REINFORCING ELEMENT OF SYNTHETIC MATERIAL FOR USE IN REINFORCED CONCRETE, MORE PARTICULARLY PRESTRESSED CONCRETE, REINFORCED CONCRETE PROVIDED WITH SUCH REINFORCING ELEMENTS, AND PROCESSES OF MANUFACTURING REINFORCING ELEMENTS, AND REINFORCED AND PRESTRESSED CONCRETE

Abstract: The invention relates to a reinforcing strip (1) applicable in reinforced concrete, more particularly prestressed concrete, formed by a matrix based on a thermosetting synthetic material in which preferably more than 15000 practically parallel continuous filaments (5) are obtained. The filaments (5) are formed from an organic polymer selected from the group of aromatic polyamides, such as polyparaphenylene terephthalamide, or from polyethylene, polyvinyl alcohol or polyacrylonitrile via solvent spinning. The matrix is formed from a synthetic material based on epoxy resin and/or bismaleimide resin. The reinforcing strip (1) has an improved resistance to alkali. The invention also relates to reinforced concrete comprising said strips (1).

Description: Reinforced concrete, more particularly prestressed concrete and processes of manufacturing reinforced and prestressed concrete The invention relates to reinforced concrete, including prestressed concrete, comprising one or more reinforcing elements, which are each formed by a matrix based on thermosetting synthetic material in which more than 5000 practically parallel continuous aromatic polyamide filaments are contained, the tensile strength of the filament band in the reinforcing element is higher than 2,0 GPa, the modulus of elasticity of the filament band in the reinforcing element is higher than 60 GPa and the elongation at rupture of the filament band in the reinforcing element is less than 6% to 7%, wherein the filaments form not more than 90% by volume of the reinforcing element and the synthetic matrix material forms at least 10% by volume thereof. The invention further comprises a process of manufacturing reinforced concrete or prestressed concrete provided with the reinforcing elements. As is known, for the reinforcement of concrete and prestressed concrete use is still mainly made of steel. The use of steel is particularly based on its favourable mechanical properties, such as high strength and high modulus of elasticity and on the fact that the more or less alkaline environment in concrete and cement mortar prevents the steel contained in the concrete from being corroded; in other words the durability of reinforced concrete in the open air especially depends on the continuous presence of said alkaline environment by which the steel reinforcement is protected from corrosion. Under the influence of CO₂ in the atmosphere, however, the free lime in the concrete will be bound, as a result of which the alkalinity will decrease. Such a process is called carbonation. A decrease in the alkalinity of the concrete, particularly as soon as the pH is lower than 10, may give rise to corrosion of the steel. From the outer surface inwards the carbonation depth increases with time and as soon as the carbonation depth has become equal to the thickness of the concrete cover, the steel reinforcement may begin to rust, which in principle may lead to considerable damage of the concrete construction and may shorten its life time. The generally known phenomenon of atmospheric pollution by, int. al., carbon dioxide and agressive sulphur, chlorine and nitrogen compounds, which has shown an upward trend over the past twenty years, may in principle lead to deterioration of the steel. This air pollution not only takes place in the immediate vicinity of the industry, but also at great distances from it. As a result, the pH of rain water has in the past 10 years decreased from 6 to 5, which means a considerable increase in environmental acidity. These environmental problems are expected to become even greater in the coming years. For more details of the problems to steel as a reinforcing material reference may be had to the article "Zelfs beton vraagt aandacht" ("Even concrete requires attention"), by Ir. W.R. de Ritter, Hollandse Betongroep N.V. Dept. S & O (see the Journal: Cement of March 1983), and CUR VB-84-6, "Agressiviteit Milieu en Duurzaamheid Betonconstructies" (Agressiveness of Environment and Durability of Concrete Structures") and CUR VB-84-1 "Corrosie van de wapening in gewapende betonconstructies" ("Corrosion of the reinforcement in reinforced concrete structures") published by the "Stichting voor onderzoek, voorschriften en kwaliteitseisen op het gebied van beton" ("Institute for tests, regulations and quality standards in the field of concrete"). Consequently, of reinforced concrete structures that have been exposed to atmospheric pollution or some agressive environments it has been found that particularly in recent years their steel reinforcement was damaged by corrosion. Durability therefore does not come up to expectations and high costs of repair must be reckoned with. To solve the above-described corrosion problems attempts have been made to find alternative reinforcing materials that display similar physical and mechanical properties and are not or less sensitive to steel corroding environments. Up to the present invention the only eligible materials of any practical value were glass or glass fibres. An attendant problem to a material such as glass is that although it does have the desired mechanical and physical properties and cannot be subject to corrosion, it generally displays insufficient chemical resistance to the alkaline environment (pH > 12) prevailing in non-carbonated concrete. Synthetic yarns that are melt spun from polymers such as polyethylene terephthalate, polyolefins and polyamide that do display the necessary chemical resistance generally have physical and mechanical properties, such as a very low modulus of elasticity, a high creep, etc., that render them totally unsuitable for use as an alternative to reinforcing and prestressing material for concrete. Research has also led to the development of non-steel reinforcing elements that have been tested on a small scale in actual practice and are formed by a matrix based on a thermosetting synthetic material in which there are more than 5000 practically parallel continuous glass filaments. Such reinforcing elements and their use in concrete and various manufacturing methods are described in the article "Kunstharz gebundene Glasfaserstäbe - eine Korrosiensbeständige Alternative zum Spannstahl" by Martin Wieser and Lothar Preis on pp. 79-85 of the book "Fortschritte im konstruktiven Ingenieurbau", published by Rolf Eligehausen and Dieter Russwurm, Verlag Ernst und Sohn, 1984, Berlin. In that article consideration is given to the replacement of prestress steel in concrete with reinforcing elements which each consist of a large number of glass filaments in a matrix of synthetic material in the form of unsaturated polyester resin. These known reinforcing elements have been successfully used outside the concrete field, especially as a result of suitable physical and mechanical properties and a good chemical resistance to various media and particularly a good resistance to acids. From the considerations on page 81 (right hand column) and page 82 in said article of Weiser and Preis, however, it appears that there are problems with regard to the resistance of these known reinforcing elements to the alkaline environment prevailing in concrete or cement mortar. Under points 4.1, 4.2 and 4.3 in said article three different solutions to these problems are discussed, mention being made of the risks and/or drawbacks to each of them. Mentioned as one of these three alternatives is a protection in the form of modifications to the synthetic matrix (of an unspecified composition) in which during loading there may be no formation of cracks down to as far as the glass filaments. Another alternative consists in providing said reinforcing element with a special sheath. As a third possibility is mentioned the use of some special injection mortar, which alternative is not only laborious but exclusively applicable in the fairly costly process of making prestressed concrete, where during the pouring of the concrete mixes room is left for channels, in which after the hardening stage the reinforcing elements are generally pretensioned, use being made of steel, corrosion sensitive anchoring elements, and the special mortar must be injected. This last-mentioned solution is so complicated and costly that instead of employing these well-known reinforcing elements of glass filaments and unsaturated polyester resin use had better be made of the less costly conventional reinforcement of prestressing steel. With respect to the state of the art reference is still made to the article: "Kunststof profielen met glasvezelwapening" (Glass fibre reinforced sections of synthetic material) in the journal: Metaal en Kunststof of 1983-02-14. Just as in the afore-mentioned article of Weiser and Preis special consideration is given to the product Polystal® of the firm of