CA1222796A - Device for determining corrosion in concrete - Google Patents

Abstract

ABSTRACT

A device for determining corrosion in reinforced concrete comprises an electrolytic half cell having an electrode (10) in contact with an electrolyte (11). The electrolyte 111) is in contact with a wheel (16), around which is provided absorbent material (15), by way of a porous plug (13). A voltmeter (14) is connected between the electrode (11) and the reinforcement. As the wheel i 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.

Description

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TITLE OF l'HE INVENTION
. .
A DEVICE FOR DETERM~NING CORROSION IN CONCRETE
TECHNICAL FIELD
This invention relates to a devlce for determining corrosion in xeinforced concrete.
BAC~CGROUND OF II~E INVENTION
me corrosion of steel in reinforced concrete is an electrochemical ; process in which iron is removed at the steel surface-(formung an an~de) and oxygen is reduced at an area formln~ a ca~hode.
- ~ ~he prDce~s creates a .
corr~si~D cell iD which anodic ~n~ cathodic ~reas in the regi~l of the steel are at ~i~ferent potentials. Current flows from the ~athode to tlle a3Anode ~cro~ ~he poten'cial gradient. ~y measuring the pDtenti~l difference it is therefore possible ~o locate ~nvdic an~ Icathodic axeas ~nd to ~stimate~ ~rom the ~otential ~radientp the ~ ; ~everity .of ~he co,rrofiion.
In order to ~ea~ure these potential ~ifferences it is known to ~ompare the ~otentlal of the ~teel with a - standard reference cell having a known ~and stable potential, with Copper~Copper Sulphate being conunonly used. By connec'cing th~e reference cell to the area of reinforcement the p~tent~al~ can be measured again~t the reference electrode.
i~6 will be appreciated, ~n electriç~l cell .
consist8 of two di~similar '~netals ' in ~ common electrolyteO If the ~wc) metals ~nd the cc:mmon ~lectrolyte ~re phy~ically 6eparated, ~wo hal~-cells are created~ ~ringing the electrolyte of the two hal~-cell6 .back into con~act re-create~ a full cell. In the ~ase o~
30 reinforced concr~te and ~ copper/copper ~ulphate half-cell, it is the reinforcing bar ~rebar) within the concrete 'electrolyte' that forms t~e seco~d half of the cellO By bringing the copper ~ulphate into contact with the ccncrete, a full cell i~ created. The fact that the electrolytes are different has a ~mall but ne~ligible effect upon the magnitude of the cell'~ potential.
In any full cell, both metals contribute to the .

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overall cell voltage. Ihe part of the potentic~l contrikuted by the copper/copper sulphate half-cell is reasonably constant with time an~ temperature. ~hus any variation in the potential of the rebar/concrete/copper sl~phate/copper cell can be attributed to the rebar/concrete half-cell. The potential c~ntributed by the rebar/concrete half-cell depends on the Istate' of the rebar, corrod mg steel having a very different potential from non-corroding 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 known apparatus for measuring potentials ccY~prises a tube having a porous wooden plug at one end thereof and a stopper at the other end thereofO Saturated Copper Sulphate solution (CuSO4) is contained in the tube with which a copper electrode is in communication. The electrode is connected to one terminal of a voltmeter whose other terminal is connec-ted to a reinforcing steel bar in the concrete. The plug is in contact wi-th the concrete.
With such a known 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.

~his kn~wn method suffers from a number of disadvantages which limit its widespread use as corrosion monitoring ~eans:
(1) ~he method is time consuming: individual spot readings must ~e taken, noted down on sheets and then analysed later in a laboratory; while this may be

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1 satisfactory for a ~mall column or beam where, ~ay, 2D0 readings are ~eguired for contouring, for a ~tructural survey of a larger structure ~uch as an offshore platform for example, the meth~d can ~nly be ~sed in lccalised areas rather han for ~eneral ~verall inspection. -2) It is p~ssible to mi~s areas of localised very high ~r l~w p~tentials, for ~xample corrosion at cracks, honeycombed areas, ~r areai of locali~ed low ~over where steel i~ ~corrodiDg, if ~ ~ide grid i~ used.
1~ 3) ~anual processing o~ ~esults is!time consuming: c~mputer programs have ~een ~evel~ped for processing re~ult~ 3but ~ei~ e is limited as data has to ~e manually ~eyed into ~he ~r~gram.

4) The meth~d is not capable of very fine resolution and therefore cannot precisely locate defects.
~ S~MMARY OF THE INVENTION
According to the present invention there is provided a device, for determining corrosion of a reinforcing ~ember in concrete, comprising means providing an ~lectrolytic hal~ cell and including a container, a fluid within the container ~nd a device which restrict~ flow of the fluid from the container to the ~urface of the ~oncrete when the ~evice i~ in use, the restricted ~l~w ~f fluid enabling ~he ~alf cell to orm ~n electr~lytic full icell with ~he concrete and the reinforcing member, the potential of the ~ull ~ell b~ing indi~ative of said corrQsion, the device al~o including an absorbent member di~po~ed .~o receive said ~estricted ~low i~ u~e ~f the device an~ ~o be in ~lidin~ or rollin~
contact with the ~urface to enable ~he device to operate during continuous movement of the device along the surf a ce .
The present invention thus uses an absorbent-member, movable along the concrete to provide an intermediate contac~ between ~he half cell and the concrete whereby continuous measurement of potential along a path of the concrete may be effected.

