SE539878C2

SE539878C2 - Process for manufacturing a floating prestressed concrete structure and such a concrete structure

Info

Publication number: SE539878C2
Application number: SE1351054A
Authority: SE
Inventors: Odhe Fredrik; Lindberg Anders
Original assignee: Sf Marina System Int Ab
Priority date: 2013-09-13
Filing date: 2013-09-13
Publication date: 2018-01-02
Also published as: RS60065B1; SE1351054A1; WO2015038060A1; CY1122811T1; PL3044085T3; PT3044085T; LT3044085T; DK3044085T3; EP3044085B1; SI3044085T1; ES2773978T3; EP3044085A1; HRP20200459T1

Abstract

11 ABSTRACT A method of manufacturing a buoyant Concrete structure, comprising the stepsof placing at least one first reinforcement bar in a mould, substantially along a Iongitudi-nal extension of the mould; pouring concrete into the mould such that the concrete co-vers the at least one fioating element and the at least one reinforcement bar; and allow-ing the concrete to cure. The at least one first reinforcement bar comprises a non- magnetic material and is prestressed. (Fig. 1)

Description

PRESTRESSED, NON-MAGNETIC REINFORCEMENT IN BUOYANT CONCRETESTRUCTURES The present invention relates to reinforcement in buoyant concrete structures suchas pontoons, piers, breakwaters, ferry Iandings, floating house platforms and bathing platforms.

Technical Background of the lnvention Concrete structures for floating applications are a common component in today'scivil construction and buoyant concrete structures have been commercially available foralmost a hundred years. Among many applications there are pontoons for boat mooring,floating breakwaters, ferry landings and bathing platforms.

Concrete has a natural weakness in tension. Therefore, concrete structures relyon the strength of embedded reinforcement. This reinforcement normally consists ofiron rods and/or nets with or without added fibres of steel or plastic. The fact that anybuoyant concrete structure partly is submersed in water poses a great challenge to anysuch construction as chloride ions from the surrounding water will penetrate the con-crete and eventually come into contact with the reinforcement, thus causing the rein-forcement to corrode. Corrosion in turn leads to failure of the reinforcement bar and de-creased protection against tensile stress, which ultimately results in cracking of the con-crete. Therefore, when the calculated migration distance of the chloride ions equals thethickness of the concrete cover, the technical lifetime of the product is reached and itmust be replaced.

Industry standard to prolong the technical lifetime of a buoyant concrete structurehas been to either increase the quality of the concrete, thus slowing the chloride iontransport in the concrete, or to cast with thicker layers of concrete between the waterand the outmost part of the reinforcement, i.e. thicker topcoat layers also called con-crete cover. lncreasing the quality of the concrete also increases the cost of manufac-ture, while thicker topcoat layers require more concrete and lead to bulkier concretestructures which alter their floating characteristics.

Other materials have been proposed to replace steel and iron reinforcement barsto provide protection against corrosion, such as fibres of polymer, glass, carbon or ara-mid. WO 2011/108941 discloses a reinforcement system for concrete structures, such as pontoons, comprising reinforcement elements made of basalt or carbon fibres. The 2 reinforcement elements are interconnected by flexible bands into flat-packed units,which are rolled out into longer lengths at the construction site. One drawback with suchother materials is that they have a poor service life in the highly alkaline environment ofconcrete. Also, the characteristics of the proposed materials with respect to strength,creep and elasticity differ from those of metals.

Another disadvantage with reinforcement made from the non-metallic materials inbuoyant concrete structures is that the concrete has been shown to be susceptible tocracking or breaking in harsh sea conditions due to incoming waves. Therefore, there isa need of developing improved reinforcement for buoyant concrete structures overcom- ing problems of corrosion whilst minimising the amount of concrete required.

Summary of the lnvention The object of the present invention is to provide systems and methods for improv-ing reinforcement for buoyant concrete structures.