Bayer. As is known, the last-mentioned product consists of a great many parallel glass filaments contained in a matrix of unsaturated polyester resin. In the first column of the article in Metaal en Kunststof it says that for the matrix material also other synthetic materials may be used and that the production process also lends itself for processing other reinforcing fibres, such as carbon or aramid fibres. The reinforcing element according to the invention consisting of the special afore-mentioned combination of PPDT, PE, PVA or PAN filaments contained in a matrix of epoxy resin and/or bismaleimide resin, and the particularly favourable use thereof in reinforced or prestressed concrete are not mentioned in said publication. Although the development of concrete reinforcement consisting of bars of glass filaments embedded in a synthetic matrix dates back to 1972 and both aramid yarns and epoxy resins were already known in themselves at that time, the use in reinforced concrete of the special reinforcing element according to the invention has not been proposed. The use of continuous glass filaments in prestressed concrete has even been known since 1954 (see the article: "A preliminary investigation of the use of fibre-glass for prestressed concrete" by Ivan A. Rubinsky and Andrew Rubinsky, Magazine of Concrete Research; September 1954, p. 77). It is believed that in the generally conservative building market the man skilled in the art has been prejudiced against the use of synthetic materials in fields where they are to satisfy high strength requirements over a long period. In the article "Lifetime Predictions for Polymers and Composites" by R.M. Christensen, Lawrence Livermore Laboratory, University of California, in Journal of Rheology, 25 (5), pp. 517-528 (1981), p. 24, mention is made of composites of aramid yarns in epoxy resin. US 4 515 636 proposes the manufacture of concrete sheets reinforced with short fibres of aromatic polyamide. The fibres used have a length of, for instance, 6 mm and are homogeneously distributed throughout the concrete matrix. Such a reinforcement is uneconomical in that it requires a relatively large amount of reinforcing fibres of which a considerable proportion is present in places where no reinforcement is required. Moreover, the strength properties of the aramid are not taken full advantage of. EP 0 127 198 describes composites for use in aircraft, automobiles and sporting goods. Those generally described composites are formed of an epoxy resin with a hardener and a fibre selected from the group of carbon, glass, silicon carbide, poly(benzothiazole), poly(benzimidazole), poly(benzoazole), alumina, titania, boron and aromatic polyamides. DE 2 653 422 describes a special process for manufacturing fibre-reinforced synthetic strips. As synthetic materials are mentioned thermoplastic and thermosetting materials and a blend of epoxy resin and phenolic resin. As fibre materials are mentioned carbon and aromatic polyamide. NL 7 108 534 describes a process of preparing reinforced, prestressed or unprestressed concrete, in which process a bundle of continuous reinforcing filaments are provided with a resin coating before they are passed into the form. It mentions various resins, viz. unsaturated polyester resin, acrylate resins, epoxy resin and polyurethane resins. As eligible filament materials are mentioned the rather conventional synthetic polymer materials, viz. polyester, polyamide and polypropylene processed by melt spinning, and polyvinyl alcohol and rayon. Although said polymers are particularly suitable for various purposes, it has been found that they are not suitable in actual practice to replace steel as reinforcing material in concrete, notably because of the fact that of the yarns described in NL 7 108 534, int. al. the physical properties, such as tensile strength and modulus of elasticity were too low and the creep was generally too high. EP 0 062 491 describes a process for the manufacture of a composite material formed from a matrix containing a reinforcing material of a polymer, the polymer being subjected to a plasma treatment in order to improve the adhesion to the matrix. As suitable reinforcing materials are mentioned, int. al., (see pages 7 and 8 of said publication) film, fibrillated film or fibres in the form of monofilaments, multifilament yarn, staple fibres, optionally in the form of a fabric. According to said publication these last-mentioned materials may consist of homo-or copolyolefins, such as polyethylene, polypropylene or a polyethylene-polyester copolymer, and also polyethylene terephthalate, nylon and aramid are mentioned. As suitable matrix materials are mentioned thermosetting and thermoplastic resins, polyvinyl chloride, inorganic cement such as Portland or other cement. As preferred thermosetting matrix resins are mentioned phenolic resin, epoxy resin, vinyl ester, polyester, etc. GB 1 425 032 describes a band of carbon filaments held in band form by a watersoluble binding material. These bands may be impregnated with matrix material such as a polymer or cement. US 4 077 577 describes an asbestos-cement pipe manufactured by winding. In addition to the wound asbestos cement layers the pipe consists of helical windings of aromatic polyamide filaments, the filament bundle being directly impregnated with cement. Japanese patent publication J 57 156 363, DE 1 925 762 and DE 2 848 731 relate to applying surface irregularities to the filaments for the purpose of improving the adhesion to a matrix. EP-A-0 170 499, which is only prior art according to Art. 54(3), relates to a reinforcing bar for concrete. Said bar comprises a plurality of threads of reinforcing fibres, which may consist of aromatic polyamide, impregnated with a hardenable material, such as epoxy resin. Said reinforcing bars have a substantially cylindrical configuration in cross-section. US 4 194 873 particularly deals with an apparatus for producing a fibre reinforced pultruded rod provided with at least one groove or protrusion. The reinforcing fibres may consist of Kevlar®. i.e. aromatic polyamide polyparaphenylene terephthalamide, embedded in a matrix of thermosetting resin, such as epoxy resin (see column 3, lines 1-8) and the rods may be used to reinforce concrete. Figs. 6-9 show rods of which the cross-section is provided with only one longitudinal groove (53) or only one longitudinal protrusion (52). Figs. 3, 4 and 5 show embodiments of rods with a number of grooves (53) and protrusions (52) to give rods having a multilobal cross-section. As appears from Fig. 1 and 2, the partially cured rods are rotated so that said grooves and/or protrusions extend helically along the rod (see Figs. 5, 7 and 9). Said rotation results in twisting of the partially cured rod (20) prior to its full cure (see column 3, lines 49, 50). Said twisting is disadvantageous because in the twisted rod the reinforcing fibres will not extend parallel to the longitudinal direction of the rod and so cannot bear the maximum load. It should also be noted that US 4 194 873 does not disclose rods having a substantially rectangular cross-section as specified in the claims of the present invention. Japanese patent application 59-58389 was published on October 15, 1985 and corresponds to US 4 648 224. This patent publication also relates to a rod or tendon formed of fibre reinforced plastics for pretensioning a concrete structure. The rods are covered by a separate plastic adhesive layer comprising inorganic particles to achieve mechanical engagement between the rod and the concrete (see Fig. 1 + claim 1). The reinforcing fibres may consist of aromatic polyamide embedded in a matrix of epoxy resin (see column 2, lines 43 and 47). Like EP-A-0 170 499 US 4 648 224 only discloses rods having a circular cross-section. EP-A-0 199 348 also relates to a structural fibre reinforced rod for reinforcing concrete. The reinforcing fibres may consist of aramid filaments embedded in a thermosetting resin (see page 5, line 13 and claim 1a). Like EP-A-0 170 499, EP-A-0 199 348 only describes the use of reinforcing rods having a substantially circular cross-section. Also, EP-A-0 199 348 is only prior art according to Article 54(3). The invention has for its object to provide a novel reinforced concrete and reinforcing element of the type mentioned in the opening paragraph which, however, no longer displays the problems encountered with the known reinforcing elements. The reinforcing element in the reinforced concrete according to the invention has physical and mechanical properties which are in the same range as that of steel. Further, the reinforcing element in the concrete according to the invention is chemically resistant to the environment in which steel corrodes. Moreover, within the life time set for concrete structures the reinforcing element according to the invention is insensitive to the alkaline environment in non-carbonated concrete, so that it can also be used in direct contact with cement or concrete mortar. The reinforced