Patent Application
20230399810
The invention relates to a soil reinforcement strip for mechanically stabilized earth structures.
The invention also relates to a reinforcement grid for soil and ground.
Furthermore, the invention also relates to applications of this reinforcement strip and grid, namely to a reinforced soil layer and to a mechanically stabilized earth structure or retaining wall.
Improved mechanically stabilized earth structures and reinforced soil layers already date back from the French patent FR 1 393 988 of Henri Vidal where the basic principle of soil stability and reinforcement was disclosed and explained. Fill material and reinforcement elements form a stable equilibrium based upon the simple principle of friction of the fill material with the reinforcement elements or within the fill material that is in contact with the reinforcement elements.
By way of example, a mechanically stabilized earth structure comprises a facing in the form of prefabricated concrete blocks, metal welded lattice panels, or woven or welded gabions of metal wires. The facing has connectors for connecting soil reinforcement strips to the facing. These soil reinforcement strips extend at one side of the facing and provide stability to the fill material that is provided at the one side of the facing.
The service life of these retaining wallsis expected to be between 75 years and 120 years. Hence, huge demands are required from the soil reinforcement strips. Firstly, the strips must provide the required friction to give stability to the fill material. Secondly, the strips must provide the required tensile strength and elongation for wall stability. Thirdly, the soil reinforcement strips must have a long service life time, i.e. they must be highly corrosion resistant since the fill material can be aggressive. Fourthly, the soil reinforcement strips must be resistant to damage possibly caused by the layout and compaction inside the backfill material.
EP-B1-3 099 860 of Terre Armée Internationale discloses a soil reinforcement strip having a polymer matrix and reinforcement fibres inside. In order to improve the adhesion between the reinforcement fibres and the polymer matrix, the polymer is at least partially functionalised, more particularly the polymer comprises a functionalised polyolefin.
A general object of the invention is to avoid or at least to mitigate the disadvantages of the prior art.
A particular object of the invention is to provide a further improvement to the soil reinforcement strips.
A more particular object of the invention is to provide improved reinforcement elements for the reinforcement strips.
Still another object of the present invention is to increase for example the stiffness, the durability and/or service life of a reinforcement strip.
According to a first aspect of the present invention, there is provided a soil reinforcement strip, for example, for mechanically stabilized earth structures. The strip has a polymer matrix and further comprises elongated steel elements inside the polymer matrix as reinforcing elements. The elongated steel elements have a combined elastic and plastic elongation at break that exceeds 4%, for example 4.5%, 5% or even 6%. and/or further preferred stays below 15%, further preferred stays below 12%, further preferred stays below 10%. This may contribute for example to an efficient mechanical stabilization of earth as well as to allow structures to settle at least to some extend and/or allow some displacement of any facing or facing elements during use to help visually diagnose any problems and correspondingly plan maintenance, while at the same time helping to avoid total failure, especially from facing elements toppling over or falling and/or from the embankment crumbling.
In an embodiment, the invention may especially for example concern a soil reinforcement strip,
In an embodiment, the invention may especially for example also concern a soil reinforcement strip,
In an embodiment, the invention may especially for example further concern a soil reinforcement strip,
The elongated steel elements may be stainless steel elements or carbon steel elements. The carbon steel elements may be coated with a material providing cathodic protection, for example, they are coated with zinc or with a zinc alloy to increase the resistance against corrosion. Alternatively, or in addition, the elongated steel elements may be in contact with a material providing cathodic protection.
The use of these elongated steel elements has a number of advantages. First of all, one is be able to benefit of the high tensile stiffness of steel material at low elongation. As a result, low elongation can be maintained during the construction and normal life of the structure. Secondly, enough ductility is kept at break in case of unexpected event on the structure approaching higher movement or failure, especially to avoid sudden failure. Steel in the sense of the present invention may thereby preferably have a nickel content <17 wt-% preferably below 15 wt-%, further below 10 wt-%, even further preferred below 5 wt-% and/or a manganese content <9 wt-%, preferably below 7 wt-%, further preferred below 5 wt-%. Steel may thereby especially for example have a Young's modulus between 180 and 200 GPa. The use of steel may thereby further contribute to reduce problems with creep, especially when physically and/or chemically coupled to the matrix.
