Research Article pubs.acs.org/journal/ascecg
Self-Healing Coatings for Steel-Reinforced Concrete Yixi Chen,†,⊥ Chris Xia,† Zachary Shepard,‡ Nicholas Smith,§ Nicholas Rice,§ Amy M. Peterson,*,† and Aaron Sakulich§ †
Department of Chemical Engineering and §Department of Civil & Environmental Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, United States ‡ Department of Natural Sciences, Assumption College, 500 Salisbury Street, Worcester, Massachusetts 01609, United States S Supporting Information *
ABSTRACT: Self-healing rebar coatings were prepared and investigated for steel-reinforced concrete, with the goal of creating coatings that can withstand construction site damage. Such damage to conventional coatings results in epoxy chipping or cracking and negates the coatings’ ability to resist corrosion. The coatings consisted of a conventional epoxy coating containing 10 wt % microencapsulated tung oil as the healing agent. Upon coating damage, the microcapsules ruptured, releasing tung oil that cured across the damaged area. In accelerated corrosion testing, the times to failure of steel-reinforced concrete with self-healing coatings were at least three times longer than steel-reinforced concrete with conventional coatings. After 150 days of accelerated corrosion testing, 83% of the samples with self-healing coatings exhibited no corrosion. No difference between undamaged and intentionally damaged specimens was observed, potentially because the damaged area was not large enough. These results suggest that natural product-based, self-healing coatings are able to passivate rebar surfaces in response to corrosion initiation, significantly increasing their corrosion protection ability. Pullout testing revealed that self-healing coatings exhibited comparable bond stresses compared to conventional coatings. In summary, this is a promising technology for extending service lives of steelreinforced concrete structures with no reduction in interfacial bonding, which could have significant implications for infrastructure resilience and CO2 emissions. KEYWORDS: Self-healing, Concrete, Microcapsules, Rebar, Corrosion, Sustainability
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further corrosion.5,6 The repair of concrete infrastructure has a significant environmental impact, both directly and indirectly. The cement industry emits >100 million tons of CO2 annually worldwide.7 Additionally, during the 4.2 billion hours that American drivers spend in maintenance-related congestion, 3 billion gallons of fuel are wasted and 30 million tons of CO2 are emitted in addition to the materials directly consumed at the construction site.8−10 The most common method of preventing corrosion is the use of epoxy-coated rebar (ECR). The epoxy-based thermoset acts as a physical barrier between aggressive media and steel that can prevent, or significantly delay, the onset of corrosion.11,12 Other methods of corrosion prevention are either significantly more expensive (e.g., stainless steel rebar) or 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 coating, which can occur during transportation or during handling at a worksite, provide aggressive media access to the reinforcing
INTRODUCTION America is facing a rapidly escalating infrastructure maintenance crisis. Over $100 billion is spent on maintaining and improving the built environment annually.1 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 of additional spending will be needed by 2020 to improve infrastructure to a satisfactory condition.2 Since 2013 alone, insufficient investments in infrastructure have translated to $700 million in environmental costs and the loss of $3,400 in disposable income annually, per U.S. household, as a result of infrastructure deficiencies.3 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 100 million m3 is added each year.4 Electrochemical corrosion, which occurs when aggressive media (salts and water) break down the protective oxide film on reinforcing steel and enable the formation of rust, is one of the most significant contributors to service life reduction in steel-reinforced concrete. As rust is produced, it expands, causing internal stresses that, once they exceed the tensile strength of concrete, cause cracking or spalling and enable © 2017 American Chemical Society
Received: December 23, 2016 Revised: February 23, 2017 Published: March 17, 2017 3955
DOI: 10.1021/acssuschemeng.6b03142 ACS Sustainable Chem. Eng. 2017, 5, 3955−3962
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ACS Sustainable Chemistry & Engineering
leading to greater protection against the ingress of deleterious species.26,27 Native to southern China, this oil has been used since antiquity as a waterproofing agent for boat hulls. Tung trees are relatively robust, bearing commercially relevant quantities of nuts roughly five years after planting and continuing to do so for at least two decades. Today, only 5000 acres of land in the United States are planted with tung trees, as the industry was nearly destroyed by Hurricanes Betsy and Camille in the 1960s and, more recently, Hurricane Katrina.28 Tung oil is still, however, available in commercial quantities, and microencapsulation methods for tung and linseed oils have been well established.29−32 Although the self-healing of bulk concrete through the hydration of unreacted cement particles exposed during cracking or the precipitation of calcium carbonate by bacteria has been explored, these mechanisms are limited to healing cracks on the order of microns, are applicable only to certain fiber-reinforced cementitious composites, and do nothing to prevent further deterioration of reinforcing steel.33 There has been no research to date on self-healing coatings for the rebar in steel-reinforced concrete, presumably due to the unique processing requirements of structural concrete, including the high pH of concrete and curing temperatures that can reach 80 °C, as well as the strict economic constraints of the construction materials industry. In the current work, self-healing coatings for rebar in steelreinforced concrete are reported for the first time. Microcapsule and self-healing coating physical properties were measured. Lifetime extension through coating healing ability was characterized with accelerated corrosion testing. The base mechanical properties of concrete reinforced with rebar coated with a self-healing coating were also measured and compared to those of uncoated and conventionally coated rebar systems.
