GB2358880A

GB2358880A - Method for reinforcing material

Info

Publication number: GB2358880A
Application number: GB0000640A
Authority: GB
Inventors: Stuart Ian Jackman
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-01-12
Filing date: 2000-01-12
Publication date: 2001-08-08
Also published as: GB0000640D0

Abstract

The method, primarily for pre-stressing or post-tensioning structural elements, involves anchoring within the body to be reinforced an elongate reinforcing means, at least a portion of which is formed from a shape memory alloy 18. The reinforcing means is then heated causing the shape memory alloy portion to shorten so that tension is applied to the reinforcing means and hence a compressive force is applied by the reinforcing means onto the body of the material. The body to be reinforced may be cast around the reinforcing means or alternatively the reinforcing means may be inserted through pre-prepared holes in the body after it has been cast. The method is primarily for use with concrete and can be used with shape memory alloys that can contract and/or lengthen and which can be controlled when secured within the body, preferably by being heated using an electric current.

Description

1 Method of Reinforcing material 2358880 The present invention relates to

an improved method of reinforcing material for use in the building trade, for example, for reinforcing concrete, brick masonry, stone masonry, rock formations and ground anchors.

Conventional methods of reinforcing material include prestressing and post-tensioning techniques, which have been used to introduce compressive stresses into, for example, concrete to increase the capacity of the concrete to withstand tensile forces, which would otherwise create unacceptable tensile stresses in the concrete. Concrete in its normal condition, can withstand compressive stresses very effectively, but has a practically negligible tensile stress capacity. Pre-stressing and posttensioning of concrete is used to introduce a permanent state of compressive stress in areas of concrete which will experience tensile stresses due to effects, such as, structural loading of a structural member. Pre-stressing and post-tensioning techniques, therefore, limit the possibility of concrete or other materials suffering distress due to the onset of tensile stresses.

Known pre-stressing techniques are usually used in pre-cast factories, not on site, a tension force is applied to a metal strand or tendon, this causes elongation strain of the strand and tensile forces in the strand. Once the required tensile stress is achieved in the metal strand, the ends of the strand are fixed to restraining elements, and concrete is cast around the metal strand to form the concrete beam. As the concrete cures, it forms a bond with the surface of the strand. The strand is then slowly released from the restraining elements and undergoes shortening strain from its elongated condition, the surrounding concrete also undergoes shortening strain and 2 an increase in compressive stress. The increase in compressive stress induced in the concrete is according to the following law:

a = E X E where a = stress, E = Young rs modulus of elasticity and g = strain. Thus, a permanent state of compressive stress is induced in the concrete in the area around the prestressing strand or tendon. The major problem with this method of pre-stressing concrete is that large tension elements are required to elongate the strand. Also, depending on the configuration of the concrete element, the compressive stress induced into the area of the concrete around the pre-stressing strand, may also cause some tensile stress in another part of the concrete element.

Post-tensioning techniques are generally applied to large structures insitu. The concrete structure is cast with ducts in it, when the concrete has cured and gained sufficient compressive stress capacity, tendons or strands are threaded through the ducts and each tendon is then tensioned by jacking against the end of the concrete element. After the tendon has been tensioned to the required tensile force, the tendon is anchored at the ends of the concrete element and the duct is filled with grout to avoid corrosion of the tendon. Post-tensioning requires the concrete element to be robustly designed to resist the high jacking and anchorage forces at the ends of the concrete element.

In known pre-stressing and post-tensioning techniques, large tensile forces have to be applied to the strands or tendons. Also, if a varying profile is required, then strand supports must be used within the concrete or mould 3 to maintain the profile of the strand during the prestressing or post- tensioning process. Strand supports must be heavy duty and strong to maintain the profile and to resist the stressing forces generated., Accordingly, the use of strand supports involves additional skill, labour, materials and expense.

It is an object of the present invention to overcome the problems of reinforcing materials of the past.

It is a further object of the present invention to provide a method of reinforcing materials that does not require the use of large and expensive equipment to tension and anchor the strand or tendon.

It is a still further object of the present invention to provide a method of reinforcing materials easily on site and with a varying profile.

According to a first aspect of the present invention there is provided a method of reinforcing material comprising a first step of forming an element of a material to be reinforced in a mould around at least one strand of a shape memory alloy (SMA), and a second step wherein the strand is heated causing the strand to shorten.

