwater bath and an electrical current was used to accelerate the corrosion processes. The results of this test indicated that steel coated with a conventional epoxy coating outperformed uncoated steel, and that the experimental coatings (those containing either 10 wt.% or 20 wt.% microcapsules) outperformed both. The time it took for the samples incorporating the experimental self-healing coatings to fail was at least three times longer than the samples incorporating the conventional coatings. There was no apparent difference between samples that were intentionally damaged with a razor and ones that were not (regardless of whether the coating was the conventional system or the self-healing system). A scenario was proposed to account for the increased corrosion resistance of the experimental selfhealing coatings. Normally, rust is created by corrosion; expands; damages the coating; and then damages the concrete cover once the pressure has built up to a sufficient level. In the systems incorporating the self-healing coating, it seems likely that corrosion is initiated; rust is produced; the rust begins to expand, damaging the coating and rupturing the microcapsules; and the tung oil that is released repassivates the steel surface, inhibiting the further creation of rust. In this scenario, while some corrosion takes place and pressures build up inside the structure, these pressures are not sufficient to crack or spall the concrete cover and destroy the specimen.
Conclusions:
A self-healing coating was developed by first creating polymer microcapsules filled with tung oil, and then incorporating these microcapsules into a conventional epoxy coating. The encapsulation of an alternative self-healing agent, 2methylbenzothiazole, proved difficult; however, as the materials related to this are more expensive and more hazardous to handle than tung oil, it is not considered a significant setback to the project. The accelerated corrosion tests indicate a significant increase in the service life of systems incorporating self-healing coatings. Although it is difficult to translate the results of an accelerated corrosion test into performance under actual, expected field conditions, this significantly increase corrosion resistance implies a substantial impact on the economic, environmental, and safety performance of steel-reinforced infrastructure.
More study is necessary to optimize the self-healing coating properties and perform scaled up corrosion testing under real world conditions. A number of questions have not yet been addressed in this project. The effects of microcapsule size and microcapsule wall thickness, which can be altered by changing variables such as the mixing speed or pH of the solution used during production was not investigated, on the performance of self-healing coating have not been investigated. Economic analyses are underway, but have been complicated by the fact that many of the experimental samples have not yet failed, despite being exposed to a corrosive environment for three times as long as the conventional samples.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
| Other project views: | All 1 publications | 1 publications in selected types | All 1 journal articles |
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Chen Y, Xia C, Shepard Z, Smith N, Rice N, Peterson A, Sakulich A. Self-Healing Coatings for Steel-Reinforced Concrete. ACS SUSTAINABLE CHEMISTRY & ENGINEERING 2017;5(5):3955-3962. |
SU835699 (Final) |
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Supplemental Keywords:
Waste minimization, accelerated corrosion testing, civil engineering, chemical engineering, life cycle analysis, epoxy resinThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.