WO2011124252A1

WO2011124252A1 - A duct for a prestressing tendon

Info

Publication number: WO2011124252A1
Application number: PCT/EP2010/054535
Authority: WO
Inventors: Hans Rudolf Ganz; Pär Bo Erik GUSTAVSSON
Original assignee: Vsl International Ag
Priority date: 2010-04-06
Filing date: 2010-04-06
Publication date: 2011-10-13

Abstract

The present invention concerns a plastic duct (103) for a prestressing tendon (105). The material of the duct (103) contains at least one volatile migratory corrosion inhibiting agent arranged to be emitted out of the plastic duct (103). The present invention also relates to a method for producing such a duct (103) and to a construction element comprising such a duct (103). The volatile and/or migratory corrosion inhibiting agent provides corrosion protection for the prestressing tendon (105).

Description

A d uct for a prestressi ng tendon

TECHNICAL FIELD

The present invention relates to the general technical field of ducts that are suitable for receiving prestressing tendons. Such ducts are used, for instance, in concrete elements that are used in numerous construction works.

BACKGROUND OF THE INVENTION

Prestressing tendons are used to overcome concrete's natural weakness in tension. The method of prestressing concrete is used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Traditional reinforced concrete is based on the use of steel reinforcement bars (rebars) inside poured concrete. Prestressing tendons, generally composed of tensile cables made of high strength steel strands or rods, are used to provide a clamping force which produces a compressive stress on the concrete member to offset the tensile stress that the concrete member would otherwise experience due to an applied load.

The prestressing tendons are generally made up of a plurality of wires, bars or strands, the strands being further made up of several twisted metal wires. These tendons may have poorly protected areas, and are consequently sensitive to the action of oxidising agents. The lifespan of these tendons can thus be adversely affected. Known strands used in prestressing tendons are generally made up of metallic wires, for example steel wires. In some applications these wires are twisted together, and are coated with a protective filler and wrapped in a protective sheath of polymeric material, which may be extruded around the bundle of twisted-together wires.

Prestressed concrete can generally be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Prestressed concrete by pretensioning is obtained by casting concrete around already tensioned tendons. This method produces a good bond between the concrete and tendon, with concrete protecting the tendon from corrosion and allowing for direct transfer of tension. The cured concrete can then adhere and bond to the tendons, and when the tension is released, the compressive stress is transferred to the concrete by bond. However, this method requires stout anchoring points between which the tendon is to be stretched, and the tendons are usually in a straight line. No ducts are needed for the tendons.

Prestressed concrete by applying the method of bonded post- tensioned concrete comprises applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic or steel duct (often curved), to follow the area where otherwise tension would occur in the concrete element. A set of tendons is fed through the duct, and the concrete is poured. The tendons may also be fed after pouring the concrete. Once the concrete has hardened, the tendons are tensioned by e.g. hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications, they are wedged in position so that the tension is maintained after the jacks are removed and the pressure is transferred to the concrete through the anchoring elements. Finally the duct is then filled with a hardening protective filler such as grout to protect the tendons from corrosion and to provide bond. This method is commonly used to create monolithic slabs for building construction and in the construction of various types of bridges.

Unbonded post-tensioned concrete differs from bonded post- tensioning by providing tendons with permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a layer of grease (usually lithium-based) and covered by a plastic sheathing formed in an extrusion process. These coated and sheathed tendons are either placed directly inside the concrete or alternatively inside a duct which is finally filled with a hardening protective filler such as grout. Alternatively, non-coated and non-sheathed tendons (same as for bonded post-tensioned concrete above) may be installed inside the duct which then may be filled with a flexible protective filler such as grease or wax to prevent bond.

Especially in the method of post-tensioned concrete, non-coated and non-sheathed tendons inside the duct are prone to corrosion before the filling of the duct with a protective filler is completed. Thus, there is need for a solution which eliminates or minimises the risk of corrosion of the tendons inside the duct between the time of installation and application of the protective filler.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a plastic duct for a prestressing tendon is provided, wherein the duct is made of a plastic material, and the duct is characterised in that the material of the duct contains at least one volatile and/or migratory corrosion inhibiting agent arranged to be emitted out of the duct.

