GB2157441A - A device for determining corrosion of reinforcing members in concrete - Google Patents

Abstract

A device for determining corrosion of a reinforcing member (7) in concrete (8) comprises an electrolytic half cell having an electrode (10) in contact with an electrolyte (11). The electrolyte (11) is in contact with a wheel (16), around which is provided absorbent material (15), by way of a porous plug (13) or other flow restricting device. A voltmeter (14) is connected between the electrode (11) and the reinforcement (7). As the wheel is rolled along the path, a continuous scan is obtained of the potential of a full cell formed by the half cell, the concrete and the reinforcement, said potential being indicative of such corrosion. <IMAGE>

Description

SPECIFICATION A device for determining corrosion in concrete This invention relates to a device for determining corrosion in reinforced concrete.

The corrosion of steel in reinforced concrete is an electrochemical process in which iron is removed at the steel surface (forming an anode) and oxygen is reduced at an area forming a cathode. Fig. 1 shows the process diagrammatically. The process creates a corrosion cell in which anodic and cathodic areas in the region of the steel are at different potentials. Current flows from the cathode to the anode across the potential gradient. By measuring the potential difference it is therefore possible to locate anodic and cathodic areas and to estimate, from the potential gradient, the severity of the corrosion.

In order to measure these potential differences it is known to compare the potential of the steel with a standard reference cell having a known and stable potential, with Copper/ Copper Sulphate being commonly used. By connecting the reference cell to the area of reinforcement the potentials can be measured against the reference electrode.

As will be appreciated, an electrical cell consists of two dissimilar 'metals' in a common electrolyte. If the two metals and the common electrolyte are physically separated, two half-cells are created. Bringing the electrolyte of the two half-cells back into contact recreates a full cell. In the case of reinforced concrete and a copper/copper sulphate halfcell, it is the reinforcing bar (rebar) within the concrete 'electrolyte' that forms the second half of the cell. By bringing the copper sulphate into contact with the concrete, a full cell is created. The fact that the electrolytes are different has a small but negligible effect upon the magnitude of the cell's potential.

In any full cell, both metals contribute to the overall cell voltage. The part of the potential contributed by the copper/copper sulphate half-cell is reasonably constant with time and temperature. Thus any variation in the potential of the rebar/concrete/copper sulphate/copper cell can be attributed to the rebar/concrete half-cell. The potential contributed by the rebar/concrete half-cell depends on the 'state' of the rebar, corroding steel having a very different potential from noncorroding steel. Thus the resulting potential produced by the rebar/concrete and copper/copper sulphate cell can be related to the 'corrosive' state of the rebar.

A typical apparatus is shown in Fig. 3, comprising a tube 1 having a porous wooden plug 2 at one end thereof and a stopper 3 at the other end thereof. Saturated Copper Sulphate solution (CuSO4) is contained in the tube 1 with which a copper electrode 5 is in communication. The electrode 5 is connected to one terminal of a voltmeter 6 whose other terminal is connected to a reinforcing steel bar 7 in the concrete 8. The plug 2 is in contact with the concrete 8.

With such an apparatus, potentials are measured at given points on a grid marked on the concrete surface by taking spot readings. The spot readings are then later contoured, using x,y coordinates of the grid, sectioned to indicate where anodic areas are located. One example of this method of recording potentials is shown in Figs. 2a and 2b, where Fig. 2a is a sample of concrete on which a grid has been marked and Fig. 2b is a contour map derived from spot readings taken at the points marked in Fig. 2a.

This known method suffers from a number of disadvantages which limit its widespread use as corrosion monitoring means: (1) The method is time consuming: individual spot readings must be taken, noted down on sheets and then analysed later in a laboratory; while this may be satisfactory for a small column or beam where, say, 200 readings are required for contouring, for a structural survey of a larger structure such as an offshore platform for example, the method can only be used in localised areas rather than for general overall inspection.

(2) It is possible to miss areas of localised very high or low potentials, for example corrosion at cracks, honeycombed areas, or areas of localised low cover where steel is corroding, if a wide grid is used.

(3) Manual processing of results is time consuming: computer programs have been developed for processing results but their use is limited as data has to be manually keyed into the program.

(4) The method is not capable of very fine resolution and therefore cannot precisely locate defects.