As will be explained hereinafter, the structural elongation is not included in the combined elastic and plastic elongation at break.
The elongated steel elements may be steel wires.
Most preferably, however, the elongated steel elements are steel cords or comprise at least one steel cord, preferably for example comprising steel filaments that are individually coated with zinc or with a zinc alloy and that are twisted together to form the steel cord. A steel cord may have the same reinforcing effect of a steel wire while being much more flexible. A steel cord may thereby been assembled from drawn filaments, whereby especially for example the heat of the drawing may influence the crystalline structure and properties of the steel.
The elongated steel elements have been preferably stress-relieved in order to reach the minimum elastic and plastic elongation. Stress-relieving the elongated steel elements is done by heating the steel cords to a temperature above 400° C. In doing so, the crystalline structure of the steel may be influenced and the tensile strength of an elongated steel element may drop for example by somewhat more than 10% but at the same time, the plastic elongation will increase very significantly so that the sum of elastic elongation and plastic elongation may exceed 4%, especially at still high values of tensile strength. Steel cords with a high tensile strength or a high cord tensile strength of for example 3000 MPa but with a very low elongation at break may thereby be assembled from drawn steel filaments. Upon stress relieving the tensile strength or a cord tensile strength may for example decrease to about 2600 MPa, while the plastic elongation increases significantly, especially to above a value dor an equivalent cord with a tensile strength or a cord tensile strength of for example about 2600 MPa, so that the sum of elastic elongation and plastic elongation may exceed 4%. This may particularly contribute for example to achieve high elongation at break and high cord tensile strength or tensile strength.
In case of steel cords, the preferred steel cords are single strand cords that can be made in one single twisting operation and with a limited number of steel filaments, e.g. between two and seven. Most preferably, the steel cords have an open construction, i.e. that polymer of the polymer matrix contacts all steel filaments and, ideally, surrounds all steel filaments individually. A full penetration of the polymer inside the steel cord and good adherence of the polymer to the steel cord is a guarantee for excellent corrosion resistance and prevents water or moisture from travelling along the length of the steel cord. Adhesion between the polymer matrix and the steel cords maybe thereby also be improved for example by physical or chemical couping for example using surface physical or chemical surface treatments and/or chemical agents such as glues, primers or adhesion promotors. This may further contribute to improve the coupling and/or load transfers between the polymer matrix and the steel cords, so as to help with mechanical stabilization. In an embodiment, at least one of the elongated steel elements or each elongated steel element may thereby comprise for example a plurality of steel filaments, wherein the steel filaments have a twisted configuration such that helical interstices are formed between at least some of the steel filaments at a periphery of the at least one of the elongated steel elements, wherein polymer material of the matrix penetrates into the helical interstices. In an embodiment, the at least one of the elongated steel elements or each elongated steel element may comprise for example a central steel filament and peripheral steel filaments stranded around the central steel filament. In an embodiment, the helical interstices at the periphery of the at least one of the elongated steel elements or each elongated steel element may reach the central steel filaments. In an embodiment, the peripheral steel filaments may be between 3 and 9, preferably between 3 and 6, peripheral steel filaments. In an embodiment, the central steel filament may have a diameter larger than at least some or all of the peripheral steel filaments. In one embodiment, the at least one of the elongated steel elements or each elongated steel element is made of a group of at least two steel filaments twisted together as a group. In one embodiment, the/said group may have at most 5 steel filaments. The use of steel cords, especially with multiple strands or filaments, which may allow for the polymer to penetrate inside the steel cords, may thereby further contribute to improve coupling and/or load transfers between the polymer matrix and the steel cords, especially by allowing the polymer to be thereby physically coupled to the steel cords in a very durable way, so as to help with mechanical stabilization, especially when in use for a very extended time period of many years.
Examples of single strand cords that offer good polymer penetration may be:
Preferably the elongated steel elements have a cord tensile strength or tensile strength above 1800, e.g. above 2000 MPa, most preferably above 2200 MPa, e.g. above 2500 MPa and/or below 4500 MPa. Such high values may thereby be unusual for steel, especially in combination with a high elongation at break that exceeds 4%, since elongation at break usually significantly decreases to very low levels as cord tensile strength or tensile strength increases. This may contribute for example to an efficient mechanical stabilization of earth, especially by the high modulus and/or high tensile strength of steel.
