JPH0328551B2 - - Google Patents

Abstract

Concrete strengthening members, particularly prestressing tendons such as strands of steel wire, are provided with a strongly adherent plastic coating which may be substantially impermeable for improved corrosion resistance, and/or which may have embedded therein abrasive or grit-form particles to provide improved bond with the concrete, and particularly to provide controllable bond transfer in prestressing tendons of the pretensioned type. The plastic coating preferably is applied electrostatically in powder form, and fusion bonded by heat. The abrasive can be applied by spraying during a viscous state of the heated resin, and can be varied as to size and spacing density so as to control the surface condition and the bonding effect. Fusion and curing heat may come from preheating of the member before application of the resin powder, which preferably is a heat curable, thermosetting epoxy. Coating thickness and grit application are readily variable to meet particular requirements. Particularly advantageous results are achievable for high strength steel strands for prestressing concrete by pretensioning, facilitating their use where previously considered impractical or impossible.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to steel members for strengthening concrete, particularly wire and strand type prestressing for prestressing by pretensioning or post tensioning. Regarding reinforcing steel, reinforcing bars, wires, etc. can be applied at several points. More specifically, the present invention resides in improving the corrosion resistance and/or bond control of the component.

BACKGROUND OF THE INVENTION Corrosion of steel reinforcement members in concrete has been a long-standing technical problem and is very sensitive. For example, it is known to coat reinforcing bars with an epoxy coating using an electrostatic spray gun;
for Testing and Materials (ASTM)
Standard Specification for Epoxy Coated Reinforcement Bars and Steel, Including Deformed and General Steel Reinforcement Bars, with a Protective Epoxy Coating Using Electrostatic Coating Spraying Methods Under ASTM A775-81 and D3963-81 was issued. This method clearly avoids bonding problems encountered with thicker thicknesses of 0.13-0.3mm.
(5 to 12 mils) is not without problems, and thinner thicknesses may result in two unpainted holes (visible to the naked eye) per linear foot of coated bar. Including coating integrity and permeability issues as indicated by the ASTM standard, which allows no pinholes. Epoxy coating materials are available on the market, especially for use in coating reinforcing bars. However, the problem remains of obtaining an adequate corrosion protection coating while maintaining a good quality bond with the concrete.

There is a corresponding problem, but an even bigger and more important one: high-strength steel wires and strands used in prestressed concrete [hereinafter referred to as
It exists in the case of PC (Prestressed Concrete) wire and strand. Of course, the strands are formed by spinning a number of wires (typically six) surrounding each other around a central core. The importance of this problem is illustrated by the fact that the use of PC wire or strands is suppressed or prohibited in areas where it could be used advantageously. Also, the U.S. Department of Transportation (US Department of Transportation)
Department of Transportation) Federal Highway Administration (1981)
The report dated February 10, 2017 was titled “Corrosion Protection of Reinforcement
The report deals with standards for the application of all bridge deck reinforcements, whether prestressed or not, that are susceptible to corrosion in anti-icing salt or salt water environments. “All conventional reinforcement must be coated with an epoxy coating, but pretensioning should not be allowed on bridge decks because there is no known way to eliminate the possibility of corrosion, and polyethylene However, in addition to grouting, provision should be made to protect post-tensioned reinforcing steel. Demonstrates that epoxy coating of reinforcing bars or rods is not the only method of corrosion protection for bridge decks.
In a follow-up dated April 14, 1981, ``There is currently no known method for epoxy-coating the strands in pretensioned materials, and the potential for corrosion exists in saltwater return areas and areas where anti-icing chemicals are used.''"It has said. Attempts have been made to develop corrosion-resistant PC reinforcing steels, some of which have been used due to the unavailability of more efficient and more economical materials. The use of galvanized strands has therefore been associated with corrosion and has often been advocated by designers who are not very familiar with the properties of galvanized strands. Galvanized strands are weaker than stress-relieved strands of the same size, and galvanized strands can be processed to have all of the desirable properties obtained by stress-relieving uncoated strands. Can not.
Galvanized strands are fairly expensive per unit of strength, their bonding properties are not constant, and chemical reactions may occur between the zinc coating and the cement paste in the concrete. Even though galvanized strands were available prior to the development of prestressed concrete, they were not commonly used. Individual, unbonded, post-tensioned strands are used in the construction of floorboards in garages, apartments and office buildings, and reinforcing steel made of several parallel wires is used in a similar manner. . These reinforcing steel materials are typically coated with corrosion-resistant grease, stored in a tube, and installed at a predetermined location, after which concrete is placed around the steel materials to create a slab. After curing the concrete, tension is applied to the reinforcing steel,
The reinforcing steel is then permanently kept under tension by means of fasteners at both ends. Currently, this type of reinforcing steel is coated with grease and placed in a plastic tube. Although this is an improvement over traditional paper wraps, typically the tube of the fixture must be removed in order for the fixture to secure the strands, so Corrosion still occurs. Additionally, the relatively thin plastic of the tube is sometimes damaged during handling.

