WO2009019464A1

WO2009019464A1 - Pre-stressing or confinement of materials using polymers

Info

Publication number: WO2009019464A1
Application number: PCT/GB2008/002651
Authority: WO
Inventors: Anthony Duncan Jefferson; Christopher Joseph; Robert John Lark
Original assignee: University College Cardiff Consultants Ltd
Priority date: 2007-08-03
Filing date: 2008-08-01
Publication date: 2009-02-12
Also published as: GB201003479D0; GB0814089D0; GB2465707A; GB0715123D0; GB2451740A

Abstract

There is disclosed a method for multi-axial confinement of materials such as cementitious composites and particulate structures, using a polymer as a composite, which can return to an original shape upon heating. The mechanical action of the polymer will cause strain, for example compression, in the material to be confined. Examples of uses include strengthening of concrete structures, repair, autogenic healing of cracks in concrete, wrapping damaged structural elements and reinforcement of particulate structures such as flood defences and railway embankments.

Description

Pre-stressing or confinement of materials using polymers

This invention relates to methods and apparatus for the pre-stressing or confinement of materials using polymers which can change shape under an external influence.

In this specification the term 'pre-stressing' refers to uniaxial compression of a material whereas 'confinement' refers to biaxial or triaxial (multi-axial) compression of material. Traditionally stressing of a settable material such as concrete is carried out by pre-stressing or post-stressing. Pre-stressing is achieved typically by encapsulation of tensioned steel members within a concrete matrix. When the matrix has cured, the tensioning force is released, causing compression of the concrete in the axis of the initial tensioning. The concrete is said to be pre-stressed in the direction of the compression. Simple beams can be prefabricated in this way for use on site, but larger structures are more difficult to produce and manoeuvre on site. Without expensive tooling, pre- stressing is limited to material compression in a single axis and for relatively short and simple structures.

Post-stressing typically involves introducing tensionable cables within apertures in a cured concrete member and then tensioning the cables. This technique is generally used for large structures such as bridge beams, and requires expensive on-site jacking equipment, and complicated cable end detailing. Moreover, difficulties arise in the safe demolition at the end of the structure's life.

Conventionally, soil or other particulate structures have been reinforced with mats during construction, so as to offer resistance to movement, should any movement occur. Structural plastics materials have conventionally been reinforced using fibres such as aramid, glass or carbon which act only to improve resistance to strain of the material in use. In the case of soil/particulate reinforcement and plastics reinforcement, the reinforcement is passive and becomes effective only when the soil etc or plastics is strained in use, thus no pre-stressing of the soil etc or plastics material has been contemplated previously.

Shape memory alloys (SMAs), for example nickel-titanium alloys, have been employed to pre-stress concrete. One such use has been disclosed in US

5093065. In that document memory alloy is formed into rods. SMA's have a memory such that when they are subjected to increased temperature they return to their memorised state. US 5093065 describes how this property is employed in a concrete structure to induce shortening of the rods when they reach a temperature which is above an activation threshold value. They remain at that reduced length and so exert internal compressive stresses to pre-stress the concrete. One problem associated with SMAs is their cost. This problem appears to be addressed in US 5093065 by using cheaper material in a portion of the length of the rod. However, larger structures will still be expensive to pre- stress in this way. Another problem is that the SMAs have relatively high activation temperatures, which may affect some materials, for example concrete and plastics, in which they are held, when activation is effected. Conventionally concrete has been pre and post stressed, that is, simple uniaxial pre-stressing of concrete has been performed to date. If biaxial or triaxial confinement could be achieved at low cost it would provide improved material characteristics at an economic cost. For example, the inventors have realised that biaxial or triaxial confinement of concrete or other settable materials improves the strength and ductility of these materials and reduces the likelihood of cracking. The inventors have realised also that even modest confinement forces give significant improvements in these materials, so stronger or thinner structures can be used. Yet further, the inventors have found that autogenic healing of cracks in cementitious materials can be enhanced under certain conditions.