According to a second aspect of the present invention there is provided a method of reinforcing material comprising a f irst step of placing at least one strand of SMA through an element of material to be reinforced, attaching each end of the strand to the element of material to be reinforced covering the length of the element to be reinforced, and a second step wherein the strand is heated causing the strand to shorten.

4 The material to be reinforced may be formed with a hollow tube running through it in which the strand of SMA is placed.

Preferably, the material to be reinforced is concrete.

Advantageously, the SMA used is a reversible type, allowing control of both contraction and lengthening of the at least one strand.

The at least one strand of SMA is preferably heated by pas s ing an electrical current through the strand, alternatively, the strand may be heated by placing the element containing the strand in an autoclave or by a method of induction heating.

According to a further embodiment of the present invention, one or more strands of the SMA are arranged in a network. The network of at least one strand of SMA may be interlinked with other reinforcing materials.

In order to aid in understanding the invention some specific embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is an angled side view of a beam; Figure 2 is a cross-sectional side view illustrating a known method of pre-stressing; Figure 3 is a cross-sectional side view illustrating the method of reinforcing material according to the present invention; Figure 4 is a plan view of a network of reinforcing materials according to the present invention.

Referring to Figure 1, there is shown a beam 1 of, f or example, concrete, supported at ends 2. and 3 by supports 4 and 5, respectively. Due to the reinforcing technique, the top section 6 of the beam 1 is under tensile stress and the bottom section 7 of the beam 1 is under compressive stress. When a load L is applied to the centre of the beam 1, the tensile and compressive stresses through the beam crosssection are re-distributed, due to both the weight of the beam and the applied load L. The top section 6 of the beam 1 becomes under compressive stress while the bottom section 7 is under tensile stress. The tensile stress induced into the concrete by the weight of the beam and the applied load L would normally exceed the tensile stress capacity of the concrete and the beam would fail as a structural element. However, the compressive stresses induced into the concrete by the reinforcing technique can counteract the effects of the loadings and maintain the concrete in compression as follows:

stresses from reinforcing + stress from resulting loading stress distribution where T = tensile stress and C = compressive stress.

Referring to Figure 2 there is shown a known method of prestressing an element 8, for example, a concrete beam. A mould or moulds 9 are positioned between two external end anchorages 10 and 11 and a metal strand or tendon 12 runs through the mould 9 out of exit points 13 and 14 in the sides of the mould 9 and through the anchorages 10 and 11. The anchorages 10 and 11 must be capable of resisting all of the f orces generated by the strand 12 and may be 6 partially below ground level 15. At one anchorage 10 the end 12a of the strand 12 is secured by means of a collett 16. At the other end 12b of the strand 12, a jacking force is applied to the strand 12 using a jack 17, causing elongation and tensioning of the strand 12. After tensioning of the strand 12, the end 12b is secured at the anchorage 11 by means of a second collett (not shown) Concrete is then cast into the mould 9 around the strand 12 which is under tensile stress. After the concrete has cured and gained sufficient strength, the strand 12 is released from the anchorages 10 and 11. On release the strand 12 tries to elastically revert to its original shorter length and the force in the strand 12 is transferred to the concrete, since the concrete surrounding the strand 12 in resisting the forces generated by the strand 12 becomes internally stressed as required by the structural design of the beam.

Referring now to Figure 3 in which common elements are indicated with the same reference numerals, there is shown a mould 9 having side exits 13 and 14, and a strand or tendon 18 running through the mould 9 and out of the side exits 13 and 14. The strand 18 may also be introduced into the material in a tube running through an element of the material, such as a concrete beam.

