Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
  • B05D7/20—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to wires
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D5/00—Processes for applying liquids or other fluent materials to surfaces to obtain special surface effects, finishes or structures
  • B05D5/02—Processes for applying liquids or other fluent materials to surfaces to obtain special surface effects, finishes or structures to obtain a matt or rough surface
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
  • B05D7/14—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies
• • D—TEXTILES; PAPER
  • D07—ROPES; CABLES OTHER THAN ELECTRIC
  • D07B—ROPES OR CABLES IN GENERAL
  • D07B1/00—Constructional features of ropes or cables
  • D07B1/06—Ropes or cables built-up from metal wires, e.g. of section wires around a hemp core
  • D07B1/0693—Ropes or cables built-up from metal wires, e.g. of section wires around a hemp core having a strand configuration
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
  • E04C5/00—Reinforcing elements, e.g. for concrete; Auxiliary elements therefor
  • E04C5/01—Reinforcing elements of metal, e.g. with non-structural coatings
  • E04C5/015—Anti-corrosion coatings or treating compositions, e.g. containing waterglass or based on another metal
• • E—FIXED CONSTRUCTIONS
  • E04—BUILDING
  • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
  • E04C5/00—Reinforcing elements, e.g. for concrete; Auxiliary elements therefor
  • E04C5/08—Members specially adapted to be used in prestressed constructions
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/02—Processes for applying liquids or other fluent materials performed by spraying
  • B05D1/04—Processes for applying liquids or other fluent materials performed by spraying involving the use of an electrostatic field
  • B05D1/06—Applying particulate materials
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D2401/00—Form of the coating product, e.g. solution, water dispersion, powders or the like
  • B05D2401/20—Aqueous dispersion or solution
• • D—TEXTILES; PAPER
  • D07—ROPES; CABLES OTHER THAN ELECTRIC
  • D07B—ROPES OR CABLES IN GENERAL
  • D07B2201/00—Ropes or cables
  • D07B2201/20—Rope or cable components
  • D07B2201/2015—Strands
  • D07B2201/2042—Strands characterised by a coating
  • D07B2201/2044—Strands characterised by a coating comprising polymers
• • D—TEXTILES; PAPER
  • D07—ROPES; CABLES OTHER THAN ELECTRIC
  • D07B—ROPES OR CABLES IN GENERAL
  • D07B2201/00—Ropes or cables
  • D07B2201/20—Rope or cable components
  • D07B2201/2083—Jackets or coverings
  • D07B2201/2084—Jackets or coverings characterised by their shape
  • D07B2201/2086—Jackets or coverings characterised by their shape concerning the external shape
• • D—TEXTILES; PAPER
  • D07—ROPES; CABLES OTHER THAN ELECTRIC
  • D07B—ROPES OR CABLES IN GENERAL
  • D07B2207/00—Rope or cable making machines
  • D07B2207/40—Machine components
  • D07B2207/404—Heat treating devices; Corresponding methods
  • D07B2207/4068—Heat treating devices; Corresponding methods for curing
• • D—TEXTILES; PAPER
  • D07—ROPES; CABLES OTHER THAN ELECTRIC
  • D07B—ROPES OR CABLES IN GENERAL
  • D07B2501/00—Application field
  • D07B2501/20—Application field related to ropes or cables
  • D07B2501/2015—Construction industries
  • D07B2501/2023—Concrete enforcements

