The present invention relates to reinforcement in buoyant prestressed concrete structures such as pontoons, piers, breakwaters, ferry landings, floating house platforms and bathing platforms.

Technical Background of the Invention

Concrete structures for floating applications are a common component in today's civil construction and buoyant concrete structures have been commercially available for almost 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 rely on the strength of embedded reinforcement. This reinforcement normally consists of iron rods and/or nets with or without added fibres of steel or plastic. The fact that any buoyant concrete structure partly is submersed in water poses a great challenge to any such construction as chloride ions from the surrounding water will penetrate the concrete 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 decreased protection against tensile stress, which ultimately results in cracking of the concrete. Therefore, when the calculated migration distance of the chloride ions equals the thickness of the concrete cover, the technical lifetime of the product is reached and it must be replaced.

Industry standard to prolong the technical lifetime of a buoyant concrete structure has been to either increase the quality of the concrete, thus slowing the chloride ion transport in the concrete, or to cast with thicker layers of concrete between the water and the outmost part of the reinforcement, i.e. thicker topcoat layers also called concrete cover. Increasing the quality of the concrete also increases the cost of manufac- ture, while thicker topcoat layers require more concrete and lead to bulkier concrete structures which alter their floating characteristics.

Other materials have been proposed to replace steel and iron reinforcement bars to provide protection against corrosion, such as fibres of polymer, glass, carbon or ara- mid. WO 201 1/108941 discloses a reinforcement system for concrete structures, such as pontoons, comprising reinforcement elements made of basalt or carbon fibres. The 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 such other materials is that they have a poor service life in the highly alkaline environment of concrete. 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 in buoyant concrete structures is that the concrete has been shown to be susceptible to cracking or breaking in harsh sea conditions due to incoming waves. Therefore, there is a need of developing improved reinforcement for buoyant concrete structures overcom- ing problems of corrosion whilst minimising the amount of concrete required.

Summary of the Invention

The object of the present invention is to provide systems and methods for improving reinforcement for buoyant prestressed concrete structures.

This is achieved by a method of manufacturing a buoyant concrete structure according to claim 1 , comprising the steps of placing at least one first reinforcement bar comprising basalt in a mould, substantially along a longitudinal extension of the mould; pouring concrete into the mould such that the concrete covers the at least one reinforcement bar; allowing the concrete to cure; and attaching at least one floating element to the concrete before or after curing to form a buoyant concrete structure. The method further comprises prestressing the at least one first reinforcement bar, before or after the concrete has cured.

In order to manufacture concrete structures with longer span, adapted to specific dimensions or requirements, prestressed (pre-tensioned or post-tensioned) concrete is often used. Pre-tensioned concrete is cast around steel tendons— cables or bars— while they are under tension. The concrete bonds to the tendons as it cures, and when the tension 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 subsequent tensioning of the tendons by means of e.g. hydraulic jacks pushing against the cured concrete structure. The post-tensioned concrete may be either bonded or unbonded, referring to whether the tendons are free to move in relation to the concrete once the concrete is cured. When sufficient tension is applied, the tendons are fastened or wedged in position to maintain the tension after the jacks are removed. Tension subsequently 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 whilst retaining the strength properties in compression and tension. However, since reinforcement bars made of steel or iron are normally used in prestressed concrete, the problem of corrosion is still present. Another problem encountered when prestressing concrete for buoyant concrete structures is that the reinforcement bars protrude from the cast after curing, thus requiring additional topcoat layers of concrete to cover.

It has been found that by prestressing reinforcement bars made from nonmagnetic material such as basalt, thus not susceptible to corrosion like metallic reinforcement bars, it is possible to achieve strong buoyant concrete structures which are able to withstand harsh sea conditions including high waves without breaking or cracking. Hence, the present invention solves the problem of protecting reinforcement in buoyant concrete structures from corrosion whilst also allowing for a considerable reduction in the amount of concrete during manufacture. The tensile strength of the buoyant concrete structure is also increased due to the resulting compression forces applied by the prestressed reinforcement bars.

In the context of the present invention, the term prestressed or prestressing comprises both pre-tensioning and post-tensioning of the reinforcement bars to create a prestressed concrete structure with a considerably increased tensile strength compared to an unstressed concrete structure. As such, it should be understood that elements and devices required for applying and maintaining the prestressing tensile forces to the reinforcement bars and the prestressed concrete structure of the present invention are implicitly included as known in the art, although not explicitly disclosed in the present description.

In 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 the present invention are non-metallic, non-conducting, non-corrosive or similar.

In an advantageous embodiment, the at least one reinforcement bar is pre- tensioned before the concrete is poured and the tension applied to the at least one first reinforcement bar is released after the concrete has cured.

In an alternative embodiment, the at least one reinforcement bar is post-tensioned after the concrete has substantially cured and the tension applied to the at least one first reinforcement bar is maintained. The non-magnetic material used for the reinforcement bars of the present invention comprises basalt. Basalt is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava. It has excellent anti-corrosive properties as well as high tensile strength. Reinforcement bars made from basalt will therefore be suitable for use in prestressed buoyant concrete structures and resist corrosion.

In an alternative embodiment, the method comprises adding reinforcement fibres made from basalt, plastic, polymers, glass, carbon, aramid or any combination thereof to the concrete. The non-magnetic fibres incorporated into the matrix of the concrete offers increased protection from cracking during pouring.

In a further preferred embodiment, the step of attaching at least one 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. By placing the floating element in the mould before pouring the concrete, the concrete structure may be adapted to wholly or partially enclose the floating element to form the buoyant con- crete structure during pouring. Alternatively, the floating element may be attached to the concrete in a known manner after the concrete has cured.

In an advantageous embodiment, the method further comprises the step of placing at least one second prestressed reinforcement bar comprising basalt substantially perpendicular to the at least one first prestressed reinforcement bar. By providing perpen- dicular prestressed reinforcement bars comprising basalt, the tensile strength of the buoyant concrete structure will be increased in the longitudinal as well as the lateral direction.

In a second aspect, the present invention relates to a buoyant prestressed concrete structure according to claim 8 comprising at least one floating element embedded in or attached to the concrete structure, and at least one first prestressed reinforcement bar embedded in the concrete structure substantially along a longitudinal extension thereof, wherein the reinforcement bar comprises basalt.

In a preferred embodiment, the concrete comprises reinforcement fibres made from non-magnetic material. Preferably, the non-magnetic material comprises fibres of basalt, plastic, polymers, glass, carbon, aramid or any combination thereof.

In a further preferred embodiment, the buoyant concrete structure comprises at least one prestressed reinforcement bar comprising basalt positioned substantially perpendicular to the first prestressed reinforcement bar. In and advantageous embodiment, the floating element has a substantially rectangular 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 rein- forcement bars comprising basalt embedded in at least one corner region of the U- shaped cross-section of the concrete structure. More preferably, the prestressed reinforcement bars are embedded in each corner region of the U-shaped cross-section of the concrete structure as well as the end region of each stem of the U-shape.

Brief Description of the Drawings

Fig. 1 shows in a perspective view a buoyant prestressed concrete structure according to the present invention in the form of a pontoon;

Fig. 2 shows in a cross-sectional view a buoyant prestressed concrete structure according to the prior art; and

Fig. 3 shows in a cross-sectional view a buoyant prestressed concrete structure according to the present invention.

Detailed Description of the Invention

Below, the buoyant concrete structure will be described more in detail, reference being made to the figures. However, the invention should not be considered limited to the embodiment or embodiments shown in the figures and described below, but may be varied within the scope of the claims.

Fig. 1 shows a perspective view of buoyant prestressed concrete structure according to the present invention, in the form of a pontoon. It should be understood that other examples of buoyant prestressed concrete structures, such as piers, breakwaters, bathing platforms, mooring jetties, bridges, floats, floating house platforms etc. may also be manufactured based on the principles of the present invention.

Normally, pontoons are manufactured by casting or moulding concrete around a floating element. The floating element may comprise closed-cell plastic or polymer foam, air-filled or inflatable containers or basically any element that is capable of providing sufficient buoyancy to the finished concrete structure. It is desirable that the pontoon has a freeboard of at least 50 cm when floating, but the freeboard may be adapted to specific conditions and requirements. The number and buoyancy force of the floating elements is adapted to the size and amount of concrete required for the pontoon to achieve the desired freeboard.

In Fig. 2, the cross-section of a pontoon 1 according to the prior art is shown. The pontoon 1 comprises reinforcement bars 2 typically made from steel embedded in the concrete structure 3 along a longitudinal extension of the pontoon. In the lateral direction, 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 invention. It may be seen that the concrete has been poured to enclose a floating element (not shown) on at least three sides of the floating element. Ideally, 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. Preferably, the stems extend further than the side of the floating element, thus creating a turbulence chamber which is beneficial for breaking and dampening incoming waves. The turbulence chamber is delimited by the stems of the U-shaped concrete structure 13 and the bottom side of the floating element.

In order to reinforce and strengthen the buoyant concrete structure, a plurality of prestressed reinforcement bars 12 comprising basalt is embedded in the concrete structure 13. In Fig. 3 it may be seen that three reinforcement bars 12 are embedded in each upper corner region 16, 17 of the concrete structure 13 as well as in the distal end re- gion 18, 19 of each stem of the U-shaped concrete structure 13. However, any number of reinforcement bars 12 is foreseen by the present invention. The reinforcement bars 12 extend in a longitudinal direction of the buoyant concrete structure 13 and are pre- tensioned before the concrete is poured. The tension is maintained while the concrete is cured such that the concrete bonds to the pre-tensioned reinforcement bars. When the concrete is cured, the tension is released which results in transfer of a compression force from the reinforcement bars 12 to the concrete structure 13. This compression force increases the tensile strength of the reinforced concrete structure 13, making it capable of withstanding stronger forces without cracking or breaking.

As an alternative to pre-tensioning, prestressing of the concrete structure may also be achieved through bonded or unbonded post-tensioning of the reinforcement bars. Here, the reinforcement bars 13 are placed in the mould and the concrete is poured and allowed to cure. In the case of unbonded post-tensioning of the reinforcement bars, each reinforcement bas is covered by e.g. a plastic sheath such that the reinforcement bar is free to move in relation to the concrete. After curing, tension is applied to the rein- forcement bars 12 e.g. by means of hydraulic jacks. When sufficient tension has been applied, the reinforcement bars 12 are wedged or fastened in position, e.g. by means of suitable anchors, such that the applied tension is maintained and transferred to the concrete structure through static friction. Both methods of prestressing concrete are en- compassed by the present invention.

In an alternative embodiment, the buoyant prestressed concrete structure 13 is manufactured as a reinforced concrete deck or slab adapted to be supported by one or more floating elements. Here, the concrete structure 13 is pre-fabricated according to the principle of the present invention using prestressed reinforcement bars embedded in a longitudinal direction of the concrete structure and subsequently attached to the floating elements. Because of the increased tensile strength due to the prestressed reinforcement bars, the deck may be made very thin and lightweight. The pre-fabricated reinforced concrete deck may be attached to already existing floating devices such as pontoons, piers, breakwaters, ferry landings, floats and bathing platforms.

The reinforcement bars used in the present invention comprise basalt which is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava. It has excellent anti-corrosive properties as well as high tensile strength (4.84 GPa), 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 from twisted basalt fibres or strands of desired lengths.

Prestressed reinforcement bars comprising basalt may also be embedded in a lateral direction of the buoyant concrete structure, perpendicular to the first set of prestressed reinforcement bars 12. This will increase the tensile strength of the buoyant concrete structure 13 also in the lateral direction.

Although the prestressed reinforcement bars in the buoyant concrete structures 13 will protrude from the concrete after casting, the anti- or non-corrosive properties of the reinforcement bars obviate the need for additional topcoat layers of concrete. Hence, the amount of concrete needed to manufacture the pontoon is dramatically reduced, in the order of 50 %. Moreover, the increased tensile strength of the buoyant concrete structure comprising prestressed reinforcement bars comprising basalt allows for further reduction in the amount of required concrete.