The strand or tendon 18 used in the present invention consists of a metal alloy, known as a shape memory alloy (SMA). The Shape memory alloy may be one of two types, irreversible or reversible, the most generally used type will be irreversible, however, the reversible type may be particularly applicable when a change is required in the loading of a structure, or for use in, for example, earthquake zones, since it can be revers.ibly activated to aid a building withstand the forces of the earthquake. Shape memory is the ability of certain alloys to recover a 7 prior deformation upon heating above a critical temperature. A suitable grade of SMA is used f or the strand 18, for example, commercially available standard forms include strands having a diameter of 1.0 mm, and 2.0 mm. The commercially available strands of SMA are frequently supplied with a plastic coating which provides a degree of self -lubrication and electrical insulation, however, in the method according to the present invention, it is preferably used with the plastic coating removed to aid bonding with the concrete and stress transfer. The strand or tendon 18 can consist or a single strand of SMA or a number of strands, for example, twisted or bundled together to form a cable. Shape memory alloys (SMA) are commercially available, for example, experiments have been carried out according to the present invention using a Nickel-Titanium superplastic shape memory alloy manufactured by Furokawa Electric Company of Japan. Typically, SMAs comprise approximately 47% Nickel, 50% Titanium and 3% Iron, and a maximum free reversible unrestrained motion of typically 7 to 8-% can be achieved with this form of SMA. The typical maximum tensile stress for SMAs is approximately 1000 N/mrr and the constrained recovery stress is very uniform over the range of recovery strains, typically from 8% to 1.5%. SMAs are advantageous for use in the method of the present invention over ordinary high tensile strands. Concrete undergoes strain shortening and creep that has the effect of shortening the path of the strand through the concrete element, with ordinary high tensile strands the resulting shortening has the effect of reducing the stress in the strand, but this effect is minimised by the use of SMAs. Further, over time the stress in ordinary high tensile strands will reduce, however, this effect is significantly less when SMAs are used. SMAs have previously been used in many applications, such as medical applications, and in forms, such as, pipe flange clamping collars, over many years. Tensile forces, elongation strain and tensile 8 stresses are applied to the strand or tendon 18 used in the present invention during the manufacturing process, these forces generated in the SMA are very uniform. During manufacture of a strand of the SMA, an- elongation strain is achieved and the elongation strain is 'locked-in, by a metalurgical process after the tensile forces are removed.

No forces are applied to the strand 18 and the material, for example, concrete is cast into the mould 9 around the strand 18. The strand 18, which in this case would be of the irreversible type of shape memory alloy, is then subjected to a heating process which unlocks the elongation strain causing the strand to attempt to return to its original length. The strand 18 may be heated in many ways, for example, by use of an electrical circuit 19, which passes an electrical current through the strand 18 and heats the SMA by means of electrical resistance, or by placing the concrete element produced in an autoclave or by induction heating. The strand 18 must be elevated to a temperature of typically 47.50C, however, this temperature can vary depending on the specific manufacturing process and alloy constituents. The strand 18 attempts to revert to its original length but is restrained by the material or concrete which bonds to the strand 18 as it cures. The stand 18 becomes stressed in tension and the surrounding concrete stressed in compression, therefore, the desired structural characteristics of the concrete element produced can be obtained.

In contrast, known techniques of reinforcing material apply large tensile forces to the strands or tendons in order to put elongation strain and tensile stresses into the strands or tendons. Additionally, strong anchorages are required against which to jack the strands.

According to the method of the present invention, the 9 application of large forces to the strands is not required. There are many industrial benefits to this, for example, as large forces do not need to be generated, anchorages are not required and equipment to generate these forces and apply them to the strands or tendons are not required. In particular, this enables concrete elements to be prepared more easily on site, rather than requiring elements to be manufactured in a factory then transported to the site.

Further, as no forces are generated in the strand before the concrete has been prepared to accomodate these forces, then, in the case of prestressing concrete, the strand can be set to varying profiles, without the use of strand supports. This permits more complicated and structurally effective pre-stressing strand profiles to be easily and economically achieved, along with many profiles that would not be practical for normal pre-stressing techniques. New structural forms would also be viable for reinforced concrete, for example, circular reinforcing around ducts perforating through concrete containment vessels in nuclear reactor systems or complex 3D shell structures.

Referring now to Figure 4, there is shown a network 20 which could be used for the construction of, for example, tensioned plate structures or a concrete dome. By tapping electrical current supply wires (not shown) into various locations along the length of the network 20, it is possible to control the forces generated within the length of the network 20 which undergoes changes in stress/strain as described previously. This ultimately enables control of the degree and location of the forces within the concrete element produced. Further, strands 18 of SMAs can contract typically up to eight times as much as conventionally used reinforcing materials 21. Accordingly, only small sections of the strands 18 of the SMAs may be required interlinked by known methods, such as a link 22, with known reinforcing materials 21 to form a network 20. Such an arrangement would be more economically viable than producing the entire network 20 out of the more expensive SMAs, although, this is obviously possible and may be required where large dynamic fluctuations are foreseen. The strand 18 of the SMA can be used as a single strand or a number of strands, for example, twisted or bundled together to form a cable. obviously, there are any number of arrangements and configurations of the materials that could be used to form the shape of network 20 required.

