EP0110542B1

EP0110542B1 - Concrete strengthening members, particularly prestressing tendons, having improved corrosion resistance and/or bonding characteristics, and methods relating thereto

Info

Publication number: EP0110542B1
Application number: EP83306399A
Authority: EP
Inventors: Frederick F. Hunt; Donald J. Gillette
Original assignee: Florida Wire and Cable Co
Current assignee: Florida Wire And Cable Inc Te Jacksonville Flor
Priority date: 1982-10-28
Filing date: 1983-10-21
Publication date: 1987-12-09
Also published as: ZA837766B; AU2055983A; JPH0328551B2; JPS59130960A; ATE31338T1; EP0110542A1; CA1228998A; ES8407543A1; AU578589B2; ES526828A0; DE3374885D1

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

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.
Corrosion of prestressing strand in concrete has long been a problem and is exemplified by the fact that use of prestressing strand is discouraged or prohibited in certain areas where it advantageously could be used. Thus, in a Memorandum dated February 10, 1981, of the Federal Highway Administration, U.S. Department of Transportation, captioned "Corrosion Protection of Reinforcement in Bridge Decks," and dealing with criteria to be applied to all reinforcement in bridge decks, prestressed or otherwise, where de-icing salts or a salt water environment present the potential for corrosion, it is suggested that all conventional reinforcement be epoxy coated, but that "Pretensioning should not be permitted in bridge decks since there is no known way of eliminating the potential for corrosion," and that "Polyethylene ducts should be provided for protection of post-tensioned tendons in addition to grouting." In a follow-up Memorandum dated April 14, 1981, indicating that epoxy coating of rebars (reinforcing bars or rods) was not intended to be the only method of corrosion protection of bridge decks, it was stated that "In pre-tensioned work, there are currently no known methods for epoxy coating the strands, and the potential for corrosion exists in a salt water environment as well as in areas where de-icing chemicals are used."
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. As 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.
According to the present invention, 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:-

passing the strand from a pay-off;
cleaning the strand;
heating the strand to a predetermined temperature;
electrostatically applying a fusible synthetic resin powder to the strand;
quenching the applied coating before it is fully cured; and
passing the coated strand to a take-up.

Preferably, the coating is a partially cured thermosettable epoxy resin.
In an embodiment of the invention, 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:-

corrosion resistance under high tension,
ductility of strand and coating,
adherence of the coating,
toughness of the coating,
integrity of the coating under stress and bending angle (an important feature because of the packaging of strand in coil packs),
controllable bond transfer, and
desired coating thickness while retaining overriding control of bond characteristics.

In prestressing concrete structures there are two important characteristics relating to the prestressing members, these are "transfer length" and "development 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. Bond or adhesion between the surface of the strand and the concrete prevents this. 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). When 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. As 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.. For a typical one-half inch (1.27 cm) diameter uncoated strand, 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.
In coating prestressing 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. Although not specifically necessary, 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.
From the rinse tank, 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. Typically 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. Preferably 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. In this coating process, 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). However, 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. Although 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. Alternatively, 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. For instance, 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, whereas 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. In general, depending on the particular characteristics desired in the fusion bonded coating, it is considered that 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. In general, 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.
Satisfactory products have been made using continuous line speeds of ten feet per minute (3.05 metres per minute) to thirty feet per minute (9.14 metres per minute), and it is anticipated that line speeds of up to 400 feet per minute (122 metres per minute) are obtainable.
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.
Although 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. Furthermore, 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.
Although electrostric application of the resin power in a fluidized bed is preferred, alternative methods of application include known electrostatic spray guns.

Claims

1. A concrete prestressing metal member having an adherent coating of synthetic resin characterised in that the metal member is a flexible strand, having one or more central core wires and a plurality of outer wires extending helically around the core, and that the synthetic resin coating 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.

2. Coated prestressing strand as claimed in claim 1 characterised in that the exterior coating is a partially cured thermosettable epoxy resin.

3. Coated prestressing strand as claimed in claim 2 characterised in that the coating is limited to approximately 80% to 90% of final cure.

4. Coated prestressing strand as claimed in claim 2 or claim 3 characterised in that the coating is between 0.26 and 1.3 mm thick.

5. Coated prestressing strand as claimed in claim 4 characterised in that the coating is between 0.52 and 1.04 mm thick.

6. Coated prestressing strand as claimed in any of claims 1 to 5 characterised in that the coating has grit-form material partially embedded therein so as to be partially exposed at the external surface thereof with substantially none of the grit-form material penetrating to contact the strand.

7. A method of coating a concrete prestressing metal member characterised in that the metal member is a flexible strand, having one or more central core wires and a plurality of outer wires extending helically around the core, the method being continuous and comprising the steps of:-

passing the strand from a pay-off;

cleaning the strand;

heating the strand to a predetermined temperature;

electrostatically applying a fusible synthetic resin powder to the strand;

quenching the applied coating before it is fully cured; and,

passing the coated strand to a take-up.

8. A method as claimed in claim 7 wherein the fusible synthetic resin is a thermosettable epoxy resin and characterised in that the period between application of the powder to the heated strand and quenching is controlled to limit curing of the coating to approximately 80% to 90% of final cure.

9. A method as claimed in claim 8 characterised in that gel time of the applied coating is prolonged so that a thinner, but still impermeable coating is obtained.

10. A method as claimed in any of claims 7 to 9 characterised in that grit-form material is applied to the coating prior to quenching, the force of application being controlled so as to partially embed grit-form material in the coating such that grit-form material is exposed at the external surface of the coating but substantially none penetrates to contact the strand.

11. A method as claimed in any of claims 7 to 10 characterised in that the coated strand is packed in coil packs for shipment.