This invention relates to a novel structure of reinforced concrete and a method for achieving this structure.

Concrete is conventionally reinforced by means of steel rods incorporated in the concrete and there are a large number of different types of structures and configurations which have been suggested and used for such purposes.

A description of conventional concrete reinforcing techniques is given in BS 8110 - Structural use of concrete, BSI, 1985; BS 8007 - Design of concrete structures for retaining aqueous liquids, BSL 1987; BS 5400 - Steel, concrete and composite bridges, BSL 1990; Eurocode 2 - Design of concrete structures, CEN 1992 and Reynolds and Steedman - reinforced concrete designer's handbook, 10th edition, C.E Reynolds and J C Steedman, Chapman & Hall 1988 (publishers).

In quality concrete, the steel reinforcing rods are protected against corrosion by a protective oxide film which forms on the surface of the rods. This oxide film is due to the alkaline conditions in concrete. In building and other structures, this oxide film may be destroyed locally or over greater surface areas due to carbon dioxide in the air penetrating to the interior of the concrete - a process known as carbonation. In applications such as roads, bridges, tunnels, piers and other coastal and off-shore structures the oxide film may be destroyed by the action of chloride ions, originating from sea-water or de-icing salts, also penetrating to the interior of the concrete. Once the oxide film has been disrupted, provided oxygen and moisture are present, corrosion of the reinforcing rods may then occur.

The presence of cracks in the concrete surface will facilitate the penetration of carbon dioxide and chlorides to the interior of the concrete; also such cracks, if they exceed a width of 0.3 mm (BS 8110 Eurocode 2) can look unsightly.

Repair of such structures is a very expensive process and ways have been sought to delay or prevent corrosion.

Increasing the thickness of the concrete cover to the reinforcing rods will lead to greater surface crack widths due to the fact that surface crack width is related to concrete cover such that the greater the concrete cover the greater the surface crack width, as referred to in BS 8110 and Eurocode 2 above. Large surface crack widths are undesirable from the viewpoint of both appearance and delaying the onset of corrosion.

Another proposed solution to this problem is to use reinforcing rods made of a material which will not be corroded by carbon dioxide or chloride salt, e.g. stainless steel or non-metallic materials such as fibreglass-reinforced plastic (FRP) or carbon fibre.

However, providing stainless steel rods is very expensive and the use of non- metallic materials such as FRP is unsuitable in structures where strength, deflection and/or crack widths are critical since non-metallic materials do not have the stiffness of conventional reinforcing rods as reported in - Kodiak: fibreglass-reinforced plastic rebar, International Grating Inc. (manufacturer's literature). Alternative, Materials for the reinforcement and pre-stressing of concrete, edited by J.L. Clarke, Chapman & Hall 1993 (publishers). Non- ferrous reinforcement for structural concrete, J.L. Clarke. In Concrete 2000, ed. R.K. Dhir and M.R. Jones, Dundee University, E & FN Spon (publishers), 1993. Designing concrete beams with FRP rebars, H.V.S. Granga Rao, S.S. Faza and J. Anderson. In Concrete 2000, ed. R.K. Dhir and M.R. Jones, Dundee University, E & FN Spon (publishers), 1993.

We have now devised a structure which reduces these difficulties.

According to the invention, there is provided a structure comprising concrete reinforced with a plurality of iron or steel reinforcing rods embedded in the concrete, there being a plurality of non-corrosive reinforcing rods positioned substantially between the iron or steel reinforcing rods and the surface of the concrete.

The invention also provides a method of making a reinforced concrete structure which method comprises placing a plurality of iron or steel reinforcing rods in position, there being a plurality of non-corrosive reinforcing rods in position and pouring concrete over them whereby when the concrete has set a concrete structure is formed comprising concrete having embedded therein at least one layer of iron or steel reinforcing rods and at least one layer of non-corrosive reinforcing rods positioned between the iron or steel reinforcing rods and the surface of the concrete.

Any conventional concrete can be used, as in BS 8110 or - ENV 206: concrete - performance, production, placing and compliance criteria, CEN 1992; or concrete made from new materials, e.g. as referred in - Concrete, Technology and Design: 1) New Concrete Materials; 2) New Reinforced Concrete; 3) Cement Replacement Material; ed. R.N. Swamy, Surrey University Press.

The iron or steel reinforcing rods can be any iron or iron based alloy of the type used in the conventional reinforcement of concrete, e.g. plain round steel bars and deformed high yield bars - BS 4449: Carbon steel bars for the reinforcement of concrete, BSL prEN 10080: Steel for reinforcement of concrete: Weldable ribbed reinforcing steel B 500; Technical delivery conditions for bars, coils and weldable fabrics; and steel fabric - prEN 10080. BS 4482: Cold reduced steel wire for the reinforcement of concrete, BSL BS 4483: Steel fabric for the reinforcement of concrete, BSI.

The arrangement of the iron or steel reinforcing rods can be in a conventional configuration, e.g. as described in Reynolds and Steedman above.