The soil reinforcement strip may comprise between 5 and 50 steel cords or steel wires, e.g. between 10 and 45 steel cords or steel wires, e.g. between 15 and 40 steel cords or steel wires, further preferred between >15 and <30 steel cords or steel wires. In an embodiment, the steel cords or steel wires are arranged inside the polymer matrix parallel to each other, preferably in a way that they do not touch each other. This may contribute to help to prevent propagation of corrosion and/or failure of steel cords or steel wires. In an embodiment, the steel cords or steel wires may be arranged inside the polymer matrix in one plane or in multiple planes, especially for example in 2, 3 or 4 planes. In an embodiment the thickness of the polymer matrix between any point at the outside of the strip and any steel cord or steel wire inside the strip is at least 100 μm, preferably between >100 μm and <500 μm, further preferred between 150 μm and 400 μm, further preferred between 160 μm and 350 μm, further preferred between 175 μm and 300 μm. This may contribute to help to prevent corrosion and/or failure of steel cords or steel wires. In an embodiment, wherein the dimension of the outline of cross-section of the matrix in one direction is bigger than the dimension of cross-section of the matrix in another perpendicular direction, preferably wherein the outline of cross-section of the matrix may thus be for example oval, rectangular or corresponds to a rectangle with rounded corners. This may help to further increase the drag on the strips and thus also for example further contribute to mechanical stabilization of earth.
In order to increase the adhesion between the polymer and the elongated steel elements, the polymer matrix may comprise, at least partially, a functionalised polyolefin like especially for example HDPE (high density polyethylene), modified polypropylene and low-density polyethylene (LDPE), linear low density polyethylene (LLDPE), very low density polyethelene (VLDPE), ultra low density polyethylene (ULDPE) and/or ethylene based copolymers like for example ethylene-propylene copolymers, ethylene-vinyl acetate copolymers (EVA) or acrylate copolymers, especially for example modified, grafted or comprising some polar function like acrylate or acetate functions. In case of partial functionalisation and/or the matrix partially comprising other materials, the functionalised polyolefin is in contact with the steel cords. This may thereby further contribute to increase coupling and/or load transfers between the polymer matrix and elongated steel elements, so as to help with mechanical stabilization.
In addition to the functionalised polyolefin, or as an alternative, the elongated steel elements may be provided with a primer or an adhesion promotor.
In a preferable embodiment of the invention, the steel elements, especially for example the steel wire or the steel cord, may be individually or together coated in advance with a thin layer of a polymer that is a functionalized polyolefin or individually or together provided with a primer or adhesion promotor before all the steel wires or the steel cords are together extruded with a common, non-functionalized polymer.
A good adhesion is needed to protect the elongated steel elements from external damage and to maximize the durability.
The soil reinforcement strip may have a strip breaking load ranging from 1 kN to 200 kN, e.g. ranging from 2 kN to 100 kN, preferably ranging for example from 10 kN to 150 kN, further preferred from 20 kN to 125 kN, even further preferred from 30 kN to 110 kN, even further preferred from 50 kN to 105 kN.
According to a second aspect of the present invention, the invention relates to a grid for soil reinforcement. This grid comprises a set of first strips in a first direction and a set of second strips in a second direction. The set of first strips and the set of second strips cross one another and are bonded or fixed to each other, e.g. by slightly heating the strips so that the polymer matrixes adhere to each other, or by means of a glue or a hot melt, or by means of stitching. The set of first strips or the set of second strips or both the set of first strips and the set of second strips may be reinforced strips according to the first aspect of the invention.
The invention strip and the invention grid can be used as reinforcement for a reinforced soil layer.
The invention strip and the invention grid can be used as reinforcement in a mechanically stabilized earth structure.
In an embodiment, one or more strip(s) may be for example arranged to be connected or may be connected to at least one facing element, preferably stabilizing earth and/or arranged to be part of an embankment, further preferred a concrete block or a gabion, especially for example so that forces exerted on the facing and/or embankment can at least partially be counter acted by drag on the strip(s). This may allow the strip(s) to contribute to mechanically stabilize the earth and/or the facing and/or the embankment.