Shape memory Polymers (SMPs) have been contemplated for use in actuators and reversible shape applications in US 2004/0197519, and for use as a stand-alone structure in WO 02/083767.

Krstulovic-Opara and A.E.Naaman- 'Self-stressing fiber composites', ACI Structural Journal, VoI 97 No 2 March-April 2000 pg 335-344, describe in detail the use of SMAs in prestressed cementitious plate composites. They mention generally that SMPs could be employed also, but no experimental data on this assumption is presented. However, the inventors have noted that the SMP's known at that time would not have had the mechanical strength to effectively confine a cementitious matrix. Thus, effective multi-axis confinement of cementitious material using SMPs has not been considered. Further, if reversible shape memory materials such as SMPs are used then there is a risk that the initial strain exerted by the SMP will be lost if the confinement process is reversed, for example if the material is heated again.

According to a first aspect, the invention provides a method for providing a confinable material comprising the following steps, in any suitable order: a) providing an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case, providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and b) disposing the material adjacent at least a part of the polymer; the method being characterised in that the polymer is arranged to extend in more than one axis to cause multi-axial confinement of the material when activated. In an embodiment the method further includes the step of: c) causing said confinement of the material by raising the temperature of the polymer to at least an activation value.

Preferably, at step b) the material is in a substantially flowable or particulate state, and prior to step c) the material is in a substantially solid state, the solid state being achieved by any one of setting, curing, hardening, settling, compaction or consolidation of the material.

More preferably the material is: cementitious or includes another mineral base; particulate based; or a plastics material, which can take said flowable/particulate and solid states. Conveniently, there is a delay between step b) and step c).

Preferably, step c) is undertaken when a crack, fracture, split, slippage, or shear dislocation takes place in part of the material.

In an embodiment, the extending of the polymer in said more than one axis is effected by either the polymer itself extending in more than one axis or multiple polymer elements positioned so as to be interlinked or juxtaposed to extend in multiple axes.

In an embodiment the polymer is formed into a shape which extends generally in three axes for providing triaxial confinement when activated. Preferably, in one or more of said three axes the polymer is one or more of: moulded, extruded, cold or hot drawn, pressed, rolled or stamped; a perforated sheet; a mesh expanded from a sheet; bidirectional stands or fibre for example a woven mat; rovings.

Alternatively, the polymer is in the form of randomly distributed fibres. Preferably, the fibres have deformed ends providing an anchor.

According to a second aspect, the invention provides a structure, including a composite material confined or confinable in at least two axes, said structure comprising a material at least partially surrounding an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature.

Preferably, said at least two axes comprise three axes for providing triaxial confinement.

Conveniently, the structure includes any one or more of a pre-cast concrete member including a concrete lintel, a stair-flight, or a barrier; an in-situ external concrete member including a multi-storey car park slab, a bridge deck or a bridge beam; an in-situ internal concrete member including a wall or partition member; an aesthetic member including a building finishing panel; a fluid retaining member including, a flood defence, a tunnel section, or a sewerage pipe section; a high strength member including an earthquake resistant building or component thereof; a ceramic material; a particulate based structure including an embankment; or a plastics member.

According to a third aspect the invention provides a method of facilitating repair of a crack or fracture in a composite material having within the material: an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature , the method including the step of changing the temperature of the polymer so as to cause strain in the polymer and thereby bring together the crack edges or compress said fracture. Preferably the repair is facilitated in more then one direction.

Conveniently, prior to or subsequently to the activation, a sealant or adhesive is introduced into or adjacent the crack or fracture.

Suitably, where said material is cementitious, the crack or fracture is at least partially autogenically restored. According to a fourth aspect, the invention provides a method of facilitating repair, strengthening, or reinforcement of a structure by confining the structure externally, the method comprising the steps of: a) covering at least a part of a surface of the structure with a confining material including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and b) causing confinement of the structure by increasing the temperature of the polymer to at least an activation value.