Definitions

• This invention relates to concrete prestressing members and particularly relates to forming an anti-corrosion plastics coating on prestressing strand.
• Prestressing strand for use in pre- or post- tensioning a concrete structure has one or more central core wires and a plurality of outer wires extending helically around the core wire or wires.
• GB Patent Specification No. 894945 published 26 April 1962 (Commonwealth Scientific and Industrial Research Organisation) describes metal reinforcing members, in the form of steel reinforcing beams, bar or wire, coated partially or wholly with a hardenable epoxy resin material with suitable granular material applied to the coated areas of the reinforcing members before the resin has finally hardened.
• the reason for the epoxy coating with applied granular material being given as providing improved bond strength between concrete and reinforcement member without the use of deformed reinforcing bars or rods or indented reinforcing wire.
• the object of the present invention is to provide an anti-corrosion coating for prestressing strand.
• prestressing strand is flexible any coating thereon has to adhere to and flex with the strand without loss of integrity if it is to maintain its anti-corrosion function.
• a flexible prestressing strand for concrete comprises one or more central core wires and a plurality of outer wires extending helically around the core with an adherent synthetic resin coating which is only partially cured so that it is flexible but continuous and substantially impermeable and has the helical configuration of the external surface of the strand evident on the external surface of the coating; the coated strand being sufficiently flexible to be coiled and uncoiled.
• a method of providing prestressing strand with an anti-corrosion coating in accordance with the present invention comprises the steps of:-
• the coating is a partially cured thermosettable epoxy resin.
• grit-form material is partially embedded in the coating so as to be partially exposed at the external surface thereof with substantially none of the grit-form material penetrating to contact the strand.
• the purpose of the grit-form material is to improve and control in known manner bond transfer between the coated strand and the concrete in which it is embedded.
• Coated prestressing strand in accordance with the present invention exhibits the features of:-
• prestressing concrete structures there are two important characteristics relating to the prestressing members, these are "transfer length” and "development length".
• transfer length In a typical precast, pre-tensioned prestressed structure, the prestressing strands are placed in empty forms, stretched to a high tension and held at the tension by temporary anchors located beyond the ends of the forms. The forms are then filled with concrete which completely surrounds each strand. When the concrete has cured to the required strength, the temporary anchors are removed, and the load in the strand that was carried by the anchors is transferred to the structure by bond between the strand and the concrete. The tension in the strand at the extreme end of the structure is zero. Within the structure the strand is trying to contract to the zero load length that existed before it was stretched.
• Transfer length As the unit strength of the bond between the strand and the concrete is small with respect to the total load in the strand, an appreciable length of contact between concrete and strand is required to transfer the full load from strand to concrete.
• the length of contact required to transfer the full load is called "transfer length"; which can be defined as the distance from the end of the structure to the point at which the full load in a fully bonded strand has been transferred to the concrete. Transfer length is influenced by strand size, shape, material and surface condition and by the consistency of the concrete placed about it.
• Tests on seven-wire strands with diameters up to and including one-half inch (1.27 cm) indicate no difference in transfer length for concrete strengths of 1700 to 5000 psi (120 to 350 kg./ sq.cm).
• a pre-tensioned bonded prestressed concrete flexural structure is loaded from its normal working load to ultimate flexural capacity, there is a large increase in the tension in the strand.
• the tension in the strand increases, the length of strand required to transfer the tension to the concrete also increases.
• the length required to develop the tension which exists at the time of ultimate flexural failure is called the "development length". For an uncoated seven-wire strand, the development length is much greater than the transfer length..
• the transfer length is computed to be approximately twenty-five inches (63.5 cm), whereas the development length is approximately eighty inches (203 cm). In most cases for a strand that is debonded at the end of the structure, the development length becomes 160 inches (406 cm).
• the size and number of strands in a particular member are frequently determined by the development length, and a more economical design can be achieved if the development length is shorter. It is probable that a much shorter development length can be obtained with a grit-coated strand in accordance with the invention.
• the process generally involves the sequential steps of cleaning the strand, heating the strand to a predetermined temperature, electrostatically coating the heated strand in a fluidized bed, optional grit application during the gel phase of the plastic coating heated by the heated strand, and quenching at a desired stage of curing of the plastic coating.
• the process preferably is a continuous one whereby strand passes from a pay-off sequentially through the various steps of the process to a take-up.
• the process line can include a known holiday detector, e.g., a sixty-seven one-half volt DC holiday detector, as specified in the previously mentioned ASTM standard specification. Such detection normally would occur at the last stage before take-up.
• the cleaning step preferably is accomplished by passing the strand from the pay-off through a known ultrasonic washer and a rinse tank. This is a known manner of cleaning using well-known equipment. Abrasive blasting is unnecessary.
• the strand passes through an induction heater where it is heated to a temperature determined by, inter alia, the fusion and curing characteristics of the plastic to be coated.
• the strand is heated to between 350°F (177°C) and 550°F (288°C), preferably 400°F (204°C) to 450°F (232°C), so as to be at a temperature of approximately 400°F (204°C) to 410°F (210°C) when contacted by resin powder as hereinafter described.
• the heated strand is contacted with resin powder and coated electrostatically immediately after leaving the induction heater.
• this is accomplished by passing the heated strand through a coater to be coated by electrostatic fluidized bed powder deposition.
• This is a known coating technique using commercially available coating equipment.
• powder particles are aerated in a fluidizing chamber and are electrostatically charged by ionized air which passes through a porous plate at the base of the chamber. As the powder particles become charged, they repel each other and rise above the chamber, forming a cloud of charged particles. When the grounded strand is conveyed through the cloud, the charged powder particles, because of their opposite potential, are attracted to the strand.
• the powder particles form a generally uniform coating, being more attracted to exposed areas than to those already insulated. Coating thickness is controlled by applied voltage to the charging media and exposure time to the cloud.
• a suitable commercially available coater is produced by Electrostatic Equipment Corporation, New Haven, Connecticut, U.S.A., designated as Model 700. The coater includes a powder management system for handling the resin powder.
• a suitable resin coating powder is SCOTCHKOTE (Trade Mark) brand 213, produced by Minnesota Mining and Manufacturing Company, Saint Paul, Minnesota, U.S.A.
• This is a fusion bonded epoxy coating comprising a one-part heat curable, thermosetting powdered epoxy coating, known for use in providing corrosion protection of pipe, girth welds and concrete reinforcing bars. It is stated to have a gel time at 400°F (204°C) of 5-8 seconds.
• the cure schedule specifies a minimum time to quench of twenty-eight seconds for an application temperature of 450°F (232°C) to 463°F (239°C).
• the epoxy is not fully cured, but limited to approximately eighty per cent to ninety per cent of final cure.
• Degree of cure can be approximated by solvent tests of the epoxy coating, and can be regulated by application temperature and time of quench.
• the SCOTCHKOTE 213 product is usable, it has been determined that a somewhat longer gel time is desirable as giving better flow of the melted epoxy powder, thus helping to avoid the occurrence of pin holes or holidays.
• Gel time can be determined to some extent by the temperature of the wire at the time of application of the epoxy powder.
• epoxy powders having longer gel times are available, and good results have been obtained using an epoxy powder of Hysol Division of Dexter Corporation, having a gel time of approximately twenty seconds. In general, prolonging the gel time facilitates obtaining a somewhat thinner, but still impermeable, coating.
• acceptable corrosion-protective coatings of approximately thirty-five mils (0.91 mm) to forty-five mils (1.17 mm) have been obtained with a gel time of approximately seven seconds
• acceptable corrosion-protective coatings of approximately twenty-five mils (0.65 mm) thickness have been obtained using a product with an approximately twenty second gel time.
• coating thicknesses of from ten mils (0.26 mm) to fifty mils (1.3 mm) are workable and obtainable, with a range of about twenty mils (0.52 mm) to forty mils (1.04 mm) being presently preferred.
• the heated strand leaves the powder coater with its epoxy coating in a viscous state, ready to receive the optional grit.
• the grit should be applied as soon as possible after the melted epoxy has flowed sufficiently to close all holidays, but while the viscosity is sufficient to prevent the grit from penetrating to the strand.
• the grit may be applied in a number of manners, but preferably it is applied by pneumatic spray guns, and four such spray guns oriented at 90° from each other have been found satisfactory.
• the spray force should be regulated in keeping with the particle sizes and the viscosity condition of the epoxy so as to partially, but firmly, embed the grit in the viscous epoxy, short of contact with the steel strand, so as to minimise the possibility of creating flow paths for corrosive elements along the interfaces of the grit particles and the epoxy in which they are embedded, such that they will have exposed surfaces to bond with the concrete.
• Grit sizes of from about seventy to about two hundred mesh Standard Tyler Series have been found to be satisfactory, depending on the bond characteristics desired.
• the grit may be of any of various materials, including glass frit or beads, or sand.
• the coated strand, with or without applied grit, is then passed through a quench tank at the desired stage of curing of the epoxy, passes therefrom optionally through a spark tester to detect pin holes and holidays, and thence to the take-up.
• Prestressing strand is typically shipped in coil packs of long length and it is possible to vary the process during treatment of a single reel, so as to provide a coil having sections with different specified characteristics.
• epoxies are the presently preferred coatings
• other plastic resins can be used. It has been found that excellent corrosion resistance under high tension is obtainable by an epoxy coating. The coating is strongly adherent to the strand, is tough, and firmly embeds the grit. It is of compatible ductility with prestressing strand. It has good abrasion resistance, and good integrity under the conditions of stress and bending angles to be encountered. Bond transfer is easily satisfactorily controllable by varying grit size and density of application, and, unlike conventional coated reinforcing bar, a coating thickness adequate to provide the desired corrosion resistance can be used without losing control of the bond characteristics, as the grit or abrasive readily compensates for the changed surface condition arising from the plastic coating.
• the coated strand handles satisfactorily in shipping and in the field, and, for post-tensioned strands that are to be grouted, avoids the problem of rust or corrosion before the grout is injected.
• Such strands are often exposed at the job site for a considerable time before placing in the concrete structure.
• most specifications require that strands be tensioned and grouted within seven days of placement in the concrete structure. This is often a severe handicap to the engineer specifying job procedure and to the contractor who cannot schedule his operation in the best manner.
• electrostric application of the resin power in a fluidized bed is preferred, alternative methods of application include known electrostatic spray guns.