In normal usage, e.g. for bridge construction using a 40 grade concrete, it is normal to have the first layer of reinforcing rods no more than 5 cm from the surface of the concrete. This is to protect the steel from corrosion and prevent unacceptable cracking of the concrete in use, although the actual thickness will depend on the concrete type, usage, etc.

In the present invention, the nearest iron or steel rods to the surface of the concrete are preferably 5 to 15 cm from the surface of the concrete, more preferably from 5 to 13 cm.

The iron or steel reinforcing rods can be treated by conventional surface treatments in order to give them increased corrosion resistance, e.g. by epoxy coating or galvanising.

By non-corrosive reinforcing rods is meant rods which are made of a material which is substantially less liable to corrosion in usage when embedded in concrete than conventional iron reinforcing rods.

The non-corrosive reinforcing rods can be made of stainless steel or known non-metallic reinforcing agents such as glass fibre reinforced plastic resins, carbon fibre reinforced resins, polyamides, polyimide, e.g. Kevlar (RTM) reinforced resins, etc. The non-metallic reinforcing agents are described in - Non-ferrous reinforcement for structural concrete, J.L. Clarke. In Concrete 2000, ed R.K. Dhir and M.R. Jones, Dundee University, E & FN Spon (publishers), 1993; Durable reinforcement for aggressive environments. British Cement Association seminar, Luton, Nov. 1990.

The dimensions of the non-corrosive reinforcing rods are not critical but preferably their average diameter is from 0.95 to 4 cm.

When the non-corrosive reinforcing rods are being used primarily to prevent crack formation or crack propagation, their length can be shorter than when their primary purpose is for load-bearing purposes. In some circumstances, random distribution of relatively short non-corrosive reinforcing rods or fibres can be used.

The distance of the non-corrosive reinforcing rods from the surface of the concrete is nor critical, preferably they should not be so near to the surface that wear in use could cause them to be exposed, nor should they be so far from the surface that unacceptable cracks are allowed to form. Preferably they are from 2 to 5 cm from the surface of the concrete.

In some applications, such as retaining walls structures are formed in which there are one or more layers of iron or steel reinforcing rods near one surface of the structure e.g. the back surface in a retaining wall, and a layer of iron or steel reinforcing rods near the other surface, in this case one of the layers can be replaced by non-corrosive reinforcing rods.

It is a feature of the present invention that it enables conventional reinforcing rods to be used in reinforcing concrete but they can be a greater distance from the surface of the concrete so that they are less liable to attack by corrosive water or carbon dioxide with the non-corrosive reinforcing rods or fibres preventing cracks appearing in the extra thickness of concrete above the iron reinforcing rods.

Example

Four reinforced concrete beams, 300mm wide, 400mm deep and 3100mm long were formed using a conventional concrete mix. Three concrete cubes of 100mm by 100mm by 100mm were also cast from each batch to measure the compressive strengths.

The beams all were reinforced as shown in the drawing. Rods 1 are high yield steel rods of diameter 12mm, rods 3 are high yield steel of 32 mm and 2 are mild steel links of 8mm spaced at 200mm.

The side and top cover to links was 40mm and the bottom cover to links was lOOmm.

Three of the beams, in addition had fibre reinforced plastic rebars 4 in them. The bottom cover to the rebars was 30mm and the side cover to the rebars was 40mm.

The rebars were Fiberglass-reinforced plastic rebars, International Gratings Inc. Houston Texas and in beam 2 were 10mm helically braided rebars, in beam 3 were 12mm sand coated rebars and in beam 4 were 20mm sand coated rebars.

The beams were allowed to cure in their moulds for two days and were then removed from their moulds and wrapped in damp hessian and polyethylene and allowed to cure for a further 26 days.

The beams were tested in bending using a four point loading arrangement; the effective span was 2.9m. The load was applied at a rate of 0.03kN/sec. generally in increments of 12.5kN up to a maximum of 210.5 kN in order to produce a design moment of lOOkN in the lm central pure bending region of the beam.

The crack widths were measured using a portable microscope.

Beam 1, containing only steel reinforcement had six cracks four of which exceeded a surface width of 0.3mm., the maximum crack width was 0.5mm.

Beam 2 containing the braided rebars had a total of seven cracks one of which exceeded 0.3mm. and the maximum crack width was 0.5mm.

Beam 3 containing 12mm sand coated rebars had 11 cracks and a maximum crack width of 0.3mm.

Beam four containing 20mm sand coated rebars six cracks and a maximum crack width of 0.25mm.

Beams 3 and 4 complied with BS8110 as none of the crack widths exceeded 0.3mm. Beam 2 also complied with BS8110 as more than 80% of the cracks had a surface width of 0.3mm or less. Beam 1 fell outside the recommendations for crack control in BS8110.

The 28 day compressive strengths of the concrete used in beams 1-4 were tested and the results were:

Beam 1 47 N/mm ²

Beam 2 53 N/mm ²

Beam 3 53 N/mm ²

Beam 4 52 N/mm ²

The beams were tested for deflection and were all found to comply with BS8110, with all the beams showing substantially similar deflection characteristics.