GB2186017A - Method and means for the preservation of reinforced concrete structures - Google Patents

Abstract

Reinforced concrete structures deteriorating from adverse chemical reaction (e.g. alkali silica reaction) within the concrete, are preserved from further deterioration by inhibiting the reaction by heating the concrete by passage of electric current along reinforcing rods 26 and links 30 of column 20 by way of electrodes 48 passing through holes 46 drilled for the purpose to make electrical contact with rod 26 located by a cover meter. Current is controlled to prevent overheating and to maintain the concrete permanently sufficiently dry to inhibit the reaction. The heating also preserves the reinforcement from corrosion. <IMAGE>

Description

SPECIFICATION Method and means for the preservation of reinforced concrete structures The present invention relates to the preservation of reinforced concrete structures, primarily (though not exclusively) for existing structures which already suffer from concrete deterioration.

Concrete is one of the most widely used construction materials. Generally concretes are strong in compression but have only a low and brittle resistance to tensile forces. To provide such resistance, concrete structural elements are commonly reinforced, as by the provision of internal steel reinforcement (mild steel or high tensile steel) embedded in the concrete and/or by prestressing the element usinghigh-tensile steel tendons external and/or internal to the concrete.

Mostly reinforced concrete is notsubjectto marked deterioration with age, butthere have been instances of serious and alarming deterioration, leading even to the sudden collapse of buildings and bridges, and entailing loss of life and injury and considerable economic loss.

Such deterioration results from attack and destruction consequent on the reaction of chemicals, whether in the solid or gaseous orfluid state. Byway of example the destruction can be caused by chemical incompatability ofthe original constituents ofthe concrete such as the Alkali-Silica Reaction (ASR) during which alkalis and silicas in the concrete react to form a gel which absorbs some ofthe pore fluid in the concrete, swells and exerts an internal pressure which can crack the concrete. It can also be caused by moisture permeating to the level ofthe reinforcement facilitating corrosion and consequent spalling ofthe concrete.The harmful reactions can be caused and/oraided bythe introduction of de-icing salts and/or by the use of inadequately washed sea-dredged aggregates containing excesses of salts and in particular NaCI. Disruption and destruction of concrete can further be caused by it being placed in an environment containing sulphates and/or by contamination of concrete by contactwith sulphates of various types such as gypsum. Damagetoconcretebymanyother chemicals is known and can occur.

These problems have been the subject of considerable study. One approach has concentrated on the selection of chemically compatible constituents not readily liable to adverse chemical reactions; this is not of any help in the management of existing structures already suffering damage from concrete deterioration. So far as the latter are concerned, attempts at repair are commonly made by replacement of the most badly damaged structural elements, and strengthening of the structure by incorporating of additional elements.

Also, based on the recognition that chemical reaction can only take place in the presence of sufficient moisture, studies have been made to see how by changing the environment of the concrete the reaction can be slowed and deterioration retarded. Thus, cladding with carefully controlled ventilation ofthe cavity has been attempted, and so has the use of external coatings to prevent ingress of moisture and salts into cracked surfaces. However, in the case of ASR, re-wetting after the concrete has dried out is liable to lead to rapid deterioration ofthe concrete and consequent rapid damage (as distinct from the much slower rate of deterioration if the concrete had not suffered such great fluctuations in relative humidity).The fact remains that structures (including bridges and largebuildings)thatare seriously affected haveto be demolished lestthey reach the stage of being in danger of collapse.

The objectofthe invention isto alleviate the problem discussed above.

According to one of its aspects the present invention provides a method for the preservation of reinforced concrete structures having structural elements whereof the concrete shows, or by reason of its composition is susceptible to, deterioration by adverse chemical reaction occurring within the concrete, the method including the steps of heating the concrete by the flow of electrical currentthereby to dry itto a level of humidity sufficiently low substantially to inhibit such chemical reactions (herein referred to as "the sufficiently dry state"), and subsequently maintaining it in the sufficiently dry state. Generally it will suffice to warm the concrete so that the temperature at its surface is slightly above ambient and/or dewpoint temperature.

Preferably the heating is such that the temperature ofthe concrete at its surface does not exceed 30 (temperatures herein are given in degrees Celsius), and preferably such that the temperature of the heat source in contact with the concrete does not exceed 55"and preferably not 50 . In the case of most concretes it is desirable to avoid heating of any substantial parts of the concrete above 30'.

