Patent Description:
In the case of concrete structures made of pre-stressed concrete, the tensioning elements, in particular tensioning cables, are embedded directly in the concrete of the structure, whereas the connection between the tensioning elements and the concrete in the case of structures made of post-tensioned concrete is only brought about subsequently by grouting with cementitious grout. As a result of the direct or indirect connection of the tensioning elements with the concrete, the incremental strain in the tensioning elements gets consistent with the concrete strain. As the pre-stressing or post-tensioning forces, respectively, can locally vary within the tendon length whilst the force at the anchorages remain unchanged, it is not possible to measure the pre-stressing or post-tensioning forces, respectively, with typical measurement systems at the anchorages. For this reason, measurement systems including optical fibers exiting at the anchorages have been developed.

The invention may, however, also be applied to unbonded tensioning elements, as well as geotechnical strand anchors and the like.

With the aim of being able to monitor the condition of a concrete structure, <CIT> discloses a tensioning cable with an optical fiber embedded therein. The optical fiber runs between the strands of the tensioning cable formed from these strands. Each of the strands in turn comprises a plurality of wires which are helically twisted together. Although the strands may be protected by a sheath, for example made of PE (polyethylene), they have an irregular and uneven surface due to the helical twisting of the wires. Thus, the optical fiber lies against the irregular and uneven surface of the strands. The resulting clamping force, which consequently varies in the longitudinal direction of the tensioning cable, makes the interpretation of the measurement results obtained by means of the optical fiber considerably more difficult.

Additionally, <CIT> discloses a cable with optical fiber for a prestressed concrete structure according to the preamble of claim <NUM>. Further, it is referred to <CIT>.

It is therefore the object of the present invention to provide a solution to this problem.

According to the invention, as defined in claim <NUM>, this object is solved by a wire strand comprising at least three wires, wherein an optical fiber is arranged in a cavity between three adjacent ones of the at least three wires, which optical fiber comprises a fiber core and at least one further layer surrounding the fiber core, wherein the diameter of the largest imaginary circle inscribable in the cross-section of the cavity is smaller than the outer diameter of the optical fiber, wherein the diameter of the fiber core is smaller than the diameter of said largest imaginary circle, and wherein the ratio of the outer diameter of the optical fiber to the diameter of that one of the three adjacent wires which has the smallest diameter is at least <NUM> and at most <NUM>.

Due to the fact that, according to the invention, the optical fiber is arranged in the tensioning strand, it is in contact with a longitudinally substantially uniform surface, namely the surface of the three adjacent wires of the tensioning strand forming the cavity, which extends substantially uniformly in the longitudinal direction of the tensioning strand, thus providing longitudinally uniform conditions for measurements. Since the diameter of the imaginary circle inscribed in the cross-section of the cavity (in the following also referred to as "cavity circle") is smaller than the outer diameter of the optical fiber, the optical fiber can be held in the cavity purely by clamping force. The additional use of adhesives is not required. Additional measures for fixing the optical fiber in the tensioning strand are not required. At the same time, the optical fiber is protected by the outer wires of the strand. Furthermore, since the diameter of the fiber core is smaller than the diameter of the cavity circle, there is no risk of damage to the fiber core, which is important for measurements. At the same time, however, the fiber core is mechanically coupled to the wires of the strand to a degree that enables meaningful measurements if the ratio of the outer diameter of the optical fiber to the diameter of that one of the three adjacent wires that has the smallest diameter is at least about <NUM> and at most about <NUM>, preferably at least about <NUM> and at most about <NUM>, more preferably at least about <NUM> and at most about <NUM>.

The wire strand according to the present invention, and in particular the optical fiber included therein, allows to measure the strain in a locally resolved manner, i.e. resolved along the length of the wire strand. The measurements may be carried out based on both Brillouin or Rayleigh radiation.

In the case of pre-stressed and/or post-tensioned concrete structures the locally resolved measurement allows to localize the position of possibly critical parts of the concrete structure. In geotechnical applications this is also true for the bonded length of the anchor. In unbonded applications at least the overall behavior of the tensioning element between its anchorages can be detected. And the same is true for the free length of geotechnical strand anchors.

In addition, it is to be noted that the invention further allows to monitor the strain of the tensioning element during the initial tensioning of the same.

Advantageously, the Young's modulus of the material from which the at least one further layer is formed may amount to between about <NUM>,<NUM> MPa and about <NUM>,<NUM> MPa.

For example, at least one of the at least one further layers surrounding the fiber core may be formed from a polyacrylate.

Additionally or alternatively, at least one of the at least one further layer surrounding the fiber core, preferably the radially outermost layer of the optical fiber, may be formed of metal, preferably steel.

