Patent Application
20130302607
The present invention relates to structural materials and, more particularly, to a new and improved glass composite developed for coating steel elements for reinforcing concrete structures.
One material very commonly selected for large-scale construction projects is reinforced concrete (RC). Several years ago, it was discovered that the use of a modified vitreous enamel improved the bond strength, and, possibly, the corrosion resistance of the steel rods reinforcing the concrete. The enamel consisted of a glass matrix embedded with reactive ceramic particles. The glass composition was found to strongly adhere to the steel, and the reactive particles were imbedded to chemically react with the surrounding cement to form another strong bond.
The materials used for these initial tests included commercial alkali-resistant groundcoat enamelss for steels used in a variety of consumer and industrial applications. The typical compositional ranges for such enamels are summarized below as Table 1.
The ratio of the Na2O, B2O3, and SiO2 components, as well as the addition of other alkali (K2O and Li2O) and alkaline earth oxides (CaO), have the greatest effect on the thermal properties of the glass. Constituents like Al2O3 are added to improve the corrosion-resistance of the glass. ZrO2 is usually added to an enamel as an opacifier to affect the visual appearance of the coating. However, zirconia has the added advantage of improving the chemical resistance of silicate glasses to attack by alkaline environments. Alkaline-resistant silicate glass fibers developed for reinforcing cement composites typically contain 10-20 wt % ZrO2, and a protective coating of Zr-oxyhydroxide forms on the glass surface when exposed to an alkaline environment, further impeding corrosion. Transition metal oxides, like MnO2, Co3O4, and NiO, are added to enamels to aid bonding to the substrate.
In general, these materials are sodium-borosilicate glasses modified with various constituents to tailor thermal and chemical properties. However, the conventional groundcoat enamels (such as the ones listed in Table 1) only offer limited protection to the underlying steel if they are cracked or otherwise damaged. If the cementitious material can directly contact the steel, such as through chips, holes or cracks in the glass or enamel coating, the steel is then locally attacked by the corrosive cementitious material. Therefore, there is a need to provide a new and improved glass or enamel composition offering enhanced corrosion resistance to the steel, as well as corrosion resistance even if cracked or broken. The present novel technology addresses these needs.
The present novel technology relates to a chemically-active glass composition for providing corrosion protection to coated steels for use in alkaline environments.
One object of the present novel technology is to provide an improved steel reinforced concrete system including the same. Related objects and advantages of the present novel technology will be apparent from the following description.
For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
In one embodiment, steel reinforcing rods 10 are coated with the novel glass composition 20 to yield coated reinforcing rods 30. The glass coating 20 is particularly suitable for coating the steel alloys used in the rods 10, as the glass coating 20 typically has a coefficient of thermal expansion close to but lower than that of the steel rods 10, such that the glass coating 20 is maintained in compression. Further, the glass coating 20 is substantially more corrosion resistant than the conventional enamel coatings known in the art. Specifically, the thermal properties of the glass coatings are tailored for the steel alloys used in RC structures, which have different thermal expansion coefficients than the alloys used in commercial and industrial applications for which the conventional groundcoat compositions were designed. Typically, the steel alloys used in the rods 10 are ASTM A 615, 706, 955, 996 or the like, which typically have thermal expansion coefficients of from about 14 ppm/° C. to about 17 ppm/° C. The glass coating 20 typically has a thermal expansion coefficient of between about 12.5 ppm/° C. and about 13.5 ppm/° C. at ambient temperatures typical of most applications.
In particular, the borate-to-silicate ratio and the fraction and type of alkali oxide of the coatings 20 has been optimized to yield coatings 20 characterized by greater CTE to improve the thermomechanical compatibility with typical reinforcing steel. In other words, the CTE of the glass coatings 20 has been raised to be closer to that of typical steel rebars 10 while remaining slightly lower than the steel CTE, such that the glass coating 20 is put into compression 20 but not so much so that it fails and disengages therefrom. Further, this CTE matching was accomplished without sacrificing chemical durability of the glass coating 20. Thus, by better matching the thermomechanical properties of the glass coatings 20 to the steel members 10, the glass coatings 20 are less prone to failure due to stresses arising from thermal cycling and thus remain on the steel members 10 where they can participate in the bonding process with a surrounding cementitious matrix material.