The method may include the monitoring of temperature adjacent the heat source and/or at the surfaces of the concrete, as by thermocouples or othertemperature gauges. If desired the moisture content within the concrete can also be measured by the use of the half cell potential method (e.g. as that test procedure is defined in ASTM C876-80) or any other means.

The currentflow may be kept on continuously to prevent the concrete from re-absorbing moisture following drying. Or it may be switched on and off manually or automatically in response to the monitoring mentioned above to prevent overheating and/orto keep the concrete in the sufficiently dry state.

Alternatively, on completion of the drying, an impermeable coating or membrane may be applied to all, or at least the surfaces of the concrete that are exposed, to prevent re-absorption of moisture and thus maintain the concrete in the sufficiently dry state.

Heat may be applied to the concrete byway of its surfaces. This may be done by wrapping it with electrical warming cables (e.g. as used for underfloor heating or de-icing busy roads), overlaid by heat insulating blankets; orthis may be done by wrapping it with heat insulating blankets containing warming cables.

According to a second aspectofthe invention (preferably used in combination with the method described as its first aspect), there is provided a method for the preservation of a reinforced concrete structure which includes the step of heating the structure by passing electrical current along metal reinforcement extending along its structural elements. For this purpose, one would generally pass current along the customary reinforcement rods (and their interconnecting links and wire ties).

By reason of their relatively great thickness and relatively low electrical resistivity, relatively high electrical currents, e.g. ofthe order of 1 to 100 amps, at relatively low voltages, e.g. of the order of 24to 60 volts, would be appropriate to achieve the necessary warming, (as distinctfrom the much lower currents as much higher voltages commonly used for underfloor heating and road de-icing).The method may include the further steps of locating from the surface of the concrete the reinforcement within (e.g. by use of a cover meter), drilling at least one hold near each of opposite extremities of a length of the structural element to a depth of contact a reinforcement rod (ora link connecting such rods), making electrical connection between the rod or link in that hole and an electrical conductorto includethe rod in an electrical circuitforpassing current along the rod to heat it. Preferably the currentflows along a multiplicity of reinforcing rods interconnected by connecting links. The heating ofthe structure, particularly if by way of passing electric current along the metal reinforcement, also helps to preserve the reinforcement itself from corrosion.

This is of considerable value, aside from inhibiting chemical reaction within the concrete.

Itwill be appreciated that the methods described above do not interrupt the use of a structure; buildings, multi-storey car parks, bridges etc. can continue in commission notonlywhilethe drying is being carried out but also whilst the means (mentioned below) for so doing is being installed.

According to a third aspect, the invention provides the combination which includes a reinforced concrete structure having structural elements whereof the concrete shows, or by reason of its composition is susceptible to, deterioration by adverse chemical reaction, and meansforheating the concrete by the flow of electrical current to dry it to the sufficiently dry state and for maintaining it in the sufficiently dry state. Some or all of the structural elements may of course extend vertically (or substantially vertically), in the nature of columns.

The combination may include monitoring means for monitoring the temperature adjacent the heat source and/or atthe surface of the concrete.

Preferably the combination includes switching devices for switching the current on and off manually orautomatically underthe control of the monitoring means, a nd device and devicesforvaryingthecurrentintensity.

The combination may include an impermeable coating or membrane applied to the surface of the concrete after drying, or at leastto those that are exposed to moisture, to preventre-absorption of moisture and thus maintain the concrete in the sufficiently dry state; examples of suitable coating materials are solvent-free slightlythixotropiccoating materials, two-component coating materials or dispersion paints all based on epoxy resins such as the SIKAGAARD range of materials supplied by SIKA LTD.

In one form of carrying out the invention, electrical warming cables (such as mentioned above) are wrapped around the surfaces of the concrete for the application of heat thereto, and heat insulating blankets are laid overthewarming cables.

Alternatively, heat insulating blankets containing warming cables are wrapped around the surfaces of the concrete.

According to a fourth aspect (preferably used in conjunction with its third aspect), the invention provides a combination of a reinforced concrete structure and meansforpassing electrical current along the metal reinforcement extending along the structural elements, in order to apply heat to the concrete. The electrical supply may be A.C. or D.C.

Preferably the electricity supply is of the order of 10 to 100 amps, at voltages of the order of 24to 60 volts.

The calculation of suitable voltages and current intensities is well within the scope of the man skilled in the relevant art (given the appropriate parameters) and does not itself form part ofthe invention.

