Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
STRAND IN GLASS AND/OR BASALT FIBERS FOR PRESTRESSED CONCRETE
Document Type and Number:
WIPO Patent Application WO/2019/162390
Kind Code:
A1
Abstract:
A composite material strand is described, having a "lxn" geometry composed of a polymeric matrix added with roving glass fibers and/or basalt fibers, wherein said polymeric matrix is formed of one or more thermoplastic resins, or of a more thermosetting resins or mixtures thereof.

More Like This:
Inventors:
NANNI, Antonio (11404 Smathers Circle, Pinecrest, Florida, 33156, US)
ARDUINI, Marco (Via Pindemonte 16, Reggio Emilia, 42100, IT)
BRUSCHI, Flavio (Via Tolmino 15, Vimercate, 20871, IT)
BALCONI, Gabriele (Via Battisti 28, Arcore, 20862, IT)
Application Number:
EP2019/054334
Publication Date:
August 29, 2019
Filing Date:
February 21, 2019
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SIREG GEOTECH S.R.L. (Via del Bruno 12, Arcore, 20862, IT)
UNIVERSITY OF MIAMI (Coral Gables, Florida, 33124, US)
International Classes:
B29C65/00; C03C25/1025; D01F9/08; D07B5/00; E04C5/00; H01B5/10
Foreign References:
RU2520542C12014-06-27
US20120298403A12012-11-29
US4015994A1977-04-05
Other References:
None
Attorney, Agent or Firm:
PETRUZZIELLO, Aldo (RACHELI S.R.L, Viale San Michele del Carso 4, Milano, 20144, IT)
Download PDF:
Claims:
CLAIMS

1. Composite strand (10) having a non-solid cross-section with a "lxn" construction, being composed of a central wire (1) and n-1 side wires (2) helically twisted around said central wire (1),

each of said wires (1; 2) having a full cross-section and being consisting of a polymeric matrix added with glass fibers in roving form and/or basalt fibers, wherein said polymeric matrix is one or more thermoplastic resins, or one or more thermosetting resins, or mixtures thereof.

2. The strand according to claim 1, wherein said glass fibers are glass fibers of grade E-CR (E-CR Fiberglass) and/or Advantex® glass fibers.

3. The strand according to claim 1 or 2, wherein said composite wires (1; 2) have the same nominal diameter.

4. The strand according to any one of the preceding claims, wherein the polymeric matrix is an epoxy vinylester resin. 5. The strand according to any one of the preceding claims, wherein the construction is "1x7" and six side wires (2) are twisted around said one core wire (1) with a helical pitch.

6. The strand according to claim 5, wherein each of the wires (1 ;2) has a diameter of 5 mm and the overall diameter of the strand is 15.2 mm.

7. The strand according to any one of the preceding claims obtained by means of pultrusion. 8. The strand according to claim 7, wherein when the polymeric matrix is a thermoplastic resins, e.g. acrylic resin, PEEK, PP or the like, a single wire is first prepared by means of pultrusion and then the side wires obtained from said first wire are helically wrapped around the central rectilinear wire in a mermofonning step. 9. The strand according to claim 8, wherein the central wire is round in cross- section while the twisted side wires are slightly ovalized along the cross-section.

10. Transportable coils of composite strand (10) as defined in any one of the preceding claims.

11. Wire-strand rope comprising one or more composite strands (10) as defined in any one of me preceding claims.

12. Use of one or more composite strands (10) as defined in any one of the preceding claims for manufacturing prestressed concrete (PC). 13. Concrete manufactured articles comprising one or more strands (10) or wire- strand ropes as defined in any one of the preceding claims, preferably selected from the group consisting of piles, sheet piles, substructure bearing piles, noise protection panels and retaining walls. 14. Manufactured articles made of concrete and reinforced with one or more strands (10) as defined above, in the form of

- structural element composed of reinforced concrete made by using sea water in the mixture and reinforced with the strand (10) as defined in any one of the preceding claims;

- reinforced concrete made of cement characterised by a pH lower than 10;

- reinforced concrete having a surface layer characterized by a high localized or distributed porosity and remforced with the strand (10) as defined in any one of the preceding claims. IS. Manufactured article in concrete in the form of a concrete element reinforced by strands (10) as defined in one of the preceding claims, characterized in that said strand is mild-prestressed by a value comprised between 30 and 50% of the ultimate load of the same strand.

