SELF-MENDING COMPOSITES INCORPORATING ENCAPSULATED MENDING AGENTS AND METHOD OF SELF-MENDING COMPOSITES

Abstract

A cement mixture is disclosed that includes an aqueous mending agent that is disbursed within but isolated from the cement mixture, wherein the aqueous mending agent will form molecular bonds with hardened cement that is formed by the cement mixture when the aqueous mending agent is permitted to flow within the hardened cement.

Claims

1. A method for self-mending a cement mixture, comprising: providing a wet cement mixture comprising cement and water; forming a plurality of microcapsules by chemical synthesis including emulsified reaction, the microcapsules each consisting of a microcapsule material encapsulating an aqueous mending agent consisting of water and sodium silicate, the microcapsules being formed by chemical synthesis including emulsified reaction to encapsulate the aqueous mending agent; disposing the plurality of microcapsules within the wet cement mixture; allowing the wet cement mixture to hydrolyze to become a hydrolyzed cement mixture; rupturing at least one microcapsule responsive to a stress applied to the hydrolyzed cement mixture, the rupturing resulting in the aqueous mending agent flowing to the hydrolyzed cement mixture damaged by the stress; and forming a covalent bond between the aqueous mending agent and the hydrolyzed cement mixture to improve a property of the cement mixture by at least 10% (ten percent) relative to the cement mixture's initial strength.

2. The method of claim 1, further comprising including metal reinforcement in the cement mixture.

3. The method of claim 2, further comprising pretreating the metal reinforcement with the microcapsules.

4. The method of claim 3, wherein the improved property of the cement mixture comprises reduction of corrosion in the metal reinforcement compared to a non-pretreated metal reinforcement.

5. The method of claim 1, further comprising incorporating a fiber into the cement mixture.

6. The method of claim 5, further comprising incorporating an effective amount of the fiber, the effective amount being an amount to strengthen the cement mixture.

7. The method of claim 1, wherein the microcapsule material comprises at least one of polyurethane, urea-formaldehyde polymer, polystyrene, polyamide, gelatin, or a combination thereof.

8. The method of claim 1, wherein the stress comprises at least one of freezing, thawing, loading, cracking, impacting, corrosion, weight, chemical, creep, expansion, or shrinkage.

9. The method of claim 1, wherein forming a covalent bond between the aqueous mending agent and the hydrolyzed cement mixture does not require a catalyst or initiator.

10. A product made by self-mending the cement mixture according to the method of claim 1.

11. A method for making a hydrated cement product, the method comprising: forming a plurality of microcapsules, the microcapsules each consisting of a microcapsule material and an aqueous mending agent encapsulated by the microcapsule material, the microcapsule material including at least one selected from the group consisting of polyurethane, urea-formaldehyde polymer, polystyrene, polyamide, and gelatin, the microcapsules being formed by chemical synthesis including emulsified reaction to encapsulate the aqueous mending agent in the microcapsule material; incorporating the plurality of microcapsules into a hydrated cement of cement and water to form the hydrated cement product; and applying mechanical stress to the hydrated cement to form one or more cracks and rupture the microcapsule material of one or more of the microcapsules to release the aqueous mending agent which flows to the one or more cracks to form covalent bonds with the hydrated cement in the one or more cracks.

12. The method of claim 11, further comprising: including metal reinforcement within the hydrated cement product and treating the metal reinforcement to enhance its binding capacity to the microcapsules.

13. The method of claim 11, wherein the aqueous mending agent consists of water and one of sodium silicate or calcium nitrite.

14. The hydrated cement product made by the method of claim 11.

15. A method of making a hydrated cement product, the method comprising: incorporating a plurality of microcapsules into a hydrated cement of cement and water to form the hydrated cement product, the microcapsules each consisting of a microcapsule material and an aqueous mending agent encapsulated by the microcapsule material, the microcapsule material consisting of at least one selected from the group consisting of polyurethane, urea-formaldehyde polymer, polystyrene, polyamide, and gelatin, the microcapsules being formed by chemical synthesis including emulsified reaction to encapsulate the aqueous mending agent in the microcapsule material.