Preferably the combination includes an electrical conductor making electrical connection with a reinforcing rod near each of opposite extremities of a length of the structural element, part of that electrical conductor extending along holes drilled to a depth to contactthat rod (or a link connecting such rods), the electrical conductor forming part of an electrical circuit for passing current along the rod to heat it.

Preferably the entire length of the structural element is included in the circuit, if appropriate by connecting in series shorter lengths that span the entire length; alternatively shorter lengths may be connected in parallel.

One form of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure I shows diagrammatically partofa reinforced concrete column in elevation, in combination with preferred means for heating it for the purpose ofthe invention, and, Figure2 shows the column in section taken on line ll-ll of Figure 1.

In the drawings, a square column generally designated 20 (ofwhich for simplicity only part is shown) extends vertically. Its concrete 22 has structural and severe surface map-cracking (not shown). The cracks are wide and still growing, clearly indicative of ASR deterioration. The column has vertical reinforcing rods 24,26,28 inset from each of its corners, and interconnected by spaced links 30 (of which only two are shown). Rods 26 overlap at 32,34 with rods 24 and 28 in the region of floor slabs 36,38. The overlapping rods are tied by tie wire (not shown), in the regions 32,34.The distance from slab top to slab top is here 3450 mm, the colums are 300 mm by 300 mm in cross section, the rods are of high tensile steel 16 mm in diameter, the links and tiewire are of mild steel, and the links are 8 mm in diameter and are set at 240 mm centres. The construction as described thus far is of a structural element forming part of a wholly conventional reinforced concrete structure.

Near opposite ends 40,42 of a length 44 of column 20, the concrete is drilled to reach the rods 26,28, the required position of the holes 46 being determined by previously locating the line ofthe rods by use of a conventional cover meter at the surface of the concrete. Electrodes 48 are inserted in the holes, connected with the rods to make good electrical contact and fixed in position.

The electrodes 48 form part of an electrical circuit which includes a control panel and box 50, ammeter 52, on-off switch 54, current source 56 (in the form of a rectifier and/or transformer), current flow and voltage control devices 58,60 and fuses 62, all interconnected by connectors 64, the current source 56 having its input connected to a 380 voltA.C. mains supply at 66and havingthevoltageofitsoutput measured by a voltmeter 68. Additionally a thermocouple 70 is fitted in a hole 72 in the concrete mid-way between the holes 46 in contact with one of the rods 26and is connected byconnector74toa thermometer dial 76 and thence to the control panel.

The ammeter, voltmeter, thermometer dial, on-off switch, voltage and current flow controllers and fuses are all housed in the control box (albeit for clarity of illustration shown separately).

In operation, the deteriorating concrete having a level of humidity such that further deterioration is liable to take place, current isturned on at the mains, the current source 56 provides a D.C. output at 50 volts, and on closure of switch 54 current is passed byway ofthe circuitry to the electrodes 48 and thence along rods disposed between the electrodes.

The tie wires and the links help distribute the current to the rods at the different corners ofthe column (thereby connecting them in parallel) and from the rods 26 to the rods 28. Although relatively thick and although of relatively low resistivity, the rods do become warm by reason of the relatively heavy electric current of the order of 80 amps. If lower current intensity is required, this is achieved by voltage reduction by way of voltage device 60 under the control of current control device 58.Conversely if at any time the current is belowthe required value, e.g. because of poor electrical contact between elements ofthe reinforcement, the voltage is increased byway of devices 58 and 60 (temporarily or permanently, as necessary). lfthetemperature of the rods as monitored bythethermocouple exceeds 50 a control device in the control panel automatically opens switch 54 to stop flow of current and conversely turns it on again when that temperature falls below 50 ; alternatively the temperature is control led byvoltage/current regulation. The effect of warming the concrete is (as explained) to drive off moisture.When the concrete has reached the sufficiently dry state, adverse chemical reaction ceases, but the drying is continued to dry out the concrete to a greater extent, and the current is then turned off. In practice monitoring of the humidity is continued and the current is turned on again and heating resumed when the level of humidity again approaches the sufficiently dry state.

But in situations of high humidity the current and consequently heating and drying oftheconrete would be maintained continuously.

In the illustratedform ofthe invention onlya relatively short length of the column is warmed. In an alternative form the entire length is warmed, the electrodes being fitted at the top and at the bottom of the column. If there is electrical discontinuity in the reinforcement, additional electrodes can be fitted in additional holes drilled in the concrete to either side of the discontinuity, interconnected to bridge the discontinuity.

The circuitry described can be used in conjunction with a single column but equally in conjunction with a greater number of structural elements.