Suitable and preferable polymers or functionalised polymers for the polymer matrix 12 may be for example polyolefins such as grafted and/or blended HDPE (high density polyethylene), modified polypropylene and low-density polyethylene (LDPE), linear low density polyethylene (LLDPE), very low density polyethelene (VLDPE), ultra low density polyethylene (ULDPE) and/or ethylene based polymers or copolymers like for example ethylene-propylene copolymers, ethylene-vinyl acetate copolymers (EVA) or acrylate copolymers, especially for example modified, grafted or comprising some polar function like acrylate or acetate functions.
The strip 10 may have a width ranging from 5 mm or 10 mm to 80 mm, preferably 20 mm to 70 mm. Especially in case of a grid, the width of the strips for example may be less than 20 mm. The strip 10 may have a thickness ranging from 2 mm to 4 mm.
Referring to
Alternative steel cords may be provided that also have a construction with one or more core filaments and an unsaturated layer.
An example is a 1×d1+6×d2 construction where the diameter d2 of the layer filaments is smaller than the diameter di of the core filament. Another example is a 1×d1+4×d2 construction with the diameter d1 of the core filament equal or somewhat thicker than the diameter d2 of the layer filaments.
The steel filaments of the steel cords preferably have a zinc or a zinc alloy coating. Most preferably, the zinc or zinc alloy coating is applied to the steel filaments by way of a hot dip operation. The thickness of the zinc coating may be smaller than four micrometer, e.g. lower than three micrometer. Preferably an alloy layer zinc-steel is present between the steel core and the zinc or zinc alloy coating. This zinc or zinc alloy coating is added to the steel wires or the steel filaments of the steel cord before any extrusion with a polymer or before any treatment with a primer or adhesion promotor.
A zinc alloy coating may be a zinc aluminium coating that has an aluminium content ranging from 2 per cent by weight to 12 per cent by weight, e.g. ranging from 3% to 11%.
A preferable composition lies around the eutectoid position: Al about 5 per cent. The zinc alloy coating may further have a wetting agent such as lanthanum or cerium in an amount less than 0.1 per cent of the zinc alloy. The remainder of the coating is zinc and unavoidable impurities.
Another preferable composition contains about 10% aluminium. This increased amount of aluminium provides a better corrosion protection then the eutectoid composition with about 5% of aluminium.
Other elements such as silicon (Si) and magnesium (Mg) may be added to the zinc aluminium coating. With a view to optimizing the corrosion resistance, a particular good alloy comprises 2% to 10% aluminium and 0.2% to 3.0% magnesium, the remainder being zinc.
An example is 5% Al, 0.5% Mg and the rest being Zn.
Other n×1 steel cord constructions may be provided with n the number of steel filaments and ranging from three to where nine. One or more of the steel filaments has a plastic deformation, for example an undulation, to avoid that all n steel filaments contact one another and form and enclose a central cavity where polymer would not be able to penetrate.
At small initial loads steel cord constructions may show in some cases a relatively large elongation As, referred to as the structural elongation. This is the first phase. The structural elongation As of the steel cord is that part of the elongation that is due to the twisted nature of the cord or due to plastic preformation of the individual steel wires. The cord is somewhat stretched, filaments come closer to each other and the twisting pitch increases somewhat. In case of an individual steel wire, the steel wire is stretched and becomes straight.
It is not necessary that this structural elongation is avoided. In some cases it may be useful to obtain a complete penetration by the polymer inside the steel cord.
In a second phase, the steel filaments in the steel cord show an elastic elongation Ae. That part is the linear part according to Hooke's law.
In a third phase, the steel filaments are plastically deformed over an elongation Ap until the steel cord breaks at an elongation of At.
According to the present invention, the sum of the elastic elongation Ae and the plastic elongation Ap must exceed 4%, e.g. is more than 4.5%, e.g. more than 5%. The structural elongation As, if any, must not be added, since this structural elongation gets lost due to embedment of the steel cord in the polymer matrix.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20290074.2 | Nov 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/080566 | 11/3/2021 | WO |