AMENDED CLAIMS received by the International Bureau on 24 May 2019 (24.05.2019)

1. Composite strand (10) having a non-solid cross-section with a "1x7" construction identical or similar to the conventional steel strands,

said strand (10) being composed of a central wire (I) and six side wires (2) helically twisted around said central wire (1) with a helical pitch

each of said wires (1; 2) having a full cross-section and being made of a composite material which consists of a polymeric matrix added with gl ass fibers in roving form and/or basalt fibers,

wherein said polymeric matrix is one or more thermoplastic resins, or one or more thermosetting resins, or mixtures thereof.

2. The strand according to claim 1, wherein said glass fibers are glass fibers of grade E-CR (E-CR Fiberglass) and/or Advantex® glass fibers.

3. The strand according to claim 1 or 2, wherein said composite wires (1; 2) have the same nominal diameter.

4. The strand according to any one of the preceding claims, wherein the polymeric matrix is an epoxy vinylester resin.

5. The strand according to any one of the preceding claims, wherein each of the wires (1 ;2) has a diameter of S mm and the overall diameter of the strand is 15.2 mm. 6. The strand according to any one of the preceding claims obtained by means of pultrusion.

7. The strand according to any one of the preceding claims, wherein when the polymeric matrix is a thermoplastic resins, e.g. acrylic resin, PEEK, PP or the like, a single wire is first prepared by means of pultrusion and then the side wires obtained from said first wire are helically wrapped around the central rectilinear wire in a thermofonning step.

8. The strand according to claim 7, wherein the central wire is round in cross- section while the twisted side wires are slightly ovalized along the cross-section.

9. Transportable coils of composite strand (10) as defined in any one of the preceding claims.

10. Wire-strand rope comprising one or more composite strands (10) as defined in any one of the preceding claims.

11. Use of one or more composite strands (10) as defined in any one of the preceding claims for manufacturing prestressed concrete (PC). 12. Concrete manufactured articles comprising one or more strands (10) or wire- strand ropes as defined in any one of the preceding claims, preferably selected from the group consisting of piles, sheet piles, substructure bearing piles, noise protection panels and retaining walls. 13. Manufactured articles made of concrete and reinforced with one or more strands (10) as defined above, in the form of

- structural element composed of reinforced concrete made by using sea water in the mixture and reinforced with the strand (10) as defined in any one of the preceding claims;

- reinforced concrete made of cement characterised by a pH lower man 10;

- reinforced concrete having a surface layer characterized by a high localized or distributed porosity and reinforced with the strand (10) as defined in any one of the preceding claims. 14. Manufactured article in concrete in the form of a concrete element reinforced by strands (10) as defined in one of the preceding claims, characterized in that said strand is mild-prestressed by a value comprised between 30 and 50% of the ultimate load of the same strand.

Description:
STRAND IN GLASS AND/OR BASALT FIBERS FOR PRESTRESSED CONCRETE

DESCRIPTION The present invention refers to a non-metallic composite strand∞ntaining glass and/or basalt fibers, with a construction 1 x n ", in particular "1 x 7" similar to that of conventional steel strands {multi-wire strands) as defined by the standards - Pr EN 10138 o equivalent ones (DM 14/01/2008 or ASTM A416/A 416 M). Particularly, the present invention refers to a strand as above defined, apt to be used in the prestressing of concrete (PC) or in the construction of geotechnical tendons, and more generally in all those applications wherein carbon steel strands of any diameter are used and the durability requirement is lower or a particularly simple removal phase is required.

More particularly, the present invention refers to the composite strands as above defined, which are apt to be used in the existing chuck systems (reusable strand chuck) and existing devices of tensioning which are normally used for carbon steel strands having the same construction, generally "1x7" type construction.