Additionally, the corrosion resistance of the glass coatings 20 is especially attractive in alkaline environments. The glass coatings 20 typically includes substantially increased concentrations of CaO, K2O and, more typically, ZrO2 at levels substantially greater than the typical enamel compositional ranges to provide increased corrosion resistance of the glass coated rods 30 in alkaline environments.
In some embodiments, as seen in
Table 2 shows the compositions of several embodiments of the glass coating 20, along with test results of the dilatometric softening point and the CTE, designated ARE-1 through ARE-5. For comparison, the composition and properties of a standard (conventional) alkali-resistant groundcoat composition is presented and designated ARG.
In another embodiment, reinforced concrete 50 was prepared by the pouring wet concrete over coated rods 30 and allowing the concrete to dry and cure to define a concrete matrix 40, yielding a reinforced concrete composite material 50. The bonding of the coated rods 30 in the concrete matrix 40 was analytically measured.
A series of pull-out tests was conducted to assess the bond strengths of the embedded coated rods 30 with several compositional embodiments of the glass coating material 20. The results of pull-out testing are shown in
Preparation of Test Mortar.
Uncoated steel rods 10 and coated rods 30 were embedded in a mortar prepared using the guidelines presented in ASTM C109, Standard Method for Determining Compressive Strength of Hydraulic Mortars. The proportion of the standard mortar was one part cement (Type I) to 2.75 parts of standard graded sand. The water-to-cement ratio was maintained at 0.485. Test cylinders were prepared for each mortar batch and tested to investigate the compressive strength at 7 and 30 days.
Preparation and Testing of Embedded Rods for Pull-out Testing.
Each uncoated 10 and glass coated test rod 30 was inserted in a 50.8-mm in diameter, 101.6-mm long plastic cylinder mold filled with fresh mortar. The respective rods 10, 30 were clamped at the top so that a 63.5-mm length of each respective rod 10, 30 was under the mortar; for the coated rods 30, the portion under mortar was glass coated. Each cylinder was tapped and vibrated to remove entrapped air and also to consolidate the mortar. The samples were kept in a 100% humidity environment at room temperature and cured, with curing times ranging from 7 days and to 60 days. After curing, the test cylinders were de-molded and the mounted in the test apparatus and the force required to pull each respective rod 10, 30 out of the mortar was measured.
The testing pin-pull results for steel after up to 60 days in mortar indicate that the bond strength of the uncoated pins decreases from about 4 MPa to about 2.2 MPa between seven days and 28 days of curing. This is consistent with reports in the literature for decreasing bond strength between cement paste and reinforcing steel with increasing curing time age, particularly from 1 to 14 days. However, due the hydration reaction of cement with the reactive Ca-silicate particles used for the glass coated samples 30, these bond strengths increase from 1.2 MPa to 6.60 kPa with an increase of curing time from three days to 60 days. Further, glass coated steel pins 30 with reactive calcium silicate have about three times the bond strength of bare steel pins after 60 days in cement.
Steel-reinforced concrete composite material 50 benefitting from increased bond strength and decreased degradation of the steel 10 from corrosive attack by the concrete matrix 40 give rise to a number of uses, such a structural material for floors and decking, hardened or reinforced civilian and military structures, sewage pipe, geotechnical anchorages, and the like. Further, the strong bond formed between the glass-coated steel 30 (with or without calcium silicate particles or the like dispersed therein as bonding enhancers) and the cementitious material 40 enables design options such as concrete-filled steel tubes or casings.
Further, the glass composition may be optimized to be self-sealing. As the glasses have relatively low softening temperatures, they are well suited for low temperature applications, such as retrofit and remediation applications. Additionally, glass-tape composites may be made with these compositions that may be wrapped around steel members and then fused thereto via the direct application of heat, such as by induction or a torch, to provide corrosion protection and/or an enhanced bonding surface.
Referring to
The desired properties of the novel glass composite include 1) a coefficient of thermal expansion (CTE) that is more compatible with the steel alloy that is to be coated, 2) a softening temperature that is relatively low (<700 degrees Celsius) to ensure low processing temperatures that do not degrade the mechanical properties of the steel, and 3) outstanding corrosion-resistance to the alkaline environment of wet cement. The novel glass composite comprises at least 4.0% (wt) K2O and at least 5.6% (wt) ZrO2, with about 4-20% (wt) of K2O, and/or about 5-20% (wt) of ZrO2, whereas both K2O and ZrO2 are significantly increased compared to the conventional groundcoats.
