Patent Application
20170044379
Steel reinforced concrete is commonly employed in civil engineering contexts, in particular with infrastructure developments such as bridges, roads and tunnels, but also in commercial construction. Ribbed steel rods, or rebar (reinforcement bar), is typically inserted into a concrete form, and concrete molded (poured) around the inserted rebar forming an encapsulated steel which complements the compressive strength of the concrete. Premature failure of reinforced concrete has often been associated with corrosion and oxidation of the embedded steel members.
In particular, steel-reinforced concrete is by far the most widely used infrastructure material, with approximately 7 billion cubic meters currently in place in the U.S. alone. An additional 380 million cubic meters are added each year. Electrochemical corrosion, which occurs when aggressive media break down the protective oxide film on reinforcing steel and enable the production of rust, is one of the most significant contributors to service life reduction in steel-reinforced concrete
A self-healing coating for reinforcing steel embedded in concrete includes emulsion derived microcapsules having a healing agent disposed in particles adapted for dispersion through a liquid, and a coating medium adapted to be disposed on a structural steel surface to form a coating for corrosion prevention. The particles (microcapsules) are dispersed in the liquid coating medium for being applied to the surface and are configured to rupture and release the healing agent onto the surface in response to a compromise of the coating, such as being dropped or dragged on a construction site. The self-healing agent, such as Tung oil, complements the protective properties of the coating medium by flowing into regions where the coating medium has been scraped off, flaked off, or otherwise undergone compromise. Alternatively, post-installation corrosive influences, such as rust and oxidation, can also cause rupture of the particles to abate corrosion in the concrete-encased steel members. Configurations herein are based, in part, on the observation that steel-reinforced concrete remains a substantial structural component for many infrastructure and commercial construction needs. The symbiotic combination of the tensile strength of steel coupled with the compressive strength of concrete provides a versatile load bearing construction medium. Unfortunately, reinforcing steel encased in the concrete, typically ribbed steel rods known as rebar, suffers from corrosion and oxidation within the concrete. Rebar is typically coated with a rust preventative, but the rough environment of construction sites coupled with course aggregate (rocks) in the concrete poured over the rebar during installation can often scratch and compromise the rebar, leaving exposed regions susceptible to corrosive elements. Accordingly, configurations herein substantially overcome the above described shortcomings of conventional rust preventative coatings and epoxies by providing a self-healing coating having microcapsules filled with a healing agent dispersed as particles throughout the self-healing coating. The microcapsules have a polymer coating responsive to physical abrasion by rupturing and distributing the healing agent around the damaged region. Following encapsulation in concrete, the microcapsule particles remain on the coating and are responsive to oxidation and corrosion by rupturing and releasing the healing agent across the oxidation region for retarding corrosion.
In an example configuration, the healing agent is Tung oil, and the self-healing coating is a liquid epoxy medium similar to conventional rust preventative coatings. Upon construction site handling, typically by being dropped or dragged in conjunction with hard surfaces, scratched or abraded epoxy regions release the healing agent from the microcapsules (particles) ruptured by the damage. Scratched epoxy regions therefore result in the healing agent filling in the scratch to compensate for the breached coating. However, any suitable healing agent may be encapsulated in the particles and other liquid mediums in addition to epoxy may be employed for coating the structural steel.
In further detail, the method for disposing a self-healing coating on reinforcing steel rebar as disclosed herein includes preparing a microcapsule emulsion for generating particles containing a healing agent surrounded by a polymer shell, and combining the particles with a liquid coating for preventing oxidation of steel members.
Either prior to or during formation concrete molds, a spray or brush process applies the coating with the particles to a structural reinforcing member (rebar). The construction process often involves introducing the coated, structural reinforcing member into a compromising environment, such that the compromising environment causes the shell to rupture and release the healing agent. The healing agent complements the corrosion prevention of the coating by flowing into and covering gaps in a compromised region of the liquid coating upon release from a ruptured shell.
The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
The figures and examples below depict an anticorrosion coating for structural steel members such as rebar that employs microcapsules containing a healing agent for releasing the healing agent in response to abrasion, damage or pH changes indicative of corrosive infiltration in reinforced concrete. The anti-corrosion coating, such as a polymer or resin based mixture, distributes poly(urea-formaldehyde) microcapsules of Tung oil or other healing agent along the surface of the rebar. The microcapsules of Tung oil define a secondary phase healing agent releasable in response to detected damage or corrosion for protecting the steel surface from corrosive infiltration from water, road salt and other foreign elements that permeate the concrete and cause degradation of the reinforcing steel.