As is known, the precompression of the concrete is used to compensate for the low tensile strength of the concrete mix.

Pre-compression is obtained by using carbon steel cables inserted in the pre-stressed state in the concrete, through different methods (pre-stretched cable systems or post- tensioning cables).

In particular, once the formwork has been laid out and before the concrete casting is carried out, the prestressing cables are tensioned between two fixed and external supports (anchors), and subsequently the concrete casting mat wraps the pretended cables, is carried out Once the concrete mix has undergone maturation, the cable is released from the anchors.

The cables for the prestressing are used in various forms depending on the needs: wire (drawn product with a solid section with notches that serve to improve the adherence with the concrete and to facilitate the anchorage); bar (laminated with a full section provided with projections to promote adhesion); strand (consisting of several wires or groups of wires helically wound, in one or more layers, around a rectilinear wire arranged along the longitudinal axis to completely cover the whole by the wires).

The strands are generally used in the construction of prestressed cement structures such as piles, sheet piles, substructure bearing piles, noise protection panels and retaining walls. Given their high mechanical strength and low cost, the most commonly used strands in prestressed concrete are those in carbon steel (CS) for pre-stress application, which however suffer from the inconvenience of being extremely sensitive to corrosion.

Less sensitive to corrosion are strands in stainless steel, but they also show corrosion problems in the long term, particularly when they are used in subtropical coastal environments or in contact with de-icing agents in the cold regions of the northern hemisphere.

When long-term resistance to corrosion is required, composite strands of thermosetting resins reinforced with aramid fibers or carbon fibers are generally used, thanks to their resistance to corrosion and to their excellent mechanical properties: however, this type of strands suffers from inconvenience of having a cost much greater than carbon steel strands. In particular, the composite strands in thermosetting resin reinforced with carbon fiber, which are marketed mainly by Tokyo Rope Manufacturing Co., notwithstanding they show a good compromise between high mechanical performance (i.e. tensile strength) and corrosion resistance, they have a very high cost, generally about 4 times greater than the strands in carbon steel, as well as some disadvantages if they are used in the field of pre-stressed concrete.

Composite strands made of aramid fibers have the disadvantage of having lower mechanical strengths man carbon fiber composite strands and a higher cost Moreover, the use of these carbon fiber composite strands is very complex on site; especially for two reasons: - given the higher cost of the product, compared to the steel strands, it is "necessary" to pretension mem to much higher loads so as to exploit the most of their high tensile strength, but this involves the need to protect the strands at the interface between strands and gripping device, thus requiring a specific gripping system (chuck) with very complex tensioning devices which are different from those used for steel strands (which have no protective interface sheath);

- the high pretension load to which they are subjected involves a high energy incorporated in the pretensioned element, which causes local damage (cracking) to the cement structure in many operating phases when this energy is freed, as well as less safe use when they are cut at the ends of the cast cement element.

Cables of power transmission or for civil constructions having strands with "lxn" construction are known, wherein said cables and/or strands contain, in addition to composite wires, also metal wires variously associated with the wires made of composite polymer material and glass fibers.

The objective of the present invention is to overcome, at least in part, the disadvantages of the prior art by providing a corrosion-resistant strand to be used mainly in the construction of prestressed cement structures, having a cost comparable to the carbon steel strands so as to avoid being preloaded to the maximum, and which can be used in the same plant of prefabricotion of steel strands.

Another objective is to provide a strand as defined above that is endowed with a good mechanical strength such as to be used in the construction of cement elements which do not require a particularly high pretension.

A further objective is to provide a strand as defined above that is also easy to produce, and with high productivity, as well as being harmless from a constructional point of view.

These and other objectives are achieved by the glass and/or basalt fiber composite strand according to the invention having the characteristics listed in the attached independent claim 1. Advantageous embodiments of the invention appear from the dependent claims. An object of the present invention relates to a non-metal composite material strand, with a "lxn" geometry, preferably 1x7, identical or similar to the conventional steel strands complying with standards Pr EN 10138 or similar (DM 14/01/2008 or ASTM A4167A 416 M standards), composed of a plurality of n wires, wherein each wire forming the strand is, generally, non-metallic as it is made of a composite material, has a solid cross-section and is composed of a polymeric matrix added with glass fibers or basalt fibers, or mixtures thereof.

