IT201800002988A1 - COMPOSITE STRAND IN GLASS FIBERS AND / OR BASALT FOR PRECOMPRESSED CONCRETE - Google Patents

Description

Description of the invention entitled:

"COMPOSITE STRAND IN GLASS AND / OR BASALT FIBERS FOR PRECOMPRESSED CONCRETE"

DESCRIPTION

The present invention refers to a composite "strand" containing glass and / or basalt fibers, having a "1 x n" geometry similar to that of conventional steel strands (multi-wire strands) as defined by the Pr EN standards 10138 or similar (DM 14/01/2008 or ASTM A416 / A 416 M).

In particular, the present invention refers to a strand as defined above, suitable for use in the prestressing of concrete (PC) or in the construction of geotechnical tie rods, but more generally in all those applications where steel strands are used. carbon of any diameter but in which the demand for durability is lower or a certain simplicity is required in the removal phase.

More particularly, the present invention relates to composite strands as defined above which are suitable for use in existing chuck systems and stringing devices, normally used for carbon steel strands.

As is known, the prestressing of the cement is used to compensate for the poor tensile strength of the cementitious conglomerate.

Prestressing is achieved through the use of carbon steel cables inserted in the prestressed state into the concrete, using different methodologies (pre-tensioned cable systems or post-tensioned cables).

In particular, once the formwork has been placed and before the concrete casting (concrete casting) is carried out, the prestressing cables are stretched between two fixed and external supports (anchors), and then the concrete casting is carried out that wraps the cables claimed. Once the cement conglomerate has matured, the cable is released from the anchors.

The prestressing cables are used in various forms according to the needs: wire (drawn product with full section with notches that serve to improve adhesion with the concrete and to facilitate anchoring); bar (full section laminate provided with projections to promote adherence); braid (consisting of groups of 2 or 3 threads wound in a helix around their common longitudinal axis); strand (consisting of several wires or groups of wires wound in a helix in one or more layers around a rectilinear wire arranged along the longitudinal axis of the whole and completely covered by the layers).

Strands are generally used in the construction of prestressed concrete structures such as piles, sheet piles far seawalls, substructure hearing piles, noise panels and retaining walls walls).

Given their high mechanical strength and low cost, the most commonly used strands in prestressed concrete are those in prestressed carbon steel (CS), which however suffer from the drawback of being extremely sensitive to corrosion.

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

When long-term corrosion resistance is required, composite strands of thermosetting resins reinforced with aramid fibers or carbon fibers are generally used, thanks to their resistance to corrosion and excellent mechanical properties: however this type of strands suffers from drawback of having a cost much higher than that of carbon steel strands.

In particular, carbon fiber composite strands, mainly marketed by, while showing a good compromise between high mechanical performance (i.e. tensile strength) and corrosion resistance, have a very high cost, generally about 4 times higher than strands. in carbon steel, as well as having some disadvantages when used in the field of prestressed concrete.

Aramidic fiber composite strands have the disadvantage of having lower mechanical strengths than carbon fiber composite strands and a higher cost.

In fact, the use on site of these composite carbon fiber strands is very complex, above all for two reasons:

given the higher cost of the product, compared to steel strands, it is "necessary" to pre-tension them to much higher loads in order to make the most of their high tensile strength, but this entails the need to protect the strands in the strand contact interface - gripping, thus requiring a specific gripping system (chuck) with very complex stringing devices different from those used for steel strands (which do not have any protective interface sheath);

the high pre-tensioning load to which they are subjected involves a high energy incorporated in the pretensioned element, which in many operating phases causes local damage (cracking) to the concrete structure when this energy is released (see the figures below reported), as well as an unsafe use when cutting at the ends of the concrete casting.

The object of the present invention is to overcome, at least in part, the disadvantages of the prior art by providing a corrosion-resistant strand for prestressed concrete, having a cost comparable to carbon steel strands so as to avoid being preloaded to the maximum, and which can be used in the same pre-stress systems as steel strands.

Another purpose is to provide a strand as defined above which has a good mechanical strength such that it can be used in the construction of reinforced concrete elements which require a not particularly high pre-tensioning.

A further object is to provide a strand as defined above which is also easy to produce, and with high productivity, as well as being safe from the construction point of view.

These and other purposes are achieved by the composite strand made of glass and / or basalt fibers 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 composite material strand, with "1 x n" geometry equal to or similar to conventional steel strands standardized according to Pr EN 10138 or similar standards (DM 14/01/2008 or ASTM A4 16 / A 416 M), where each wire forming the strand has a full cross-section and composed of a polymeric matrix added with glass fibers or basalt fibers, or mixtures thereof.

The term "strand" here is intended to identify an element with a non-full cross-section, composed of a plurality of round wires, each with a full section, helically wound around a central straight wire arranged along the longitudinal axis of the strand and completely covered by the remaining outer threads.