The invention extends to a method as set out above wherein activation or shrinking of the polymer is caused by heating using thermal energy applied to the polymer or by internal heating of the polymer resulting from electrical current flowing through the polymer.

The invention extends also to a polymer when used in any of the methods mentioned above.

Preferably, said polymer mentioned immediately above is doped with a conductive material in order to produce resistive electrical conduction and consequential heating.

Preferably, said polymer is a heat shrink polymer.

Preferably the heat shrink polymer comprises one or more of polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET); polyamide (PA), e.g. Nylon; Polybutylene terephthalate (PBT); Polytrimethylene terephthalate (PTT); or Polymethylene terephthalate (PMT).

The polymer may fall within one of the classes 1 to 4 as defined below.

The polymer may be a shape memory polymer.

According to a fifth aspect, the invention provides a composite material having a first component of the material including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and which extends in more than one axis, and a second component at least partially surrounding the first component, the second component being capable of taking a flowable state and a substantially solid state.

Preferably the second component is concrete, a plastics material, a particulate based material or a ceramic material.

According to a sixth aspect, the invention provides a plastics, ceramic or particulate based material composite including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature, which extends in or adjacent the material in at least one axis.

The invention extends to a method, structure, or material substantially as described herein, optionally with reference to the drawings. The inventive concepts mentioned can be put into effect in numerous ways, examples of which are described below, with reference to the drawings, wherein, Figure 1a shows a cross section through a structural element having a three dimensional polymer grid in a cementitious matrix;

Figure 1b shows a section along line 1b-1b shown in Figure 1a; and Figure 2 shows a cross section through a further structural element having an array of polymer elements in a cementitious matrix. Referring to Figure 1 , there is shown a structural element 5 formed from a cast cementitious matrix 20, in this case a standard concrete mix. The matrix has an internal lattice 10 which has been cast within the element 5. The lattice 10 is in the form of a rigid three dimensional grid formed from intertwined rods of polymer based reinforcement, in this case a glass fibre reinforced polymer such as polypropylene or polyethylene terephthalate formed into a twisted strand rod.

This material had an initial shape and has been deformed (for example elongated). In this case the deformation was carried out after the material was heated and then cooled whilst holding the material in the deformed state, but that heating and cooling cycle is not essential. It is possible to deform without heat, herein called cold deformation. The deformation can be at least partially reversed on (further) heating so that when the material is heated above an activation temperature it will strain toward an initial shape. The material will be 'frozen' in the deformed state until it is heated so in effect it will have a memory so that it can return to its original shape (e.g. pre-elongated shape). On heating above an activation temperature, not necessarily the initial temperature used during deformation, the material's memory will cause it to strain to return to its memorised shape (for example to shrink again back to the pre-elongated shape), thus providing a heatable polymer which will strain (for example shrink) when heated.

For ease of reference the term 'heat shrinkable polymer' or simply 'polymer' is used herein to describe at least the effect mentioned immediately above. It should be noted that the above definition includes material which would expand as well as shrink on activation. In the embodiment, the polymer is doped with carbon to provide a resistive electric path throughout the grid. The lattice is held in place during the pouring of the concrete by means of tensioned ties attachable to formwork (not shown).

An electrical supply interface 30 is in electrical communication with the grid so that electricity can be caused to flow through the grid.