Hie term "strand" here is intended to identify a non-full cross-section element, composed of a plurality of n round/elliptical/ovalized wires, preferably all having the same nominal diameter, each with a solid section, where n-1 wires are helically wound/twisted around 1 (one) central rectilinear wire (core) arranged according to the longitudinal axis of the strand and completely covered by the remaining side wires. Glass fibers used in the present invention can be of any type and grade available on the market, preferably long continuous fibers (continuous filament fibers in the form of continuous filament) or also defined as "roving" fibers: the Applicants have in fact found that such roving fibers are more easily usable in the construction of wires which form the strand of the present invention.

As glass fibers can be mentioned for example E-glass, E-CR-glass, A-glass, C-glass, D-glass, R-glass, S-glass (with high tensile strength) fibers.

Preferably, the glass fibers used are the glass fibers of grade E-CR {E-CR Fiberglass), Advantex® glass fibers from the company Owens Coming, more preferably E-CR Fiberglass Roving and/or Advantex® glass fibers.

The E-CR fibers, defined by the ASTM D 578 standard {Standard Specification for Glass Fiber Strands), section 4.2.4, are similar to the glass fibers of E grade but modified as free from boron, and therefore endowed with an improved corrosion resistance to most acids: therefore, glass fibers of grade E-CR are suitable for use in acidic environmental conditions (aggressive environment).

Each of the roving fibers comprises a surface treatment called sizing which is specifically designed and suitable for chemical bonding with specific matrices: in the case of the roving fibers of E-CR grade, the Applicants have found that they have a surface treatment suitable for pultrusion and chemically compatible with epoxy vinylester resin or with thermoplastic resin.

The temperature range with which the fiber comprising sizing is processed in the pultrusion process is between 40 and 150°C.

In fact, glass fibers of E-glass grade are used where environmental and processing conditions are not problematic/critical. The Advantex® glass fibers, which also comply with the aforementioned ASTM D578 standard, section 4.2.4, have high chemical resistance similar to that of E-CR fibers but exhibit improved mechanical properties with respect to E and E-CR glass fibers. The polymer matrix can be formed of one or more thermoplastic resins, or one or more thermosetting resins, or their mixtures.

An example of a thermosetting resin is a vinyl ester resin, preferably an epoxy vinyl ester resin.

An example of a thermoplastic resin is a resin of Acrylic type, or PEEK or PP.

The thermoplastic resins are preferred. Thermoplastic resins are particularly advantageous as they make the strand completely recyclable (100% of recyclability, even if the resin is added with, for example, ABS for the creation of ABS and glass fiber granules), thus making the product more environmentally friendly. Moreover, such thermoplastic resins are advantageous as they allow the mermoforming of the strand after pultrusion.

In one embodiment according to the invention, the composite material of the present strand is an epoxy vinyl-ester resin, reinforced with E-CR grade roving glass fibers.

It should be noted that the roving fibers of the E-CR type, in addition to being a product widely used and easily available today, are able to meet the latest ASTM D S78 standards, besides the specific regulations regarding FRP bars used as reinforcements for concrete, as they have proven to guarantee the durability required by structural applications in reinforced concrete (up to 125 years).

The amount of glass and/or basalt fibers with respect to the polymeric matrix is generally between 60 and 75% by weight, but also up to 80%.

The glass and/or basalt fiber composite wires of the present strand are assembled with a continuous production process: a pultrusion line allows to obtain a continuous composite wire to be cut into various elements of a given length, which will then be helically wound/twisted around the central rectilinear wire (core) arranged along to the longitudinal axis of the strand. In particular, in the case of thermoplastic resins, a single wire is prepared in a first step and in a second step the wires thus obtained from said first wire are men helically wrapped around the central rectilinear wire (core) in mermoforming so as to obtain the strand according to the invention. The term "pultrusion" here means to identify a continuous extrusion process used to produce reinforced polymeric profiles/section bars with a constant transverse cross- section, wherein the reinforcing fibers, taken from the creel in the dispersed phase, are passed through the impregnation bath in which they are bonded to the resin matrix, then passed through a comb, into a preforming station, and then into a heated mold (forming and curing die) having the shape of the desired end product section.