Thus, when a prestressing tendon is inside the duct, the corrosion inhibiting agent migrates (through solid or gas) to the steel surface and deposits on the surface of the tendon. In the absence of a protective filler, the corrosion inhibiting agent sublimates and deposits on the surface of the tendon. This has therefore the advantage that the tendon can be protected against corrosion. This is especially advantageous for the corrosion protection of the tendon during the time period when the duct is not yet filled with protective filler because during that time period the prestressing tendon is especially vulnerable to corrosion due to the absence of the protective filler.

According to a second aspect of the invention, a construction element comprising the plastic duct according to the first aspect of the present invention is provided, the construction element further comprising a prestressing tendon arranged to traverse the duct in a longitudinal direction of the duct and wherein the volatile and/or migratory corrosion inhibiting agent is arranged to deposit on the surface of the tendon.

According to a third aspect of the invention, a method of producing a plastic duct for a prestressing tendon is provided, the method being characterised by:

• feeding plastic material and at least one volatile and/or migratory corrosion inhibiting agent into an extrusion machine,

• melting the plastic material, and • extruding the melted plastic material and the at least one volatile and/or migratory corrosion inhibiting agent for obtaining the duct of a desired shape, wherein the volatile and/or migratory corrosion inhibiting agent is arranged to be emitted out of the plastic duct.

Other aspects of the invention are recited in the dependent claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of a non-limiting exemplary embodiment, with reference to the appended drawings, in which:

- Figure 1 is a simplified perspective view of a concrete beam including a duct and a prestressing tendon when not tensioned ; and

- Figure 2 is a cross-sectional view of the duct according to another variant of the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An embodiment of the present invention will be described in the following in more detail with reference to the attached figures. Identical functional and structural elements which appear in the different drawings are assigned the same reference numerals.

Figure 1 shows a concrete beam 101 , where a duct 103 made of plastic traverses the beam 101 in a given direction. In this example the duct 103 is made of thermoplastic polymer, such as high density polypropylene (HDPP) or high density polyethylene (HDPE). The concrete 101 has been cast around the duct 103. In this figure one prestressing tendon 105 is shown that, in turn, traverses the duct 103 in the longitudinal direction of the duct 103. In this figure the tendon 105 is not yet tensioned. The tendon 105 can be fed through the duct 103 either before the concrete is poured around the duct 103 or, alternatively, the tendon 105 can be fed through the duct 103 after this, even after the concrete has hardened.

As explained earlier, the tendon 103 is especially vulnerable to corrosion while the tendon is inside the non-filled duct 103. The protective filler reduces the risk of corrosion, but does not completely eliminate the risk. In the context of this application, the term corrosion is used to mean any process, for example electrochemical, which can have a deleterious effect on the chemical integrity, and hence the mechanical properties, of the tendon.

According to the present invention, the material of the plastic duct 103 contains at least one volatile and/or migratory corrosion inhibiting (VMCI) agent or component. Such corrosion inhibiting agents are known by the acronyms "VPI" (Vapour Phase Inhibitor) or "VPCI" (Vapour Phase Corrosion Inhibitor) or VCI (Volatile Corrosion Inhibitor) or MCI (Migratory Corrosion Inhibitor). In this application, the term VMCI is used to describe the family of such agents. The material of the duct 103 may contain VMCI substance up to 40 % of its total weight. Advantageously this percentage is between 10 and 20. It has been discovered that this concentration provides a sufficient vapour pressure for the wires of the tendon to be sufficiently protected. VMCI agents have a vapour pressure in the range of 10^"4 to 10^"6 mm Hg, for example, and are designed to sublimate or otherwise migrate out of the substrate in which they are contained, over weeks, months or years. The VMCI agents generally also contain corrosion inhibiting agents that are not volatile. In this case, the corrosion inhibiting agent is made from a mixture of volatile and non-volatile corrosion inhibiting agents.