Glass fibers can be of any type and grade on the market, such as E-Glass, E-CR-glass, A-glass, C-glass, D-glass, R-glass, S-glass (with high resistance traction).

Preferably, the glass fibers used are roving fibers, for example grade E-C (EC Fiberglass Roving).

The polymeric matrix can be formed by one or more thermoplastic resins, or by one or more thermosetting resins, or by their mixtures.

An example of a thermosetting resin is a vinyl ester resin.

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

The quantity of glass and / or basalt fibers with respect to the polymeric matrix is generally between 60 and 75% by weight.

The glass fiber and / or basalt composite yarns of the present strand are assembled with a continuous production process, in particular by means of a pultrusion line.

The term "pultrusion" here is intended to identify a continuous extrusion process used to produce reinforced polymeric profiles, where the reinforcing fibers, taken from the creel in the dispersed phase, are passed through the impregnation bath where they are bound to the resin matrix. , then passed, through a comb, into a preforming station, and subsequently into a heated mold (curing die) having the shape of the section of the desired final product.

The preferred construction Ixn of the strand of the present invention is the 7-wire one where around the central wire (indicated with 1 in the above formula) the remaining 6 wires (indicated with n in the above formula) are spirally wound ): this type of strand has the advantage of being a redundant system, compared to the single cable (bar), of showing a high tensile strength, but of being a flexible strand as it is bending stiffness linked to the single wire and not to the strand , as well as showing high adhesion to concrete thanks to both the spiraling of the strand itself, similar to that of the carbon steel strand.

In an example of such a 7-wire construction, the wires each have a diameter of 5mm while the overall diameter of the 7-wire strand is approximately 15.2mm.

Other types of constructions are possible according to the present invention by suitably varying the number of wires, their diameter, and the windings around the central wire per unit of length and the cross-sectional area to obtain the desired strength and flexibility.

The strand according to the present invention is suitable for being used both as a pre-tensioned cable or as a post-tensioned cable in the prestressed concrete sector, but also in the construction of geotechnical tie rods, wire ropes and the like.

It should be noted that this strand, in any type of construction, and / or the threads that compose it, does not need to be wrapped with textile threads (yarn) or sheaths, contrary to what is necessary for composite carbon fiber strands. Further features of the invention will become clearer from the detailed description that follows, referring to a purely exemplary, and therefore non-limiting, embodiment illustrated in the attached drawings, in which:

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

Figure 2 illustrates an embodiment of a composite 7-strand glass fiber strand (G-FRP) according to the invention;

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

Figure 4 illustrates the cracks as a result of the release of carbon fiber composite strands of the known art;

Figure 5 illustrates a gripping cone used for steel strands;

Figure 6 illustrates a composite fiberglass strand in accordance with the invention associated with the wedges 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 as a whole with the numerical reference 10, is formed by a central wire, indicated in the figure with the numerical reference 1, and by six wires, indicated in the figure (see also fig. 6) with the numerical reference 2 , lateral (indicated with the letter n in the aforementioned wording) which are arranged twisted around the central thread 1 with a spiral step.

The abbreviation "1 x n" here means the typical construction of a strand where 1 represents the central wire while n represents the number of side wires that are twisted around the central wire with a spiral step.

Said composite strand 10 according to the invention, has a non-full cross section, contrary to the wires that compose it, and with empty spaces that determine the high flexibility of the strand and its winding in coils.

As illustrated in fig. 6, the composite fiberglass strand according to the invention is associated with a wedges gripping system 20 which will fit into the pre-tensioning jack.

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

Furthermore, it has been found that the present strand exhibits comparable characteristics, if not better for some parameters, than the known strands, in terms of

- high tensile strength,

- lightness (lightweight),

- high corrosion resistance,

- magnetic transparency,

- electrical resistivity since there is no electrical conductivity, - low linear coefficient of expansion rate ',

- Ease of cutting for temporary applications.