The preferred lxn construction of the strand of the present invention is the 7-wire (wires) construction where each wire is preferably formed of a resin reinforced with "roving" glass fibers of E-CR grade.

In said 7-wire construction, around the central wire (indicated with 1 in the formula above) the remaining 6 wires are wound in spiral: this type of strand has the advantage of

being a redundant system, compared to the single cable (bar),

- showing a high tensile strength while being a flexible strand since the flexural stiffness is associated to the single wire rather than to the strand, and showing high adherence to the concrete thanks to the coiling of the strand, similar to that of a carbon steel strand.

In an example of such a 7-wire construction (7-wire strand), each of the wires has a diameter of 5 mm while the overall diameter (nominal) of the 7-wire strand is about 15.2 mm.

In another example of this 7-wire construction (7-wire strand) according to the invention, each of the wires has a diameter of 5.3 mm while the overall (nominal) diameter of the 7-wire strand is approximately 15.7 mm.

Further examples of 7-wire strands are strands with a total nominal diameter of 6.40 mm, 7.90 mm, 9.53 mm, 11.11 mm, 12.70 mm, 13.20 mm, 14.20 mm, 17.78 mm. Other types of constructions are possible according to the present invention by suitably varying the number of wires, their diameter, and the number of windings around the central wire per unit of length and the cross-sectional area to obtain the desired strength and flexibility. It should be noted that the strand consisting of 7 wires as described above is preferred in mat the resulting strand shows a bending stiffness lower man a product consisting of a single wire having a cross-sectional area equivalent to the total area of the 7 wires: this lower rigidity is fundamental for making the strand woundable in a coil transportable which has a maximum external diameter less than or equal to 2.5 meters.

Providing coils of strand and thus assuring a continuity thereof without connecting elements, is the advantage of the present composite strand compared to the steel strands having the same construction geometry. It should be noted that the steel strand known in the art of prestressed concrete works as:

- it's a redundant system: 7 wires are associated together,

- high tensile strength, but it is a flexible element since the flexural stiffness is associated to the single wire and not to the strand;

- adherence to the concrete is guaranteed by the helical wounding principle. These advantages are therefore also present in the composite strand according to the invention.

The strand according to the present invention is suitable for use either as a pre- stretched cable or as a post-tensioned cable in the prestressed reinforced concrete sector, but also in the construction of geotechnical tendons, wire ropes and the like.

It should be noted that the present strand, in any type of construction, and/or the wires mat compose it, does not need to be covered by textile yarns or sheaths, contrary to what is necessary for the carbon fiber composite strands.

Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non-limiting embodiment thereof, illustrated in the appended drawings, in which:

Figure 1 is an example of a 7-wire strand of steel of the prior art for prestressed concrete, also shown in section;

Figure 2 shows an embodiment of a 7-wire glass fibers composite strand according to the invention;

Figure 3 shows a conventional apparatus for the pre-stressing of steel strands known in the art;

Figure 4 shows the cracks as a consequence of the release of carbon fiber composite strands of the prior art;

Figure 5 illustrates a gripping chuck used for steel strands;

Figure 6 shows a glass fiber composite strand according to the invention associated with the wedge-shaped gripping system.

As illustrated in Figure 1, the typical construction of a strand, including the composite strand of the present invention, is of the "1 x n" type.

In particular, the present strand indicated with the numerical reference 10, is formed by a central wire, shown in the figure with the numerical reference 1, and by six side wires, shown in the figure (see also Fig. 6) with the numerical reference 2 which are helically twisted around the central wire 1 with a spiral pitch. With the abbreviation "7 x rT here it is intended the typical construction of a strand where 1 represents the central wire while n-1 represents the number of side wires that are twisted around the central wire with a spiral/helical pitch. Said composite strand 10 according to the invention, has a non-full cross-section, in contrast to the wires which compose it, and has empty spaces which determine the high flexibility of the strand and its capability to be wounded in coil.