This is achieved by a method of manufacturing a buoyant concrete structure ac-cording to claim 1, comprising the steps of placing at least one first reinforcement bar ina mould, substantially along a longitudinal extension of the mould; pouring concrete intothe mould such that the concrete covers the at least one reinforcement bar; allowing theconcrete to cure; and attaching at least one floating element to the concrete to form abuoyant concrete structure. The at least one first reinforcement bar comprises a non-magnetic material and is prestressed before or after the concrete is poured, and thetension applied to the at least one first non-magnetic, reinforcement bar is released afterthe concrete has cured. ln order to manufacture concrete structures with longer span, adapted to specificdimensions or requirements, prestressed (pre-tensioned or post-tensioned) concrete isoften used. Pre-tensioned concrete is cast around steel tendons-cables or bars-whilethey are under tension. The concrete bonds to the tendons as it cures, and when thetension is released it is transferred to the concrete as compression by static friction.Post-tensioned concrete is cast around steel tendons and is allowed to cure before sub-sequent tensioning of the tendons by means of e.g. hydraulicjacks pushing against thecured concrete structure. When sufficient tension is applied, the tendons are fastened orwedged in position to maintain the tension after the jacks are removed. Tension subse-quently imposed on the concrete is transferred directly to the tendons. As a conse-quence, it is possible to manufacture longer concrete structures with reduced thickness 3 whilst retaining the strength properties in compression and tension. However, since re-inforcement bars made of steel or iron are normally used in prestressed concrete, theproblem of corrosion is still present. Another problem encountered when prestressingconcrete for buoyant concrete structures is that the reinforcement bars protrude fromthe cast after curing, thus requiring additional topcoat layers of concrete to cover. lt has been found that by prestressing reinforcement bars made from non-magnetic material, thus not susceptible to corrosion like metallic reinforcement bars, it ispossible to achieve strong buoyant concrete structures which are able to withstandharsh sea conditions including high waves without breaking or cracking. Hence, thepresent invention solves the problem of protecting reinforcement in buoyant concretestructures from corrosion whilst also allowing for a considerable reduction in the amountof concrete during manufacture. The tensile strength of the buoyant concrete structureis also increased due to the resulting compression forces applied by the prestressed,non-magnetic reinforcement bars. ln the context of the present invention, the term non-magnetic material is to be in-terpreted as any material which is not or only negligibly affected by magnetic fields.Secondary definitions of materials to be used as reinforcement bars or elements in thepresent invention are non-metallic, non-conducting, non-corrosive or similar. ln an advantageous embodiment, the at least one non-magnetic reinforcement baris pre-tensioned before the concrete is poured and the tension applied to the at leastone first non-magnetic, reinforcement bar is released after the concrete has cured. ln an alternative embodiment, the at least one non-magnetic reinforcement bar ispost-tensioned after the concrete has substantially cured and the tension applied to theat least one first non-magnetic, reinforcement bar is maintained. ln a preferred embodiment, the non-magnetic material comprises basalt. Basalt isa common extrusive igneous (volcanic) rock formed from the rapid cooling of basalticlava. lt has excellent anti-corrosive properties as well as high tensile strength. Rein-forcement bars made from basalt will therefore be suitable for use in prestressed buoy-ant concrete structures and resist corrosion. ln an alternative embodiment, the concrete comprises reinforcement fibres madefrom non-magnetic material. Preferably, the non-magnetic material comprises basalt,plastic, polymers, glass, carbon, aramid or any combination thereof. The non-magneticfibres incorporated into the matrix of the concrete offers increase protection from crack- ing during pouring. 4 ln a further preferred embodiment, the step of attaching at least one floating ele-ment to the concrete comprises placing the at least one floating element in the mouldadjacent the at least one reinforcement bar before pouring the concrete. By placing thefloating element in the mould before pouring the concrete, the concrete structure maybe adapted to wholly or partially enclose the floating element to form the buoyant con-crete structure during pouring. ln an advantageous embodiment, the method further comprises the step of placingat least one second prestressed, non-magnetic reinforcement bar substantially perpen-dicular to the at least one first prestressed, non-magnetic reinforcement bar. By provid-ing perpendicular prestressed, non-magnetic reinforcement bars, the strength of thebuoyant concrete structure will be increased in the longitudinal as well as the lateral di-rection. ln a second aspect, the present invention relates to a buoyant concrete structureaccording to claim 9 comprising at least one floating element embedded in or attachedto the concrete structure, and at least one first reinforcement bar embedded in the con-crete structure substantially along a longitudinal extension thereof, wherein the rein-forcement bar comprises a non-magnetic material and is prestressed. Preferably, thenon-magnetic material comprises basalt. ln a further advantageous embodiment, the at least one non-magnetic reinforce-ment bar is pre-tensioned before the concrete is poured and the tension applied to theat least one first non-magnetic, reinforcement bar is released after the concrete hascured. ln an alternative embodiment, the at least one non-magnetic reinforcement bar ispost-tensioned after the concrete has substantially cured and the tension applied to theat least one first non-magnetic, reinforcement bar is maintained. ln a preferred embodiment, the concrete comprises reinforcement fibres madefrom non-magnetic material. Preferably, the non-magnetic material comprises fibres ofbasalt, plastic, polymers, glass, carbon, aramid or any combination thereof. ln a further preferred embodiment, the buoyant concrete structure comprises atleast one prestressed reinforcement bar positioned substantially perpendicular to thefirst prestressed reinforcement bar. ln and advantageous embodiment, the floating element has a substantially rectan-gular cross-section and the concrete structure has a substantially U-shaped cross-section such that it substantially encloses at least three sides of the floating element.