Post-tensioned and grouted reinforcing materials have a service life as long as the prestressed concrete itself. The reinforcing steel may be threaded through a cavity cast into the concrete, or the reinforcing steel may be placed into an oversized flexible metal tube or other type of tube before the concrete is placed. After the concrete has cured, tension is applied to the reinforcing steel, and the cavity around the reinforcing steel is filled with cement liquid using a pump.
The reinforcing steel materials are properly surface-treated and assembled.
The cavity will also be filled if the cement solution is properly injected. However, in actual operation, this is not the case, leaving areas in the cavity susceptible to corrosion.

A typical reinforcing material in the precasting of pretensioned parts is strands of seven wires that are tensioned and secured during formation. Concrete is then poured around the strands, and once the concrete has hardened, the strands are removed from the fixings on the outside of the strands and the prestressing forces in the strands are transferred to the concrete by means of the bond between the steel and the concrete. Therefore, with such pretensioned PC reinforcing steel materials, there is not only the problem of corrosion protection, but also the problem of prestress transfer due to the bond between the pretensioned PC reinforcing steel material and concrete.

Patent technology amply discloses various solutions to the problem of corrosion protection and/or bonding properties, including the same disclosure, such as Billner US Pat. No. 2,319,105; No. 2414011 describes thermoplastic and thermosetting coatings for bonding concrete bodies and concrete reinforcement. Simonsson U.S. Patent No. 2,591,625 and No.
No. 2,611,945 includes a coating that includes a silicon-containing material.
Wijard, U.S. Pat. No. 3,030,664 discloses a suspension of hydraulic cement and rubber in suitable proportions in which steam curing of the concrete converts the reinforcing components to the concrete into a strong layer with good adhesion. A reinforcing member is disclosed with a coating consisting of a liquid suspension and possibly also serving as a rust protection film. US Pat. No. 3,293,811 to Rice discloses an epoxy resin coating on a PC strand to protect a serrated tooth notch supported by a fixture wedge. Mager
U.S. Pat. No. 3,377,757 relates to a tank prestressed with a plastic-coated reinforcing steel disposed around a steel storage tank, the plastic coating being for the purpose of protecting the reinforcing steel from corrosive factors. Lang U.S. Patent No. 3513609
The No. 1 relates to reinforcing steel of the post-tensioned type, including one embodiment in which the reinforcing steel includes a hardenable plastic material, such as an epoxy resin, between the wire or strand and an outer plastic coating, The resin cures while the wire is held under tension, thereby securing the wire to the outer plastic sheathing and thus to the concrete structure along the length of the wire. The curable resin initially provides a lubricating effect and, after curing, a bonding effect. The resin is cured by passing an electric current through the core wire. Lang, US Pat. No. 3,596,330, is for the same material. U.S. Pat. No. 3,596,330 to Scotto discloses a structural tension member made of steel wire or the like with a sheath or coating of polypropylene or other impermeable corrosion-resistant material. Lang
U.S. Pat. No. 3,646,748 discloses a PC strand encased in a corrosion inhibiting material and surrounded by a seamless plastic jacket that tightly encloses the encased strand. U.S. Pat. No. 3,755,003 to Palm discloses a concrete reinforcing steel member coated with a coating consisting of an intimate mixture of powdered metal and organic components plus residue from a composition containing a hexavalent chromium donor. It is stated that the coating provides corrosion resistance and also enhances the adhesion of the concrete to the coated component. U.S. Pat. No. 3,922,437 to Kitta includes an internal resin layer and then a coating of the PC strands with a lubricant-containing thermoplastic, and states that the internal resin layer provides increased corrosion protection.