According to the present invention there is provided a device, for determining corrosion of a reinforcing member in concrete, comprising means providing an electrolytic half cell and including a container, a fluid within the container and a device which restricts flow of the fluid from the container to the surface of the concrete when the device is in use, the restricted flow of fluid enabling the half cell to form an electrolytic full cell with the concrete and the reinforcing member, the potential of the full cell being indicative of said corrosion, the device also including an absorbent member disposed to receive said restricted flow in use of the device and to be in sliding or rolling contact with the surface to enable the device to operate during continuous movement of the device along the surface.

The present invention thus uses an absorbent member, movable along the concrete to provide an intermediate contact between the half cell and the concrete whereby continuous measurement of potential along a path of the concrete may be effected.

It will be appreciated that the term "reinforcing member" used herein includes any member wholly or partially embedded in concrete, and, in particular, includes prestressing and other fortifying members.

In one embodiment, the absorbent member is carried around the periphery of a rotatable member. An array of such devices, mounted on a suitable framework, wide enough to cover for example a lane of a road, could be towed behind a vehicle to provide a rapid means for measuring 'corrosion' potentials of, for example, bridge deck concrete. No marking of a grid would be necessary, the spacing between the devices determining the resolution widthways and an 'infinite' resolution being obtained in the direction of travel. Distance measurement, in the direction of travel, could be accomplished by attaching a shaftencoder to an axle of one of the devices or to an additional wheel.

It would also be possible to obtain a continuous potential scan by pulling the cell shown in Fig. 3 along the surface of the concrete.

However, this would not give very satisfactory results since: the porous wooden plug would wear quickly and would affect the concrete surface; and it is difficult to control the amount of fluid seeping through the plug if it is in continuous contact with the ground.

To overcome these difficulties, it is possible to provide an absorbent member, which may in its simplest form be a "shoe", between the plug and the concrete. The absorbent member may be made of a foam rubber and provide a low friction moving contact with the surface of the concrete, kept moist as a result of its absorbent properties. The absorbent member is preferably kept small to approximate as far as possible to a point contact.

Thus, according to another aspect of the present invention there is provided a method of determining corrosion of a reinforcing member in concrete, the method comprising: moving a device including an electrolytic half cell continuously along the surface of the concrete so that a portion of the device is always in contact with the surface to provide a pathway for fluid from the half cell to the surface so that there is formed an electrolytic full cell comprising the reinforcing member, the concrete, the fluid and the half cell; and measuring the potential of the full cell so formed to indicate such corrosion.

There follows a brief description of the accompanying drawings: Figure 1 is a diagram of the electrochemical process of corrosion in reinforced concrete; Figures 2a and 2b illustrate one example of a known method of recording potentials; Figure 3 is a diagram of a known apparatus for measuring potential; Figure 4 is a diagram of a device according to one embodiment of the present invention; Figure 5 is a diagrammatic front view of a device according to another embodiment of the present invention; Figure 6 is a diagrammatic view from the side of the device shown in Fig. 5; Figure 7 is a section through the device shown in Figs. 5 and 6; Figures 8a to 8e illustrate the conditions and results of a first test carried out using the device; Figures 9a to 9c illustrate the conditions and results of a second test;; Figures 10a to 10e illustrate the results of a third test; Figures 11 and 12 are side views of a further embodiment of the present invention; and Figure 13 is a section through a further embodiment of the present invention.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to Figs. 4 to 1 2.

Fig. 4 illustrates diagrammatically one embodiment of the present invention. The device comprises a Cu/CuSO4 half-cell consisting of a pure copper rod 10 immersed in a saturated solution of copper sulphate 11 as an electrolyte. An electrical connection 1 2 to a voltmeter 14 is made to the copper rod 10, and a porous wooden plug 1 3 provides contact between the copper sulphate electrolyte 11 and the concrete surface 8 by way of an absorbent material 1 5 mounted around the periphery of a wheel 16.

In this embodiment, the device has a perspex (R.T.M.) wheel 1 6 with a plastic foam rim forming the absorbent material 1 5. To use the device the absorbent material is damped with water and the half-cell 10, 11 is placed in contact therewith. The wet absorbent material provides an electrically conductive path between the top 1 3a of the half-cell and the concrete surface 8. However, with this embodiment, difficulties arise in keeping the absorbent material 1 5 damp and in reducing the wear to the absorbent material 1 5 due to frictional contact of the probe 1 3.

An improved embodiment of the device of the present invention which overcomes these difficulties is shown in Figs. 5 to 7. This device uses, as an alternative to the liquidstate or 'WET' half-cell used in the device of Fig. 4, a solid-state or 'DRY' half-cell e.g. a cell consisting of silver and silver chloride.