As illustrated in fig. 6, the glass-fiber composite strand containing E-CR Fiberglass Roving according to the invention is associated with a wedge-shaped gripping system 20 which will fit into the pre-tensioning jack.

The Applicants have found that the aforesaid composite strand 10 according to the invention is a low cost effective reinforcement, easy to handle and to be pre-stressed with the standard pre-stress means used with the steel strands, and it does not need to be pre-stressed at high levels, given its lower cost compared to the known carbon fiber strands, and therefore the level of elastic energy stored in the composite strand of the invention, during the pre- stressing phase, does not cause damage to the structure in concrete, contrary to what happens with the carbon fiber strands.

In particular, it is possible to use the strand of the present invention with innovative concretes such as for example:

- a concrete with CSA cement since the pH of the manufactured article would be compatible with the strand according to the invention;

- a concrete where the mixture is made using sea water,

- a concrete where the mix is made using sea aggregates (sand) without a prewashing.

Furthermore, it has been found that the present strand exhibits comparable characteristics, even better for some parameters, than the known strands in terms of

- high tensile strength,

- lightweight,

- high resistance to corrosion,

- magnetic transparency,

- electrical resistivity as there is no electrical conductivity,

- low coefficient of thermal expansion; - ease of cutting for temporary applications.

In particular, the characterization tests of the material have determined the following properties:

- nominal guaranteed tensile strength (fib*) = 800 MPa,

- nominal tensile module (Ef) = 46000 MPa.

However, given the lower limit of the strength of creep rupture compared to carbon fiber and stainless steel composite strands, even suitable for some applications, the present strand can be used more advantageously where a high level of cement prestressing, like the one to which the carbon fiber composite strands are subjected, is not required (mild prestress); for example the present strand is useable in structural elements or prestressed concrete articles such as piles, sheet piles, substructure bearing piles, noise protection panels and retaining walls, whose level of prestressing is typically around 4.8 MPa (700 psi) which is well sustainable by the present strand.

As far as the Applicants knows, a non-metallic composite strand of roving glass fibers and/or basalt fibers with a "lxn" construction has never been described or produced in the art: in feet, rods and bars made of glass fiber composite material are known in the art as reinforcements for concrete, which however cannot be used in pre- stressing applications due to their low flexibility and consequently their impossibility to be rolled into coils.

This makes the present strand unusable in continuous and very long quantities, for example with lengths of 1000 m, as necessary in the prefabrication plants where it is necessary to guarantee the continuity of the cables without junction elements.

Moreover, the present strand can be used in turn to make ropes (wire strand ropes) composed of a central core formed by the present strand which other strands of the present invention are helically wound around.

In particular, in a preferred embodiment it is possible to provide a rope with 1x7 geometry wherein each single "wire" of the rope consists of a 1x7 strand according to the present invention as described above. The pretensioning of the present glass and/or basalt fiber composite strand is generally set to be less than 50% of the guaranteed strength of the material as such. Hie present strand shows numerous advantages.

Given mat the construction geometry is the same of the steel strands, it is possible to use prestress equipment existing for the steel strands in prestressed concrete, including the chuck devices: mis makes the present strand useable immediately.

In fact, the gist of the present invention resides in the fact of providing a composite strand which is standardized like a steel strand and which is suitable for being used in the same standard equipment used in the prestress industry, thanks to this standardization and configuration, thus using the same gripping cone (Fig. 5) and the same prestressing hydraulic jack (Fig.3).

Another advantage is that the glass fiber composite is "immune" from corrosion, and has a cost comparable to that of traditional carbon steel cables, while it shows a greater strain at failure (2.0% ultimate elongation) and a significantly lower elastic modulus (about 7 Ksi) compared to stainless steel and carbon fiber composites.

It should be noted that the lower elastic modulus is advantageous in the manufacture of prestressed concrete (greater and more controllable displacement during prestressing).