It is clear from the foregoing that the problem to which the present invention is directed is a long-standing and important one, and that various solutions and measures have already been proposed. but,
To our knowledge, the solution and advantages provided by the present invention are not known in the prior art;
and cannot be reasonably derived from the prior art.

A strand for concrete prestressing having an adhesive coating of synthetic resin, the strand being a flexible strand having one or more central core wires and a plurality of outer wires twisted around the core wires in a spiral shape, The synthetic resin coating is a partially cured thermosetting epoxy resin, which is flexible but continuous and substantially impermeable, and the outer surface of the coating clearly shows the spiral shape of the outer surface of the strands. The object of the present invention is to provide a strand for concrete prestressing, which is characterized in that the coated strand is sufficiently flexible when being wound or unwound, and a method for producing the same. The basic idea of the present invention involves improving the steel for PC reinforcement, which allows obtaining an impermeable coating and at the same time controlling the bonding properties. It is thought that this method can be applied to conventional steel reinforcing bars and wire reinforcing materials. The plastic coating is preferably applied by electrostatic coating from a cloud of electrostatically charged resin particles produced by aeration of the resin powder and fusion bonding by heating. The abrasive material is preferably applied by spraying while the heated resin is in a viscous state at the time the resin is fused into the combined coating, and the size and spacing are adjusted to control the surface condition and bonding effect. Density can be changed. The characteristics that can be achieved by the present invention in various aspects are corrosion resistance under high tension conditions, ductility of the strands and coatings, adhesion of the coatings, toughness of the coatings, abrasion resistance of the coatings, resistance under stress and bending angle conditions. Obtaining the desired thickness while prioritizing coating integrity (an important feature for coiled and packaged fixation of strands), controllable bond force transfer, and control of bond properties.

3. Improved PC strands according to the present invention
form is provided. Firstly, a corrosion-resistant strand is provided which is designed primarily for post-tensioning using a plastic coating alone (in this case no consideration is given to transfer of bonding forces). Second, it provides a strand with greatly improved corrosion resistance + force transfer properties equal to or greater than that of the base strand, and therefore an easily controllable force transfer property into a strand with good corrosion resistance. give. Third, when corrosion resistance is not a primary consideration, the improvement in corrosion resistance is relatively modest, but provides a strand with easily controllable bond force transfer characteristics.