16. The method of claim 15, wherein the hydrated cement of cement and water includes one or more cracks; and wherein the aqueous mending agent, which is encapsulated by the microcapsule material of one or more of the microcapsules incorporated in the hydrated cement of cement and water, flows to the one or more cracks to form covalent bonds with the hydrated cement of cement and water in the one or more cracks.

17. The method of claim 15, wherein the hydrated cement of cement and water includes one or more cracks; and wherein one of more of the plurality of microcapsules, which are incorporated in the hydrated cement of cement and water, are ruptured to release the aqueous mending agent, which is encapsulated by the microcapsule material of the ruptured microcapsules incorporated in the hydrated cement of cement and water, to flow to the one or more cracks.

18. The method of claim 15, wherein the hydrated cement of cement and water includes one or more cracks; and wherein one to some of the plurality of microcapsules, which are incorporated in the hydrated cement of cement and water, are ruptured to release the aqueous mending agent to flow to the one or more cracks to form covalent bonds with the hydrated cement of cement and water in the one or more cracks.

19. The method of claim 15, wherein the microcapsules are formed by chemical synthesis of sorbitan trioleate, polyethylene glycol (PEG), toluene, methylene diisocyanate, dibutyl tin dilaurate, and water.

20. The method of claim 15, wherein the microcapsules are formed by emulsified reaction of sorbitan trioleate, polyethylene glycol (PEG), toluene, methylene diisocyanate, dibutyl tin dilaurate, and water.

21. The hydrated cement product made by the method of claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention may be further understood with reference to the following drawings in which:

[0033] FIG. 1 shows an illustrative diagrammatic representation of adding microcapsules containing the aqueous mending agent into a concrete mixture, as well as the application of a load to hardened concrete and the rupturing of the capsules, releasing the mending agent that can repair the cracks responsive to the mechanical stress from the application of the load;

[0034] FIGS. 2 and 3 show illustrative graphical representations of load versus displacement (extension) for flexural strength characterization of a control material and a material containing microcapsules in accordance with an embodiment of the invention; and

[0035] FIG. 4 shows an illustrative graphical representation of open circuit potentials versus time for corrosion and capsule samples in an electrochemical experiment.

[0036] The drawings are shown for illustrative purposes only.

DETAILED DESCRIPTION

[0037] In accordance with the present invention, it has been discovered that a self-mending process may be employed that is targeted to areas of hardened cement that have undergone stress, by incorporating microcapsules within the cement mixture. The microcapsules contain mending agents that when ruptured (by stress within the hardened cement) release the aqueous mending agent in the area of the stress. The aqueous mending agent naturally and covalently bonds with the by-products of concrete hydration in the damaged concrete, improving one or more properties of the concrete as well as reducing corrosion of metal reinforcement incorporated in the concrete.

[0038] The encapsulated aqueous mending agent is one that does not require any additional factors such as catalysts or initiators and has a natural cement bond with an endogenous cement hydration by-product in damaged targeted areas. The invention further provides that corrosion of metal reinforcement material encased by the cement matrix may be reduced. The invention additionally provides for a method of pretreatment of metal reinforcement material at any time prior to the addition of the cement matrix to reduce corrosion in microcracks formed by external stress.

[0039] The present invention involves the discovery that an aqueous mending agent may be encapsulated into microcapsules and incorporated into a cement matrix, whereby upon targeted release from microcapsules caused by stress the aqueous mending agent reacts with endogenous products of cement hydration in the damaged concrete to produce a natural cement bond that mends the damaged concrete.

[0040] A targeted event producing a microcrack may be caused by a variety of stress factors that act as a trigger for the self-mending process to occur in the localized region. Preferably, microcapsules containing an aqueous mending agent core are incorporated within the concrete matrix in the absence of any other additives, such as catalysts or activators, to promote or stimulate mending. When mechanical or other stress is applied, the microcapsules rupture releasing the aqueous mending agent into the microcracks formed by stress and the aqueous mending agent bonds with the damaged concrete improving at least one of its physical properties.

