Technical Background The surfaces of concrete and other building materials can look monotonous, especially when they form an extensive wall. The present invention allows patterns to be created in such surfaces, especially patterns that can be varied and selected. This opens the possibility of forming moving or changing patterns on such surfaces and even forming visual displays, for example, working clock faces or addressable alpha numeric characters in such building materials.

Dyes, inks and pigments (which will hereafter be referred to as"pigments") are known that can change colour when heated. Such pigments are incorporated, for example, into moulded plastic kettles bodies to indicate that the kettle is still hot.

US-6281 165 and GB 2325182 describe the application of thermochromic inks onto the surface of crockery, e. g. a mug or a teapot, that changes colour when the crockery is hot. US-3214283 describes a photochromic cement that darkens when illuminated with ultraviolet light.

Disclosure of Invention According to the present invention, there is provided a method of producing patterns in building materials, e. g. concrete and synthetic stone, which building material has a

surface layer in which a pigment has been dispersed, which pigment has different visible light transmission properties at temperatures above a threshold temperature as compared to its visible light transmission properties below that threshold temperature, the building material also including at least one heating element arranged to heat the surface layer, wherein the method comprises energising the heating element to heat the surface layer in the vicinity of the heating element to a temperature in excess of the pigment threshold temperature, whereby the said heated surface layer has a different visual appearance as compared to material that has not been heated to above the threshold temperature. Thus the heating elements can heat selected local areas of the surface layer to above the threshold temperature while the surrounding surface layer remains below the threshold temperature and hence takes on a different visual appearance in comparison to the surrounding surface layer.

The present invention also provides a structure including the said pigment and the heating element and a building material that can be formed to shape including the pigment.

The pigment is preferably chosen such that the threshold temperature is not below 30°C since the pigment might otherwise change colour on a hot day and is preferable less than 60°C so that excessive energy is not needed to create the pattern and so that someone touching the heated wall is not burnt ; the threshold temperature preferably lies in the range of 35-45°C, e. g. about 40°C.

The heating elements may, for example, be heating wires within the material which, when current is passed through them, heat the surrounding material to a temperature above the threshold at which the pigment changes colour. Heating can alternatively be brought about by methods other than electrical heating, for example by passing heated fluids, e. g. water, through pipes embedded within the building material but heated fluids are much harder to control than electricity and, accordingly, electricity is the preferred source of heating energy.

The patterns that can be formed in the building material are governed by the shape of the heating elements in the building material. However, the shape of the heating elements can be chosen at will and may be an abstract pattern, a graphic pattern depicting, for example a logo, picture, letters and/or numbers. The patterns may be static or move or alter and, as discussed in further detail below, the pattern may be a general display of data and/or images and the data displayed may be changed so that the patterns of the present invention can form general information displays.

When several heating elements are used, it is possible that the individual heating elements can be heated separately from some or all of the other elements so that only selected areas of the overall pattern are visible. For example, an array of heating elements can be embedded within the building material, the individual elements extending radially outward to form the possible positions of hands of a clock. The appropriate element (s) could then be energised to indicate the time; the passage of time can thus be indicated by energising successive elements to form patterns on the building material that look like the hands of a clock as time passes.

In an alternative embodiment, individually addressable heating elements can be grouped together such that, by heating individual elements within the group, patterns representing letters and/or numbers are visible. Different letters or numbers could be selected for display by changing the elements within the group that are heated. For example, an array of seven-element arranged in the shape of a number"8"can be used to display any integer from 0 to 9 by energising selected elements, in the same manner as a liquid crystal display. In the same way, heated elements can be arranged in groups such that, when selected elements within the group are heated, a letter can be formed. It is therefore possible to include within a wall of a building, a display giving information, for example, temperature and/or time or more specific information e. g. on the use of the building.

The time taken for a pattern to develop will depend on several factors, including the depth of the heating element under the surface of the building material, the energy supplied to the heating element and the ambient temperature of the building material.

An area of the surface surrounding a heating element will be heated up above the threshold temperature and will change colour and so will form the or part of a pattern.

