This invention relates to a method to create prestressed concrete structural elements in new constructions (poured on-site at the construction site) or in the prefabrication as well as for the subsequent reinforcement of existing structures by means of cement-bound mortar in which profiles made from shape-memory alloys, among experts often referred to as shape-memory alloy profiles, or SMA-profiles in short, are placed for the purpose of prestressing. This prestressing system also makes it possible to attach subsequent additions to an existing structure under prestress. Additionally, the invention also relates to a concrete structure that was built or subsequently reinforced by using this method and where additions were docked to, respectively, according to this method. A special feature hereby is the fact that steel-based shape-memory alloys are used in the form of profiles to generate a prestress.
A prestress within a structure generally increases its fitness for use in that cracks become smaller or the formation of cracks is actually prevented. Such a prestress is already being used today for reinforcement against the bending of concrete parts or for strapping purposes, for instance, of columns to increase the axial load and to strengthen the shear, respectively. Another application of the prestress of concrete are tubes to transport liquids and silos and tanks, respectively, which are tied up to generate a prestress. Round steel or cables are placed in the concrete or afterwards externally secured on the tensile side on the surface of the structural element in prior art for prestressing purposes. The anchoring and transmission of power from the prestressed element to the concrete is very expensive in all these known methods. High costs are incurred for anchoring elements (anchor heads). As far as external prestress is concerned, prestressed steel and cables, respectively must also be protected against corrosion by means of a coating. This is necessary because traditionally used steel is not corrosion-resistant. When the prestressed cables are placed in the concrete, they must be protected against corrosion at a high cost by means of cement mortar that is inserted in the duct through injection. An external prestress is also generated in prior art with fibre-reinforced composites which are affixed to the surface of concrete. In this case, the fire protection is often very expensive since the adhesives exhibit a low glass transition temperature. The corrosion protection is the reason for the fact that a minimal covering of the steel reinforcements of approx. 3 cm must be adhered to in traditional concrete. As a result of environmental influences (namely CO2 and SO2 in the air), carbonation occurs in concrete. The basic environment in concrete (pH-value 12) drops to a lower value, i.e. a pH-value of 8 to 9, due to this carbonation. The corrosion protection of the traditional steel is no longer guaranteed if the internal reinforcement lies in this carbonated area. Accordingly, the 3 cm thick covering of the steel guarantees a corrosion resistance for the internal reinforcement during a service life of the structure of approx. 70 years. The carbonation is substantially less critical when using the novel shape-memory alloy since the novel shape-memory alloy exhibits a clearly higher corrosion resistance in comparison with common construction steel. Due to the fact that the concrete part and mortar, respectively, are prestressed, cracks are closed and the penetration of pollutants is sharply reduced accordingly. The concrete covering can be greatly reduced with the new development and, accordingly, structural elements such as balcony projections, balcony parapets, pipes, etc. can have thinner dimensions. Consequently, the structural elements become lighter and more economical in their use.
Hence, the task of the present invention is to create a method to prestress new concrete structures and concrete structural elements or cement-bound mortar mixes for the reinforcement of existing structures and, alternatively, for the purpose of improving the fitness for use and stability of the structure, to guarantee a more flexible use of the building for subsequent projecting additions or to increase the durability as well as fire resistance of the structure. In addition, the task of the invention is to specify a concrete structure that exhibits generated prestresses or reinforcements by applying this method.
The task is initially solved by a method to create prestressed concrete structures by means of profiles made from a shape-memory alloy, be it of new concrete structures and concrete structural elements or of cement-bound mortar mixes for the reinforcement of existing structures, characterised by the fact that profiles made from steel-based shape-memory alloy of polymorphic and polycrystalline structure with ribbed surface or with a thread-shaped surface, which can be brought from its state as martensite to its permanent state as austenite by increasing its temperature, are placed in the concrete or the cement-bound mortar mix and, alternatively, with additional end anchors so that these generate contraction force and thus tension either as a result of a subsequent active and controlled input of heat with heating media or through the impact of heat in case of fire and, accordingly, generate a prestress on the concrete and mortar mix, respectively, whereby the power is transmitted through the surface structure of the profile and/or through the end anchors of the profile.