Although described in relation to a column, the circuitry is equally applicable to other structural elements, including for example roof slabs, floors, beams, walls, bridge piers, jetties, etc.

Depending on the size and configuration of a structure and the outcome of calculations applicable thereto, as well as economic considerations, a structure can be dried using a single electrical circuit interconnecting a numberorall its elements, ora multiplicity of circuits may be required or preferred for different parts of the structure. If desired,the drying can be confined to those parts of a structure exposed to moisture or already affected by concrete deterioration.

As regards reinforced concrete elements that are prestressed, these in addition to the prestressing reinforcement, normally contain other ordinary reinforcement as well, either high tensile or mild steel. Such elements can be treated in like manner as described above, with the electrodes connected to the ordinary reinforcement and notthe high tensile stressed tendons.

It will be appreciated that many modifications may be madetothespecificform of the invention described above, without departing from the scope ofthe invention.Thus, e.g. a readilytransportable welding generator would, for many applications, be quite suitable in place of a rectifier and/or transformer.

Claims (20)

1. Methodforthe preservation ofareinforced concrete structure having structural elements whereofthe concrete shows, or by reason of its composition is susceptible to, deterioration by adverse chemical reaction occurring within the concrete, the method including the steps of heating the concrete by the flow of electrical current thereby to dry itto the sufficiently dry state (as that phrase is hereinbefore defined), and subsequently maintaining it in the sufficiently dry state.

2. Method according to claim 1 whereinthe heating is such that the temperature of the concrete at its surfaces does not exceed 30".

3. Method according to claim 2 wherein the heating is such that the temperature of the heat source in contact with the concrete does not exceed 55 .

4. Method according to any preceding claim which includes the steps of monitoring the temperature adjacentthe heat source and/or atthe surfaces ofthe concrete, and varying the heating in dependence on such monitoring.

5. Method according to any one ofthe preceding claims which includes the steps of monitoring the moisture content within the concrete, and varying the heating in dependence on such monitoring.

6. Method according to any one of the preceding claims wherein the current is switched on and offto prevent overheating and/or to keep the concrete in the sufficiently dry state.

7. Methodforthe preservation of a reinforced concrete structure which includes the step of heating thestructure by passing electrical current along metal reinforcement extending along itsstructural elements.

8. Method according to claim 7 which includes the further steps of locating from the surface of the concrete the reinforcement within, making at least one hole near each of opposite extremities of a length of the structural element to a depth to contact a reinforcement rod (or a link connecting such rods), and making electrical connection between the rod or linkinthat hole and an electrical conductortoinclude the rod in an electrical circuitfor passing current along the rod to heat it.

9. Method according to claim 7 or claim 8 wherein the currentflows along a plurality of reinforcing rods interconnected by connecting links.

10. Method according to any one of claims 1 to 6 in combination with any one of claims 7 to 9.

11. The combination which includes a reinforced concrete structure having structural elements whereof the concrete shows, or by reason of its composition is susceptible to, deterioration by adverse chemical reaction, and means for heating the concrete by the flow of electrical current to dry it to the sufficiently dry state and for maintaining it in the sufficiently dry state.

12. Combination according to claim 11 also including monitoring means for monitoring the temperature adjacentthe heat source and/or atthe surfaces of the concrete, and means for varying the heating in dependence on such monitoring.

13. Combination according to claim 12 including control devices for varying the current intensity.

14. Combination according to claim 12 or claim 13 including automatic control means underthe control ofthe monitoring means and arranged to vary the current intensity and or to switch the current on and off automatically.

15. Thecombinationofa reinforced concrete structure and means for passing electrical current along the metal reinforcement extending along its structural elements, in order to apply heat to the concrete.

16. Combination according to claim 15including an electrical conductor making electrical connection with a reinforcing rod near each of opposite extremities of a length of a structural element, parts of that electrical conductor extending along holes made to a depth to contactthat rod (or a link connecting such rods), the electrical conductor forming part of an electrical circuit for passing current along the rod to heat it.

17. Combination according to claim 15 or claim 16 wherein the entire length of the structural element is included in the circuit by connecting in series shorter lengths making up the entire length.

18. Combination according to any one of claims 11 to 14 in combination with any one of claims 15to 17.

19. Method forthe preservation of a reinforced concrete structure comprising the steps substantially as hereinbefore described with reference to the accompanying drawings.

20. The combination of a reinforced concrete structure and means for heating it substantially as shown in, and hereinbefore described with reference to, the accompanying drawing.