Before describing the illustrative preferred embodiment of the present invention, a second
It seems appropriate to discuss two well-known concepts or characteristics: "transition length" and "ultimate tension development length." In typical precasting, the pretensioned PC member, i.e. the strand or wire to be prestressed, is placed in an empty mold, stretched to high tension, and tensioned by temporary fixing devices placed outside the ends of the mold. It is kept in the hung state. The mold is then filled with concrete to completely surround each reinforcing steel member. When the concrete has hardened to the required strength, the temporary fixing device is removed and the load in the reinforcing steel supported by the fixing device is transferred to the concrete member by the bond between the reinforcing steel and the concrete. PC
The tension in the reinforcing steel is zero at the tip of the concrete member. Inside the PC concrete member, the reinforcing steel tends to contract to its zero-load length before being stretched. This is prevented by the bonding force or adhesion between the reinforcing steel surface and the concrete. Because the strength per unit length of the bond between reinforcement steel and concrete is small relative to the total load on the reinforcement steel, a considerable contact distance between the reinforcement steel and the reinforcement steel is Required to transfer loads. The contact length required to transfer the full load is called the "transfer length." The "transfer length" can be defined as the distance from the end of the PC concrete member to the point where the full load of the fully bonded reinforcing steel is transferred to the concrete. "Transition length" is the size of the reinforcing steel material,
It depends on the shape, material and surface condition and on the concrete and consistency placed around the reinforcing steel. 7 for diameters up to 1.27 cm (1/2 inch)
The test on a strand consisting of book wire is
There was no difference in transition length for concrete strengths from 1195 t/m ² to 3515 t/m ² (1700 psi to 5000 psi). The second concept or characteristic is known as the "ultimate tension development length." When pretensioned bonded prestressed concrete flexures are loaded from normal service loads to ultimate bending capacity, the tension in the strands increases significantly. As the tension in the strands increases, the length of the strands required to transfer the tension to the concrete also increases. The length required to generate the tension present at ultimate bending failure is called the "ultimate tension generation length." For a strand of seven uncoated wires, the ultimate tension development length is significantly larger than the transition length. For a typical 1/2 inch diameter uncoated strand, the transition length is calculated to be approximately 63.5 cm (25 inches), whereas the ultimate tension development length is approximately 203 cm (80 inches). . The ultimate tension development length for most unbonded strands at the ends of a PC concrete member is 406 cm (160 inches). The dimensions and number of strands in an individual PC concrete member are often determined by the ultimate tension length, and more economical designs can be achieved if the ultimate tension length is shorter. Much shorter ultimate tension lengths can be obtained with coarse-grained coatings according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production of improved reinforcing steels or other components according to the present invention, processing generally involves washing the reinforcing steel, heating the reinforcing steel to a predetermined temperature, or heating the reinforcing steel in a fluidized bed. electrostatically painted coatings on reinforcing steel materials, appropriate provision of coarse particles in the gel phase of epoxy resin coatings heated by heated reinforcing steel materials, and successive cooling at desired stages after curing of the epoxy resin coatings. Including process. Preferably, the process is continuous, with the reinforcing steel or strand being passed from an unwinding device through the various stages of the process to a winding device. There is no special need, but if the processing system is
Known blank hole detection devices may be included, such as a 67.5 volt DC blank hole detection device, as specified in ASTM standards. This detection is usually done in the last step before winding.

Preferably, the cleaning step is carried out by passing the strand from the unwinding device through a known ultrasonic cleaner and rinsing tank. This is a well known cleaning method using well known equipment. No abrasive spraying required.

From the rinse tank, the strand passes through an induction heater and the strand is heated to a temperature determined by, among other things, the fusing and curing properties of the plastic to be coated. Typically, strands are 177~
288℃ (350~550〓), preferably 204~232℃
(400~450〓), and the temperature is about 204~210℃ (400~410〓) upon contact with the resin powder.

The heated strand comes into contact with the resin powder and is coated with an electrostatic coating immediately after leaving the induction heater. Preferably, this is done by passing the heated strand through a coating device which applies the coating by electrostatic fluidized bed powder deposition. This is a known coating technique using commercially available coating equipment. In this coating process, powder particles are foamed in a fluidization chamber and electrostatically charged by ionized air passing through a perforated plate at the bottom of the fluidization chamber. When the powder particles are charged, they repel each other and rise to the top of the fluidization chamber, forming a cloud of electrostatically charged particles. When a grounded strand or wire is carried through the cloud, charged powder particles are attracted to the strand or wire due to their opposite charge. The powder particles generally form a uniform coating and are more attracted to areas that are still bare than to areas already insulated with powder particles. The thickness of the coating is controlled by the voltage applied to the charged medium and the time of exposure to the cloud. A suitable commercially available coating device is the Model 700 manufactured by Electrostatic Equipment Corporation, New Haven, Connecticut, USA. The coating device includes a powder processing device for processing resin powder.