In another embodiment, as shown in
The chemically-active glass compositions 100 provide added protection for steel rods 10 used in RC structures. The glasses 100 may be used as the frit component in a reactive enamel formulation for enhanced bond strengths between the concrete 40 and steel 10. The glasses 100 contain relatively high concentrations of P2O5 (typically between about 5 and about 20 weight percent, more typically between about 9 and about 18 weight percent), far in excess of the concentrations than used in the conventional groundcoat compositions (typically about 0.5-1 weight percent, see Table 1). In the present novel technology, P2O5 is released as phosphate ions when attacked by a corrosive aqueous alkaline solution. The released phosphate anions react with calcium cations, and other constituents in the corrosive environment, to form a stable hydroxyapatite (HAp) coating 110 on exposed metal, such as steel reinforcing rods 10. This HAp coating 110 provides protection of the metal 10 against corrosion. Representative compositions of phosphate-releasing glasses 100 are given in Table 2. The glasses 100 are designated as ‘ARE-xP’ glasses, where ‘x’ is a composition number, and ‘P’ signifies a phosphate-releasing composition. The coefficient of thermal expansion (CTE) and dilatometric softening temperature for each glass is also listed. These properties were determined by dilatometry.
Microscopic and spectroscopic analyses of the surfaces of the steel 10 enameled with phosphate-releasing glasses, such as the ARE-8P composition, as well as phosphate-free glasses, such as the ARE-4 composition, show that the phosphate-releasing glass 100 provides enhanced corrosion protection to the steel 10, even in regions where glass 100 did not initially coat the steel 10. For example, micro-Raman spectroscopy revealed that holes 125 in the ARE-8P enamel coating 100 of up to several hundred microns were filled with HAp after 24 hours in Lawrence solution, a simulated cement effluent, whereas only rust (iron oxide) was detected in similar defects in an ARE-4 enamel on steel 10. Similar results were noted when the same samples were exposed to a wet salt environment.
In general, the glass coating 100 has a composition of
(100−x)(0.25Na2O.0.25B2O3.0.5SiO2).xP2O5 in mole percents,
where x is typically between about 5 and 15, more typically between about 5 and 13, and still more typically between about 5 and 9. The glass formulation typically includes small amounts of one or more of the following oxides, most typically present in amounts of less than about 6 mole percent: K2O, CaO, Al2O3, ZrO2, MnO2, NiO, and/or CoO.
In operation, one or more (typically a plurality) steel reinforcing members 10 are coated with phosphate-rich glass layers 100 for emplacement into a cementitious matrix. The respective steel reinforcing members 10 have a first coefficient of thermal expansion and wherein the respective glass layers 100 have a second coefficient of thermal expansion, typically generally matched to the first coefficient of thermal expansion. A cementitious matrix 40 is formed around each steel reinforcing member. Once placed into the cementitious environment 40, phosphate is released from the glass coatings 100. The phosphate reacts with the cementitious material 40 to form a hydroxyapatite layer 110 on steel surfaces 10 without glass coatings 100. The glass coatings 100 typically have compositions, in weight percent, of about 40-45% SiO2, 3-5% Al2O3, 5-25% B2O3, 3-15% K2O, 5-25% Na2O, 4-7% CaO, 1-2% ZrO2, 0-2% NiO, 0-2% CoO, and 5-20% P2O5 and each coating 120 typically has a coefficient of thermal expansion between about 12.5 and 13.5 ppm per degree Celsius.
While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.
This patent application claims priority to co-pending U.S. patent application Ser. Nos. 13/040,781, filed Mar. 4, 2011, which claims priority to Ser. No. 12/623,236, filed Nov. 20, 2009, and issued on Mar. 8, 2011 as U.S. Pat. No. 7,901,769, which claims priority to U.S. Provisional Patent Application Ser. No. 61/199,901, filed Nov. 21, 2008.
The invention was made in part from government support under Grant No. W911NF-07-2-0062 from the Department of the Army. The U.S. Government has certain rights in the invention.