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• Engineering & Computer Science (AREA)
• Architecture (AREA)
• Civil Engineering (AREA)
• Structural Engineering (AREA)
• Life Sciences & Earth Sciences (AREA)
• Wood Science & Technology (AREA)
• Reinforcement Elements For Buildings (AREA)
• Application Of Or Painting With Fluid Materials (AREA)
• Laminated Bodies (AREA)
• Wire Processing (AREA)
• Curing Cements, Concrete, And Artificial Stone (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Ropes Or Cables (AREA)
• Moulds, Cores, Or Mandrels (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

Country Status (8)

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• 1983
  • 1983-10-19 ZA ZA837766A patent/ZA837766B/xx unknown
  • 1983-10-19 CA CA000439302A patent/CA1228998A/en not_active Expired
  • 1983-10-21 DE DE8383306399T patent/DE3374885D1/de not_active Expired
  • 1983-10-21 EP EP83306399A patent/EP0110542B1/en not_active Expired
  • 1983-10-21 AT AT83306399T patent/ATE31338T1/de not_active IP Right Cessation
  • 1983-10-25 AU AU20559/83A patent/AU578589B2/en not_active Expired
  • 1983-10-27 ES ES526828A patent/ES526828A0/es active Granted
  • 1983-10-28 JP JP58201130A patent/JPS59130960A/ja active Granted

Patent Citations (1)

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