steel and negate the protection that the coating would otherwise have provided. With this in mind self-healing coatings, capable of sensing and repairing damage to ECR prior to placement and during the service life of a steel-reinforced concrete structure without need of external stimuli, are a compelling concept to reduce CO2 emissions and improve the safety, sustainability, and resilience of concrete infrastructure. Self-healing materials in general can be divided into two classes based on their healing mechanism. In one class, healing is achieved through the inherent dynamic nature of bonds in the system, allowing for network remodeling at the damage site. The other class of selfhealing materials uses a secondary phase healing agent that is released upon damage initiation.13,14 Systems capable of network remodeling have been investigated for healable anticorrosion coatings. Oxetanesubstituted chitosan incorporated within polyurethane networks and polymers containing thermoreversible cross-links, such as Diels−Alder bonds, have been shown to repair cracks.15,16 Healing can be achieved numerous times in the same area; however, an external healing stimulus is required, which makes this concept impractical for self-healing coatings on steel embedded in concrete. Upon damage initiation, polyelectrolytes in polyelectrolyte multilayer-based coatings will rearrange to cover a scratch and also buffer the local pH, preventing corrosion; however, this strategy is best suited to an aqueous environment.17,18 Healing of coatings via secondary phase healing agents can take a number of forms. The healing agent can be a polymer resin that will fill a crack with replacement material;19−21 a corrosion-inhibiting molecule that passivates any exposed metal upon release;22,23 or a solution containing compatibly functionalized molecules that form covalent bonds across a crack.24 The versatility and scalability of the secondary phase approach make it the most promising option for self-healing rebar coatings. In the current work, tung oil, a drying oil obtained from the nut of the tung tree (Aleuritis fordii), was selected as a secondary-phase healing agent. Tung oil is a triglyceride that consists primarily (∼84%) of alpha-eleostearic acid (Scheme 1).25 When drying oils are exposed to air for a sufficient amount of time, they polymerize through an autocatalytic oxidation reaction and form a hard, waterproof coating; conjugated oils such as tung oil (containing partially or fully conjugated double bonds) display a greater reactivity than nonconjugated oils such as linseed oil and, thus, react more quickly and completely,
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EXPERIMENTAL SECTION
Materials. Urea, formaldehyde, ammonium chloride, resorcinol (also known as 1,3-benzenediol and 3-hydroxyphenol, among others), and tung oil were purchased from Sigma-Aldrich. Ethyl maleic anhydride (EMA) was purchased from Vertellus. Commercially available Type I/II portland cement and appropriate aggregates were obtained from Bond Sand and Gravel; #4 rebar was purchased from Sullivan Metals. Sodium Chloride was purchased from Fisher Scientific. All chemicals were used as received. Microcapsule Synthesis. Tung oil microencapsulation was based on the method described by Samadzadeh et al.30 A 200 mL portion of deionized water, 25 mL of 2.5 wt % EMA solution, 0.5 g of resorcinol, 0.5 g of ammonium chloride, and 5 g of urea were mixed with a high speed homogenizer (IKA Werke) at room temperature in a 500 mL beaker until a homogeneous solution was formed. Following this, the pH of the solution was adjusted to 3.5 using a dilute sodium hydroxide solution. The pH-adjusted solution was placed in a 25 °C water bath and stirred at 400 rpm as 50 mL of tung oil was slowly added to the solution. The resulting mixture was stirred at 400 rpm for 10 min to form a stabilized emulsion, after which 13 g of 37 wt % formaldehyde solution was added. The temperature of the solution was raised to 60 °C for 4 h at 400 rpm to facilitate the polymerization of urea and formaldehyde. The solution was then removed from the oil bath and stirred as it cooled to room temperature over 6 h. To extract the microcapsules, the product was vacuum filtered with coarse filter paper, then washed twice each with deionized water and acetone. Microcapsules were air-dried for 48 h prior to storage. Microcapsule Characterization. Microcapsules were imaged using scanning electron microscopy (SEM, FEI Quanta 200 FEG MKII). The thermal stability of microcapsules was evaluated via thermogravimetric analysis (TGA, Netzsch TG 209 F1 Libra) under a nitrogen atmosphere at a heating rate of 10 °C/min.
Scheme 1. Alpha-eleostearic Acid, the Primary Component of Tung Oil
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DOI: 10.1021/acssuschemeng.6b03142 ACS Sustainable Chem. Eng. 2017, 5, 3955−3962
Research Article
ACS Sustainable Chemistry & Engineering Coating Characterization. To confirm that the self-healing coatings autonomously repaired damage, healing of the coatings was observed using optical microscopy. For this test, glass slides were coated on one side with either a control epoxy coating or a self-healing coating with 10 wt % microcapsules. Coatings were cut in a straight line with a razor blade and the damage site was imaged at various times after damage using an inverted optical microscope in transmission mode. This is comparable to the damage induced on the rebar coating for the “damaged” condition. Rebar Coating. Microcapsules were mixed into a two-part epoxy (Rust-Oleum Parks) to form self-healing coatings in a planetary centrifugal mixture (Thinky ARE-31) for 2 min at 200 rpm and 30 s at 400 rpm. These times and speeds were sufficient for thorough mixing and degassing. The mixing conditions were optimized to ensure good dispersion while avoiding microcapsule rupture. Control (nonselfhealing) coatings were prepared with the two-part epoxy. Rebar sections were cleaned and dip-coated in either a control coating or self-healing coating containing 10 wt % microcapsules. Coatings were cured for 72 h at room temperature, according to supplier’s instructions. Images of coated rebar are provided in the Supporting Information and confirm that microcapsules are welldispersed in the coating. Damaged coating samples were prepared by using a utility knife to make a 7.62 cm × 0.05 cm (3 in. × 0.02 in.) cut in the coating at a distance of 2.54 cm (1 in.) from the bottom of the rebar section. The blade that was used had a thickness of 0.10 cm (0.04 in.), and even pressure was applied while damaging the sample. To provide consistency, the same person damaged all samples. Coating Impact Damage. Impact testing was performed to evaluate the ability of coatings to withstand damage, which relates to the strength of the coatings as well as adhesion of the coatings to rebar. After rebar coating and curing, as described above, samples were impacted with a 5.0 kg (11 lbm) weight that was released from a distance of 7.62 cm (3 in.) above the sample using an Instron Dynatup impact tester. Three samples were tested for each condition. Photographs of the damaged area were then analyzed using the ImageJ image analysis program. Mortar Preparation. Given the relatively small size of the samples produced, a concrete mix without large aggregate (i.e., a mortar) was selected. Large aggregates in a properly proportioned, conventional concrete do not commonly play a major role in the electrochemical corrosion processes under investigation, which is distinct from other deleterious processes, such as alkai/aggregate reactions, in which aggregates may play a role.34 45 vol % cement and 55 vol % fine aggregate were mixed to form the dry mixture. A 3:10 water:cement ratio was determined to be adequate in terms of both workability of the wet mixture as well as the strength of the resulting material. On a per kilogram basis, 346 g of portland cement and 550 g of fine aggregate were mixed for 2 min in a commercial-grade mixer, then 104 g of water was slowly added while mixing, followed by an additional 4 min of mixing. The mortar mix was allowed to rest for 1 min before being turned by hand and poured into the appropriate mold. Three different types of samples were prepared: mortar cubes for compression testing, mortar cylinders with embedded rebar for pullout testing, and mortar cylinders with embedded rebar for accelerated corrosion testing. In all cases, prior to the mix being placed in the mold, the mold was lightly wiped down with WD-40 as a release agent. Molds were filled one-third of the way and rodded 12 times. This was repeated twice, at which point the mold was filled. This process of partial filling and rodding helped remove air bubbles from the mortar mix. The sample was then placed in a sealed plastic bag (intended to keep the moisture content in the sample stable for the initial curing period) and moved to the curing room (23 ± 2 °C, ≥95% R.H.). Twenty-four h after pouring, samples were demolded and returned to the curing room for 1, 7, or 28 days for compression testing or for 28 days for pullout or accelerated corrosion testing. Compression Testing. In a steel-reinforced concrete structure, concrete is primarily used to provide compressive strength. Therefore, the evolution of compressive strength during curing is an appropriate way to evaluate the quality of a mortar. Compression testing was performed in accordance with ASTM C39/C39M-15a at a loading rate
of 8.9 kN/min (2000 lbf/min) until failure. Three samples were tested per time point. Rebar Pullout Testing. Rebar, which provides tensile strength in steel-reinforced concrete, is grooved to facilitate a strong physical bond between the reinforcement and concrete. Coatings change the rebar surface, which may alter the shear bond strength between the rebar and concrete, possibly increasing the potential for the rebar-concrete interface to fail when the rebar is placed under stress.35 In order to quantify the effect of the self-healing and control coatings on bond strength, pullout tests were performed. Rebar sections 91.44 cm (3 ft) in length were suspended in a 10.1 cm × 20.2 cm (4 in. × 8 in.) cylinder mold into which mortar was placed, and extended out of the top of the mortar to form a “lollipop”. After full mortar curing, each cylinder was placed in a 200 kN (45 000 lbf) capacity Tinius Olsen universal testing machine with the rebar clamped to the bottom platen and the concrete cylinder braced to the upper platen. The rebar was pulled away from the concrete at a uniform rate of 35.6 kN/min (8000 lbf/min) until failure. Three samples were tested for each condition. Accelerated Corrosion Testing. The accelerated corrosion test was based on the procedure first reported by Ahmad and previously used by the authors in an investigation of unconventional corrosion inhibitors.36,37 Samples for accelerated corrosion testing were produced in the same way as the pullout samples, with the exception that the rebar was 30.48 cm (1 ft) in length and suspended 1.9 cm (0.75 in.) from the bottom of a 7.62 cm × 15.24 cm (3 in. × 6 in.) mold. Samples were submerged in a 5 wt % sodium chloride solution with the waterline just below the top of the concrete. Two stainless steel plates were placed in the solution on opposite sides of each sample. A 30 V power supply was used to apply a current through the system with the stainless steel plates attached to the negative leads and the rebar protruding from the sample attached to the positive lead (Figure S1). A data logging program (TracerDAQ) tracked the current in the circuit at regular intervals of 100 s. A spike in current was observed when the first full crack propagated through the concrete due to significant loss in electrical resistance as the rebar came in direct contact with the salt solution. The time at which this spike was observed was recorded as the time to failure. While this method of accelerated corrosion testing is not fully analogous to the natural environment, it allows for the qualitative comparison of different systems in a corrosive environment. Three samples were tested for each condition.