[0041] As diagrammatically shown in FIG. 1, for example, the mending agent 10 is encapsulated in microcapsules 12 in a cement mixture 14, of sand, Portland cement and water. The cement mixture is poured and hardened to provide a hardened cement structure 16. A load is applied (as indicated at 18), and small cracks 20 appeared in the structure 16. The mending agent is released from the microcapsules 12 when the stress is applied and the cracks form. The mending agent flows into the small cracks 20 and bonds at a molecular level to the surfaces of the cracks as diagrammatically shown at 22 to provide a mended structure 24.

[0042] In one example of the invention, sodium silicate reacts with calcium hydroxide, a product of cement hydration, and produces a calcium-silica-hydrate (C—S—H) gel—a binding material natural to concrete. A key aspect of the invention is that the mending agent resides in an aqueous environment within the microcapsule and the water facilitates the hydration of the damaged cement and subsequent bonding of the mending agent. The C—S—H gel (x.(CaO.SiO.sub.2).H.sub.2O) fills the crack and allows recovery of strength. The relevant chemical reactions are shown below:

Na2O.SiO2+Ca(OH)2x(CaO.SiO2).H2O+Na2O

x(CaO.SiO2).H2O+Na2O+CO2 CaCO3+SiO2+2NaOH

[0043] C—S—H is a complex product that often has varying C/S ratios present and may differ slightly in nanostructure. It has been demonstrated in hydrated cement and is described as a network of nanoparticles. For this invention, only the first reaction forms the product rapidly. It is the newly formed C—S—H gel that will act as a binder and mender in cracks and pores, bridging the gaps in the material and ultimately improving its strength. The second reaction is a longer time scale. Sodium-silica-hydrate (N—S—H) is observed in concrete as a result of the reaction between sodium hydroxide and silica. The long-term products initiated by the presence of the aqueous mending agent provides further integrity of the concrete. In addition to sodium silicate, other mending agents include calcium nitrite.

[0044] This present invention is not limited to the use of microcapsules as the aqueous mending agent may be delivered using hollow fibers or other shapes provided that the mending agent may be incorporated in the shape and that stress may be used to release the mending agent. The size of the shaped enclosures may be on the order of 10 to 1000 microns. The desired properties of microcapsules may depend upon a variety of attributes that include, but are not limited to, resistance to aggregation, whether they become uniformly dispersed in a cement mixture, temperature stability, long shelf life, capsule wall thickness, and resistance to mixing when added to the cement mixture.

[0045] The mixture for forming microcapsules containing a mending agent includes at least an aqueous mending agent solution, a surfactant, and a polymerizer. This invention preferably excludes the use of any other agents, catalyst or activators, or external stimulus that initiate the mending action. The mending agent solution is an aqueous solution of the mending agent having a concentration of 0.1% to 10% weight of mending agent per volume of water. Polymers used for production of microcapsules may include a polyurethane precursor such as a diol, a diisocyanate, and/or a monomer containing both alcohol and isocyanate functional groups. In other examples of polymers, the polymer precursor may include a urea-formaldehyde polymer precursor, such as urea and/or formaldehyde. In yet other examples of polymers, a polystyrene precursor, such as styrene and/or divinylbenzene; or a polyamide precursor, such as an acid chloride and/or a tramine. This invention is not limited to the microcapsules prepared as described herein and includes any and all materials regardless of composition and shape that provide for the containment of the aqueous mending agent that may be released upon stress.

[0046] Microcapsule properties such as the walls of the capsules or aggregation may be adjusted using ionic surfactant, such as a cationic surfactant, an anionic surfactant, or an amphoteric surfactant or non-ionic surfactant. The process of dispersing the mixture may use a variety of protocols including mechanical agitation, magnetic stirring, vortexing, and high-pressure jet homogenizing. Additional methods may be employed during or after the production of microcapsules to provide a more uniform diameter using either controlled processing or selection of microcapsules sizes using centrifugation, sonication, or other post-production methods.

[0047] The present invention incorporates aqueous mending agent microcapsules into the cement matrix by blending microcapsules into the wet cement mixture and constitute between 0.5% to 25.0% of the total weight of the cement mixture. The aqueous mending agent microcapsules may be added initially or just prior to pouring the cement mixture into the molded article.

[0048] Preferably, the incorporation of the aqueous mending agent microcapsules restores at least one of the cement properties to a level of at least 10% compared to the non-incorporated control cement, more preferably at least 25%, and most preferably at least 50%.