Such a 'DRY' half-cell 1 7 consists of a silver rod coated with fused silver chloride salt. The silver/silver chloride cell has a much faster dynamic, electrical response than the cop per/copper sulphate cell. Water, or a weak saline solution, is then used as an intermediate contact between the silver chloride of the half-cell and the concrete surface 8; it is important that the water does not come into contact with the silver rod. The introduction of water between the concrete and silver chloride adds a negligibly small amount to the overall potential measured. Where water is used, the potential of the Ag/AgCI half cell may vary, and it is therefore preferable to use a solution of an ionic chloride to ensure stable operation of the cell.

In Figs. 5 to 7, like parts to those shown in Fig. 4 are designated by primed like numerals. The device comprises a handle 1 8 having a forked end which supports an axle 1 9 of the wheel 16'. The axle 1 9 is hollow and is provided with an opening 20 to allow water to flow, via a plastic tube 21, to the hub of the wheel 16'. The hub of the wheel 16' communicates with the wheel rim, which is surrounded by an absorbent material 1 5' such as felt by way of hollow spokes 22. In this way, water fed to the device in the direction of arrow W, passes to the absorbent material 15' around the wheel rim. O-ring seals 23 are provided so that as the wheel 16, rotates about the axle 1 9 no water can leak sideways through a hub/axle bearing.

The 'DRY' half-cell 1 7 is arranged so that only its tip, that is a region of AgCI, is in contact with the water flow W through the device. The half cell 1 7 is connected to a voltmeter, by way of a cable 24, as illustrated in Fig. 4.

The use of felt as the absorbent material is not entirely satisfactory as, even with low water supply pressures, the absorbency of the felt is not sufficient to prevent water from flooding the surface of the reinforced concrete whose potential is to be measured. To overcome this a more viscous fluid such as a solution mentioned above may be used in place of the water to provide an electrical contact between the half-cell and the surface of the reinforced concrete, and/or a denser more absorbent material may be used in place of felt.

It will be appreciated that it is possible to use the device singly or in a multiple device assembly, e.g. a hand operated device of about eight wheels mounted side-by-side with 100 mm spacing. This would enable a number of devices to be towed behind a vehicle and provide a means of rapidly scanning reinforced concrete roadways and/or bridge decks etc. Such a trailer assembly might require the provision of a source of water to wet the roadway ahead of the wheels in order to maintain the tyre rims of the wheels saturated with water.

It is also possible to construct a device to be drawn by or pushed by a motorised vehicle.

With the device of the present invention, to make a continuous measurement of reinforcement potential of reinforced concrete, the wheel 16' is rolled across the concrete surface 8 and a continuous potential scan made. A small distance recorder can be placed at the axis of the wheel 16' to record distance travelled.

A further embodiment of the present invention is shown in Figs. 11 and 1 2.

A wheel 31, having a circumference of 600 mm is free to rotate about a bearing coupled to a shaft 32. The rotating wheel 31 drives a shaft-encoder 33 which generates 1 50 pulses/rev. Around the rim of the wheel is a tyre 34 of water absorbent foam plastics material.

A sintered silver/silver chloride half-cell 3 (for example such as is described in British Patent Application No. 84 29046) is on one side of the shaft, in a plastic holder 36 which screws into the body of a plastic cell chamber 37 so that only the silver chloride tip is in contact with the electrolyte contained in the chamber 37.

There is also located within the chamber a length of sintered water absorbent nylon stylus (the tip 38 of which can just be seen in Fig. 11) which projects from the chamber 37 to make a frictional contact with the foam plastic tyre 34. The sintered nylon stylus may be pushed out from the body of the chamber 37 by means of a screw 39.

On the other side of the shaft 32 (Fig. 12) there is a reservoir chamber 40 in the form of a recess in the body of the shaft 32 and having a clear plastics cover. Holes are drilled through the shaft such that the electrolyte within the reservoir chamber 40 can flow into the cell chamber 37. The reservoir chamber is so designed as to maintain the cell chamber 37 full of electrolyte whatever the attitude of the whole device. The reservoir chamber is able to be filled through an orifice (not shown) in the shaft of the device, which is then sealed when the chamber is full.