Suitable resin coating powders are available from the Minnesota Mining and Manufacturing Company.
Company (Saint Paul, Minnesota) under the trade name SCOTCHKOTE 213.
It produces a one-pack thermoset thermoset powdered epoxy fused bonded epoxy coating and is known to be used to protect pipes, girth wells and concrete reinforcing bars from corrosion. There is. 5~ at 204℃ (400〓)
It is said to have a gel time of 8 seconds. The curing procedure specifies a minimum cooling time of 28 seconds for a coating application temperature of 232-239°C (450-463°C). However, for the purposes of this invention the epoxy is not fully cured;
It is rather preferred that the cure be limited to about 80-90% of the final cure. The degree of cure can be estimated by solvent testing of the epoxy coating and can be adjusted by the operating temperature and cooling time. Although Scotchicoat 213 products can be used, we have found that a slightly longer gel time is desirable to provide better flow of the molten epoxy powder and thus avoid the occurrence of pinholes or bleed holes. Gel time is determined in part by the temperature of the wire at the time of application of the epoxy powder. Alternatively, epoxy powders with longer gel times may be useful, such as Hysol Division of Dexter Corporation, which has a gel time of approximately 20 seconds.
Good results have been obtained using epoxy powders from the Company. In general, longer gel times tend to yield somewhat thinner, but still impermeable, coatings. For example about 0.89 to 1.1
A satisfactory corrosion-resistant coating of 35-45 mils can be obtained with a gel time of about 7 seconds;
Satisfactory corrosion-resistant coatings with a thickness of 100 mils are obtained by using a product with a gel time of about 20 seconds. Generally to determine the desired properties of the fusion bond coating
Coating thicknesses of 0.25-1.3 mm (10-50 mils) are available and obtainable; approximately 0.51-1.0 mm (20-50 mils)
40 mils) is considered preferred by the present invention.

The heated strand leaves the powder coating equipment with a sticky epoxy coating and is ready to accept sand as appropriate. In general, sand should be applied as soon as possible when the molten epoxy has flowed sufficiently to fill all unfilled holes and the viscosity is such that the sand does not penetrate into the metal. Sand can be applied in a number of ways, but is preferably applied by an air spray gun;
Four spray guns at 90° angles to each other have been found satisfactory. Depending on the viscosity of the epoxy resin and its viscosity conditions, the spraying force allows only some of the sand particles to settle firmly on the sticky epoxy, but does not come into contact with the steel strands, etc. The sand particles along the interface with the applied epoxy should be adjusted to minimize the possibility of creating channels for corrosive components that expose the outer surface for bonding with the concrete. A sand size of about 70 to 200 mesh standard Tyler sieves has been found to be sufficient depending on the desired binding properties. The sand may be glass frit or beads or any of a variety of materials including sand. Of course, for strands where corrosion is not a major consideration, contact between the sand and the steel strand need not be considered, and the epoxy coating only needs to be thick enough to firmly implant the sand for bond transfer purposes.

The coated strand with or without sand applied is then passed through a cooling tank at the desired stage of curing of the epoxy, from which it is optionally passed through a spark tester to detect pinholes and unfilled holes.
From there it enters the winder.

Satisfactory product is 3.05-9.15 m/min (10-30
built using a continuous working speed of 1 ft),
It is anticipated that operating speeds of up to 400 feet per minute will be achieved.

Particularly in the case of PC strands, which are typically shipped in long length coil packaging, processing can be modified between single reel processing to provide coils with zones that give different properties. can.