The most common method of preventing steel corrosion in reinforced concrete 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. This project reports the first use of self-healing coatings for rebar in steel-reinforced concrete. When damage occurs in self-healing coatings, microcapsules rupture and healing agent passivates the surface and restores the physical barrier to corrosive species. Based on preliminary results, the self-healing coatings may extend infrastructure lifetimes threefold. Since the disclosed coatings are applied similarly to conventional epoxy coatings, usage is less expensive than stainless steel rebar, which is preferentially used over ECR for its improve corrosion resistance.
Conventional approaches suffer from the shortcoming of phase change, allowing only a one-time healing action, or a vascular approach, which includes a fluidic network. One-time healing approaches impose that once the healing agent has reacted and gone from a monomer (liquid) to a polymer (solid), there can be no more healing in that location. Vascular approaches are a strategy for compensating for the lack of a healing agent in secondary phase-based approaches, by including a network through which healing agent can flow. However, this is not feasible in a coating. Secondary phase approaches employ a coating material to which a second phase is added, such as fluid contained capsules, however sometimes there can be natural phase separation of the healing agent from the coating matrix, rendering a capsule unnecessary.
Depicted below is a particular configuration of the self-healing coating for reinforcing steel embedded in concrete, such as rebar.
Referring to
In sum, structural steel members such as rebar encounter many manipulations which can cause physical damage or discontinuities in the rebar coating. In conventional approaches, the discontinuities represent paths for corrosion and degradation. In contrast, in configurations described below, physical agitation of the coating ruptures the microcapsules and releases the healing agent.
Referring to
The healing agent encapsulated in the microcapsules may be any suitable substance that promotes passivity, longevity and/or mitigates corrosion or compromise of the structural steel member. In the example configuration, Tung oil has shown to be an effective healing agent, and the Tung oil microcapsules are generated as described below. The microcapsules are formed from an emulsion, generally regarded as a mixture of two or more liquids that are normally immiscible. The formed microcapsules are particles, nanoparticles or other particles dispersed throughout a liquid medium for application as a coating on reinforcing members embedded in concrete prior to curing.
The procedure used for encapsulating Tung oil was begins with an oil-in-water emulsion to which the following components were added: ethyl maleic anhydride (EMA) solution as a surfactant, resorcinol to stabilize the solution, ammonium chloride to provide a pH buffer, and urea reacting with formaldehyde to form the polymer shells. At room temperature, 200 mL 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 fully in a 500 mL beaker. Following this, the pH of the solution was adjusted from 2.7 to 3.5 using a dilute sodium hydroxide solution in order to control the morphology of the polymer shells. This solution was placed into a room temperature water bath and stirred at 400 rpm as 50 mL of Tung oil was slowly added into the solution. The resulting mixture was mechanically stirred at 400 rpm for 10 minutes 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 hours at 400 rpm to facilitate the polymerization reaction between urea and formaldehyde. The solution was then removed from the oil bath and stirred as it cooled to room temperature over 6 hours.
To extract the microcapsules, the mixture was vacuum filtered with coarse filter paper, then washed with deionized water and acetone, respectively. Finally, the microcapsules were air-dried for 48 hours before they could be used. Both poly[(phenyl isocyanate)-co-formaldehyde] (isocyanate pre-polymer, number of reactive groups per molecule˜3.0, MW˜375) and poly(vinyl alcohol) (PVA, MW-9,000-10,000, 80% hydrolyzed) were obtained. 2-methylbenzothiazole, ethylenediamine, and tetraethylenepentamine (TEPA) were procured from various sources. All chemicals were used without any purification.
Encapsulation of Tung oil is achievable using the same approach as that used for Encapsulation of 2-methylbenzothiazole, as follows. At room temperature, 40 mL of deionized water, 5 mL of 2.5 wt.% EMA solution, 0.1 g of resorcinol, 0.1 g of ammonium chloride, and 1 g of urea were mixed fully in a 500 mL beaker. Once the solids were completely dissolved, the solution was adjusted to a pH of 3.5 using dilute sodium hydroxide. This was placed into a room temperature water bath and stirred at 400 rpm as 5 mL of 2-methylbenzothiazole was slowly added to the solution. The resulting mixture was mechanically stirred at 400 rpm for 10 minutes to form a stabilized emulsion, after which 2.6 g of 37 wt. % formaldehyde solution was added. The temperature of the solution was raised to 60° C. for 4 hours at 400 rpm to facilitate a polymerization reaction. The resulting solution was filtered using vacuum filtration and rinsed with deionized water and acetone.
While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 62/204,006, filed Aug. 12, 2015, entitled “SELF HEALING STEEL COATING” incorporated herein by reference in entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62204006 | Aug 2015 | US |