A handle 41 of the device is hollow and able to accommodate a small encapsulated, high input impedance, buffer amplifier. The input to this amplifier is terminated in a small smb socket 42, into which is plugged an smb plug 43 on the end of half-cell cable 47. The cable 47 of the half-cell is screened by the output signal from the buffer amplifier.

An output cable 44 from the shaft encoder 33 passes through a gland in the handle 41 of the device, on the other side to that bearing the smb socket 1 2. Power and signal cables for the buffer amplifier and shaft-encoder are terminated within the handle by a 6pin chassis mounting plug 45. External circuitry (to be described later) is connected to this plug 15.

The handle 41 of the device may be unscrewed from the shaft 32 of the device, providing access to the buffer amplifier. A locking ring 46 is screwed on to lock the hollow handle onto the shaft of the device.

In operation, a conductive path from the half-cell 35 to the surface of the concrete is effected by the electrolyte within the cell and reservoir chamber, the sintered nylon stylus 38 extending from the half-chamber to the wheel tyre 34, and the water or solution absorbed by the tyre.

The device could be provided with a control unit to enable the shaft of the device to maintain a constant angle to the concrete surface, thus eliminating errors in the distance measurement which would arise due to the shaft rotating about the wheel rather than the wheel rotating at the end of the shaft. A support structure would be provided to enable the device to be extended and used overhead, or up and down vertical walls.

There is provided a + 5 volt power source for the buffer amplifier, and a + 5 volt source for the shaft-encoder 33 connected to the device via the plug 45. When rotated, the shaft-encoder 33 produces two pulse trains 90 out of phase with each other. If the rotational sense of the wheel is reversed one waveform reverses through 180 relative to the other. Signal conditioning electronics can be used to combine the two pulse trains in a suitable manner to result in a pulse train of 600 pulses/rev (4 X 150, the number of pulses/rev of each waveform) and a directional sense signal. The electronics can be designed to produce this pulse train for only one direction of rotation of the wheel. If rotated in the reverse direction no pulse train would then be produced.The directional sense may however, be reversed by a switch. With the shaftencoder and associated circuitry producing 600 pulses/rev. of the wheel and with a wheel circumference of 600 mm, 1 pulse is produced for every millimetre travelled. The circuitry can include a-1 0 counter to reduce the number of pulses to 60/rev thus giving an alternative output of 1 pulse/cm. Analogue circuitry may also be included to provide a low-pass filter of 1 OHz pass-band and 40dB attenuation at 50Hz. The signal from the Ag/AgCI half-cell 35 is fed to the buffer amplifier, the output of which is fed to a potential measuring unit (not shown) which is also connected, via a cable (not shown), to the reinforcing bar in the concrete (as in Fig.

4).

Data output from the potential wheel is recorded using a chart reader (which uses a stepper-motor, driven by a train of pulses, to drive the paper chart) with a remote drive input. The train of pulses from the shaftencoder and associated circuitry is fed into the remote drive input so that the chart paper is effectively driven forward in relation to the rotation of the wheel. Using a chart recorder that steps the paper forward at 0.1 mm/pulse, the paper may be moved forward 100 mm or 10 mm per metre travelled by the wheel, depending upon whether the circuitry is arranged to generate 1 pulse/mm or 1 pulse/cm. The analogue voltage derived from the cell (buffered and filtered by the analogue circuitry) is used to drive a pen of the chart recorder.Thus, as the wheel is rolled across the surface of the concrete a chart is produced of the measured 'corrosion voltage' versus distance travelled by the wheel.

It is also possible to record data using a microcomputer with interface circuitry to digitize the buffered analogue voltage from the cell for every pulse generated by the shaft encoder. An 8-bit digital word is stored in a buffer memory and output down a serial line to the microcomputer. The digital input data may fill up the buffer memory at a faster rate than it is emptied down the serial line. This wll allow data to be collected by the wheel rapidly whilst controlling the serial output rate to match the capability of the microcomputer.

The memory buffer will be capable of storing enough data from one scan of the potential wheel and of giving an indication of the buffer space available at any time, to warn a user of the device when the buffer memory capability is about to be exceeded.

The results of test made using one embodiment of the device are given below: Example I An area A of a concrete block 8' was ponded in seawater to artifically produce an effect similar to that produced by corrosion due to reinforcing bars. The block is shown in Fig. 8a, with dimension 1 being 350 mm and the ponded area representing a circle in the centre of the block of approximately 75 mm diameter.