concrete, more particularly prestressed concrete, according to the invention is in the first place characterized in that the section transverse to the longitudinal direction of the reinforcing element is substantially rectangular, particularly with the ratio of thickness to width being in the range of 1:2 to 1:90. Important features of the reinforcing element in the concrete according to the invention are: the endless filaments are formed from aromatic polyamides, such as polyparaphenylene terephthalamide, the matrix is formed from a synthetic material based on epoxy resin and/or bismaleimide resin, the resistance to alkali of the reinforcing element determined by the method defined in the description is such that after 180 days at 80°C the residual strength of the filament band in the reinforcing element is more than 40% of the initial strength, the filaments form not more than 90% by volume, more particularly 40 to 70% by volume, of the reinforcing element and the synthetic matrix material forms at least 10% by volume, more particularly 60 to 30% by volume thereof. The alkali resistance of the reinforcing element in the concrete according to the invention, also when it is in direct contact with the environment of non-carbonated cement or concrete, is such that the residual strength of the filament band in the reinforcing element is higher than 40% of the initial strength, measured as indicated in the description. By extrapolation it may be inferred therefrom that after 50 years at 20°C the residual strength of the filament band in the reinforcing element will also be higher than 40% of the initial strength. Surprisingly, it has even been found that said alkali resistance of the reinforcing element in the concrete according to the invention is such that after 180 days at 80°C the residual strength of the filament band is 60-100%, more particularly about 80-100% of the initial strength. Further, the reinforcing element in the concrete according to the invention is characterized in that the tensile strength of the filament band in the reinforcing element is 2,2-4 GPa, preferably about 3 GPa, the modulus of elasticity of the filament band in the reinforcing element is 100-200 GPa, the elongation at rupture of the filament band in the reinforcing element is higher than 1,5%, and is preferably about 2,0-4%. If the filaments consist of polyparaphenylene terephthalamide (PPDT), then according to the invention the shear strength of the filament band in the reinforcing element is higher than 30 MPa and preferably about 45 MPa. Of the reinforcing element in the concrete according to the invention the relaxation is less than 10%, but more particularly the relaxation is 3-5%. According to the invention the reinforcing element is preferably characterized in that the epoxy resin is of the novolak type or is formed of a resin based on diglycidyl ether of bisphenol A or a tetrafunctional epoxy resin, such as N,N,N'N'-tetraglycidyl-4,4'-methylene bisbenzenamine, the epoxy resin being hardened by an amine curing agent, such as a cycloaliphatic amine, a dicyandiamine, an aromatic amine or a polyamine. It is also possible to apply catalytic hardening with a curing agent based on BF₃. According to the invention an accelerator may be added to the synthetic matrix, such as an accelerator based on BF₃, imidazole or dimethyl urea. The synthetic matrix based on epoxy resin according to the invention may in addition to the epoxy resin contain a limited amount of adjuvants, such as particular elastomeric or other thermoplastic substances or adjuvants in an amount of, say, not higher than 20% by weight, calculated on the weight of the resin, which substances may serve, for instance, to improve the elasticity of the matrix. As examples of said adjuvants may be mentioned butadiene/styrol or substances such as polysulphone, polyether sulphone, polycarbonate or polyester. The thermosetting resin also may consist of a mixture or a reaction product of separate components. The resin also may consist of a mixture of various epoxy resins or a mixture of epoxy resin and bismaleimide resin. Or the resin may consist of a mixture of resins capable of forming interpenetrating networks. The reinforcing element according to the invention is characterized in that the bismaleimide resin is a resin based on 4,4'-bismaleimidodiphenyl methane. According to the invention it is preferred that in addition to 4,4'-bismaleimido diphenyl methane the synthetic matrix should contain some amount of allyl phenol, for instance in the ratio of 100:75 parts by weight. Referred to as the XU 292 type, this last-mentioned resin system is elaborately described in the article "High-Performance Matrix Resin System" by T.J. Galvin, M.A. Chaudhari and J.J. King of Ciba-Geigy Corp. on pp. 45-48 of Chemical Engineering Progress January 1985. It is of course also possible to include the above-mentioned adjuvants in a matrix of bismaleimide resin. Favourable results are obtained with a reinforcing element which is characterized according to the invention in that the diameter of the filaments is 5-20 µm, preferably about 12 µm. The filaments are so closely surrounded by the special matrix resin that the reinforcing element according to the invention is characterized in that in any random section transverse to the longitudinal direction of the reinforcing element the volume of hollow space is less than 1%, which means that said hollow space is practically nil and the internal transmission of force is therefore optimal. In the present reinforcing element with the more or less flattened, approximately rectangular cross-section the ratio of the thickness to the width is less than 1:2. With advantage, however, the ratio of the thickness to the width of the reinforcing element is in the range of 1:8 to 1:90, preferably 1:8 to 1:20. According to the invention the width of the reinforcing element may be in the range of 10 to 50 mm, and is preferably about 20 mm, and the thickness may be in the range of 1 to 3 mm, and is preferably about 1,5 mm, and viewed in transverse direction the number of filaments is 3000 to 20 000 per mm, preferably about 5000-10 000 filaments per mm. The specific mass of the reinforcing element according to the invention is 1100 to 1500 kg/m³, preferably about 1300 kg/m³. In addition to the favourable physical and mechanical properties required for use in reinforced concrete the reinforcing element according to the invention surprisingly displays the desired chemical resistance. Particularly favourable is the resistance of the reinforcing element according to the invention to the strongly alkaline environment prevailing in the fresh concrete and in the cement mortar. The reinforcing element according to the invention also displays a good resistance to an acid environment. Because of these properties the use of reinforcing elements according to the invention make it possible to obtain reinforced concrete, more particularly prestressed concrete, which on the strength of favourable test results is expected to have a long service life free of costly repairs in any environment. Particularly, the chemical process taking place in the concrete as a result of air pollution and acid rain will cause no damage to the prestressed or non-prestressed concrete provided with the reinforcing elements according to the invention. Further, the reinforcing elements according to the invention are totally insensitive to electric or magnetic currents, as a result of which reinforced or prestressed concrete according to the invention can be used in cases where this has so far been impossible because of the sensitivity of steel, e.g. in electric traction. An additional advantage to the reinforcing elements according to the invention is that due to their low specific mass, i.e. a few times lower than that of steel and also lower than in the case of the known reinforcing elements of glass filaments in a matrix of polyester resin, they are easy to handle in the building industry. This contributes to somewhat lighten the generally hard working conditions in the building industry. The reinforcing elements of the invention formed by relatively thin strips can in a simple way be sawn to size manually or by machine. An important advantage of the special, substantially flat and rectangular shape of the cross-section of the reinforcing elements according to the invention consists in that the adhesion required for the transmission of force from the cement or concrete mortar to the reinforcing element, or conversely, is considerably better than in the case of a circular cross-section. The use of the non-circular, flattened, approximately rectangular shape of the cross-section transverse to the longitudinal direction of the reinforcing elements according to the invention permits realizing 100% transmission of force over a very limited distance both in the concrete and in the anchoring construction. Such a transmission of force has been found impossible or in any case costly and complicated if in the