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

- guaranteed nominal tensile strength (ffu *) = 800 MPa; - nominal tensile modulus (Ef) = 46000 MPa. ; However, given the lower breaking strength limit for creep compared to composite carbon fiber and stainless steel strands, even if suitable for some applications, the present strand can be used more advantageously where a high level of prestress is not required. cement such as that to which the composite strands in carbon fibers are subjected, for example usable in structural elements or prestressed concrete products such as piles, sheet piles for seawalls, piles of supporting structures (substructure bearing piles) ), noise panels and retaining walls, whose prestressing level is typically around 4.8 MPa (700 psi) which is well sustainable by the present strand. ; As far as the Applicants are aware, a composite strand made of glass fibers or basalt has never been described or produced in the art: in fact, rods and bars made of glass fiber composites are known as reinforcements for concrete, which however cannot be used in prestressing applications due to their low flexibility and consequently their impossibility to be rolled into coils. ; This makes them unusable in continuous and very long quantities, for example with lengths of 1000 m, as necessary in the prestress constructions of precast systems where it is necessary to guarantee the continuity of the cables without joining elements. Moreover, the present strand can in turn be used to make ropes (wire strand rope) composed of a central core formed by the present strand on which other strands of the present invention are wound in a helix. ; The pre-tensioning of this composite glass fiber and / or basalt strand is generally set to be less than 50% of the guaranteed strength of the material as it is (GFRP - glass fiber reinforced polymer). ; The present strand exhibits a number of advantages. ; Given the geometry equal to that of the steel strands, it is possible to use the existing prestress equipment for the steel strands used in prestressed concrete, including the stringing devices (chuck): this makes this strand usable immediately. ; In fact, the heart of the present invention lies in the fact of having made a composite strand, standardized like steel strands, which thanks to this standardization and configuration is suitable for use in the same standardized equipment used by industry in prestress, using the same gripping cone (gripping cone - fig. 5) and the same prestressing hydraulic jack - fig. 3). ; Another advantage is that the glass fiber composite (GFRP) is immune to corrosion, and has a cost comparable to that of traditional carbon steel cables, while exhibiting greater strain at failure; 2.0% ultimate elongation ) and a significantly lower modulus of elasticity (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 GFRP has a lower limit of creep breaking strength than carbon fiber and stainless steel composites, which does not allow it to be pretensioned to the same levels, it is nevertheless useful when a lower prestress is required to reduce losses due to the cement creep during the service life of prestressed concrete. ; It is understood that it is also possible to provide for the formation of a composite strand of glass and / or basalt fibers according to the invention, but having a geometry different from 1 x n, with the threads intertwined according to different schemes, for example braids of strands consisting of wires of different diameters, without thereby departing from the scope of the present invention. The present invention is not limited to the particular embodiments previously described and illustrated in the annexed drawings, but numerous detailed modifications can be made to it, within the reach of the person skilled in the art, without thereby departing from the scope of the invention itself. as defined in the appended claims. Some illustrative but non-limiting examples of the present invention follow. ; EXAMPLES; Example 1; A 7-wire strand of the dimensions as indicated above with reference to the preferred embodiment was prepared. ; This strand was applied to a prestressed concrete element in the form of a sheet pile, 200 mm (8 inch) long and with a cross section of 139x10 <3> mm <2>, and subjected to a series of tests such as load tests . ; The same was repeated using three other strands of the prior art, having the same geometry of the strand according to the invention, and substantially the same diameter of the wires and the same diameter of the strand, but made of carbon steel (HSCS) , stainless steel (HSSS) and carbon fiber composite (CFRP). ; The properties of the four prestressed concrete samples reinforced with the four different strands are reported below in the following table: ;; ;; where:; ff, pull = Pretension Stress; Ppull = Pretension Load; Ae = Initial Strain; Pi = Initial Load; σc, l = Initial Prestressing; Δ75y = Strain at 75 years; P 75y = Load at 75 years; σc , 75y = Prestressing at 75 years; ffu * = characteristic tensile strength.

Example 2

A strand according to the invention (indicated in the graph with the initials SIR # 1.50-U), formed by 7 wires with the dimensions as indicated above in example 1, was subjected to a tensile test in accordance with the ASTM D7205 standard, obtaining the strain-stress data reported in the following graph:

Claims (10)

CLAIMS 1. Composite material strand, with "1 x n" geometry composed of a polymeric matrix added with glass fibers and / or basalt fibers, in which said polymeric matrix is formed by one or more thermoplastic resins, or by one or more resins thermosets, or from their mixtures. 2. Strand according to claim 1 wherein the glass fibers are selected from E-Glass, E-CR-glass, A-glass, C-glass, D-glass, R-glass, S-glass grade glass fibers , preferably E-C grade roving fibers (. EC Fiberglass Roving). The strand according to claim 1 or 2 wherein the polymeric matrix is formed from a vinyl ester resin. 4. Strand according to any one of the preceding claims in which the geometry is 1 x 7, where a central wire (1) is wound by six lateral wires (2), arranged twisted around said central wire (1) with a spiral pitch . The strand of claim 4 wherein in 7-wire strand geometry, the wires each have a diameter of 5 mm and the overall diameter of the strand is 15.2 mm. 6. Strand according to any one of the preceding claims obtained by pultrusion. 7. Coils of composite strand as defined in any one of the preceding claims. 8. Wire strand ropes comprising one or more composite strands as defined in any one of the preceding claims. 9. Use of one or more composite strands as defined in any one of the preceding claims in the prestressing of cement (PC). 10. Prestressed concrete products comprising one or more strands or ropes as defined in any one of the preceding claims, preferably selected from piles, sheet piles for seawalls, substructure hearing piles, noise panels and retaining walls.