After the concrete matrix 20 has cured, the polymer grid 10 is connected to an electrical supply, for example from an on-site generator, via the interface 30 and an electric current is passed through the grid 10. The current will pass in a series/ parallel path throughout the grid and in so doing will generate heat in the grid 10. This heat can be controlled accurately by regulating the current supplied to the grid to produce a temperature in the grid material above the activation temperature and in so doing cause the polymer of the grid 10 to strain towards its memorised shape and confine the concrete in three dimensions. Control of the current flowing in the grid can prevent excessive heat in the grid and so keep the overall temperature of the matrix 20 below a potentially damaging level. Also, effective temperature regulation can allow only partial activation of the polymer in the grid 10, for example where only a limited confinement is required. This technique provides accurate control of the activation of the polymer as well as low energy requirements. An activation temperature of 50-60^°C avoids the risk of activation taking place as a result of the heat generated from initial hydration of the cementitious matrix. The cost of the polymer confinement is far less than it would be if SMAs were used, or if conventional pre or post stressing were employed in 3 axes.

Figure 2 shows a second embodiment of the invention. In this case a geometrically more complex structure is shown, in the form of a casing 45 for example for an underground tunnel wall. A cementitious matrix 50 is used which contains heat shrinkable polymer fibres 60 having barbs 65 at each end (only some of which are referenced). In this instance these barbs are formed by melting the ends of the fibre during cutting to form bulbous ends. Alternatively they may be formed by deforming one end of the fibre to form an 'L' or hooked shape, splitting the ends of the fibres to form a 'V, or attaching a barb to the ends of the fibres. The fibres are randomly dispersed in the matrix and are sufficiently numerous so that they generally overlap and thereby interlock. The barbs aid interlocking and inhibit slip at the polymer 60 and matrix interface.

The structural element 45 is cast, typically in a mould, and allowed to cure. The element is then heated, for example by means of an electrically heated jacket to the activation temperature of the polymer 60 and maintained at that temperature until the element is heated throughout. The heating can be done under factory conditions or after the structural element is cast on site.

In the embodiments above activation of the polymer in the grid 10 and the polymer 60 has been carried out immediately after curing to provide improved structural strength and ductility. However, activation could be done partially, immediately after curing allowing further partial activation later, or activation could be delayed, perhaps for months or years, until required. For example when the structure has cracked it may be then desirable to 'self-heal' the crack and this can be done for example by further (not necessarily complete) activation of an initially partially activated heat shrinkable polymer, or complete activation of an initially non-activated polymer. The advantage of 'self healing' a structure is that the magnitude of confining force required to achieve crack closure is less than that required for the pre-tensioning methods described above, and so if the polymer cannot generate sufficient pre-tensioning force for a specific function, it can be used for crack closure with good effect, be that by partial or complete activation of the polymer.

In the embodiments above the heat shrinkable polymer has been cast within material. This is the favoured technique for adding strength and ductility to materials like concrete, ceramics, and plastics which can be moulded or formed at relatively low temperatures. However, single axis tie rods, two dimensional mesh, three dimensional grids or the like can be incorporated into other materials or structures such as plastics parts, road or railway embankments, or flood defences formed from soil, sand and or other particulate material. Activation of the polymer will cause confinement of the material and add cohesion to such a structure. The activation can be caused by heat generated in the polymer from electrical resistance or external thermal energy input.

In addition, confinement of a material can be brought about by externally wrapping a structure with a material which includes a heat shrinkable polymer. For example where a bridge pillar or the like has been damaged due to collision, explosion, environmental erosion or 'blowing¹ of concrete caused by corroding internal reinforcement, a blanket of heat shrinkable polymer can be wrapped tightly around the pillar and activated. This will confine the material of the pillar at least temporarily until a permanent repair can be effected. The following description and experimental data is provided as a supplement to the description above- It is generally known that thermo-plastic polymers comprise long chain molecules, which are normally oriented in a random, tangled fashion. However, the molecules become more orientated when a polymer is stretched or deformed, as occurs in the process of drawing. As a consequence, drawing polymers to a high draw ratio very significantly increases their strength and stiffness in the direction of stretch. On the application of heat, the oriented molecules tend to revert to their original state, which leads to shrinkage of the material, or, if the material is restrained, the generation of shrinkage stresses. This behaviour is akin to a shape memory effect and herein all polymer materials that exhibit such behaviour will be classed, in a general sense, as heat shrinkable polymers. The shrinkage stress achievable depends on the molecule linearity, crystallinity and level of orientation, of which the latter is governed by the draw ratio. Polymers have a mixture of amorphous and crystalline regions and, in general, it has been found that polymers with a higher amount of crystallinity and with relatively linear molecules, such as polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET) and polyamide/Nylon (PA), will achieve significant shrinkage stresses, and so these are favoured for use as heat shrink polymers in the examples described above. In addition other polymers will exhibit heat shrinkable properties include:

Polybutylene terephthalate (PBT); Polytrimethylene terephthalate (PTT); and Polymethylene terephthalate (PMT). In all cases the polymer is either cold drawn or heated, stretched, and held in tension while the polymer cools, so that it is frozen in the stretched state, ready to shrink when heated again. The temperature at which shrinkage occurs depends on several factors including crystallinity, glass transition temperature, melting temperature and draw ratio. Shrinkage tends to occur in the amorphous regions but fully amorphous polymers cannot be oriented successfully. Therefore, semi- crystalline polymers with a significant proportion of amorphous regions are suitable. PP, PET and Nylon have distinct amorphous regions and so shrink significantly at temperatures below their respective melting temperatures, whereas PE is highly crystalline so does not shrink until very close to its melting point. Shrinkage stresses in oriented PP have been found to increase with draw ratio, with peak values at a draw ratio of twelve_. The onset of shrinkage tends to occur at approximately 60⁰C.

The inventors have tested various semi-crystalline polymers in a thermal chamber connected to a tensile loading machine. Polyethylene terephthalate (PET) showed excellent shrinkage stress developed following heating. Strips of PET material 6mm wide and 0.045mm thick were tested. The strips were preheated, extended and held in that extended state until cool. The strips were bought 'off the shelf in that state. The strips were then grouped into bundles of 25 and 50 in order to give sufficient cross-sectional area of material for testing. The PET strips were tested on their own and also inside hollow plain mortar prisms.

The PET strips were restrained so they could not shrink in a tensile testing machine whilst heating the material in a thermal chamber. The strips were further heated to a temperature of 90⁰C for a period of 30min and then cooled to ambient room temperature over a period of 60min. The maximum average stress developed within the PET was approximately 30MPa during the heating phase and approximately 35MPa following subsequent cooling. These tensile stresses evolved due to the shrinkage forces acting on the restrained PET strips (i.e. restrained at a constant length, and therefore not allowed to shrink).

Tests have also been completed on the combination of PET strips within a mortar material. This was done in order to prove the concept that some form of heat shrinkable polymer could confine a cementitious material and also assist in the crack closure and subsequent autogenic healing of a cementitious material. Hollow mortar specimens were prepared. The specimens were 145mm long, and 25mm x 25mm cross-section and were cast with a 10mm x 10mm cavity located in the centre. A notch was cast into the centre of the specimen across its width. The mortar mix used was a standard mortar mix with a maximum aggregate size of 1mm. After one week of curing in air, fifty side by side strips of heat shrinkable PET were inserted into the 10mm x 10mm cavity. These were then pulled taught and clamped against a thin metal end plate at either end of the specimen. The cross-sectional area of the PET strips was about 13.5mm². This equates to a reinforcing percentage of approximately 2.5%. (i.e. sectional area of PET to sectional area of mortar).

Each specimen was then placed in a three point bend testing rig. The action of an external force at the centre of the top of the specimen and the reaction supports on the bottom of the specimen on either side of centre caused a tensile force to be created on the bottom face of the specimen. Due to the stress concentrations around the pre-fabricated notch a tensile crack began to open up at the tip of the notch and gradually propagated upwards under increasing prescribed vertical displacement. The opening of the crack was measured using a crack mouth opening displacement (CMOD) gauge.