[0049] The present invention further provides for reducing the corrosion of metal reinforcement incorporated in the cement for strengthening properties. Examples of metal reinforcement include, but are not limited to, rebar or metal mesh or any other type of non-cement structure that provides additional strength to the hardened cement structure. Metals include any and all metals that are reactive to corrosion. When stressed, the mending agent is released and some of the aqueous mending agent deposits on the metal reinforcement bars (rebars) traditionally used in concrete. The formation of a passive film on the surface of the metal will provide protection of the metal reinforcement from corrosion. Additionally, the mending of the targeted area will reduce water transport through the concrete matrix due to reduced porosity and decreased interconnectivity of cracks. The mending of the cracks will inhibit the ingress of damaging chlorides, thereby reducing the rebar corrosion rate.

[0050] A further aspect of the present invention involves the treatment of metal reinforcement prior to the addition of the cement matrix. Microcapsules may be produced, selected, and/or additionally treated to provide for enhanced surface and physical properties to adhere to the metal reinforcement. Further, the metal reinforcement may be pretreated to enhance its binding capacity or properties to the microcapsules. Preferably, the pretreatment of the metal reinforcement reduces corrosion by at least 10% compared to the non-treated metal reinforced cement, more preferably at least 25%, and most preferably at least 50%.

[0051] The in situ synthesis using an interfacial polymerization is described in the following steps. 4.202 mL of sorbitan trioleate (Span 85) and 2.116 mL of polyethylene glycol (PEG) were dissolved in 90 mL of toluene. A 15 mL aliquot was taken from this solution and placed into a separate beaker (referred to as E.sub.1). 0.682 mL of methylene diisocyanate (Basonat®) and 0.0469 mL of dibutyl tin dilaurate was dissolved in E.sub.1. This blend was mixed at 350 rpm to ensure a homogenous mixture and set aside. The original mixture (Span 85, PEG, and toluene) was combined with 30mL of water, stirring at 8000 rpm in a homogenizer or blender. Finally, E.sub.1 was added to this primary emulsion and stirred at 700 rpm for 10 minutes at room temperature. The speed was reduced to 350 rpm at 63° C. and allowed to react for 4 hours. Polyurethane microcapsule sizes varied in size from 40-800 microns.

[0052] Concrete samples were prepared to the specifications of ASTM C-109 with a mix containing 1375 grams of Ottawa C-109 sand, 500 grams of Type VII Portland cement and 242 mL of water. For samples containing the polyurethane microcapsules, the capsules were added to the mix water at 2% volume and prepared identical to the control samples. Molds of dimensions 160 mm×40 mm×20 mm (for flexural strength) and 500 mm×500 mm×500 mm (for compressive strength) were used. After being stripped from the molds, the specimens were submerged in water for two days then contained in a 95% constant humidity environment for 28 days to ensure full curing.

[0053] For mechanical strength testing, randomized internal microscale damage was induced with an applied load to incipient failure, to mimic realistic cracking patterns. For the corrosion testing, one large crack was induced directly to the iron wire to ensure a common path between samples.

[0054] For the flexural strength tests, approximately 160 mm×40 mm×20 mm samples were used. Each sample was subjected to an applied load of 0.25 mm/min to induce microcracking within the sample. The cracking was minor and internal only and meant to mimic microscale damage and deformations that occur within the concrete after applied or natural stress, and prior to catastrophic failure. After one week, these samples were retested to see how much strength has been recovered after the initial damage.

[0055] For the corrosion experiments, a larger, single crack was induced to give the sodium chloride solution a direct and common path to the iron wire in each sample. This was achieved by subjecting the 160 mm×40 mm×20 mm samples to a three-point bend test so that a crack propagated directly to the wire upon failure.

[0056] The experimental procedure to determine the compressive strength of each specimen was adapted from ASTM C-109. Each sample was centered between the two parallel discs. The strain rate is 1 mm/min. For the first test, the load was stopped after the sample had reached a maximum load and shows a gradual descent, but was not allowed to reach failure. After the short-term mending time had passed, each sample was retested to failure. Only the results of the retested samples are presented herein.