In order to be able to prevent a harmful influence of the metal layer on the steel of the tension wires, it is proposed, that the standard potential of the metal of the at least one further layer surrounding the fiber core is lower than or equal to the standard potential of the wires of the strand. Here, the standard potential is the electrochemical potential, the value of which provides information about the ability of the metal to accept electrons.

In a further development of the invention, it is proposed that the wire strand comprises a center wire and a plurality of outer wires, preferably helically wound around the center wire, wherein at least six of the outer wires are contacting the center wire. Typically, a wire strand used in connection with the present invention may comprise a center wire surrounded by six outer wires. Such a strand has at least six cavities around the center wire in which optical fibers can be disposed. The diameter of the center wire can be up to <NUM>% larger than the diameter of the outer wires.

For such a wire strand it is further suggested that a further optical fiber may be arranged in a further cavity formed by the center wire and two adjacent outer wires, at least one further layer surrounding the fiber core of the further optical fiber being formed by a gel. In this way, even if the further optical fiber additionally also has a cladding layer, for example formed of metal, mechanical decoupling between the fiber core and the cladding or the wires of the stranded wire can be ensured, so that the further optical fiber can be used, for example, as a temperature sensor. Such a temperature sensor can facilitate the evaluation of the measurement signals originating from the first optical fiber. It is to be understood that the outer diameter of the further optical fiber can be smaller than the diameter of the cavity circle.

It should be added that the aforementioned optical fibers have, on the one hand, the advantage of alkali resistance, which is important with respect to the cement contained in the concrete, and, on the other hand, the advantage of a service life that usually exceeds the service life of the building structure, so that the condition of the building structure can be monitored even toward the end of the service life of the building structure.

A tensioning strand of the type mentioned above is neither anticipated by the aforementioned <CIT> nor by any other prior art document. <CIT> discloses a measuring system comprising an optical fiber, but deals almost exclusively with metrological aspects. The optical fiber known from <CIT> runs in a straight line on the outer surface of a pipeline. Similarly, the optical fiber known from <CIT> runs on a frame embedded in concrete.

According to a second aspect, the present invention relates to a tensioning cable including at least one wire strand according to the present invention.

According to a third aspect, the present invention relates to a concrete structure including at least one wire strand according to the present invention or at least one tensioning cable according to the present invention, said concrete building structure preferably being made from pre-stressed concrete and/or post-tensioned concrete.

In the following the invention will be explained in more detail referring to the attached drawings, in which.

In <FIG>, a wire strand according to the invention is generally designated <NUM>. The wire strand <NUM> includes a central wire <NUM> and six outer wires <NUM> surrounding the central wire <NUM>.

A cavity <NUM> is formed between the central wire <NUM> and each two adjacent outer wires <NUM>. A hypothetical or imaginary circle <NUM> may be inscribed in this cavity <NUM>, which contacts all three wires <NUM>-<NUM>-<NUM>. The diameter of this circle <NUM>, shown as dotted circle in <FIG>, is denoted D1 in <FIG>.

At least one of the cavities <NUM> may be used to receive an optical fiber <NUM>. At the left side of <FIG>, at the same level as the circle <NUM>, an optical fiber is shown for merely explanatory reasons. In particular, it is shown that the optical fiber <NUM>, in a state not received in the cavity <NUM> and therefore undeformed state, has a diameter D2 which is greater than the diameter D1 of the imaginary circle <NUM>. For this and merely explanatory reasons, the outline of the optical fiber <NUM> is shown to intersect the outlines of the wires <NUM> and <NUM>.

The actual installed state of the optical fiber <NUM> is shown centered between the undeformed fiber <NUM> (left) and the imaginary circle <NUM> (right). As is shown, the clamping forces of the wires <NUM>, <NUM> have deformed the outer surface of the optical fiber <NUM> so that the optical fiber <NUM> is securely held between the wires.

As shown in <FIG>, the optical fiber <NUM> includes a fiber core <NUM> and at least one further layer surrounding the fiber core <NUM>. In the illustrated embodiment, two further layers are provided, namely a layer <NUM> directly surrounding the fiber core <NUM>, which may for example be formed from a polyacrylate, and an outer (protective) layer <NUM>, which may for example be formed from a metal, preferably steel.

As the inventors have recognized, by simply clamping the optical fiber <NUM> between the wires <NUM>, <NUM>, i.e., without using any additional adhesive, it is possible to achieve that, on the one hand, the fiber is mechanically connected to the wires <NUM>, <NUM> so tightly that it can provide information about their state of elongation and thus also about the state of the concrete structure, but that, on the other hand, the fiber core <NUM> is not subjected to such high pressure that its light conduction properties and thus the function of the optical fiber <NUM> as a whole would be compromised. It is to the credit of the inventors to have recognized that this objective can be achieved, in particular, if the ratio of the outer diameter D2 of the optical fiber <NUM> to the diameter D3 of an outer wire <NUM> is at least about <NUM> and at most about <NUM>, preferably at least about <NUM> and at most about <NUM>, more preferably at least about <NUM> and at most about <NUM>.