According to the method of the present invention it is possible to vary the strength of the stress created in the concrete by controlling the amount of contraction of the tendon. Different strengths of stresses being suitable for different applications. Reversible and non-reversible forms of the SMAs are available enabling further control of the stresses to be applied to the concrete. In particular, the strand 18 can be tapped at various positions along its length with electrical current supply cables (not shown) and thus applying electrical resistance heating to controlled portions of the strand 18. Accordingly, it is possible to control the degree of reinforcement achieved by controlling the amount and position of contraction or relaxation of the strand 18 which is not possible with conventional reinforcing techniques.

Also, it is known that concrete actually contracts over time, therefore, a conventional reinforcing tendon gradually loses much stress. Advantageously, SMAs can maintain the same force in the concrete because they can contract far more than known reinforcing materials.

Further uses of the present invention include the possibility of leaving "dormant" strands in concrete structures, these can then be "activated" at a future date 11 to accomodate further development of the structure, or as a safety feature, for example, against possible failure of bridge support piers. The creation of intelligent, structures is possible by the application of the present invention to a building constantly monitored by a computer system, linked to movement sensors, to control contraction and relaxation of the tendons, in this case being of the reversible type of SMA, to enable the building to withstand, for example, high winds and earthquakes. The reversible form of the SMA would be particularly applicable to such intelligent, buildings.

It will also be understood that various alterations and modifications may be made to the above embodiments without departing from the scope of the invention. For instance, the method of the present invention is applicable to both a pre-stressing and a post-tensioning technique and to other structural situations, for example, for reinforcing rock formations in tunnels and to glued segmental post-tensioned bridges.

12

Claims

CLAIMS:

1. A method of reinforcing material comprising the steps of anchoring within a body of material to be reinforced, an elongate reinforcing means at least a portion of which is comprised by an element formed of a shape memory alloy, and heating said element of shape memory alloy in order to cause the same to shorten and apply tension to the reinforcing means anchored within the material to be reinforced, whereby a compressive force is applied from the reinforcing means to the body of material.

2. A method as claimed in Claim 1, wherein said reinforcing means is anchored within the material to be reinforced by casting the body of material to be reinforced around the reinforcing means.

3. A method according to Claim 1, wherein the reinforcing means is anchored within the reinforcing material by placing the reinforcing means through an aperture within an element of. the material to be reinforced, and anchoring ends of the reinforcing means to the element of material to be reinforced.

4. A method as claimed in Claim 3, wherein the material to be reinforced is formed with an aperture by casting a body of material around a hollow tube intended to receive the reinforcing means.

5. A method according to any one of Claims 1-4, wherein the material to be reinforced is concrete.

6. A method according to any one of Claim 1-5, wherein the shape memory alloy forming said element is of a reversible type wherein both contraction and lengthening of the element of shape memory alloy can be controlled whilst 13 in place in said body of material.

7. A method as claimed in any one of Claims 1-6, wherein said elongate reinforcing means is comprised by at least one strand of shape memory alloy.

8. A method according to Claim 7, wherein said shape memory alloy is heated by passing an electrical current through the said at least one strand.

9. A method according to any one of Claims 1-6, wherein said elongate reinforcing means includes components interconnected by said element of shape memory alloy, whereby said interconnected components are placed under tension upon contraction of the shape memory alloy when heated.

10. A method according to Claim 9, wherein said reinforcing means comprises a network of reinforcing members interconnected by intermediate elements of shape memory alloy.

11. A method of reinforcing material as claimed in Claim 1, substantially as described herein.

12. A structural element comprising a body of material incorporating a reinforcing element anchored thereto, at least a portion of said reinforcing element being formed of a shape memory alloy.

13. An element according to Claim 1, wherein said reinforcing means is anchored within said element by having been cast within said element during formation thereof.

14. An element according to Claim 12, wherein said reinforcing means is located within an aperture formed in 14 said element and has ends thereof anchored to said element.

15. A structural element according to any one of Claims 12-14, wherein said reinforcing means comprises a network of interconnected members extending through said structural element, members of said reinforcing means being interconnected by links of shape memory alloy.

16. A structural element according to any one of Claims 12-15, wherein said element or elements of shape memory alloy have been tensioned by heating to cause contraction thereof.

17. A structural element comprising a reinforcing means incorporating a shape memory alloy and substantially as described herein with reference to Figure 3 or 4 of the accompanying drawings.