[0057] The results of the compressive strength re-test for 5 control samples and 5 samples containing microcapsules are presented in Table 1. No loss was found in compressive strength for the capsule-containing samples.

TABLE-US-00001 TABLE 1 Compressive Strength in Control and Microencapsulated Samples. Sample Control, strength (ksi) With 2% vol. microcapsules (ksi) 1 2.279 2.253 2 2.283 2.307 3 2.247 2.875 4 2.244 2.579 5 2.276 2.718

[0058] The microcapsules proved to be a highly effective way of encapsulating the mending agent for a targeted release. The results from the compressive strength tests show that the capsules do not interfere with the cementitious matrix.

[0059] The experimental procedure to determine the flexural strength of each specimen was adapted from ASTM C348-97. The flexural strength was measured by means of a three-point bend test. Samples were supported by two parallel beams and compressed by one central beam. The load was set to move at 0.25 mm/min. For the first test, the load was stopped after the sample had reached a maximum load and showed a sharp descent, but was not allowed to reach failure. After the mending time had passed, each sample was retested to failure.

[0060] The subsequent experiment was used to evaluate whether the material was able to recover some of its strength after acquiring some minor, microscale damage. First, the sample was loaded to incipient failure, indicated by the sharp decrease in the load-displacement curve. The samples were then left to mend for one week. During this time period, the aqueous mending agent that was released from the capsules had time to react with the calcium hydroxide to form the C—S—H, filling some of the cracks that have formed.

[0061] The results for 5 control samples and 5 samples containing the microcapsules are shown at 40 and 42 respectively in FIGS. 2 and 3, and are summarized in Tables 2 and 3.

TABLE-US-00002 TABLE 2 Flexural Strength in Control Samples. Initial Max Max Load After Recovered, Sample Load, N Damage, N % 1 512.9 46.7 9.11 2 490.7 57.6 11.7 3 541.9 76.6 14.1 4 470.6 66.4 14.1 5 525.8 69.6 13.2

TABLE-US-00003 TABLE 3 Flexural Strength in Microencapsulated Samples. Initial Max Max Load After Recovered, Sample Load, N Damage, N % 1 495.7 124.4 25.1 2 416.7 85.9 20.6 3 476.6 125.2 26.2 4 513.8 127.5 24.8 5 528.2 130.5 24.7

[0062] Strength recovery was reported as a percentage of the maximum strength reached after minor damage has been induced compared to the maximum strength in the initial test. The control samples had about 10%-14% of its initial strength left after microscale damage had occurred. The samples containing the microcapsules restored 20%-26% of its flexural strength after the damage. Compared to control samples, the aqueous mending agent microcapsules restored 43% to 260% more of cement flexural strength. This was indicative of the capsules rupturing where the cracks were initiated, partially mending them, and providing more strength to the samples in the second test. Ultimately, this type of mending is desired to promote a longer life of the material since it is prolonging the time to failure.

[0063] The area under the stress-strain curve was obtained to provide a measure of the toughness of the material and is summarized in Tables 4 and 5. Compared to the average of the control samples, concrete samples containing the aqueous mending agent microcapsules restored 85% more of cement toughness.

TABLE-US-00004 TABLE 4 Toughness of Control Samples. Toughness (N-mm), Toughness (N-mm), Control initial damage Failure 1 31.5 11.6 2 24.0 4.6 3 42.6 20.1 4 28.9 7.8 5 31.5 11.8 Average 31.7 11.2

TABLE-US-00005 TABLE 5 Toughness of Capsule-Containing Samples. Toughness (N-mm), Toughness (N-mm), Capsule initial damage Failure 1 35.9 24.4 2 27.4 14.9 3 22.6 20.8 4 34.1 21.2 5 39.9 22.9

[0064] High strength concrete exhibits a brittle behavior in which cracks quickly propagate. This was displayed in the initial test of the flexural data in which a linear relationship was interrupted by a sharp decrease in the load. After the initial damage has been done, the material exhibits a much more ductile behavior. This was more evident in the capsule-containing samples, and results in higher toughness than the controls.