reinforcing elements with this type of matrix resin use is made of the circular cross-section commonly employed in steel reinforcement. Although the reinforcing element according to the invention satisfactorily adheres to the concrete matrix, the adhesion can still be further improved if according to the invention the outer surface of the reinforcing element is rough and contains a great many irregularities introduced into it for instance by rolling. Alternatively, the outer surface of the reinforcing element may contain a great many projecting fine-grained particles, in particular of inorganic material, such as particles based on silicon oxide, titanium oxide or aluminium oxide. It has been found that the total tensile strength of the filament band in the reinforcing element according to the invention is 5 to 20% higher than the tensile strength of non-embedded filament band. The invention also comprises a simple process of manufacturing the reinforcing element according to the invention, in which process more than 5000, more particularly more than 15 000 practically parallel filaments are collectively embedded in a liquid synthetic material which is to serve as matrix, which is subsequently cured, particularly by subjecting it to a heat treatment, use being made of filaments having the desired mechanical properties and formed from a polymer selected from the group of aromatic polyamides, such as polyparaphenylene terephthalamide, and use being made of a matrix formed from a synthetic material based on epoxy resin and/or bismaleimide resin, more particularly an epoxy resin of the novolak type or an epoxy resin based on diglycidyl ether of bisphenol A or a tetrafunctional epoxy resin, such as N,N,N'N'-tetraglycidyl-4,4'-methylenebisbenzene amine. A favourable embodiment according to the invention is characterized in that the liquid epoxy resin in which the filaments are embedded contains an amine hardener, such as a cycloaliphatic amine, a dicyanodiamine, an aromatic amine or polyamine, the ratio of the amounts by weight of epoxy resin and amine hardener being in the range of 100:25 to 100:40. According to a favourable embodiment use is made of a bismaleimide resin which is formed by a resin based on 4,4-bismaleimidodiphenyl methane supplemented with some amount of allyl phenol, for instance in the ratio of 100:75 parts by weight. The process according to the invention is advantageously characterized in that said embedding is effected by passing a filament bed having a width of at least 5 mm and a thickness of preferably not more than 3 mm under one or more preferably trough-shaped metering devices from which the mixture of liquid matrix resin is fed to the filament bed and in that the thus impregnated filament bed is passed through a curing zone for the resin preferably while subjected to a heat treatment. To reduce the viscosity the resin may be preheated in the metering device before it is discharged therefrom. According to the invention the filament bed provided with resin is heated to a temperature of 35°-70°C before it reaches the curing zone. It has been found that the process for manufacturing the reinforcing element according to the invention is of particular importance for obtaining a proper embedment of the filaments in said resins. Optionally, the resin-hardener mixture may contain a so-called accelerator, by which the curing of the epoxy resin may be speeded up. For a proper embedment of the filaments it is also of importance that the process should be so carried out, for instance by embedding the filaments in vacuo, that the chance of air being entrapped in the ready reinforcing element is nil. Notably, proper embedment of the filaments in matrix resin as carried out in accordance with the invention is considered to result in obtaining a reinforcing element of optimum properties and quality. If during the embedment the underside of the filament bundle is free, the chance of air being entrapped will be reduced. According to the invention the reinforcing element can in a simple manner be given the thickness desired with a view to its end use by attaching the widest side face of a formed, at least partly cured strip-shaped reinforcing element to one or more, preferably two, other identical strip-shaped reinforcing elements, preferably by means of the matrix resin. Thus, according to the invention two at least partly cured or uncured strip-shaped reinforcing elements may with advantage each be attached to a different side of a reinforcing element by means of a still wet, practically uncured resin, after which the three reinforcing elements thus joined are passed through a curing zone. According to the invention the reinforcing element should prior to being completely cured be gauged more particularly by means of transporting gauging rolls which are provided with recesses that correspond to the desired cross-section of the reinforcing element. The at least partly cured reinforcing element can be wound onto a reel having an original diameter of, say, 100 cm. A large number of reinforcing elements can be collectively placed in an oven for completely curing the matrix resin for several hours. The concrete according to the invention is characterized in that the ratio of the modulus of elasticity of the concrete matrix to the modulus of elasticity of the filament band in the reinforcing element is in the range of 1:2 to 1:6, preferably about 1:4. A favourable embodiment of the reinforced concrete according to the invention is characterized in that prior to curing the concrete mortar there is added to it a chloride-containing curing accelerator, for instance: 0,5 to 7% by weight of CaCl₂, preferably 2 to 5% by weight, calculated on the cement weight in the concrete matrix. Adding CaCl₂ to the concrete mortar or cement mortar will cause the curing process to accelerate, which permits removal of the form work at an earlier stage and generally contributes to faster and more efficient building. When use is made of a reinforcement of steel, the addition of CaCl₂ is generally undesirable and virtually prohibited in the concrete specifications. CaCl₂ promotes the corrosion of steel, as is explained in CUR VB-84-1 published by the "Stichting voor onderzoek, voorschriften en kwaliteitseisen op het gebied van beton" ("Institute for tests, regulations and quality standards in the field of concrete"). For under alkaline conditions the chloride ions may break through the protecting passivating film on the steel. The reinforcing elements according to the application are properly resistant to the action of chloride ions. This has moreover the advantage that also after a number of years the concrete provided with the reinforcing elements according to the invention will not be subject to any damage either when at some later stage chloride ions will penetrate into the concrete, which may happen for instance under the influence of seawater or road salt. Consequently, the formerly often used chloride-containing hardening accelerators, whose use in steel reinforcement is severely restricted if allowed at all because of its often leading to damage, can to a practically unlimited extent be employed in the manufacture of the concrete according to the invention, as a result of which a considerable economy can be achieved. The reinforced concrete according to the invention is also characterized in that the covering or covering thickness of the concrete matrix measured between the outer surface of the concrete matrix and the circumferential surface of the reinforcing element can be practically reduced to nil and, more particularly, need be as little as 0 to less than 15 mm, preferably about 2-5 mm. Such a thin covering usually suffices to permit the transmission of the forces in the concrete to the reinforcing element and conversely. Use of the conventional steel reinforcement requires a covering partly in order to protect the steel from corrosion, for instance as a result of carbonations and/or penetration of chloride ions. In the case of steel a covering layer of 15 mm or more need be applied and in the case of prestressing steel a layer of 25 mm or more; and in an agressive environment a covering of respectively 30 and 40 mm must be used. Since in the reinforced concrete according to the invention a thin layer of concrete may be applied, the present invention makes it possible for prestressed or non-prestressed concrete structures, beams, flat or corrugated sheets, respectively for floors and roofs, or other concrete elements to be manufactured particularly efficiently, considering the savings that can thus be made on material, space, future maintenance and particularly the own weight. The reinforced concrete according to the invention is advantageously characterized in that a number or a group of reinforcing elements extend parallel to and at some distance from each other and substantially rectilinear in practically the same plane in the concrete matrix. There may optionally be provided a second group of such reinforcing elements in such a way that the reinforcing elements of the first and the second groups extend at right angles to each other in two parallel planes. The invention also comprises a simple process of preparing reinforced concrete, particularly prestressed concrete, the reinforcement being placed in a form into which subsequently the concrete mortar is poured, which process is characterized in that the reinforcement is formed by one or more of the reinforcing elements according to the invention and the concrete mortar is brought into direct contact with the reinforcing elements. This is possible in that also when they are in direct contact with cement mortar or concrete mortar the reinforcing elements according to the invention are properly resistant both to non-carbonated concrete (alkaline environment) and to carbonated concrete. The invention also comprises a process for the preparation of prestressed concrete which is so carried out that prior to the curing of the concrete matrix each of the reinforcing elements is pretensioned while subjected to an external tensile load, said external tensile load being removed after the curing of the concrete matrix, so that in the concrete an artificial compressive stress is set up, which process is characterized in that an external tensile load of such magnitude is applied that in the cured concrete matrix the tensile stress in each reinforcing element is 40 to 70% preferably about 50%, of the tensile strength of the filament band in the reinforcing element. The invention will be further described with reference to a few schematic drawings. Figure 1 is a view in perspective of the reinforcing element according to the invention. Figure 2 shows an apparatus for manufacturing the reinforcing element according to the invention. Figure 3 shows a variant apparatus for manufacturing the reinforcing element according to the invention. Figures 4 and 5 are perspective views of slabs of reinforced concrete according to the invention. Figure 6 shows a non-reinforced concrete slab. Figure 7 shows the set-up used in the four-point flexural strength test. Figure 8 shows the load-deflection diagram. Figure 9 is a view in perspective of an I-section of reinforced concrete according to the invention. Figure 10 is a view in perspective of a corrugated sheet of reinforced concrete according to the invention. Figures 11-16 show various embodiments for the surface of the reinforcing element according to the invention. Figure 17 is the Arrhenius diagram for determining the residual strengths after various residence periods in an alkaline environment. Figure 18 shows the residual strength in an alkaline medium as a function of time. Figure 1 is a view on a highly enlarged scale of a reinforcing element 1 according to the invention, of which the rectangular cross-section 2 has a thickness 3 of, say, 1,5 mm and a width 4 of, say, 15 mm. The cross-section need of course not be exactly rectangular. The invention not only comprises rectangular, but also more or less flattened or approximately elliptical cross-sections and the wording substantially or practically rectangular used in the claims should therefore be interpreted as such. The cross-section 2 consists of a very large number of PPDT filaments 5 having a diameter of 12 µm, as shown in part of the cross-section. The continuous filaments 5 extend uninterruptedly in longitudinal direction of the reinforcing element. The space between the filaments 5 is entirely filled with epoxy resin serving as a synthetic matrix. If the reinforcing element 1 is not unduly thick and therefore sufficiently flexible, it can be marketed in the form of a roll. The length of reinforcing material 1 wound into such a roll may amount to a few hundred metres. The length of a reinforcement element required for a particular concrete structure may then be unwound from the roll and sawn off. Instead of on a roll the reinforcing material 1 may of course also be supplied in the form of strips of a particular length. Figure 2 is a schematic representation of an apparatus for the production of the reinforcing element 1 of the type shown in Figure 1. In a framework (not shown) are placed a large number, say 33, of, for instance, 2 kg packages 6 of PPDT-filament yarn. Figure 2 shows only three of the yarn packages 6. The PPDT yarns 7 are of the dtex 1610/f 1000 type, which means that each yarn 7 is made up of 1000 filaments measuring 12 µm in diameter. The yarns 7 moving in the direction indicated by the arrow first pass over a guiding means 8 and subsequently a comb 8, so that the filaments will come to lie exactly parallel to each other. Subsequently, the filament bed is passed between a pair of brake and spread drums 11, by which the filaments are given the same tension, after which they pass under a metering slit 12 of the mixing and metering device 13 for the epoxy resin. The mixing and metering device 13 is filled with epoxy resin of the novolak type and a hardener of aromatic amine in the weight ratio resin-amine of 100:38. At the location of the metering slit 12 the filament bed 10 is free at its underside, as that under the action of gravity the resin can properly penetrate into the space between the filaments and the entire filament bed 10 is completely impregnated with resin. To improve such impregnation the mouth of the metering slit may still be provided with a heating device (not shown), by means of which the viscosity of the liquid epoxy resin is temporarily decreased. For the same purpose a heating zone with infrared radiators 14 by which the filament bed is heated to a temperature of 40°-70°C is provided downstream of the metering slit 12. For a further improvement of impregnation the filament bed may also be preheated, for instance to a temperature of 30°-70°C, before the resin comes into contact with the filament bed. Then the filament bed impregnated with epoxy resin is covered on its upper and under side with embossed or non-embossed paper release strips 15 and 16 and subsequently passed into a heated curing zone 17, in which the impregnated filament bed is heated to a temperature of about 120°C. The length of the curing zone 17 must be such that at its exit the resin is partly cured. At a travelling speed of 5 m/min the length of the curing zone 17 must be approximately 10 m. After the filament bed has left the curing zone 17, the release strips 15 and 16 are removed from the already fairly hard resin impregnated filament bed, which is then practically in the form of the reinforcing element 1 of the present invention. In the curing zone there are pairs of gauging and guiding rolls 18, 19, 20 for fixing the proper dimensions of the cross-section of the reinforcing element. The reinforcing element 1 is forwarded through the apparatus by means of a driving unit 21 which exerts a tensile force on the reinforcing element. Downstream of the driving unit 21 is a take up device 22 on which a large length of the produced reinforcing element 1 can be wound. Alternatively, the reinforcing element can be sawn into straight pieces of the desired length and collected. Subsequently, the reinforcing element must still be cured, to which end several rolls or a large number of straight pieces of reinforcing material are collectively left in an oven for, say, 8-10 hours, during which time they are subjected to a temperature of about 120°C to 180°C, depending on the type of resin, after which the reinforcing elements 1 according to the invention are completely ready for use and possess their final properties. If the filaments are not of polyparaphenylene terephthalamide but of polyethylene, polyvinyl alcohol or polyacrylonitrile, a similar manufacturing process may be used. To obtain a reinforcing element of optimum quality it is of great importance that the filament bed 10 should be completely impregnated with resin. Therefore, the thickness of the filament bed passing under the metering slit 12 should be relatively small. As a result, the thickness of the reinforcing element 1 to be produced in a single pass will be somewhat restricted. Thicker reinforcing elements 1, however, can be made in a simple manner by bonding together two, three or more partly cured reinforcing elements 1, to which end again use is made of the matrix resin as adhesive. Alternatively, one filament bed in which the resin is still wet and practically uncured may be provided between two already partly cured reinforcing elements. The resulting combination of two, three or more layers of elements must then be adequately cured. In this way the reinforcing elements according to the invention can be made to have practically any desired thickness. The quality of the multi-layer reinforcing element 1 according to the invention is such that the behaviour of the endproduct is identical with that of a single layer reinforcing element. If a reinforcing element 1 according to the invention is to be composed of several layers in the way as described, then use may also be made of a continuous production apparatus. To that end for instance several of the production lines schematically indicated in Fig. 2 may be superimposed and the separate layers will then have to be joined and bonded together in a suitable device. If in the described described way two relatively thin layers of 33 000 filaments each are combined with a layer of 34 000 filaments, a final reinforcing element with in all 100 000 filaments will be obtained. In principle it will be possible to manufacture a reinforcing element according to the invention containing 400 000 to 600 000 or 1 000 000 or more filaments, depending on the application envisaged. Figure 3 shows a somewhat different production process, the parts corresponding to those in Figure 2 being referred to by like numerals. Three superimposed groups of PPDT filament yarns are impregnated in heatable baths 23 containing a mixture of liquid epoxy resin and hardener. After leaving the impregnating bath 23 each of the three filament beds passes through a pair of squeezing rolls 24 and subsequently through a heated precuring zone 25. After leaving the precuring zone 25 the three preheated elements 26 are joined by means of a pair of pressure and gauging rolls 27 and passed as one element through a communal, heated postcuring zone 28. In the first part of the postcuring zone 28 there may be provided a special device (not shown) for feeding (in the direction of the arrows 29) sand, a mixture of sand and resin or some other agent to the element 1 in order to obtain a reinforcing element 1 according to the invention with a rough outer surface. After leaving the postcuring zone 28 the reinforcing element is wound up or cut into straight pieces of some limited length. There is again provided a driving unit 21, with which the reinforcing element 1 is pulled through the curing zone 28. Then the freshly produced reinforcing element is still to be finally hardened, to which end a large number of straight pieces are collectively placed in the oven. If the three groups of starting yarns each contain 50 000 filaments, then the reinforcing element 1 produced in accordance with the schematically indicated process of Figure 3 will contain in all 150 000 filaments. Figures 4 and 5 are perspective views of concrete slabs B and C prestressed with reinforcing elements 1 according to the invention. As the unreinforced slab A of Figure 6, said two concrete slabs in reality measure 1,70 × 0,20 × 0,04 m. The slabs B and C are merely practical examples of prestressed concrete slabs according to the invention. The slabs B, C and A according to Figures 4, 5 and 6 were made in reality and were tested by subjecting them to the four-point bending test, which is schematically illustrated in Figure 7, in which test as a function of the load 2P in Newton the deflection f in mm in the various stages was measured. Of each of the types B and C two slabs were made and tested. The slabs B according to Figure 4 are centrially pretensioned with 8 single reinforcing elements 1 (cross dimensions 20 × 0,25 mm and 22 000 filaments of  12 µm). The total initial prestressing force was 8 × 3000 N = 24 000 N. The slabs C according to Figure 5 are eccentrically pretensioned with four single reinforcing elements 1 (cross dimensions 20 × 0,25 mm and 22 000 filaments of  12 µm). The total initial prestressing force was 4 × 3000 N = 12 000 N. During the pretensioning of the reinforcing elements 1 for the slabs B and C the loss of prestress was measured for 24 hours via a load cell with T.N.O.-calibration certificate (measuring accuracy ± 0,2%). The trend of the prestress losses was recorded. Immediately upon being pretensioned, all the reinforcing elements 1 were sanded over a distance of 200 mm from the ends of the slabs. Sanding was effected by using a hardwood lacquer (varnish) mixed with sand (particle size 0,125 to 0,250 mm), which was applied by brush. Or the reinforcing elements were first treated with lacquer, which was subsequently sprinkled with sand. Immediately before casting the concrete (24 hours after pretensioning) the loss of prestress (3 to 4%) was made up to the desired prestressing level. The ends of the reinforcing elements were anchored outside the concrete element. The same anchoring used in earlier tensile tests resulted in a force of 100% of the theoretical tensile strength of a single strip. In the pouring and curing operations for the slabs A, B and C the following procedure was used: All the slabs were compacted by setting the form work into vibration. For each slab 3 cubes with an edge of 158 mm were made. They were used for determining the cube compressive strength in the various stages of the hardening process and for determining the 28 days' splitting tensile strength. Also determined were the water/cement ratio of the concrete mortar used and the slump. All the relevant concrete data were recorded. Following the pouring operation the slabs were cured in the laboratory for 2 × 24 hours, during which periods they were covered with a plastics sheet to prevent dehydration. The temperature in the laboratory ranged from 10° to 16°C. After demoulding (after 2 days' curing) the slabs were stored in a conditioning room at a temperature of 20°C ± 2°C and a relative air humidity of ≧ 95%. The moment the prestress was released the reinforcing elements 1 were not found to display any slippage. The concrete mortar for the test slabs A, B and C was composed as follows: Choice of the nominal particle size In accordance with NEN 3880 (VB 1974/1984) section 603.5.1. ¾ of the smallest distance between the reinforcing elements. The smallest distance between the reinforcing elements is 22 mm (centrically pretensioned slab B) ¾ × 22 = 16,5. An aggregate mixture having a nominal particle size of 16 mm is chosen. Grading of aggregate The aggregate mixture is such that the resulting mixture displays a grading curve which falls between the boundary lines A and B according to NEN section 603.5.3. Cement content In accordance with NEN 3880, section 603.8.2 the minimum cement content for B 22,5 class I, consistency range 2 (slump 50-90 mm) and the grading curve between the boundary lines A and B: 320 kg/m³. Increase due to particle size of 16 mm is 10%: 320 + 10% = 352 kg/m³. Use was made of: 352 kg/m³ class B Portland cement . Consistency In order to compact the test slabs by vibration the concrete mortar was controlled to a slump of 50-90 mm (consistency range 2) after the addition of 3% of superplasticizer Melment LIO, based on the weight of the cement. Total swelling calculation (per m³) Controlling the amount of fines In accordance with NEN 3880 section 603.6 the minimum amount of fines < 0,250 mm for a nominal particle size of 16 mm = 135 litres/m³. which is consequently sufficient. The concrete slabs A, B and C shown in fig. 6, 4 and 5 and made and composed in the afore-described manner were subjected to two types of loading tests on the 4-point bending tester according to Figure 7. In the first series of tests all slabs A, B and C were subjected to a bending load only up to the occurrence of visible cracking. The unreinforced slab A cracked immediately. In the second series of tests the slabs B and C were subjected to a bending load up to the occurrence of failure. Results of loading up to cracking The load at which the first crack became visible was determined with the aid of calibrated weights. The loading was increased in steps of 49,05 N. The loading was raised (after every 2 or 3 minutes) when the deflection no longer increased. Table 1 gives a summary of the results. Results of loading up to failure After a few weeks the test slabs were loaded up to the occurrence of failure. The load was increased every 5 minutes. The graph in Figure 8 shows the relationship between loading and deflection. It appears for instance that after the formation in it of cracks the structure can still take up a high additional load. The deflection will strongly increase then, which is a warning of overloading. Besides the prestressed concrete slabs Band C according to the invention shown in Figures 4 and 5, with which also the afore-described flexural tests were carried out, various other prestressed or non-prestressed reinforcing concrete structures can be realized within the scope of the present invention. It is possible, for instance, to make prestressed or non-prestressed reinforced concrete sections, such as the I-beam 31 shown in Figure 9, which is provided in its flanges 32 with a number of reinforcing elements 1 according to the invention which extend in longitudinal direction of the beam 31. Figure 10 illustrates a different construction in the form of a kind of prestressed or non-prestressed reinforced concrete corrugated sheet 33, which in the lower half to be loaded is provided with the reinforcing elements 1 according to the invention. Figures 11-16 are very schematic views in perspective of the reinforcing element 1 according to the invention, provided with different outer surfaces for improving the adhesion to the concrete matrix. In Figure 11 the reinforcing element 1 is on both sides provided with ribs 34 which are staggered relative to each other. In Figure 12 both sides of the reinforcing element 1 are entirely in the form of a serrated surface 35. Figure 13 shows a reinforcing element 1 which is provided with pyramidal projections 36. Figure 14 shows a reinforcing element 1 of which the surface contains a large number of sand granules schematically indicated with dots. Figure 15 shows a reinforcing element 1 whose surface is provided with studs 37. Figure 16 shows an embodiment of a reinforcing element 1 provided with a grid-shaped pattern of ribs 38, which may be introduced by rolling. As the reinforcing elements 1 according to the invention are particularly insensitive to corrosion, they need be covered with only a very thin layer of concrete, which leads to a considerable saving on weight and cost of material. The invention is not at all limited to the concrete elements shown in the Figures. The scope of the present invention allows of many other concrete constructions and concrete elements. As mentioned before, an important feature of the reinforcing elements 1 according to the invention consists in that they display a particularly good resistance to the action of an alkaline environment. By the alkaline resistance mentioned in the claims is meant a property which is determined in the following manner: An adequate number of test specimens of the reinforcing elements according to the invention are placed freely in a liquid bath of a saturated Ca(OH)₂ solution and a temperature of 80°C. After a period of 180 days at least 6, but preferably 10 test specimens are taken out of the bath. Then these test specimens are washed in water, dried to air of 55°C and subsequently stored in a conditioned room having a normalized climate (23°C, 65% relative humidity). Following the conditioning of the test specimens the tensile strength of the filament band contained in it is determined in conformity with ASTM 3039/76. From the values found the average tensile strength is calculated. This average tensile strength is referred to as the residual strength. The residual strength is expressed as a percentage of the tensile strength referred to as the initial strength of the reinforcing element not exposed to any environment. Said initial strength must be determined sufficiently accurately and in the same way, i.e. in conformity with ASTM 3039/76, on reinforcing elements that have not been exposed to any environment and are of the same composition as regards the filaments and the matrix and made in the same way as the reinforcing elements that were exposed to said saturated Ca(OH)₂ solution. On the strength of experiments the alkaline resistance of the reinforcing element according to the invention is expected to be such that after 180 days at 80°C the residual strength of the filament band in the reinforcing element will be more than 80% of the initial strength. If after 180 days at 80°C the residual strength of a filament band in the reinforcing element is more than 40% of the initial strength, then the reinforcing element has the alkaline resistance according to the invention. Insight into the reinforcing element 1 's alkaline resistance after a very long time, after for instance 50 or 100 years, is obtained by carrying out the following experiments: A number of test specimens are placed freely in several liquid baths which all contain a saturated Ca(OH)₂ solution. The baths have temperatures of 20°C, 40°C, 60°C, 80°C and 95°C. After certain periods, viz. after 45, 90, 180 and 360 days at least 6, but preferably 10 test specimens are taken from each bath. Subsequently, these test specimens are washed with water, dried to air of 55°C and then stored in a conditioned room having a normalized climate (23°C, 65% relative humidity). Following the conditioning of the test specimens the tensile strength of the filament band contained therein is determined. Of each series of test specimens the average tensile strength is determined (also in accordance with ASTM 3039/76). This average tensile strength is referred to as residual strength. The residual strength is expressed as a percentage of the tensile strength (referred to as initial strength, determined as described before) of the reinforcing element that has not been exposed to any medium. The percentages thus found are plotted in a so-called Arrhenius graph, which is given in Figure 17. On one axis in Figure 17 is plotted the log of the time in days, years and hours. On the other axis in Figure 17 is plotted on a linear scale the factor 1 / T × 1000, where T is the temperature in degrees Kelvin. For convenience, also the corresponding values in °C are given. So on the 20°C-line in Figure 17 are four dots I-IV at the ends of periods of 45, 90, 180, 360 days, respectively. Also on each of the lines for 40°C, 60°C, 80°C and 95°C are four dots, so that in the Arrhenius graph of Figure 17 there is obtained a grid of in all 5 × 4 = 20 dots. Each of the 20 dots of the grid represents a particular (mean) residual strength expressed as percentage of the initial strength of the starting material not exposed to a medium and/or an increase in temperature. To find out what dots in the graph represent a residual strength of 95%, 90%, 85%, 80%, etc., use is made of a model in accordance with which the residual strength r is a particular function of the time t in days and the temperature T in degrees Kelvin, such that the contour lines or percentage residual strength lines (lines with constant r) in the Arrhenius graph are parallel straight lines. The model contains a number of unknown parameters which are so determined that the values of the percentage residual strength r predicted with the model will fit as nearly as possible (minimum sum of squares of deviations) to the measuring values of the residual strength. These last-mentioned measuring values are the empirically determined percentage residual strengths in said 5 × 4 = 20 grid dots I-IV. Thus, the contour lines or lines of constant percentage residual strength for r = 95%, 90%, 85%, 80%, etc. are fixed and are drawn in the graph of Figure 17. In the graph of Figure 17 these contour lines in the zone beyond the longest time (360 days) measured are extended to the drawn 50-year and 100-year lines. The parallel lines thus drawn represent the trends of the percentages residual strength at lower temperatures and/or longer periods. In the graph of Figure 17 the dot X is now sought that corresponds to a temperature of 20°C and a period of 50 years. As appears from Figure 17, the dot X lies between the residual strength lines of 90% and 95%, so that it may be concluded that of the reinforcing element 1 for which the graph of Figure 17 is constructed the expected, extrapolated residual strength is still about 93% after 50 years at 20°C. Should the 40% residual strength line be above the X dot, then the extrapolated residual strength after 50 years would be higher than 40%. Should the 40% residual strength be below the X dot, then the extrapolated residual strength after 50 years would be less than 40%. In the graph of Figure 17 the position of the residual strength lines was calculated with said model and the measuring values are based on measurements conducted on a reinforcing element of only 1000 PPDT filaments embedded in epoxy resin. For all eight grid dots I-IV of the test specimens from baths of 20°C and 40°C a residual strength of on average about 100% was found, which percentages are mentioned at said dots in the graph of Figure 17. At 60°C the average residual strength values are successively about 100%, 100%, 95% and 90%, after 45, 90, 180 and 360 days, respectively. At 80°C the residual strength values are successively about 95%, 88%, 83% and 77%, after 45, 90, 180 and 360 days, respectively. At 95°C the residual strength values are successively about 85%, 80%, 75% and 70% after 45, 90, 180 and 360 days, respectively. The lines of identical residual strength values were determined in the above described way. If the residual strength is determined on a reinforcing element according to the invention containing more than 5000 filaments, for instance: 100 000 to 1 000 000 filaments, then the residual strength will be higher and therefore more favourable than in the case of only 1000 filaments. It should be added that due to inevitable measuring errors and normal tolerances the dots for the measured percentage residual strength values need not necessarily lie exactly on the corresponding contour lines. On the basis of the data in Figure 17 the Y line in Figure 18 represents the residual strength at 20°C (as a percentage of the initial strength) as a function of time for a reinforcing element with 1000 filaments. Figure 18 also gives a Z curve for the residual strength of a reinforcing element prestressed at a load of 50% of the tensile strength. It surprisingly shows that the residual strength of a prestressed reinforcing element is even more favourable and the alkaline resistance of prestressed