<FIG> shows a schematic representation of a pre-stressed concrete structure <NUM> according to the invention.

In this pre-stressed concrete structure <NUM>, a plurality of pre-stressed strands is embedded directly in the concrete, at least one of these strands being a strand <NUM> according to the invention with an optical fiber <NUM>. After curing of the concrete, the strands may remain in the concrete without anchoring by a mechanical anchorage. In this case, the optical fiber <NUM> can be led out of the strand <NUM> according to the invention in a simple manner at one of its ends and fed to a measuring apparatus (not shown).

<FIG> shows a schematic representation of a post-tensioned concrete structure <NUM> according to the invention.

In this post-tensioned concrete structure <NUM>, a sheathing tube <NUM> is embedded in the concrete, into which a plurality of strands can be drawn after or before curing of the concrete, including at least one strand <NUM> according to the invention. After tensioning the strands and anchoring same by means of mechanical anchorages <NUM>, the interior of the sheathing tube <NUM> remaining between the strands is filled with grout <NUM>. After hardening, this provides the necessary bond between the strands and the surrounding concrete of the post-tensioned concrete structure <NUM>.

According to <FIG>, showing the example of an unbonded concrete structure <NUM>, it is also conceivable in accordance with the invention to omit the step of filling the cladding tube <NUM> there with grout. In this case, at least the overall condition of the unbonded concrete structure <NUM> between the anchorages <NUM> can be detected by means of the strand <NUM> provided with the optical fiber <NUM>.

Finally, <FIG> shows a schematic representation of a geotechnical strand anchor <NUM> according to the invention. For the installation of this anchor <NUM>, a borehole <NUM> is made in the underground U in a manner known per se. The anchor <NUM> is inserted into this borehole <NUM>. Tthe tensioning element <NUM> of the anchor <NUM> comprises a plurality of strands, including at least one strand <NUM> according to the invention comprising an optical fiber <NUM>. Grout <NUM> is then filled into a portion of the borehole <NUM>, which, after curing, holds the anchor <NUM> in the borehole <NUM>. Outside the borehole <NUM>, the anchor <NUM> can then be tensioned in a manner known per se. In this process, the optical fiber <NUM> is fed out of the anchor <NUM> at the free end of the tensioning element <NUM> of the anchor <NUM> and fed to a measuring apparatus (not shown).

It is to be added that in addition to the optical fiber <NUM> intended for strain measurement, another optical fiber <NUM> can be provided (see <FIG>), which can be used as a temperature sensor. This temperature measuring fiber <NUM> preferably does not have a metal cladding, but is in contact with the wires <NUM>, <NUM> only via a gel in order to mechanically decouple it from the latter.

It should further be added that two methods are conceivable for inserting the fiber <NUM> and optionally the fiber <NUM> into the strand <NUM>.

According to a first method, in a first step one of the outer wires <NUM> is removed from the strand <NUM>. Subsequently, in a second step, the optical fiber <NUM> is inserted. Finally, the previously removed wire is added back to the strand <NUM> in a third step.

According to a second method, the strand <NUM> is spliced between two clamping jaws. The optical fiber <NUM> then is inserted into the strand <NUM> through the gap created thereby. Further, the splice is allowed to travel continuously along the length of the strand <NUM> or the strand <NUM> is allowed to travel continuously thorough the splicing device.

Below specific values for three embodiments are shown. Examples <NUM> and <NUM> resulted in measurements which were meaningful for the strain of the concrete structure, while the negative example resulted in unsuable measurements.

Claim 1:
A wire strand (<NUM>) comprising at least three wires (<NUM>, <NUM>),
wherein an optical fiber (<NUM>) is arranged in a cavity (<NUM>) between three adjacent ones of the at least three wires (<NUM>, <NUM>), which optical fiber (<NUM>) comprises a fiber core (<NUM>) and at least one further layer (<NUM>, <NUM>) surrounding the fiber core (<NUM>),
wherein the diameter (D1) of the largest imaginary circle (<NUM>) inscribable in the cross-section of the cavity (<NUM>) is smaller than the outer diameter (D2) of the optical fiber (<NUM>),
wherein the diameter of the fiber core (<NUM>) is smaller than the diameter (D1) of said largest imaginary circle (<NUM>),
characterized in that the ratio of the outer diameter (D2) of the optical fiber (<NUM>) to the diameter (D3) of that one of the three adjacent wires (<NUM>, <NUM>) which has the smallest diameter is at least about <NUM> and at most about <NUM>.