The area of the surface that will be caused to change colour by a heating element will depend on many factors, including: a) the time that energy has been supplied to the heating element; the greater the time that energy is supplied, the greater will be the area of the material that is heated above the threshold temperature, until a steady state is reached, b) the ambient temperature of the building material; if the building material is held just below the threshold temperature at which the pigment changes colour, then a relatively small increase of temperature will cause the pattern to appear but if the temperature is just below the threshold temperature then a broader pattern will develop for a given energy input than if the temperature is substantially below the threshold temperature, c) The power supplied to the heated element and the depth of the heating element below the surface of the building material will also be relevant to the factors in the area of the building material that is patterned and the time that it takes to become patterned as a result of the heating of the heating element. However, with the appropriate control of the wire current, a rapidly appearing line can be produced.

The pattern will disappear when the temperature of the building material falls below the threshold temperature of the dye or pigment. This time will depend on many factors, principally: (a) the difference in temperature between the building material and the threshold, (b) the ambient temperature of the surroundings and (c) the geometry of the building surface.

It is possible to reduce the time taken for a pattern to disappear by incorporating pipes within the building material and circulating cooling fluid in the pipes in order to carry energy away from the patterned area and hence speed up the rate at which a pattern disappears.

The types of building material to which the present invention applies can be any materials in which heating elements and a temperature-changeable pigment can be incorporated. This will include cast material such as concrete and other cement-based materials; material that can form a surface layer, e. g. plaster; and material that is made by solidifying material, e. g. aggregate material embedded within a matrix, for example, a resin. The building material includes materials that are cast in situ while the building is being built or pre-formed and transported to the building. For example, heating elements may be incorporated within pre-formed blocks that are used to build a wall. If it is desired to form a pattern that extends across one or more blocks, then suitable connectors for the heating elements between the blocks should be included. The building material is preferably white or as pale a colour as possible. If the ink is black at room temperature and colourless above 40°C and the basic building material is white, the unheated surface will be a pale grey colour and the heated pattern will be white.

It is not necessary for the whole of the building material used to make a wall to include the thermochromic pigment so long as the visible surface of the material includes such pigment. Thus, it is possible to incorporate the pigment only in the surface layer of a wall. Indeed, it is possible that the heating element is embedded in one layer and the pigment is contained in an outer layer forming the visible surface, e. g. by applying the pigment as a layer on top of the concrete or other building material. A transparent lacquer may be applied on top of the pigment-containing layer to protect the pigment layer from damage. Obviously, any such protective layer should be able to withstand the elevated temperatures to which the pigment-containing layer is heated.

The invention can be applied to free-standing structures or articles, for example, a free- standing concrete slab or plinth.

Brief Description of the Drawings The present invention will now be displayed, by way of example, by reference to the following Examples and drawings in which:

Figure 1 is a cross sectional view through a wall according to the present invention incorporating heating elements; Figure 2 is a view of the first of the wall shown in Figure 1 ; Figure 3 is a view of a circuit for selectively energising the heating elements shown in Figures 1 and 2; and Figure 4 is a cross sectional view through a wall according to the present invention incorporating heating elements.

Best Mode of Putting Invention into Operation Referring initially to Figure 1, a wall (10) is formed from cast concrete having a surface (12). Embedded within the wall are seven lengths of nickel chromium heating element wire (see Figure 2) to form seven heating elements. The wire (15) at the ends of the heating elements (14) are connected to a driving circuit described below in connection with Figure 3 so that each element (14) can be individually energised. In this way, the group of heating elements (14) can be used to display numeric characters, as will be described below.

The cast concrete wall (10) includes a thermochromic dye or pigment that changes colour at the threshold temperature. A suitable material is commercially obtainable under the trade name Chromicolor AQ ink, which can be obtained from Matsui Shikiso Chemical Company Limited of Kyoto, Japan. This material is soluble in water and can be included in the water used to form a concrete mix. The amount of ink or pigment incorporated will depend on the pigment concerned and on the depth of colour change required. Chromicolor AQ ink changes from a colour to being transparent at approximately 40°C. The pigment manufacturer offers the Chromicolort) AQ ink in a variety of different possible colours. The whole of the wall (10) can be made from concrete containing Chromicolor AQ ink or it is possible that only a surface layer (18) (indicated by dotted line) includes the ink and the surface layer is applied as a render to the basic wall.

Several methods are envisaged for forming the heating elements within the wall (10) : 1. The nickel chromium wire (21) can be wound around bolts (20) (see Figure 4) that are held by shuttering (22) during the casting of the wall. The bolts can be removed by unscrewing them after the wall has set, leaving the elements (14) embedded within the wall and connecting wires (15) extend through the holes left by the bolts for connection to a circuit, as discussed below in connection with Figure 3. Instead of removable bolts, it is possible to hold the wire (21) for forming the elements (14) by means of conductive pins that remain embedded in the concrete (i. e. they are not removed) and that form contacts for supplying electrical current to the elements (14).

