Use of cellulose fibers to form a thermally stable and cyclically loadable UHPC or HPC concrete component

Abstract

Use of cellulose fibers and / or regenerated cellulose fibers, the volume of which decreases when heated, for producing a thermally stable and cyclically stressable UHPC or HPC concrete component, the microstructure of which has cavities which are formed by a heating-induced volume reduction of the cellulose fibers and / or regenerated cellulose fibers and which are at least partially connected to the environment, wherein the proportion of cellulose fibers and / or regenerated cellulose fibers in relation to the volumetric solids content of the concrete component is between 0.3 and 1.2 vol.%, and wherein the concrete component is produced from a solid mass which, in addition to the fibers, has at least one hydraulic binder and at least one aggregate.

Description

The present invention relates to the use of cellulose fibers and / or regenerated cellulose fibers, the volume of which decreases upon heating, for forming a thermally stable and cyclically stressable UHPC or HPC concrete componentThe abbreviation UHPC is used below for ultra-high-strength concrete and the abbreviation HPC for high-strength concrete. The abbreviations UHFB and HFB are also known for such concretes.HPC or also UHPC concretes are sufficiently known from the prior art. UHPC or HPC concretes differ from conventional concretes in that they have a higher strength. In particular, a UHPC is referred to as a UHPC if it has a strength of more than 130 N / mm 2 at a water cement content of <25. Both the UHPC concretes and the HPC concretes are distinguished by a dense microstructure with a very low capillary porosity.However, UHPC and HPC concretes are in the rarest cases fully hydrated. When such a concrete component made of an HPC or also a UHPC concrete is heated, a partial water vapor pressure is produced from free, originally physically and chemically bound water. The partial water vapor pressure cannot escape because of the dense microstructure of the concrete. If the partial pressure of water vapor exceeds the tensile strength of the matrix, the concrete fails. This can lead to chipping of larger concrete parts, or else to explosive phenomena with simultaneous strong development of steam, in which gravel-like residues remain.To counteract this phenomenon, there would be the possibility of increasing the gas permeability of the concretes. This could be achieved, for example, by adding polypropylene fibers to the solid mass, which expand in the range from 100° C. to 140° C. when heated and then melt, whereby a so-called pore network is formed within the concrete. The partial pressure of water vapor can be discharged through this pore network. However, it has been found that the concrete matrix is damaged due to the thermal expansion coefficient of the polypropylene fibers which is about 10-fold higher than the thermal expansion coefficient of the concrete. This means that polypropylene fibers in dense concretes and especially in high-performance concretes lead to crack induction and microcrack formation in the longitudinal direction of the fibers when the temperature is increased. These cracks, which are formed in addition to the voids created by the polypropylene fibers, ultimately ensure a loss of the partial pressure of water vapor. However, the combination of hollow spaces, due to the melting of the polypropylene fibers and the formation of cracks due to the increase in volume of the polypropylene fibers before their melting, entails the risk that the formation of cracks does not take place in a targeted manner, but rather more or less randomly. This means that the crack formation can also take place in such a way that the strength is significantly reduced when the concrete component is stressed by the crack formation. Another high risk of hindering the permeability of the component to water is that the cracks are clogged due to melting of the polypropylene fibers. This means that the partial pressure of water vapor that occurs cannot then be discharged. If this pressure is higher than the tensile strength of the matrix, the concrete may also fail due to the use of PP fibers.Recently concrete slabs have also been used in baking ovens. These are exposed to high thermal loads since these concrete slabs are heated and cooled in high cycles. The temperatures reached by a furnace are up to 300°. In this case, the polypropylene fibers melt, which has the consequence that corresponding gases escape during melting of the polypropylene fibers due to the formation of cracks, which is to be avoided during the production of foods in an oven.WO 2011 / 042294 A1 discloses concrete compositions comprising hydraulically setting binders, fillers, water and optionally further additives, wherein the concrete compositions additionally comprise one or more silanes and one or more polymers based on ethylenically unsaturated monomers in order to increase durability. In addition, fibers can be used for producing the concrete compositions, such as, for example, natural, modified natural or artificial fibers, in particular steel fibers, glass fibers, carbon fibers or fibers made of polypropylene, polyethylene, polyvinyl alcohol, polyacrylonitrile, polyester, polyamide or else cellulose fibers or modified cellulose fibers.The object on which the invention is based is to provide a solid mass for producing an HPC or UHPC concrete which can be subjected to high thermal load and can furthermore be subjected to a changing load, that is to say can be subjected to cyclic load.To achieve the object, the invention proposes the use of cellulose fibers and / or cellulose regenerated fibers, the volume of which decreases upon heating, for producing a thermally stable and cyclically stressable UHPC or HPC concrete component, the structure of which has cavities which are formed by a heating-induced volume reduction of the cellulose fibers and / or