One specimen was loaded until a crack opening of 0.35mm was recorded, and then the load was removed. The specimen was then placed in an oven at 90⁰C for 12 hours in order to activate the shrinkage of the PET. Following removal of the specimen from the oven, the crack was observed to have completely closed up. This was confirmed by measurements taken between fixed datum points on the underside and side of the specimen. Subsequent testing of the specimen 48 hours later confirmed the presence of confining stresses from the PET strips on the mortar specimen. Evidence was also obtained that showed that some degree of autogenic healing had also already occurred. This autogenic healing effect is due to the continuation of the hydration process occurring within the cementitious material, and appears to be most effective on cracks in the order of 0.05mm or less. The crack closure and confining pressure on the crack faces during this healing period increases the rate of strength recovery of the material.

It will be apparent to the skilled addressee that many alternatives, variants, and modifications to the embodiments above are possible within the ambit of the invention. For example many different heat shrinkable polymers could be used, provided they exhibit some degree of shape memory. The inventors have realised that polymers can be categorised by their performance into four general categories when they are used with settable materials. In a first category (class 1) the polymer provides the lowest confining ability which is sufficient to close a crack or the like when the polymer is activated. In a second category (class 2) the polymer provides a higher confining ability which is sufficient not only to close cracks but also to prevent cracks etc from forming when activated. In a third category (class 3) the polymer provides a yet higher confining ability which, as well as the abilities of classes 1 and 2, when activated, begins to enhance the structural behaviour (e.g. ductility) of the material to which it is applied. In a fourth category (class 4) the polymer performs to the abilities of classes 1 , 2 & 3 and additionally provides full confinement of the material to which it is applied, which means that the material has increased strength and ductility by the application of an active pre-stress in more than one axis.

The word polymer is used to describe a broad range of plastics and thermo plastics materials and includes materials which are not wholly formed from polymers for example the polymer could be a composite material which has mixed polymers or one or more polymers mixed with other materials such as glass or carbon fibres, or a filler material.

In a specific example the polymer could be an elongated (stressed) glass or carbon fibre cord held in its extended form by a thermoplastic, which itself may not have a memory. The fibre can then contract when the thermoplastic is heated, becomes weak, or contracts if it has a memory, and is then unable to hold the fibre in its extended state. Thus the term 'polymer or heat shrink polymer includes known polymers that are processed to shrink on heating and any polymer composite that can change shape, for example contract, without external force, when triggered by a change in temperature or other external influence.

The polymer can take any physical form and the multi axial confinement can be brought about by the polymer extending independently in those axes or, as described above, the polymer can be intertwined, interlocking, or some other way interengage so that activation in the axes is achieved.

Claims

1. A method for providing a confinable material comprising the following steps, in any suitable order: a) providing an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case, providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and b) disposing the material adjacent at least a part of the polymer; the method being characterised in that the polymer is arranged to extend in more than one axis to cause multi-axial confinement of the material when activated.

2. A method as claimed in claim 1 further including the step of: c) causing said confinement of the material by raising the temperature of the polymer to at least an activation value.

3. A method as claimed in claim 2 wherein at step b) the material is in a substantially flowable or particulate state, and prior to step c) the material is in a substantially solid state, the solid state being achieved by any one of setting, curing, hardening, settling, compaction or consolidation of the material.

4. A method as claimed in claim 3 wherein the material is: cementitious or includes another mineral base; particulate based; or a plastics material, which can take said flowable/particulate and solid states.

5. A method as claimed in claim 2 wherein there is a delay between step b) and step c).

6. A method as claimed in claim 5 wherein step c) is undertaken when a crack, fracture, split, slippage, or shear dislocation takes place in part of the material.

7. A method as claimed in any one of the preceding claims wherein the extending of the polymer in said more than one axis is effected by either the polymer itself extending in more than one axis or multiple polymer elements positioned so as to be interlinked or juxtaposed to extend in multiple axes.

8. A method as claimed in claim 1 wherein the polymer is formed into a shape which extends generally in three axes for providing triaxial confinement.

9. A method as claimed in claim 8 wherein, in one or more of said three axes the polymer is one or more of: moulded, extruded, cold or hot drawn, pressed, rolled or stamped; a perforated sheet; a mesh expanded from a sheet; bidirectional stands or fibre for example a woven mat; rovings.

10. A method as claimed in any one of the preceding claims wherein the polymer is in the form of randomly distributed fibres.

11. A method as claimed in claim 10 wherein the fibres have deformed ends providing an anchor.

12. A structure, including a composite material confined or confinable in at least two axes, said structure comprising a material at least partially surrounding an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature.

13. The structure of claim 12 wherein said at least two axes comprise three axes for providing triaxial confinement.

14. The structure of claim 12 or 13, that structure including any one or more of a pre-cast concrete member including a concrete lintel, a stair-flight, or a barrier; an in-situ external concrete member including a multi-storey car park slab, a bridge deck or a bridge beam; an in-situ internal concrete member including a wall or partition member; an aesthetic member including a building finishing panel; a fluid retaining member including, a flood defence, a tunnel section, or a sewerage pipe section; a high strength member including an earthquake resistant building or component thereof; a ceramic material; a particulate based structure including an embankment; or a plastics member.

15. A method of facilitating repair of a crack or fracture in a composite material having within the material: an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature , the method including the step of changing the temperature of the polymer so as to cause strain in the polymer and thereby bring together the crack edges or compress said fracture.

16. A method of facilitating repair of a composite material as claimed in claim 15 wherein the repair is facilitated in more then one direction.

17. A method as claimed in claim 16 wherein, prior to or subsequently to the activation, a sealant or adhesive is introduced into or adjacent the crack or fracture.

18. A method as claimed in claim 16 or 17 wherein, said material is cementitious and the crack or fracture is at least partially autogenically restored.

19. A method of facilitating repair, strengthening, or reinforcement of a structure by confining the structure externally, the method comprising the steps of: a) covering at least a part of a surface of the structure with a confining material including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and b) causing confinement of the structure by increasing the temperature of the polymer to at least an activation value.

20. A method according to any one of the preceding method claims wherein activation or shrinking of the polymer is caused by heating using thermal energy applied to the polymer or by internal heating of the polymer resulting from electrical current flowing through the polymer.

21. A polymer when used in any of the methods mentioned above.

22. A polymer as claimed in claim 21 wherein the polymer is doped with a conductive material in order to produce resistive electrical conduction and consequential heating.

23. A polymer as claimed in claim 21 or 22 wherein the polymer is a heat shrink polymer.

24. A polymer as claimed in claim 23 wherein the heat shrink polymer comprises one or more of polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET); polyamide (PA), e.g. Nylon; Polybutylene terephthalate (PBT); Polytrimethylene terephthalate (PTT); or Polymethylene terephthalate

(PMT).

25. A polymer as claimed in any one of claims 20 to 24 wherein the polymer falls within one of the classes 1 to 4 as defined herein.

26. A polymer as claimed in claims 21 or 22 wherein the polymer is an shape memory polymer.

27. A composite material having a first component of the material including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature; and which extends in more than one axis, and a second component at least partially surrounding the first component, the second component being capable of taking a flowable state and a substantially solid state.

28. A composite material as claimed in claim 27 wherein the second component is concrete, a plastics material, a particulate based material or a ceramic material.

29. A plastics, ceramic or particulate based material composite including an activateable polymer which has an original shape, has been heated, strained to change said shape whilst heated, and has been cooled whilst said strain is maintained or which has been plastically strained in an unheated form, in each case providing a material which can be activated to return toward its original shape when subjected to a change in temperature, which extends in or adjacent the material in at least one axis.

30. A method, structure, or material substantially as described herein, optionally with reference to the drawings.