2. If the surface layer is applied as a render on a base made, for example from concrete, it is possible to install the heating elements on the surface of the concrete base; the holes are drilled through the concrete after it has set and lead wires (15) are fed through the drilled holes. The render containing the thermochromic pigment is then applied to the concrete base to cover the base.

3. It is possible to include the heating elements on the very surface of the wall; if desired, channels can be cut to accommodate the lead wires (15). However, it is preferred that the wires are embedded within the wall, preferably to a depth of 1-20 millimetres preferably about 3 mm; a larger depth will mean that the change of colour will take a long time to develop and a depth of under one millimetre will risk the wire being exposed to possible damage and possible overheating.

4. The ink may be applied as a wash to the surface of a wall after it has set; although this arrangement will use less ink than the arrangement in which the ink is dispersed within the concrete, care must be taken to apply the pigment evenly, which may be difficult to achieve. Also, this arrangement can only be used where the outer surface of the wall is absorbent.

The surface of the finished wall is preferably treated with a waterproofing lacquer, e. g.

UVproof.

Referring now to Figure 3, there is shown a drive element for selectively energising one of the various elements (14) of the group shown in Figure 2. A driver chip (30) having the same design as is widely used to drive liquid crystal displays is connected to a FET (32) the other terminals of which are connected between a power source (34) and the element (14). The driver chip (30) is likewise connected to the other six elements (14) of the group shown in Figure 2. The current supply to the heating elements will depend on the heating required but we have found that a low voltage direct current, for example, 10 volts producing a current of approximately 2 amps is sufficient to provide the developments of the colour change within a period of less than 30 seconds. The power supply can be obtained by rectifying mains AC current.

In addition to providing patterns on walls, it is also possible to include patterns on floors and ceilings; for example, a pattern could be laid across a floor to indicate a path to be followed across a room.

Instead of incorporating the pattern into a building, it is possible to have a free-standing element, e. g. a block or plinth of building material that can incorporate the present invention. For example, the block could form a free-standing clock.

Example Various test concrete compositions were made up, as set out in Table 1 below. The following raw materials were used: Cement: Snoworete@-white portland cement obtained from Blue Circle Cement Co.

Aggregate: Standard limestone aggregate.

Pigment: Chromicolor AQ ink obtainable from Matsui Shikiso Chemical Company Limited, Kyoto, Japan.

Nickel chromium wire: 24 gauge Nichrome Type C (60% Ni, 15% Cr, Balance Fe)

The aggregate, sand and cement were mixed together, the aggregate to cement ratio being as stated in Table 1. Chromicolol4 AQ ink was dissolved in water in the proportion as shown in Table 1. The water/ink mixture was incorporated into the cement/sand/aggregate mix to form a slurry which was cast. Nickel chromium wires were stretched through the mould so that it lies approximately 2 millimetres below the mould surface. The concrete was allowed to set.

The test pieces were examined by passing a current of 2 amps at 10 volts through the nickel chromium wire.

Table 1 Test Aggregate Water to Ink to Comments No. to Cement Cement Water Ratio Ratio Ratio 1 4 : 1 0.7 : 1 1: 5 Colour disappeared as the concrete dried and returned after 24 hours. 2 4: 1 0.7 : 1 1: 5 Colour disappeared as the concrete dried and returned after 24 hours. 3 2: 1 0.7 : 1 3: 10 Colour disappeared as the concrete dried and returned after 24 hours. The concrete bleached in the surface above the wire when a current was passed through the wire. 4 2: 1 0.7 : 1 3: 10 Colour disappeared as the concrete dried and returned after 24 hours. The concrete bleached in the surface above the wire when a current was passed through the wire. 5 2: 1 0.7 : 1 3: 10 6 3 : 1 0. 7 : 1 3 : 10 7 3: 1 0.7 : 1 3: 10 Colour disappeared as the concrete dried and returned after 24 hours. The concrete bleached in the surface above the wire when a current was passed through the wire.

Test 7 was deemed to be the best composition. That composition was used to make up some test pieces using the following quantities, by weight in grams: Ink 344 Cement 1639 Aggregate 4914 Water 1147