Furthermore, the task is solved by a concrete structure, which is built by using one of the preceding methods, characterised in that it contains profiles made from a shape-memory alloy in new concrete or in an applied mortar mix as reinforcement layer of an outside of the structure, which run along the outside of the structure within the mortar mix and/or reinforcement layer and are prestressed or are prepared for a prestress through the input of heat, in that electrical cables run from their end areas from the mortar mix and reinforcement layer, respectively, or their end areas are accessible by removing inserts.
The method is described and explained on the basis of drawings. Applications in new construction and in prefabrication, respectively, as well as applications for the subsequent reinforcement of existing concrete constructions are described and clarified.
The figures show the following:
At first, the nature of shape-memory alloys must be understood. These are alloys that exhibit a certain structure that changes depending on the heat but returns to its original state after heat is released. Just like other metals and alloys, these shape-memory alloys (SMA) contain more than just a crystalline structure. They are polymorphic and thus polycrystalline metals. The dominant crystalline structure of the shape-memory alloys (SMA) depends on its temperature, on the one hand, and on the external stress, on the other hand, be it tension or compression. The alloy is called austenite when the temperature is high and martensite when the temperature is low. The particular aspect of these shape-memory alloys (SMA) is the fact that they assume their initial structure and shape after increasing the temperature during the high temperature phase even when they were previously deformed during the low temperature phase. This effect can be utilised to apply prestress forces in building structures.
When no heat is artificially inserted into or released from the shape-memory alloy (SMA), the alloy is at ambient temperature. The shape-memory alloys (SMA) are stable within a specific temperature range, i.e. their structure does not change within certain limits of mechanical stress. Applications in the outdoor building sector are subject to the fluctuation range of the ambient temperature from −20° C. to +60° C. The structure of a shape-memory alloy (SMA) that is used here should not change within this temperature range. The transformation temperatures at which the structure of the shape-memory alloy (SMA) changes can vary considerably depending on the composition of the shape-memory alloy (SMA). The transformation temperatures are also load-dependent. Increasing mechanical stress of the shape-memory alloy (SMA) also implies increasing transformation temperatures. These limits must be given serious consideration when the shape-memory alloy (SMA) should remain stable within certain stress limits. If shape-memory alloys (SMA) are used for building reinforcements, it is imperative to consider the fatigue characteristics of the shape-memory alloy (SMA) in addition to the corrosion resistance and relaxation effects particularly when the loads vary over time. A differentiation is made between structural fatigue and functional fatigue. Structural fatigue relates to the accumulation of microstructural defects as well as the formation and expansion of superficial cracks until the material finally breaks. Functional fatigue, on the other hand, is the result of gradual degradation of either the shape-memory effect or the absorption capacity due to microstructural changes in the shape-memory alloy (SMA). The latter is associated with the modification of the stress-strain curve under cyclic load. The transformation temperatures are also changed in the process.
Shape-memory alloys (SMA) are suitable for absorbing permanent loads in the building sector on the basis of iron (Fe), manganese (Mn) and silicium (Si) wherein the addition of up to 10% of chrome (Cr) and nickel (Ni) makes the SMA react similarly against corrosion like stainless steel. Literature provides us the information that the addition of carbon (C), cobalt (Co), copper (Cu), nitrogen (N), niobium (Nb), niobium-carbide (NbC), vanadium-nitrogen (VN) and zirconium-carbide (ZrC) can improve the shape-memory characteristics in different ways. A shape-memory alloy (SMA) made from Fe—Ni—Co—Ti exhibits particularly good characteristics because it can absorb loads of up to 1000 MPa, is highly resistant to corrosion and its top temperature to change to the state of austenite is approx. 100° C.