[0065] A critical ability of the invention was demonstrated in testing the flexural strength after inducing microcracks, where the presence of the microcapsules restored the material performance by at least 10% compared to the control samples.

[0066] A 0.5M solution of sodium chloride was used to represent the ingress of chlorides to the steel reinforcement bars in concrete. An acrylic well was adhered to the surface of each rectangular sample with 3M 5200 Adhesive Sealant to ensure a tight, waterproof seal. The cylindrical well is 3 centimeters in diameter and was located directly over the wire present at the center of each sample. The bottom face opposite the well was fixed with a piece of Parafilm and all other surfaces were sealed with duct tape. A piece of sandpaper was used to sand off any rust or impurities that may have built up on the iron wire during curing and to ensure a good connection of the voltmeter to the wire. A potassium chloride reference electrode was placed in the empty well. The sodium chloride was poured into the small well and allowed to travel through the pores and crack to reach the iron wire. The voltage of the wire was recorded over time until the wire was corroded internally.

[0067] The most common and routine inspection of reinforced concrete was used to monitor the open circuit potential of the rebars to monitor and detect corrosion. A voltage potential higher than −0.200V implied a low risk of corrosion. If the potential was between −0.200V to −0.350V, it was an intermediate level of corrosion. If the potential dropped below −0.350V, there was a high risk of corrosion. Finally, reaching a potential of −0.500V was indicative of severe corrosion. For the results presented here, each sample was subjected to the sodium chloride solution until it displayed severe corrosion, or the potential reached −0.500 volts. A graphical representation of the results is shown at 50 in FIG. 4.

[0068] A rapid decrease in potential was observed when the sodium chloride was first poured into the well. In less than 40 seconds, each of the control samples had reached a potential near 0.350V, which indicated there was already a high risk of corrosion to the wire. Control 1, 2 and 3 lasted 96 s, 118 s and 212 s, respectively, after which they all were severely corroded.

[0069] Capsule samples 1, 2 and 3 also showed a rapid decrease in potential initially, similar to the control samples. The potential reached −0.350V in 86 s, 30 s and 40 s, respectively. Beyond this point, however, the capsule containing samples showed a significant difference from the control samples. The potential was sustained at this intermediate corrosion level. The voltage very gradually decreased to −0.400V in 276 s, 200 s and 124 s, respectively. The time taken for these samples to reach a voltage of −0.500 volts was indicated in the figure. Capsule sample 3 exhibited the shortest time period, going from −0.400 to −0.500 volts in 15.6 minutes. Capsule sample 2 followed with 18.5 minutes until severe corrosion and finally, the first sample lasted the longest with 19 minutes of elapsed time before severe corrosion was observed. The key observation was a significant retardation in corrosion in the capsule containing samples.

[0070] Two mechanisms for corrosion inhibition are therefore presented herein in accordance with various embodiments of the present invention. The first involves the formation of a passive layer to protect the metal. In the second, the ruptured capsule would fill the cracks and reduce porosity and interconnectivity to decrease the solution introduction rate. The initial corrosion rates were very similar, shown by a sharp, sudden decrease in the potentials of each sample. The control samples exhibited uniform corrosion. The chlorides permeate through the concrete quickly and severe corrosion is observed. The capsule-containing samples were able to sustain the intermediate potential. This behavior was explained by a combination of both the mending properties and passive layer. First, there was a presence of a thin passive layer that formed from the effects of the ruptured capsules. The chlorides moved quickly through the path of least resistance, which was the large, induced crack directly to the wire, and affected any of those areas not protected by the passive film. These areas would be easily corroded, explaining the similarity in the initial corrosion rate and potentials. With some passive layer present, however, it would take the chlorides longer to have a similar effect on the wire compared to the control samples, which explains why the time taken for the potential to reach the intermediate level of corrosion at −0.350 volts was longer. The results for the capsule-containing samples showed a significant amount of corrosion inhibition compared to the control samples. With increased capsule loading (optimized for strength), more silicates can be deposited onto the wire to form a passive layer that could protect it for greater time. An added approach is the pre-treatment of metal reinforcement with microencapsulates optimized for adherence to the metal reinforcement surface. An ideal application for this system would be as an added aid for corrosion inhibition in an already protected structure.

[0071] Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.