cellulose regenerated fibers and which are at least partially connected to the environment, wherein the proportion of the cellulose fibers and / or cellulose regenerated fibers in relation to the volumetric solids proportion of the concrete component is between 0.3 and 1.2% by volume, and wherein the concrete component is produced from a solids mass which, in addition to the fibers, comprises at least one hydraulic binder, for example cement, and at least one aggregate, e.g. sand (SiO 2). The reduction in volume of such fibers when heated produces a pore structure in the concrete, which opens up the possibility of discharging the partial pressure of water vapor formed in the concrete without resulting in a microcracks structure which leads to destruction of a concrete component produced from the solid mass according to the invention.It is now well known from the prior art according to US 2012 / 0328821 A1 to use such cellulose regenerated fibers in concretes. However, it is not known to use such cellulose fibers or else cellulose regenerated fibers in conjunction with HPC or UHPC concretes. Finally, it is assumed that the use of such cellulose fibers or cellulose regenerated fibers is therefore not possible for HPC or UHPC concretes, since these are highly hygroscopic. This against the background that the hydrogen partial pressure in the concrete would then be even higher than it is even without these fibers in the matrix. However, it has been found that this is not the case, but rather that the additional water escapes on the one hand, but on the other hand also leads to a further hydration of the concrete, which is due to the fact that HPC and UHPC concretes are not hydrated through. This means that upon further hydration, the water liberated leads to an increase in the strength of the concrete. Conventional concretes, on the other hand, are hydrated through, which has the consequence that here an increase in strength does not take place, even when water is liberated in the microstructure.In summary, it is therefore to be stated that the use of cellulose fibers and of cellulose regenerated fibers can further increase the already good durability and mechanical properties of the HPC or UHPC concrete. This is also particularly because a regenerated cellulose fiber has a higher tensile strength of 918.1 MPa compared to 247.2 MPa and a higher modulus of elasticity 6.753.8 MPa compared to 1158.6 MPa compared to polypropylene fibers. Since the water absorption of such cellulose or cellulose regenerated fibers is up to 10%, a considerable increase in the strength of the concrete when such concrete is heated is also to be expected.Advantageous features and embodiments of the invention can be found in the dependent claim.Thus, it is provided in particular that the proportion of fibers reducing their volume when heated is 0.6% by volume with respect to the volumetric solids proportion of the concrete. The fibers advantageously have a length of about 0.5 to 20 mm and a diameter of about 5 to 200 μm, preferably about 12 μm.In particular, a UHPC or HPC concrete component with a solid mass can be produced using cellulose fibers and / or cellulose regenerated fibers according to claim 1 or 2 according to the invention. In particular, it is advantageously further provided here that the component has a structural structure in which the cavities produced by the fibers are at least partially connected to the environment in order to allow the escape of the hydrogen partial pressure, i.e. of the water which is not used for the further hydration of the concrete. The further hydration or the pozzolanic reaction by the water vapor in principle leads to an increase in strength at specific temperature ranges.Such an HPC or UHPC concrete component is suitable for use as a fire protection element, as a furnace plate as a natural stone substitute, for example for speckle stone, or else for producing industrial floors with high mechanical and simultaneously cyclical thermal loading.An example of a solid mass for producing a UHPC concrete component is given below.Water184,20184,1981,00Pore Space15,00-0,00BinderCEM I 52.5 R HS-NA (Holcim Sulfo)251,42779,3893,10Sika Silikol P uncompacted75,10165,2162,20Additive AdditivesSika Viscocrete 281022,0723,6151,0700,000,0000,00Additives / AdditivesSilica flour Millisil W1273,34194,3532,650,000,0002,65Silica sand G32 0.125 mm / 0.5 mm355,98943,3392,650,000,0003,000,000,0003,000,000,0003,000,000,0002,65FibersMicrowire fibers 0.20 mm / 13 mm (Stratc)16,9000132,6657,85CR Fibers6,009,0001,500,000,0000,00Sum:1000,0002422,775Concrete components can be produced with such a solid mass, which have a strength of about 165 N / mm 2 after 28 days of standard storage. Such a concrete component can also be subjected to high thermal loads, since it can be heated to approximately 500° C. almost as often as desired and cooled to room temperature without the primary function of the concrete component or concrete being impaired.

Claims

Use of cellulose fibres and / or cellulose regenerated fibres, the volume of which decreases upon heating, for producing a thermally stable and cyclically stressable UHPC or HPC concrete component, the structure structure of which has cavities which are formed by a heating-induced volume reduction of the cellulose fibres and / or cellulose regenerated fibres and which are at least partially connected to the environment, wherein the proportion of the cellulose fibres and / or cellulose regenerated fibres is between 0.3 and 1.2% by volume with respect to the volumetric solids proportion of the concrete component, and wherein the concrete component is produced from a solids mass which comprises, in addition to the fibres, at least one hydraulic binder and at least one aggregate substance.Use of cellulose fibres and / or regenerated cellulose fibres according to claim 1, characterised in that the fibres have an average length of 0.5 to 20 mm and / or a diameter of 5 to 200 μm.