Although the glass fiber composite has a resistance to creep rupture lower man carbon fiber and stainless steel composites, which does not allow the same levels of pretension, it is however useful when a lower prestress is required to reduce losses due to creep of concrete (hiring the service life of prestressed concrete.

It is to be understood that it is also possible to form a glass and/or basalt fiber composite strand according to the invention with a geometry different from lxn, having the wires intertwined according to different constructions, for example strands consisting of wires of different diameters, without thereby departing from the scope of the present invention.

Therefore the strand according to the present invention can be used to make the following manufactured articles: - structural element composed of reinforced concrete made by using sea water in the mixture, and reinforced with the strand of the present invention;

- reinforced concrete made of cement with a pH lower than 10;

- reinforced concrete having a surface layer characterized by a high localized or distributed porosity and reinforced with the strand of the present invention.

Another type of manufactured article according to the invention is a reinforced concrete element reinforced with the present strand containing E-CR glass fibers and weakly pre-stressed (mild-prestress).

Unlike the steel strand, the strand according to the invention is suitable for applications characterized by a weak pre-stress between 30 and 50% of the ultimate load of the strand and therefore with a prestressing strain of 0.6% and 1.1%. This solution is specific to the present invention since the concrete does not need to avoid any type of cracking (as the concrete with steel strand indeed); in addition, since the fiberglass has known creep problems in long-term, the maximum load thereof in operation must be limited. A weak pre-compression of an element with moderate stiffness (with respect to steel) has the considerable advantage of facilitating the cutting operations of the exceeding portion of the strands during the removal phase, since the energy stored by the element is moderate. This allows safe operations and allows not to damage the concrete. Moreover the moderate stiffness allows a less loss of prestressing. Tests made by the Applicants show the possibility of pretensioning at 50% using standard anchors for steel strands, for a period longer than 18 hours: this makes the product perfectly usable within an industrial prefabrication process.

It should be noted that in the case of a strand produced from thermoplastic resin and through two distinct phases, the final strand is characterized by 6 wires twisted around a central wire, where the central wire is perfectly round while the 6 twisted side wires are slightly ovalized. The side wires have therefore an almost elliptical solid cross-section, with a minor axis directed towards the geometrical center of the strand and the major axis is perpendicular thereto. This geometry is due to the tensions necessary for the twisting phase and the plastic phase of the matrix during the thermofonning. It is therefore generated a surface at least 1 mm wide and as long as the strand, rather than a contact line between the side wires and the central wire. This surface is characterized by a zero adhesion (fundamental in order to obtain a strand with limited flexural stiffness).

This configuration allows to increase the overall area of the strand thus increasing the resistant section and the ultimate resistance of the element Furthermore, the outer surface of the side single wire also allows better contact with the gripping systems since the contact point is characterized by a greater radius.

The present invention is not limited to the particular embodiments previously described and illustrated in the accompanying drawings, but it can be made numerous modifications of detail, within the scope of the person skilled in the art, without thereby departing from the scope of the invention, as defined in the attached claims.

Below are some illustrative but not limitative examples of the present invention. EXAMPLES

Example 1

A 7-wire strand having the dimensions as indicated above with reference to the preferred embodiment has been prepared. This strand has been applied to a prestressed concrete element in the form of a sheet pile, having a length of 200 mm (8 inch) and a cross section of 139x103 mm2, and has been subjected to a series of tests such as load tests.

The same test has been repeated using three strands known in the art, having the same geometry and diameter of the strand according to the invention, with wires having the same diameters but made of carbon steel (HSCS), stainless steel (HSSS) and carbon fiber composite (CFRP).

The properties of the four pre-stressed concrete samples reinforced with the four different strands are shown below in the following table:

wherein :

Example 2

A strand according to the invention (indicated in the graph with the abbreviation SIR # 1.50-U), made up of 7 wires with the dimensions as indicated above in Example 1, has been subjected to a tensile test according to the standard ASTM D7205, obtaining strain stress data (strain-stress) reported in the following graph: