Strand with sensor

Abstract

The present invention concerns a strand for stressing concrete having a diameter ranging from 1 mm to 100 mm and having a tensile strength higher than 1500 MPa, wherein the strand comprises at least 3 strand elements comprising at least one central strand element, at least one intermediate strand element and at least one outer strand element, wherein further at least one strand element is a sensor and the sensor is an intermediate strand element.

Description

Strand with sensor[1] The present invention concerns a strand for stressing concrete having a diameter ranging from 1 mm to 100 mm and having a tensile strength higher than 1500 MPa,wherein the strand comprises at least 3 strand elements comprising at least one central strand element, at least one intermediate strand element and at least one outer strand element,wherein further at least one strand element is a sensor and the sensor is an intermediate strand element. The present invention further concerns a measuring method using a strand according to the invention. [2] Prestressed concrete is a form of concrete that is compressed by the application of tension to strengthen it against tensile force it will experience during service. Prestressed concrete thereby comprises pre-tensioned concrete and / or post-tensioned concrete. For pre-tensioned concrete tension is applied before the concrete is cast, while for post-tensioned concrete tension is applied after the concrete is cast and / or after the concrete is hardened. [3] Tension may thereby especially be applied using tendons or strands, especially steel strands or synthetic strands. [4] However, there may thereby be temperature or force variations outside specifications during the casting, the curing or service life of a concrete structure that can be detrimental and service life may be difficult or impossible to estimate. [5] It would thus be desirable to monitor the force applied to the concrete and / or the temperature of the concrete to avoid the above mentioned problems and ensure the quality and / or integrity and / or durability of a stressed concrete structure. Indeed, the present invention could help to determine temperature or force variations outside specifications during the casting, the curing or the service life of a concrete structure that can be detrimental and to estimate service life. Moreover, the present invention could for example help to control shrinkage and / or creep of concrete, to detect the occurrences of fires, to monitor integrity and / or tensile strength of a strand, to adjust strain, to detect overloads, to trigger and / or schedule repair works and / or inspections, to record the strain history of a structure and / or its reinforcing elements, especially to obtain an estimate of effective life or service time . [6] This becomes possible with a strand according to the present invention, namely a strand for stressing concrete having a diameter ranging from 1 mm to 100 mm and having a tensile strength higher than 1500 MPa,wherein the strand comprises at least 3 strand elements comprising at least one central strand element, at least one intermediate strand element and at least one outer strand element,wherein further at least one strand element is a sensor and the sensor is an intermediate strand element. [7] A strand according to the present invention may thereby be a group of two or more wires, filaments or cords. At least two wires, filaments or cords of said strand may for example be twisted together with a, preferably only one, given strand twist pitch. [8] A strand element in the sense of the invention may thereby be for example a wire, a filament or any other component of a strand including a sensor. [9] A strand according to the present invention thereby comprises inner, outer and intermediate strand elements. An inner strand element of a strand is thereby located inside said strand and / or preferably at the centre of said strand and thus surrounded by other strand elements of said strand, which may thus be outer strand elements. On the other hand, an outer strand element in the sense of the invention is located at the outside of a strand facing the exterior and thus in contact with the surrounding of said strand. Moreover, one or more inner strand elements located at the centre of a strand or alternatively opposite to one or more other outer strand elements may be one or more central strand elements. An intermediate strand element in the sense of the invention may thereby be located in between one or more inner strand element(s) or central strand element(s), on one hand, and one or more outer strand element(s), on the other hand.  

[10] A strand according to the present invention may comprise at least three strand elements comprising at least one central stand element, at least one intermediate strand element and at least one outer strand element,wherein further at least one strand element is a sensor and the sensor is an intermediate strand element. The position of the sensor according to the invention may thereby contribute to more accurate and / or reliable measurements as well as contribute to minimize the impact on mechanical properties of the strand. 

[11] A strand according to the invention may have for example a tensile strength higher than 500 MPa, higher than 1700 MPa, preferably higher than 1800 MPa, preferably higher than 1900 MPa, preferably higher than 2000 MPa, further preferred between 1800 MPa and 4000 MPa. This may contribute to allow for example for structural reinforcement and easy installation. 

[12] In an embodiment, a strand according to the invention may comprise a central strand element. This may contribute to allow to make such a strand more stable and / or to avoid deformation of the strand, especially when higher number of strand elements is used. Indeed, for more then 5 strand elements the construction of the strand can become unstable and a central strand element can stabilize such construction by blocking the central position and thus avoiding other strand elements move to the central position. 

[13] In an embodiment of the invention, a strand may have for example a maximum breaking load of higher than 190 kN, preferably higher than 195 kN, preferably higher than 200 kN, preferably higher than 220 kN, further preferred between 192 kN and 600 kN, further preferred between 195 kN and 350 kN, further preferred between 200 kN and 10000 kN. This may contribute to allow for example for structural reinforcement and easy installation. 

[14] In an embodiment of the invention, a strand may comprise non-steel strand elements, such as for example synthetic filaments or synthetic fibres, such as for example polyethylene (PE), polypropylene (PP), aramid, polyester, polycarbonate, polyamide (PA), aromatic polyamide and / or polyoxymethylene (POM) filaments and / or carbon fibres and / or glass fibres. In an embodiment of the invention, a strand may comprise steel strand elements, especially for example coated or uncoated steel wires and / or steel cords, which may be groups of steel wires preferably twisted together with a, further preferred only one, steel cord pitch. In an embodiment of the invention, a strand may comprise both non-steel strand elements and steel strand elements. In a further embodiment of the invention, a strand may thereby for example comprise a majority of steel strand elements and / or a majority of non-steel strand elements. Preferably non-steel strand elements and / or steel strand elements in the sense of the present invention are different from sensors. This may contribute to allow for example for structural reinforcement and easy installation. In an embodiment of the invention, the central strand element(s) and the outer strand element(s) may be steel strand elements. Moreover, a strand according to the invention may preferably not comprise Z-shaped steel wires and / or steel wires with a high vanadium content. A high vanadium content may thereby be a vanadium content of > 0.15 %, preferably > 1% %, even further preferred > 4%).  

[15] In an embodiment of the invention, a strand may comprise for example 2, 3, 7, 3x1+6, 1+6+12 or 7x7 strand elements, especially steel strand elements and / or non-steel strand elements. This may contribute to allow to make such a strand for example more flexible, especially as the number of strand elements increases. A strand according to the invention with 1+6 strands elements, especially steel strand elements may thereby be preferred. 

[16] In an embodiment of the invention, a strand may comprise or consist of n-1, n-2, n-3 or n-4 strand elements that are non-steel strand elements and / or steel strand elements, with n being to total number of strand elements of said strand. In such case, the remaining strand elements of said strand may preferably be for example sensors. This may contribute to allow for example to make accurate and / or reliable measurements and / or to measure multiple parameters and / or to measure with different methods and / or to ensure redundancy. 

[17] In an embodiment of the invention, the outer strand elements may be helicoidal and / or the central strand element(s) may be essentially straight. Two or more outer stand elements may for example be twisted together with a, preferably only one, given strand twist pitch. On the other hand one or more inner strand element(s) and / or central strand element(s) may be essentially straight. Preferably the outer strand elements may thereby be twisted together in a helicoidal way, further preferred for example in a helicoidal way around one or more inner strand element(s) and / or central strand element(s). Essentially straight in the sense of the present invention may thereby preferably mean not twisted with at least one given twist pitch. This may contribute to allow to make such a strand more stable and / or to avoid deformation of the strand. 

[18] In an embodiment of the invention, a / the sensor(s) may be selected for example from: an optical sensor, a conductivity sensor, an optical fiber, a glass fiber, coaxial cable, a carbon element, a copper or copper alloy wire. In an embodiment of the invention, a / the sensor maybe especially for example a strain sensor and / or temperature sensor. In an embodiment of the invention, a / the sensor may be configured to compensate strain measurements based on temperature data. In an embodiment of the invention, one sensor may measure both strain and temperature or two different sensors may measure temperature on one hand and strain on the other hand. In an embodiment of the invention, a / the sensor may be especially for example a fiber bragg grating sensor or an optical fiber. This may contribute to allow for example to make accurate and / or reliable measurements and / or to measure multiple parameters and / or to measure with different methods and / or to ensure redundancy. 

[19] In an embodiment of the invention, a / the sensor(s) may be straight or helicoidal. The sensor(s) may thereby be twisted together in a helicoidal way with another outer strand element, further preferred for example in a helicoidal way around one or more inner strand element(s) and / or central strand element(s). The sensor(s) may thereby be also be straight, preferably located for example in between one or more central strand element(s) and one or more outer strand element(s). This may contribute to allow to make such a strand more stable and / or to avoid deformation of the strand. This may also contribute to allow for example to make accurate and / or reliable measurements and / or to measure multiple parameters and / or to measure with different methods and / or to ensure redundancy. 

[20] In an embodiment of the invention, a / the sensor(s) may be coated with at least one polymer or comprise(s) at least one polymer jacket. In an embodiment of the invention, the material selected for said polymer coating and / or said polymer jacket comprises or consist of at least one of: polyvinyl chloride (PVC), glass fiber reinforced polymer, polyester, polyimide, polyethylene (PE), polypropylene (PP), thermoplastic elastomer, thermo-plastic polyester elastomer or a high density polyethylene (HDPE). This may contribute to allow to make accurate and / or reliable measurements as well as avoid damage to the sensor(s). In an embodiment of the invention, the sensor(s) may be coated with a polyester, a polyimide or a glass fiber reinforced polymer at the outside, whereby this may contribute to improve the mechanical interlocking of the sensor with at least one other strand element. This may contribute to allow for example to make accurate and / or reliable measurements. In an embodiment of the invention, the sensor(s) may be coated with a PVC, a polyethylene (PE), a high density polyethylene (HDPE), a polypropylene (PP), a thermoplastic elastomer or a thermo-plastic polyester elastomer at the outside, whereby this may contribute to reduce transversal forces, especially for example from mechanical interlocking, and / or to minimize the risk of damage to sensor. In an embodiment of the invention, the sensor(s) may be coated with a polymer material at the outside that has a Vickers hardness (measured for example with a DURAMIN-10 AC tester and an indent time of 10 sec.) between 1 and 10 HV0.01, preferably 2 and 8 HV0.01, further preferred 3 and 7 HV0.01, even further preferred 3.5 > and < 6.5 HV0.01. This may contribute to improve the mechanical interlocking of the sensor with at least one other strand element. Furthermore, this may contribute to allow for example to make accurate and / or reliable measurements. 

[21] In an embodiment of the invention, further at least two strand elements may be sensors and / or both sensors may be intermediate strand elements. In an embodiment of the invention, said two sensors may be preferably located for examples at opposite sides of a / the strand and / or next to each other, further preferred forming an angle of between 100° and 200° and / or an angle of between 10° and 90°. In an embodiment of the invention, one sensor may be a central strand element. This may contribute to allow for example to make accurate and / or reliable measurements and / or to measure multiple parameters and / or to measure with different methods and / or to ensure redundancy. In an embodiment of the invention, one strand element being a sensor, preferably a strain sensor, may be an intermediate strand element being a sensor, which may be fixed to at least one of the other strand element by mechanical interlocking and a second strand element being a sensor, preferably a temperature sensor, may not be fixed to at least one of the other strand element by mechanical interlocking. This may contribute to allow for example to make accurate and / or reliable measurements and / or to measure multiple parameters, especially strain and / or temperature.  

[22] In an embodiment of the invention, the intermediate strand element(s) being a sensor may be fixed to at least one of the other strand elements with an adhesive and / or by softening a polymer coating or polymer jacket. In an embodiment, at least some of the, preferably all of the, central, intermediate and outer strand elements are fixed to each other, preferably by an adhesive and / or by softening a polymer coating or polymer jacket. In an embodiment, an intermediate strand element being a sensor may be fixed to at least one of the other strand elements with an adhesive and / or by softening a polymer coating or polymer jacket and / or by mechanical interlocking. In an embodiment, mechanical interlocking can be used especially for example to fix a sensor as an intermediary strand element to and / or between at least three of the other strand elements. Mechanical interlocking can thereby for example contribute to minimize or reduce transversal forces and / or reduce the need for adhesive and / or other compounds to be used, which may influence and / or disturb the mechanical properties of the strand. This may also contribute to allow to make accurate and / or reliable measurements for example by minimizing bias as well as avoid damage to the sensor(s). The ratio of the diameter of the sensor over the diameter of the outer strand element with the smallest diameter may thereby preferably be > 0,180, preferably > 0,185, further preferred > 0,190, even further preferred < 0,250 and / or between > 0,180 and < 0,250. This may contribute to improve mechanical interlocking and / or may also contribute to make accurate and / or reliable measurements. In an embodiment of the invention, the diameter of the strand elements, especially the steel strand elements, is between 2 and 10 mm, preferably 3 and 7 mm and / or the diameter of the sensor being a strand element is between 0.8 and 1.5 mm, preferably > 0.9 and < 1.3 mm. This may also contribute to improve mechanical interlocking and / or may also contribute to make accurate and / or reliable measurements In an embodiment of the invention the diameter of the inner strand element may be more than 3% bigger than the diameter of the smallest outer strand element and / or may be more than 3 % bigger than the diameter of each of the outer strand elements. This may contribute to allow the inner strand element to support the strand. A strand according to the present invention can thereby be obtained for example as follows. A sensor being a strand element can be introduced into the strand at two different steps in the production process, namely during stranding or in the greasing and sheathing process. When a sensor is introduced during stranding this may be done preferably before a subsequent heat treatment.On the other hand, when a sensor is introduced in the greasing and sheathing process this requires to open the strand and to close it again prior to the grease application and sheathing. In both cases, the quick closing of the strand is important, so that the strand elements, especially the steel strand elements, do not loose grip on the sensor when being introduced, especially at the start up when only a short length may have been introduced, and to make sure that the sensor remains safely positioned in the designated spot and doesn't jump another spot. The latter could lead for example to destruction of the sensor once the strand is put under tension Feeding and guiding of the sensor can be achieved for example through:a strand opener, which may be a rotating plate with the designated number of holes of the appropriate diameters and at the appropriate location for the required number of strand elements and / or for the sensor(s) that shall be introduced into the strand.  a weight balanced feeder system for the sensor(s) suitable to hold the spool of the sensor and ensure the proper tensioning of the sensor, while it is introduced in the strand, and suitable to rotate freely and smoothly around the strand, especially for example without abrupt movements (e.g. by using counterweights to balance the masses) synchronizing the feeding and the introduction of the sensor(s) with the direction the strands elements are twisted (lay direction) and the distance it takes for one strand element to be twisted by 360° (lay length) so that the sensor(s) can be positioned in a designated void as an intermediate strand element. The strand opener may be integrated with a feeding device and may serves as the synching and propelling unit for the rotating part of the feeder system. In addition, a strand closer, which may for example for example be a ring or any other element that brings the strand elements back in close proximity to each other to close the strand back up, may be used, whereby it is positioned as close as possible to the exit of the strand from the feeder system so that the sensor is gripped and placed properly as soon as possible. The invention thus further concerns a method for producing a strandcomprising for example:opening a strand using a strand openerfeeding a sensor being a strand element using a feeder suitable to rotate freely and smoothly around the strandclosing the strand using a strand closerand / or wherein the feeding and introduction of the sensor(s) is synchronized with the direction the strands elements are twisted and the distance it takes for one strand element to be twisted by 360°. As a result the sensor may thereby possibly be experiencing some torsion. 

[23] The present invention further concerns a rope comprising for example two or more strands according to the invention. In an embodiment one or more sensors can thereby been foreseen for example in between the strands of the rope. 

[24] The present invention further concerns a measuring method,wherein a strand according to the invention may be used to measure directly or indirectly (i.e. including for example via reflected wavelengths) at least one parameter selected from: a force and / or strain and / or a temperature,during the casting of a concrete structure and / orduring the curing of a concrete structure and / orduring the lifetime of a concrete structure and / orwherein the measurement may be carried out continuously along the length of a sensor or at discrete intervals along the length of a sensor. That the measurement may be carried out at discrete intervals along the length of a sensor in the sense of the present invention may thereby especially for example be the case when a fiber bragg grating sensor is used, since in such case the measurement may be carried out for example at the bragg gratings foreseen at discrete intervals along that length of said sensor. This may also be the case is a sensor comprises multiple measuring sections foreseen at discrete intervals along that length of said sensor. This may also contribute to allow to make accurate and / or reliable measurements as well as avoid damage to the sensor(s). This may also contribute to ensure the quality and / or integrity and / or durability of a stressed concrete structure. 

[25] Examples: Different strands according to the invention were compared as follows. An Instron 8803 servohydraulic test system (300 kN limit) was used to record force in kN as a function of deformation in microstrain (1 / 1000000) in a force controlled measurement with a test speed of 12,5 kN / min. The strand is fixed by two identical standard barrels with wedges to the head of the machine and the frame. One or more sensors being strand elements, which are optical fibers, fiber bragg grating sensors, are connected to an interrogator BG-Scan 90X from FBGS Technologie GmBH in Germany and wavelength is then converted to microstrain. Fig. 1 shows a comparaison between two strands according to the invention. The first strand A (dotted line) is a strand with 6+1 steel strand elements, with the outer steel strand elements having a diameter of 5.2 mm and a sensor being an intermediate strand element and an optical fibers, preferably a fiber bragg grating sensor, having a polyethelene, especially a high density polyethylene, coating at the outside and a diameter of 0.9 mm. The central steel strand element thereby has a diameter of 5.4 mm. The ratio of the diameter of the sensor over the diameter of the outer strand element with the smallest diameter is < 0,180, namely 0.173. The second strand B (thick line) is a strand with 6+1 steel strand elements, with the outer steel strand elements having a diameter of 5.2 mm and a sensor being an intermediate strand element and an optical fibers, preferably a fiber bragg grating sensor, having a glass fiber reinforced polymer coating at the outside and having a diameter of 1 mm. The central steel strand element thereby has a diameter of 5.4 mm. The ratio of the diameter of the sensor over the diameter of the outer strand element with the smallest diameter is thus > 0,180, namely 0.192. All other parameters being the same. The measurement and graph the force / microstrain curve recorded thereby shows that the the measured strain for sensor B (higher thick line) is closer to the strain as applied and / or calculated (thin line) compared tp the strain for sensor A (lower dotted line). This may contribute to more accurate measurements. Fig. 2 shows a comparaison between two strands according to the invention. The first strand A (dotted line) is a strand with 6+1 steel strand elements, with the outer steel strand elements having a diameter of 5.2 mm and an intermediate strand element being a sensor and an optical fibers, preferably a fiber bragg grating sensor, that is fixed with an adhesive to at least one of the other strand elements. The central steel strand element thereby has a diameter of 5.4 mm. The second strand B (thick line) is a strand with 6+1 steel strand elements, with the outer steel strand elements having a diameter of 5.2 mm and a sensor being an intermediate strand element and an optical fibers, preferably a fiber bragg grating sensor, that is fixed to at least one of the other strand elements by mechanical interlocking and / or whereby mechanical interlocking is used to fix said sensor as an intermediary strand element to and / or between at least three of the other strand elements. The central steel strand element thereby has a diameter of 5.4 mm. All other parameters being the same. The measurement and graph the force / microstrain curve recorded thereby shows that the the measured strain for sensor B (lower thick line) is closer to the strain as applied and / or calculated (thin line) compared to the strain for sensor A (higher dotted line). This may contribute to more accurate measurements. Fig. 3 shows a comparaison between two different positions of sensor in a strand with 6+1 steel strand elements. The first sensor A (dotted line) is an intermediate strand element and an optical fibers, preferably a fiber bragg grating sensor according to the invention, being mechanically interlocked. The second sensor B (thick line) is an outer strand element being a sensor and an optical fibers, preferably a fiber bragg grating sensor that is fixed by an adhesive to at least one of the other strand elements. The ratio of the diameter of the sensor over the diameter of the outer strand element with the smallest diameter is thereby < 0,180, namely 0.173 in both cases and all other parameters are the same. The measurement and graph the force / microstrain curve recorded thereby shows that the the measured strain for sensor A (higher dotted line) is more reliable over sensor B (lower thick line), since if anything it slightly over estimates strain compared to the strain as applied and / or calculated (thin line) , while sensor B on the other hand may rather underestimates strain compared to the same. This may contribute to more reliable measurements. Fig. 4 shows a schematic representation of a strand with 6+1 steel strand elements (C) and two different positions of a sensor. The first sensor A is an intermediate strand element according to the invention. The second sensor B is a a sensor being an outer strand element. The intermediate strand element being a sensor A may thereby fixed to at least one of the other strand elements by mechanical interlocking and / or mechanical interlocking is used to fix a sensor as an intermediary strand element to and / or between at least three of the other strand elements. On the other hand, the outer strand element being a sensor may be fixed to at least one of the other strand elements with an adhesive. 

Claims

1. A strand for stressing concrete having a diameter ranging from 1 mm to 100 mm and having a tensile strength higher than 1500 MPa,wherein the strand comprises at least 3 strand elements comprising at least one central stand element, at least one intermediate strand element and at least one outer strand element,wherein further at least one strand element is a sensor and the sensor is an intermediate strand element.

2. The strand according to claim 1,wherein the strand has a tensile strength higher than 500 MPa, preferably higher than 1800 MPa, preferably higher than 1900 MPa, preferably higher than 2000 MPa, further preferred between 1800 MPa and 4000 MPa.

3. The strand according to claim 1 or 2,wherein the strand has a maximum breaking load of higher than 190 kN, preferably higher than 195 kN, preferably higher than 200 kN, preferably higher than 220 kN, further preferred between 192 kN and 600 kN, further preferred between 195 kN and 350 kN, further preferred between 200 kN and 10000 kN.

4. The strand according to any of the preceding claims,wherein the strand comprises non-steel strand elements and / or wherein the strand comprises steel strand elements and / or a majority of steel strand elements and / or a majority of non-steel strand elements and / or wherein the strand does not comprise Z-shaped steel wires and / or steel wires with a high vanadium content.

5. The strand according to any of the preceding claims,wherein the strand comprises 2, 3, 7, 3x1+6, 1+6+12 or 7x7 strand elements, especially steel strand elements and / or non-steel strand elements and / or wherein the diameter of the steel strand elements is between 2 and 10 mm, preferably 3 and 7 mm and / or wherein the diameter of the sensor being a strand element is between 0.8 and 1.5 mm, preferably > 0.9 and < 1.3 mm and / or wherein the ratio of the diameter of the sensor(s) over the diameter of the outer strand element with the smallest diameter is > 0,180, preferably > 0,185, further preferred > 0,190, even further preferred < 0,250 and / or between > 0,180 and < 0,250 and / or wherein the diameter of the inner strand element is more than 3% bigger than the diameter of the smallest outer strand element and / or is more than 3 % bigger than the diameter of each of the outer strand elements.

6. The according to any of the preceding claims,wherein n-1, n-2, n-3 or n-4 strand elements are steel filaments or steel cords and / or non-steel filaments, with n being to total number of strand elements and / or wherein the central strand element(s) and the outer strand element(s) may be steel strand elements.

7. The strand according to any of the preceding claims,wherein the outer strand elements are helicoidal and / or the central strand element is / are straight.

8. The strand according to any of the preceding claims,wherein the sensor(s) is / are selected from: an optical sensor, a conductivity sensor, an optical fiber, a glass fiber, coaxial cable, a copper or copper alloy wire and / or wherein the sensor is a strain sensor and / or temperature sensor and / or wherein the sensor is configured to compensate strain measurements based on temperature data and / or wherein one sensor measures both strain and temperature or two different sensors measure temperature on one hand and strain on the other hand and / or wherein the sensor is a fiber bragg grating sensor or optical fiber.

9. The strand according to any of the preceding claims,wherein the sensor(s) is / are straight or helicoidal.

10. The strand according to any of the preceding claims,wherein the sensor(s) is / are coated with at least one polymer or comprise(s) at least one polymer jacket and / or wherein the material selected for the polymer coating and / or polymer jacket comprises or consist of at least one of: PVC, glass fiber reinforced polymer, polyester, polyimide, PE, PP, thermoplastic elastomer, thermo plastic polyester elastomer or HDE and / or wherein the sensor(s) is / are coated with a polyester, a polyimide or a glass fiber reinforced polymer at the outside and / or wherein the sensor(s) is / are coated with a PVC, a polyethylene (PE), a high density polyethylene (HDPE), a polypropylene (PP), a thermoplastic elastomer or a thermo-plastic polyester elastomer at the outside and / or wherein the sensor(s) may be coated with a polymer material at the outside that has a Vickers hardness between 1 and 10 HV0.01, preferably 2 and 8 HV0.01, further preferred 3 and 7 HV0.01, even further preferred 3.5 > and < 6.5 HV0.01.

11. The strand according to any of the preceding claims,wherein further at least two strand elements are sensors and / or both sensors are intermediate strand elements, preferably located at opposite sides of the strand and / or next to each other, further preferred forming an angle of between 100° and 200° and / or an angle of between 10° and 90°.

12. The strand according to any of the preceding claims,wherein the intermediate strand element(s) being a sensor may be fixed to at least one of the other strand elements with an adhesive and / or by softening a polymer coating or polymer jacket and / or wherein at least some of the, preferably all of the, central, intermediate and outer strand elements are fixed to each other, preferably by an adhesive and / or by softening a polymer coating or polymer jacket and / or wherein the intermediate strand element being a sensor is fixed to at least one of the other strand elements with an adhesive and / or by softening a polymer coating or polymer jacket and / or by mechanical interlocking and / or wherein mechanical interlocking is used to fix a sensor as an intermediary strand element to and / or between at least three of the other strand elements.

13. A Method for producing a strand according to any of the claims 1 to 12,wherein the method comprises:opening a strand using a strand opener,feeding a sensor being a strand element using a feeder suitable to rotate freely and smoothly around the strand,closing the strand using a strand closerand / or wherein the feeding and introduction of the sensor(s) is synchronized with the direction the strands elements are twisted and the distance it takes for one strand element to be twisted by 360°.

14. A rope for stressing concrete,wherein the rope comprises two or more strands according to claims 1 to 12.

15. Measuring method,wherein a strand according to claims 1 to 12 or a rope according to claim 14 is used to measure at least one parameter selected from: a force and / or a strain and / or a displacement and / or a temperature,during the casting of a concrete structure and / or during the curing of a concrete structure and / orduring the lifetime of a concrete structure and / orwherein the measurement is carried out continuously along the length of a sensor or at discrete intervals along the length of a sensor. 

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