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RESULTS AND DISCUSSION Microcapsule Analyses. Microcapsule synthesis produced discrete, slightly yellow particles, the shapes of which could be easily observed with the naked eye (Figure 1a). When ruptured, a light yellow liquid was observed to be released from microcapsules, consistent with the color of tung oil. For each batch of microcapsules synthesized, SEM micrographs were collected and average microcapsule diameter was determined using the ImageJ (National Institute of Health) image analysis program (Figure 1b). It was observed that slight variations in solution pH during synthesis led to significant variation in microcapsule diameter (Figure S2). To ensure consistency throughout testing, four batches of microcapsules were combined, leading to a broader size distribution (Figure 2, mean = 401 μm, median = 412 μm, standard deviation = 103 μm). TGA was performed on intact and ruptured microcapsules (Figure 3). In both cases, a slight mass loss was observed starting at 250 °C, at which point the microcapsule shell degraded, followed by a significant mass loss between 350 and 480 °C, at which point the tung oil degraded. The similarities between intact and crushed microspheres are due to the tendency of tung oil to polymerize through an autocatalyzed oxidation at increased temperatures instead of evaporate.26 While no boiling point is reported for tung oil, the supplier 3957
DOI: 10.1021/acssuschemeng.6b03142 ACS Sustainable Chem. Eng. 2017, 5, 3955−3962
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ACS Sustainable Chemistry & Engineering
Figure 3. TGA of as-synthesized (intact) and crushed microcapsules.
microcapsules shows much more char than crushed microcapsules, indicating that intact microcapsules are more thermostable than crushed microcapsule. Coating Analyses. Upon damage of self-healing coatings, tung oil was released from microcapsules and filled the damage site (Figure 4). Additionally, a newly formed film of oxidized
Figure 1. (a) Microencapsulated tung oil in a 150 mm diameter Petri dish. (b) SEM of microencapsulated tung oil. Figure 4. Optical microscope images of damaged self-healing coating. (a) 0 and (b) 5 days after coating damage. The arrow indicates the location of damage.
tung oil was observed in the vicinity of the damage. After 5 days, the cut could be observed on the surface of the control coating, whereas self-healing coatings with 10 wt % microcapsules exhibited a band of material with a rough texture along the cut. This band did not exist immediately after the sample was damaged and was only observed near cut microcapsules. It should be noted that the band did not completely spread along the cut, suggesting that even the prepared self-healing coatings may not completely heal this type of damage, depending on the size, orientation, and location of the damage site. The relative thicknesses of the coatings were considered as a potential variable during the accelerated corrosion tests. Since the coatings, both control and self-healing, are waterproof and nonconductive, an increased coating thickness could be expected to provide proportionally more protection to the rebar. In order to determine if the coatings with microcapsules were thicker, calipers were used to measure the thickness of coated rebar before it was encased in mortar. Rebar exhibits significant variation in diameter between samples (d = 12.2 ±
Figure 2. Distribution of microcapsule diameters, as measured via SEM.
reports a flash point of 289 °C.38 Char results from incomplete combustion of carbon-based materials is represented in Figure 3 by the mass remaining at 500 °C. TGA of intact 3958
DOI: 10.1021/acssuschemeng.6b03142 ACS Sustainable Chem. Eng. 2017, 5, 3955−3962
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ACS Sustainable Chemistry & Engineering 0.3 cm, n = 200, based on five measurements each from 40 pieces of rebar), but far less variation within a single length of rebar (standard deviation of 160 μm). Pieces of rebar were therefore measured, then coated, and remeasured to determine the thickness of self-healing and control coatings (Figure 5).
was observed outside of the area from which the coating was completely removed. The 10 wt % microcapsule coatings exhibited larger debonding zones and additional regions where coating had delaminated but was still attached to the rebar. Images of the impact-damaged rebar are available in the Supporting Information. These results indicate that adhesion of the self-healing coatings to rebar and/or coating hardness is diminished by the presence of microcapsules. Reduction in mechanical properties has been observed at some loadings with the addition of a secondary phase healing agent, and an optimal healing agent loading is often reported that balances mechanical properties and healing efficiency.13,39,40 Ongoing work is investigating accelerated corrosion of self-healing specimens after impact damage, strategies to improve coating hardness and adhesion, and microcapsule loading optimization. Mechanical Testing of Concrete and Steel-Reinforced Concrete. Ordinary portland cement is composed mainly (>70 wt %) of tri- and dicalcium silicates that react with water to form calcium silicate hydrate (C−S−H), the primary strength bearing phase in concrete. Tricalcium silicate is the more reactive of the two, and thus is responsible for early age (∼