The present reinforcement system takes advantage of the characteristics of shape-memory alloys (SMAs) and preferably those of a shape-memory alloy (SMA) based on considerably more corrosion-resistant steel in comparison with structural steel because such shape-memory alloys (SMAs) are considerably less expensive than some SMAs made from nickel titanium (NiTi). The steel-based shape-memory alloys (SMAs) are used in the form of round steel with raw surfaces, for example with coarse thread surfaces and are embedded in a mortar mix, i.e. a mortar layer, which functions as a reinforcement layer afterwards because of an indentation with concrete beneath it. The alloy contracts permanently to its original state on dissipation of heat. SMA-profiles will assume their original form and will also retain it under load when they are heated to the temperature that changes them to the state of austenite. The effect that is obtained here is the fact that the shape-memory alloy profiles, which have been casted into the mortar mix and mortar layer, respectively, generate a prestress on the entire hardened mortar mix and mortar layer, respectively, after being heated as a result of the reverse formation of its shape-memory alloy (SMA) that is prevented by embedding in concrete, wherein this prestress extends evenly and linearly, respectively, to the entire length of the shape-memory alloy profile.
In principle, a shape-memory alloy steel profile, an SMA steel profile in short, preferably made from round steel with a ribbed surface or with a coarse thread as surface is used in new construction or in prefabrication instead of traditional reinforced steel or, in addition to that, is placed in the concrete according to this method. The power supply heats the SMA steel profile after the concrete has hardened. This results in a shortening of the SMA steel profile and causes a prestress on the hardened concrete part accordingly. Subsequent reinforcement is obtained by installing the SMA steel profile in any direction but primarily in the tensile direction towards the roughened surface of the concrete structure and is dowelled with the same and afterwards enclosed and covered over the entire surface with cement mortar or shotcrete. After the cementitious mortar mix and mortar layer, respectively, have hardened, the SMA steel profiles are heated by means of electricity, which results in the shortening of these SMA steel profiles. This shortening causes a prestress of the cementitious mortar mix and mortar layer, respectively. The forces are then transmitted from the mortar layer into the existing concrete as a result of the raw surface of the concrete structure and adhesion.
The prefabrication of armoured concrete parts, for example balcony or façade slabs or pipes in which the novel SMA steel profiles are placed and prestressed, offers further advantages. The cross-sections of the structural element can be reduced thanks to the prestress of these prefabricated concrete structural elements. Since the structural element is designed free from cracks as a result of internal prestress, it is a lot more protected against the penetration of chloride and carbonation, respectively. That is, such structural elements become not only lighter but also a lot more resistant and durable accordingly.
The invention can also be used to better protect a structure in case of fire which is why the direct contraction of the SMA steel profiles due to the input of heat is at first consciously omitted. However, the built-in SMA steel profiles contract because of the effect of heat from a fire. Consequently, a concrete building envelope that was reinforced with SMA steel profiles, automatically generates a prestress in case of fire and results in an improvement of the resistance to fire.
The method is described and explained hereinafter on the basis of figures. For this purpose,
Next, the SMA profiles, as shown in
As shown in
Number | Date | Country | Kind |
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732/13 | Apr 2013 | CH | national |
Filing Document | Filing Date | Country | Kind |
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PCT/CH2014/000030 | 3/17/2014 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2014/166003 | 10/16/2014 | WO | A |
Number | Name | Date | Kind |
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5093065 | Creedon | Mar 1992 | A |
20050231071 | Magnussen | Oct 2005 | A1 |
20150218797 | Scherer | Aug 2015 | A1 |
Number | Date | Country |
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2358880 | Aug 2001 | GB |
9612588 | May 1996 | WO |
Entry |
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International Search Report for PCT/CH2014/000030 in German, mailed Feb. 4, 2015. |
Written Opinion for PCT/CH2014/000030 in German. |
International Search Report for PCT/CH2014/000030 in English, mailed Feb. 4, 2015. |
Written Opinion for PCT/CH2014/000030 in English. |
Number | Date | Country | |
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20160053492 A1 | Feb 2016 | US |