reinforcing elements according to the invention is even better than that of non-prestressed reinforcing elements according to the invention. It should be added that Figure 18 still contains an S line which represents the expected variation of stress with time in a reinforcing element 1 according to the invention which is contained in concrete and which initially has a prestress of the order of 50% of the initial tensile strength. As to the Arrhenius graph of Figure 17 it should still be noted that the model mentioned with respect to it was as follows: For constant temperature it was assumed that dr / dt = Ar c ; A > 0; C ≧ 0 and r = 1, for t = 0 where A is the Arrhenius constant, c the reaction order and t the time (in days). This leads to and r = e for c = 1. For various temperature A is a paricular function of the temperature: a₁ + a₂ ( 1 / T - g).10³ A =e where T is the absolute temperature in degrees Kelvin and g is the mean of the inverse of the absolute temperatures, i.e. Fitting the model to the measuring values r i will be such that the sum of squares of the deviations Σ / 1 (r i - r and i )² is minimal. Here r and i is the value calculated with the model at the same point where r i was measured. In this fit estimates a and₁, a and₂ and c and of the parameters (constants) a₁, a₂ and c, respectively, are obtained. The calculations can be made with a computer program for non-linear regression analysis, such as the "Statistical Software Package BMDP, Program 3R". The equations of the contour lines or lines for constant percentage residual strength are Using the above model and on the basis of the empirically determined measuring values at the 5 × 4 = 20 grid dots in Fig. 17 the following coordinates were calculated of two dots of each contour line in Figure 17 of constant residual strength values of 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45 and 40%. The tensile strength, the elongation at rupture and the modulus of elasticity of the filament band were determined in accordance with ASTM-D 3039/76, use being of a tensile rate of 5 mm/min and fixed hydraulic grips. At the grip faces protecting strips (tabs) are provided having a thickness of 1,5-4 times the thickness of the test specimen. The shear strength of the reinforcing element is determined in accordance with ASTM-D 2344-84, using a span length/thickness ratio of 7:1. The aromatic polyamides according to the invention are polyamides that are entirely or substantially built up of repeating units of the general formula wherein A₁, A₂ and A₃ represent the same or different divalent one or more aromatic rings-containing rigid radicals in which there may be a heterocyclic ring, of which radicals the chain extending bonds are in a position para to each other or are parallel and oppositely directed. As examples of such radicals may be mentioned, 1,4-phenylene, 4,4'-biphenylene, 1,5-napthalene and 2,6-napthalene. They may or may not carry substituents, such as halogen atoms or alkyl groups. In addition to amide groups and the above-mentioned aromatic radicals the chain molecules of the aromatic polyamides may optionally contain 50 mole % of other groups, such as m-phenylene groups, non-rigid groups, such as alkyl groups or ether, urea of ester groups, such as 3,4'-diaminodiphenyl ether groups. It is preferred that the yarn according to the invention should entirely or substantially consist of poly-p-phenylene terephthalamide (PPDT). The manufacture of PPDT yarns is described in US 4 320 081. The manufacture of polyethylene filaments by solvent spinning may be carried out as described in, for instance, GB 2 042 414, GB 2 051 667 or EP 64 167. The manufacture of filaments of polyacrylonitrile by solvent spinning may be carried out as described in, for instance, EP 144 983 or JP Patent Application 70 449/83. The manufacture of filaments of polyvinyl alcohol by solvent spinning may be carried out as described in, for instance, US 4 440 711. The term concrete as used in the present description refers both to concrete containing natural aggregates (gravel and/or sand) and concrete containing synthetic aggregates. The concrete according to the invention also may contain synthetic additives. The creep is determined by subjecting a reinforcing element according to the invention to a constant load. Prior to being loaded, the length of the test specimen is accurately determined. Following loading the length of the test specimen is measured after t=0,1; t=1; t=10; t=100; and t=1000 hours. Plotting the logarithm of the time on the abscissa and the %-elongation on the ordinate generally results in a straight line. In this way the creep per decade can be given (a decade is a period in which the period of time increases tenfold (e.g., from 100 to 1000 hours). The relaxation is determined by loading a reinforcing element according to the invention in such a way that the length of the test specimen remains constant. To keep this length constant the force must be continuously reduced. By measuring the force at fixed moments of time the force can be plotted as a function of time. The relaxation is espressed as loss of force (in %) in a certain period, viz. from 0,1 to 1000 hours. It should be added that the invention is of particular advantage in the case of very thin reinforced concrete elements, for instance thinner than 3 cm. Because of the insensivity to corrosion and the atmosphere such thin concrete elements can be excellently provided with the reinforcing elements according to the invention. Such thin concrete elements can actually not be reinforced with steel, unless use is made of very special and costly provisions, such as stainless steel. An important advantage of the reinforcing elements according to the invention is that they can also be used for reinforcing or prestressing cement or concrete products which for some reason are porous or waterpermeable. Mention may be made in this connection of, for instance, concrete containing aggregates such as pumic concrete or cellular concrete, woodwool cement plates, etc.

Claims: Reinforced concrete, including prestressed concrete, comprising one or more reinforcing elements, which are each formed by a matrix based on thermosetting synthetic material in which more than 5000 practically parallel continuous aromatic polyamide filaments are contained, the tensile strength of the filament band in the reinforcing element is higher than 2,0 GPa, the modulus of elasticity of the filament band in the reinforcing element is higher than 60 GPa and the elongation at rupture of the filament band in the reinforcing element is less than 6% to 7%, wherein the filaments form not more than 90% by volume of the reinforcing element and the synthetic matrix material forms at least 10% by volume thereof, characterized in that the section transverse to the longitudinal direction of the reinforcing element is substantially rectangular, particularly with the ratio of thickness to width being in the range of 1:2 to 1:90. Concrete according to claim 1, characterized in that the ratio of the modulus of elasticity of the concrete matrix to the modulus of elasticity of the filament band in the reinforcing element is in the range of 1:2 to 1:6. Concrete according to claim 1, characterized in that prior to curing there is added to the concrete mortar a chloride-containing curing accelerator. Concrete according to claim 3, characterized in that an amount of 0,5 to 7% by weight of CaCl₂, calculated on the cement weight in the concrete matrix, is added to the concrete mortar. Concrete according to claim 1, characterized in that the covering thickness of the concrete matrix measured between the outer surface of the concrete matrix and the circumferential surface of the reinforcing element is in the range of 0 to less than 15 mm. Concrete according to claim 1, characterized in that a number of reinforcing elements extend parallel to and at some distance from each other and substantially rectilinear in practically the same plane in the concrete matrix. A process of preparing reinforced concrete including prestressed concrete, according to claim 1, characterized in that the concrete mortar is brought into direct contact with the reinforced elements. A process according to claim 7, characterized in that the concrete mortar contains a chloride-containing hardening accelerator, more particularly 0,5-7% by weight of CaCl₂, calculated on the cement weight in the cured concrete matrix. A process for the preparation of prestressed concrete according to claim 1, characterized in that an external tensile load of such magnitude is applied that in the cured concrete matrix the tensile stress in each reinforcing element is 40 to 70%, preferably about 50%, of the tensile strength of the filament band in the reinforcing element.

Inventors: GERRITSE, ARIE, METS, LAMBERTUS CORNELIS, SCHUERHOFF, HANS-JUERGEN, Schürhoff, Hans-Jürgen
Applicants: AKZO N.V., HOLLANDSCHE BETON GROEP N.V.
Application year: 1986
Main classification: E04C5
IPC codes: E04C2, C04B32, E04C5