Preferably, the buoyant Concrete structure comprises a plurality of prestressed, non-magnetic reinforcement bars embedded in at least one corner region of the U-shapedcross-section of the concrete structure. More preferably, the prestressed, non-magneticreinforcement bars are embedded in each corner region of the U-shaped cross-sectionof the concrete structure as well as the end region of each stem of the U-shape. ln a third aspect, the present invention re|ates to the use of non-magnetic, pre- stressed reinforcement bars in manufacture of buoyant concrete structures.

Brief Description of the Drawings Fig. 1 shows in a perspective view a buoyant concrete structure according to thepresent invention in the form of a pontoon; Fig. 2 shows in a cross-sectional view a buoyant concrete structure according tothe prior art; and Fig. 3 shows in a cross-sectional view a buoyant concrete structure according to the present invention.

Detailed Description of the lnvention Below, the buoyant concrete structure will be described more in detail, referencebeing made to the figures. However, the invention should not be considered limited tothe embodiment or embodiments shown in the figures and described below, but may bevaried within the scope of the claims.

Fig. 1 shows a perspective view of buoyant concrete structure according to thepresent invention, in the form of a pontoon. lt should be understood that other examplesof buoyant concrete structures, such as piers, breakwaters, bathing platforms, mooringjetties, bridges, floats, floating house platforms etc. may also be manufactured based onthe principles of the present invention.

Normally, pontoons are manufactured by casting or moulding concrete around afloating element. The floating element may comprise closed-cell plastic or polymerfoam, air-filled or inflatable containers or basically any element that is capable of provid-ing sufficient buoyancy to the finished concrete structure. lt is desirable that the pontoonhas a freeboard of at least 50 cm when floating, but the freeboard may be adapted tospecific conditions and requirements. The number and buoyancy force of the floatingelements is adapted to the size and amount of concrete required for the pontoon toachieve the desired freeboard. 6 ln Fig. 2, the cross-section of a pontoon 1 according to the prior art is shown. Thepontoon 1 comprises reinforcement bars 2 typically made from steel embedded in theconcrete structure 3 along a longitudinal extension of the pontoon. ln the |atera| direc-tion, a metal net or mesh 4 is embedded in the concrete structure 3 to add strength.

Fig. 3 illustrates a cross-section of a pontoon 10 according to the present inven-tion. lt may be seen that the concrete has been poured to enc|ose a floating element(not shown) on at least three sides of the ﬂoating element. ldeally, the concrete struc-ture 13 is substantially U-shaped placed upside-down, with the stems 14, 15 of the U-shape extending vertically downwards when the pontoon 10 is floating in water. Prefer-ably, the stems extend further than the side of the floating element, thus creating a tur-bulence chamber which is beneficial for breaking and dampening incoming waves. Theturbulence chamber is delimited by the stems of the U-shaped concrete structure 13and the bottom side of the floating element. ln order to reinforce and strengthen the buoyant concrete structure, a plurality ofprestressed, non-magnetic reinforcement bars 12 is embedded in the concrete structure13. ln Fig. 3 it may be seen that three reinforcement bars 12 are embedded in each up-per corner region 16, 17 of the concrete structure 13 as well as in the distal end region18, 19 of each stem of the U-shaped concrete structure 13. However, any number ofre-inforcement bars 12 is foreseen by the present invention. The reinforcement bars 12 ex-tend in a Iongitudinal direction of the buoyant concrete structure 13 and are pre-tensioned before the concrete is poured. The tension is maintained while the concrete iscured such that the concrete bonds to the pre-tensioned, non-magnetic reinforcementbars. When the concrete is cured, the tension is released which results in transfer of acompression force from the non-magnetic reinforcement bars 12 to the concrete struc-ture 13. This compression force increases the tensile strength of the reinforced concretestructure 13, making it capable of withstanding stronger forces without cracking orbreaking.

As an alternative to pre-tensioning, prestressing of the concrete structure may alsobe achieved through post-tensioning the non-magnetic reinforcement bars. Here, the re-inforcement bars 13 are placed in the mould and the concrete is cured and allowed tocure. After curing, tension is applied to the reinforcement bars 13 e.g. by means of hy-draulic jacks. When sufficient tension has been applied, the reinforcement bars 13 arewedged or fastened in position, e.g. by means of suitable anchors, such that the applied 7 tension is maintained and transferred to the Concrete structure through static friction.Both methods of prestressing concrete are encompassed by the present invention. ln an alternative embodiment, the buoyant concrete structure 13 is manufacturedas a reinforced concrete deck or slab adapted to be supported by one or more floatingelements. Here, the concrete structure 13 is pre-fabricated according to the principle ofthe present invention using prestressed, non-magnetic reinforcement bars embedded ina longitudinal direction of the concrete structure and subsequently attached to the float-ing elements. Because of the increased tensile strength due to the prestressed, non-magnetic reinforcement bars, the deck may be made very thin and lightweight. The pre-fabricated reinforced concrete deck may be attached to already existing floating devicessuch as pontoons, piers, breakwaters, ferry landings, floats and bathing platforms.

One exemplary material for the non-magnetic reinforcement bars is basalt which isa common extrusive igneous (volcanic) rock formed from the rapid cooling of basalticlava. lt has excellent anti-corrosive properties as well as high tensile strength (4.84GPa), high elastic modulus (89 GPa) and excellent specific tenacity (1790 kNm/kg) -three times higher than that of steel. The basalt reinforcement bars are made fromtwisted basalt fibres or strands of desired lengths.

Prestressed, non-magnetic reinforcement bars may also be embedded in a lateraldirection of the buoyant concrete structure, perpendicular to the first set of prestressed,non-magnetic reinforcement bars 12. This will increase the tensile strength of the buoy-ant concrete structure 13 also in the lateral direction.

Although the prestressed, non-magnetic reinforcement bars in the buoyant con-crete structures 13 will protrude from the concrete after casting, the anti- or non-corrosive properties of the reinforcement bars obviate the need for additional topcoatlayers of concrete. Hence, the amount of concrete needed to manufacture the pontoonis dramatically reduced, in the order of 50 %. Moreover, the increased tensile strengthof the buoyant concrete structure comprising prestressed, non-magnetic reinforcement allows for further reduction in the amount of required concrete.

Claims

1. Method of manufacturing a buoyant Concrete structure, comprising the steps: - placing at least one first reinforcement bar in a mould, substantially along a lon-gitudinal extension of the mould; - pouring concrete into the mould such that the concrete covers the at least onereinforcement bar; - allowing the concrete to cure; and - attaching at least one floating element to the concrete to form a buoyant con-crete structure, characterised in that the at least one first reinforcement bar comprises a non- magnetic material and is prestressed.

2. Method according to claim 1, wherein the at least one non-magnetic reinforcementbar is pre-tensioned before the concrete is poured and the tension applied to the atleast one first non-magnetic, reinforcement bar is released after the concrete has cured.

3. Method according to claim 1, wherein the at least one non-magnetic reinforcementbar is post-tensioned after the concrete has substantially cured and the tension appliedto the at least one first non-magnetic, reinforcement bar is maintained.

4. Method according to any preceding claim, wherein the non-magnetic material comprises basalt.

5. Method according to any preceding claim, wherein the concrete comprises rein- forcement fibres made from non-magnetic material.

6. Method according to claim 5, wherein the non-magnetic material comprises basalt,plastic, polymers, glass, carbon, aramid or any combination thereof.

7. Method according to any preceding claim, wherein the step of attaching at leastone floating element to the concrete comprises placing the at least one floating element in the mould adjacent the at least one reinforcement bar before pouring the concrete. 9

8. Method according to any preceding claim, further comprising the step of placing atleast one second prestressed, non-magnetic reinforcement bar substantially perpendic- ular to the at least one first prestressed, non-magnetic reinforcement bar.

9. Buoyant concrete structure comprising: - at least one floating element embedded in or attached to the concrete structure;and - at least one first reinforcement bar embedded in the concrete structure substan-tially along a longitudinal extension thereof, characterised in that the reinforcement bar comprises a non-magnetic material and is prestressed.

10. Buoyant concrete structure according to claim 9, wherein the at least one non-magnetic reinforcement bar is pre-tensioned before the concrete is poured and the ten-sion applied to the at least one first non-magnetic, reinforcement bar is released afterthe concrete has cured.

11. Buoyant concrete structure according to claim 9, wherein the at least one non-magnetic reinforcement bar is post-tensioned after the concrete has substantially curedand the tension applied to the at least one first non-magnetic, reinforcement bar is main-tained.

12. Buoyant concrete structure according to claim 7, wherein the non-magnetic mate-rial comprises basalt.

13. Buoyant concrete structure according to claim 7 or 8, wherein the concrete com- prises reinforcement fibres made from non-magnetic material.

14. Buoyant concrete structure according to claim 9, wherein the non-magnetic mate-rial comprises fibres of basalt, plastic, polymers, glass, carbon, aramid or any combina-tion thereof.

15. Buoyant Concrete structure according to any of claims 7 - 10, further comprising atleast one prestressed, non-magnetic reinforcement bar positioned substantially perpen- dicular to the first prestressed, non-magnetic reinforcement bar.

16. Buoyant concrete structure according to any of claims 7 - 11, wherein the floatingelement has a substantially rectangular cross-section and the concrete structure has asubstantially U-shaped cross-section such that it substantially enc|oses at least three sides of the floating element.

17. Buoyant concrete structure according to claim 12, comprising a plurality of pre-stressed, non-magnetic reinforcement bars embedded in at least one corner region of the U-shaped cross-section of the concrete structure.

18. Use of non-magnetic, prestressed reinforcement bars in manufacture of buoyant concrete structures.