Figs. 8b to 8e illustrate the resulting traces along lines i to iv in Fig. 8a. It will be noted that, although the active portion of the corrosion A was small relative to the area of the block 8, the scan located the active portion A successfully.

Example II A site test was carried out on a concrete 'pier' wall 8" in Fig. 9a, having a height, h, of about 1 m. Figures 9b and 9c illustrate the resulting traces of scans along line v and vi.

It will be noted that the scans are virtually identical, the potential gradients are substantially the same, and the most negative potentials were recorded at the base of the concrete pier wall (ground level).

Example 111 Samples of a reinforced concrete beam exposed in the splash zone facility at Portland harbour were inspected using the device.

The surface of the beam was subject to four parallel scans along lines of equidistant spacing.

The four potential profiles for the beam are shown in Figs. 10a to 1 Od: Fig. 10e shows conventional 'spot' data (converted from Cu/ CuSO4 to Ag/AgCI).

Figs. 1 0a to 1 Od show a very localised anode on the beam. This anode is not indicated with such clarity by conventional spot data in Fig. 10e.

The results obtained from this test indicated the following advantages over conventional techniques: (a) the continuous monitoring along the length of the beam has much greater resolution than the spot reading system; (b) it is possible to miss a very negative reading using the spot technique if the area is very localised; and (c) it is possible to survey an area in about one tenth of the time taken using a conventional spot reading technique.

Fig. 1 3 shows a further embodiment of the present invention, in which the absorbent member comprises a "shoe" of a foam plastics material. This embodiment has a support 57 for keeping the device directly over the concrete under test. A wheel 51 is rotatably mounted to the support, the shaft of the wheel 51 having a shaft encoder 52 as before. A fluid chamber 56 houses the fluid for a silver/silver chloride half cell 53. An absorbent wick 54 restricts the flow of fluid from the chamber to the "shoe" 55. As the wheel is rolled along the concrete surface, the shoe 55 is drawn along to enable a potential scan to be obtained as described earlier.

Claims (11)

1. A device, for determining corrosion of a reinforcing member in concrete, comprising means providing an electrolytic half cell and including a container, a fluid within the container and a device which restricts flow of the fluid from the container to the surface of the concrete when the device is in use, the restricted flow of fluid enabling the half cell to form an electrolytic full cell with the concrete and the reinforcing member, the potential of the full cell being indicative of said corrosion, the device also including an absorbent member disposed to receive said restricted flow in use of the device and to be in sliding or rolling contact with the surface to enable the device to operate during continuous movement of the device along the surface.

2. A device as claimed in claim 1, wherein the absorbent member comprises absorbent material supported for rotation.

3. A device as claimed in claim 1 or 2, further comprising a rotatable member mounted for rotation with respect to a handle of the device.

4. A device as claimed in claims 2 and 3, in which the absorbent member is provided about the periphery of the rotatable member.

5. A device as claimed in claim 4 in which the device for restricting flow is provided by a duct extending from the container to the periphery of the rotatable member.

6. A device as claimed in claim 3, 4 or 5, further comprising means responsive to rotation of the rotatable member to determine distance travelled by the device along the concrete.

7. A device as claimed in any preceding claim in which the half cell comprises a dry cell, and the fluid comprises a polar or ionic compound.

8. A device as claimed in Claim 7, in which the half cell comprises silver/silver chloride, and the fluid is a solution containing an ionic chloride compound.

9. A device as claimed in any preceding claim including means for converting an analogue value of said potential to a digital value, and a buffer for storing successive digital values from the analogue to digital converting means and supplying them to a microprocessor at a rate dependent on the operating capability of the microprocessor.

1 0. A device for determining corrosion of a reinforcing member in concrete substantially as hereinbefore described, with reference to, and as shown in, Fig. 4 or Figs. 5 to 7, of the accompanying drawings.

11. A method of determining corrosion of a reinforcing member in concrete, and method comprising: moving a device including an electrolytic half cell continuously along the surface of the concrete so that a portion of the device is always in contact with the surface to provide a pathway for fluid from the half cell to the surface so that there is formed an electrolytic full cell comprising the reinforcing member, the concrete, the fluid and the half cell; and measuring the potential of the full cell so formed to indicate such corrosion.

1 2. A method as claimed in claim 11, further comprising the step of positioning at said portion of the device an absorbent member capable of absorbing said fluid to provide said pathway.

1 3. A method of determining corrosion in concrete, substantially as hereinbefore described with reference to Figs. 4 to 1 2 of the accompanying drawings.