Epoxies are preferred coatings according to the invention.
It has been found that excellent corrosion resistance under high tension conditions can be obtained with epoxy coatings. The coating adheres strongly to the strands or other components, and is tough and firmly anchors the sand. The coating is ductile and compatible with PC strands and wires as well as reinforcing bar materials. The coating has good abrasion resistance and also has good integrity under stress bending angle conditions encountered. Bond force transfer is easily and satisfactorily controllable by varying the sand size and density used, and unlike conventional coated reinforcing bars, the coating thickness is sufficient to provide the desired corrosion resistance. Sand, sand or abrasives can be used without losing control of bonding properties since they readily offset changes in surface conditions resulting from plastic coatings. Indeed, sand-based plastic coatings offer a new range of bond force control or transfer length. The coated strands can be handled satisfactorily during transport and in the field, avoiding problems of rust or corrosion prior to grouting in the case of grouted post-tensioned strands. The strands are often exposed to the field for a considerable period of time before being placed in a concrete structure. Furthermore, most specifications require that the reinforcing steel or strands be tensioned and grouted (injected with liquid plaster) within seven days of being placed in the concrete structure. This poses a severe obstacle to the aforementioned field engineers and contractors who are unable to schedule their work in the best possible manner.

It is to be understood that the foregoing description is illustrative of preferred embodiments of the method of the invention and that various modifications may be made. For example, the application of electrostatic coating of resin powder in a fluidized bed is preferred, although known electrostatic spray gun coating, liquid application methods, etc. are also included. The preferred embodiments described above are preferred because of their practicality, controllability, and product excellence. It is also possible to powder coat the unheated reinforcing steel and then melt and harden the coating in a hardening furnace. However, this seems to be less practical and less effective.

Claims (1)

[Scope of Claims] 1. A strand for concrete prestressing having a synthetic resin adhesive coating, the strand having one or more central core wires and a plurality of outer wires twisted around the core wires in a spiral shape. It is a flexible strand, and the synthetic resin coating is a partially cured thermosetting epoxy resin, and is flexible, but
Continuous and substantially impermeable, the outer surface of the coating clearly reflects the spiral shape of the outer surface of the strands, and the coated strands are sufficiently flexible to be wound and unwound. A strand for concrete prestressing characterized by: 2. The strand of claim 1, wherein the coating is limited to a cure of 80-90% of the final cure. 3. A strand according to claim 1 or claim 2, wherein the coating is between 10 and 50 mils thick. 4. The strand of claim 3, wherein the coating is between 20 and 40 mils thick. 5. Claims 1 to 5, wherein the coating has sand-like particles partially embedded in the coating such that the coating does not substantially contact the strands and is partially exposed on the outer surface of the coating. The strand according to any of Item 4. 6. Pull out the strand from the feeding device, wash the strand, heat the strand to a predetermined temperature, and electrostatically apply a meltable thermosetting epoxy resin powder to the strand, until the resin is completely cured. A strand for concrete prestressing having an adhesive coating of synthetic resin, comprising previously cooling the applied coating and winding up the coated strand, the strand having one or more center cores and around the cores. A flexible strand having a plurality of external lines twisted together in the form of chives, the synthetic resin coating consisting of a partially cured thermosetting epoxy resin, which is flexible but continuous and substantially impermeable. For concrete prestressing, the outer surface of the coating clearly shows the spiral shape of the outer surface of the strand, and the coated strand is sufficiently flexible when being rolled or unwound. Method of manufacturing strands. 7. Claim 6, wherein the interval between the application of the resin powder to the heated strand and the cooling is adjusted to limit the curing of the coating to between 80 and 90% of the final cure. Manufacturing method described. 8. Process according to claim 7, in which the gel time of the applied coating is increased so that a thin but still impermeable coating is obtained. 9 sand-like particles are applied to the coating before cooling, such that the sand-like particles are partially embedded in the coating such that they do not substantially contact the strands and are partially exposed on the outer surface of the coating; A manufacturing method according to any one of claims 6 to 8, wherein the applied force is adjusted.