For example, the VMCI substance impregnated in the duct may be a product such as that produced by the company Cortec under the trade name VpCI®, and in particular the type M-129 for HDPE and M-121 for HDPP, which are typically supplied in the form of HDPE or HDPP plastic pellets to be added in the duct manufacturing process. Once the duct 103 is formed, molecules of the volatile and/or migratory corrosion inhibiting agents can travel by

sublimation into and through the materials of the substance (air) between the duct 103 and the tendon 105 and, when they reach the surface of the

prestressing steel of the tendon 105, they become adsorbed on the steel surface, and create a hydrophobic barrier at the surface, thus improving the corrosion resistance of the tendon 105. The VMCI agents can migrate out of the duct 103, sublimate and form a tight nitrogen bond to the steel surface which provides anodic-cathodic corrosion protection even in the presence of moisture and chlorides. The longer the steel is exposed to the corrosion inhibiting vapour, the more effective the corrosion protection is. By sublimation is understood a transition of a compound from the solid state to the vapour state with no intermediate liquid stage.

In this example, the plastic duct is produced by extrusion. Extrusion is a process used to create objects that usually have a fixed cross-sectional profile. In a first example, a material to be extruded is pushed or drawn through a die of the desired cross-section. Plastic extrusion commonly uses plastic chips or pellets, which are usually provided in a hopper of the extrusion machine before going to the feed screw. The chips or pellets are heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the plastic material through the die, forming the material into the desired shape. The extrudate is then cooled and solidified while it is pushed or pulled through the die or water tank.

According to a second example, the extrusion process uses two chains formed by moulds, each mould forming one half of the cross section. These chains join together over a certain length where the closed cross section is formed and where the plastic is injected into the moulds (under vacuum or blow moulding to form a layer of constant thickness on the mould surface). The two moulds later open and let the duct 103 exit. Cooling of the melted plastic can be integrated into the moulds (channels through which air or cold water is circulated). The advantage of this method is that ducts with repetitive surface deformations, such as corrugations/ribs, can be produced to improve bond of the duct 103 to the concrete.

In accordance with the present invention, the VMCI agents may be added to the polymer material in powder or granulate form, during or before the polymeric duct material is extruded. For instance, in one solution, plastic pellets containing the VMCI agents are poured into and mixed in the hopper of the extrusion machine. Alternatively, or in addition to having the VMCI agents in the form of pellets, the VCMI agents can be poured into the hopper as a powder. In this case a more or less homogeneous mixture of plastic material containing VMCI agents is obtained once the plastic pellets melt. It is also possible to have two types of pellets: some plastic pellets containing the VCMI agents and some other plastic pellets not containing the VMCI agents.

A further variant of the duct 103 in accordance with the present invention is shown in Figure 2. In this solution, the duct 103 consists of an inner layer 201 and an outer layer 203. The second, outer layer 203, which is less permeable than the first layer 201 , can be extruded around the first layer (inner layer) 201 . The purpose of this outer layer is to prevent or reduce migration of the VMCI substance outside the duct 103, and thereby maximise the protective effect within the duct 103. The inner layer can advantageously be co-extruded with the outer layer, the material of the inner layer containing the VMCI agents, and the material of the outer layer not containing any VMCI agents. The transverse width, i.e. the thickness, of the inner layer 201 can for instance be 2 mm and the transverse width of the outer layer for instance 2 mm. The exact thickness of the layers determines the balance between protective and mechanical properties of the duct. For instance, a too thin duct may result in mechanical damage to the duct, or alternatively a too thick duct would result in a poor bond for bonded post-tensioned concrete.

In the co-extrusion process according to the first example, two or more materials are pushed through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling. Each material is fed to the die from a separate extruder. In the co-extrusion process it is also possible to use the process in accordance with the second example by using different moulds. The advantage of co-extrusion is that each ply of the laminate imparts a desired characteristic property, such as stiffness, sealing against migration of VMCI, concentration of the VMCI agents. The two layer structure makes it easier to obtain an ideal combination of these properties.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not limited to the disclosed embodiment. Other embodiments and variants are understood, and can be achieved by those skilled in the art when carrying out the claimed invention, based on a study of the drawings, the disclosure and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

1 . A duct (103) for a prestressing tendon (105), the duct (103) being made of a plastic material, characterised in that the material of the duct (103) contains at least one volatile and/or migratory corrosion inhibiting agent arranged to be emitted out of the duct (103).

2. A duct (103) according to claim 1 , wherein the duct (103) comprises an inner layer (201 ) comprising the at least one volatile and/or migratory corrosion inhibiting agent and an outer layer (203) not containing a volatile or migratory corrosion inhibiting agent.

3. A duct (103) according to claim 2, wherein the transverse width of the outer layer (203) and the transverse width of the inner layer (201 ) are determined by an optimal balance between protective properties and mechanical properties of the duct (103).

4. A duct (103) according to any of the preceding claims, wherein the concentration of the volatile and/or migratory inhibiting agent in the material of the duct (103) is in the range of 1 -40 % of the weight of the duct (103).

5. A duct (103) according to any of the preceding claims, wherein the concentration of the volatile and/or migratory inhibiting agent in the material of the duct (103) is in the range of 10-20 % of the weight of the duct (103).

6. A duct (103) according to any of the preceding claims, wherein the volatile and/or migratory corrosion inhibiting agent is arranged to sublimate from the duct (103).

7. A construction element comprising the duct (103) according to any one the preceding claims and further comprising a prestressing tendon (105) arranged to traverse the duct (103) in a longitudinal direction of the duct (103) and wherein the volatile and/or migratory corrosion inhibiting agent is arranged to deposit on the surface of the tendon (105).

8. A construction element according to claim 7, wherein the volatile and/or migratory corrosion inhibiting agent is arranged to deposit on the surface of the tendon (105) by sublimation.

9. A method of producing a plastic duct (103) for a prestressing tendon (105), characterised by: feeding plastic material and at least one volatile and/or migratory corrosion inhibiting agent into an extrusion machine, melting the plastic material, and extruding the melted plastic material and the at least one volatile and/or migratory corrosion inhibiting agent for obtaining the duct (103) of a desired shape, wherein the volatile and/or migratory corrosion inhibiting agent is arranged to be emitted out of the plastic duct (103).

10. A method of producing the duct (103) according to claim 9, wherein the plastic material and the at least one volatile and/or migratory corrosion inhibiting agent are fed into the extrusion machine in the form of pellets so that individual pellets are a mixture of the plastic material and the at least one volatile and/or migratory corrosion inhibiting agent.

1 1 . A method of producing the duct (103) according to claim 9, wherein the plastic material and the at least one volatile and/or migratory corrosion inhibiting agent are fed into the extrusion machine in the form of pellets so that a first type of pellets comprises the plastic material without the volatile and/or migratory corrosion inhibiting agent and a second type of pellets comprises the plastic material with the volatile and/or migratory corrosion inhibiting agent.

12. A method of producing the duct (103) according to claim 1 1 , wherein the first type of pellets and the second type of pellets are fed into the extrusion machine in a predetermined ratio for obtaining desired properties of the duct (103).

13. A method of producing the duct (103) according to claim 9, wherein the at least one volatile and/or migratory corrosion inhibiting agent is fed into the extrusion machine in the form of a powder.

14. A method of producing the duct (103) according to any one of claims 9 to 13, wherein the duct (103) is formed by co-extruding an inner layer (201 ) containing the at least one volatile and/or migratory corrosion inhibiting agent, and an outer layer (203) not containing a volatile or migratory corrosion inhibiting agent.

15. A method of producing the duct (103) according to any one of claims 9 to 14, wherein the extrusion is done by extruding the plastic material through a die.

1 6. A method of producing the duct (103) according to any one of claims 9 to 14, wherein the extrusion is done by injecting the plastic material into a tube formed by at least two moulds that are pressed together during the injection, and opening the moulds to eject the obtained duct (103) once the plastic material has cooled sufficiently inside the tube.