- 3a 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 mambers.
In one embodiment, the absorbent member is carried 7~1~

1 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 wouid 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 ~f travel, could be accomplished by attaching a shaft-encoder to an axle o~
one of the devices or to an additional wheel.
It would also be possible to obtain a continuous potential ~can by pulling the k~n apparatus for measur~ng poten~als along the surface of the concrete. However, this would not give very sati6factory results since:
the porous wooden plug would wear ~uickly 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 mémber, which may in its si~plest form be a ~shoe", between the plug and the concrete. The absorbent member may be made of a foam rubber and provide a low frictivn moving contact with the surface of the concrete, kept moist as a result of its absorbent properties. The absorbent member is preferably kept ~mall 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 CQn~aCt with the surface to provide a pathway for fluid from the half _ 5 _ ~ ~227~6 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.
BRIEF DESCRIPTION OF THE DR~WINGS
There follows a brief description of the accompanying drawings:-Figure 1 is a diagram of the electrochemical process of corrosionin reinforced concrete;
Fi~ures 2a and 2b illustrated 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 diagra~matic view from the side of the device shown in Figure 5;
Figure 7 is a section through the device shown in Fisures 5 and 6;
Figures 8a to 8e illustrate the conditions and results of a first test carried out using ~he 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 ll and 12 are side views of a further emhod.i~ent of the present invention; and Fig~e 13 is a section through a fur~her embod.iment of the present invention.

-5a--DEgCRlPT.ION OF Tl-E PREFE$RlD EMBODIM~T

Fiyure 1 diagrammatically shows the electrochemical process of the corrosion of steel in reinforced concrete in which iron is removed at the steel surface.

A known ap~aratus is shown in Figure 3, comprising a tube 1 having a porous wooden pluy 2 at one end thereof and a stopper 3 at the other end -thereof. Saturated Cbpper Sulpl~ate solution (C~1SO4) is contained in the tube 1 with which a copper electrode

5 is in con~nication. 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. qhe plug 2 is in ~)ntact with the concrete 8.
Using the kno~l apparatus potentials are measured.
One example of this method of reo~rding potentials is shown in Figures 2a and 2b, where Fi~ure 2a is a sample o~ concrete on which a grid has been marked and Figure 2b is a contour map derived Ercm spot readings taken at the points marked in Fig~e 2a.

Eor 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 Figures 4 to 12.
Fi~ure 4 illustrates diayrammatically one embodir~nt of the present invention. Ihe device comprises a Cu CuSO~ half-cell o~nsisting oE a pure copper rod 1 immersed in a saturated solution of copper ~L222~7~6

- 6 -1 sulphate 11 as an electrolyte. An electrical connection 12 to a voltmeter 14 is made to the copper rod 10, and a porous wooden plug 13 provides contact between the copper sulphate electrolyte 11 and the concrete surface 8 by way of an absorbent material 15 mounted around the periphery of a wheel 16.
In this embodiment, the device has a perspex wheel 16 with a plastic foam rim forming the absorbent material 15. 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 13a of the half-cell and the concrete surface 8. However, with this embodiment, difficulties arise in keeping the absorbent material 15 damp and in reducing the wear to the absorbent material 15 due to frictional contact of the probe 13.
An improved embodiment of the device of the present invention which overcomes these difficulties is shown in Figures 5 to 7. This device uses, as an alternative to the liquid-state or 'WET' half-cell used in the device of Figure 4, a solid-state or 'DRY' half-cell e.g. a cell consisting of silver and silver chloride. Such a 'DRY' half-cell 17 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 copper~copper sulphate cell.
Water, or a weak saline solution, is then used as an intermediate contact between the silver chloride of the 3~ 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/AgCl half cell may vary, and it is therefore preferable to use a solution of an ionic chloride to ensure stable operation of the cell.

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, 7 1 In Figures 5 to 7,, like parts to those shown in Figure 4 are designated by primed like numerals. The device comprises a handle 18 having a forked end which supports an axle 19 of the wheel 16'. The axle 19 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 15' 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 lS' around the wheel rim. 0-ring seals 23 are provided 50 that as the wheel 15' rotates about the axle 19 no water can leak sideways through a hub/axle bearing.
1~ The 'DRY' half-cell 17 is arranged so that only its tip, that is a,region of AgCl, i6 in contact with the water flow W through the device. The half cell 17 is connected to a voltmeter, by way of a cable ~4, as illustrated in Figure 4.
The use of felt as the absorbent material is not entirely ~atisfactory as, even with low water supply pressures, the absorbency of the felt is not sufficient to prevent water from flQoding 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 proYide 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

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8_ l to wet the roadway ahead of the wheels in order to maintain the tyre rims of the wheels ~aturated 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 xolled across the concrete surface B and a continuous potential scan made.
l~ A small distance recorder can ~e placed at the axis of the wheel 16' to record distance travelled~
A further embodiment of the present invention is shown in Figures ll and 12.
A wheel 31, having a circumference of 600 mm is free to rotate about a beariny coupled to a shaft 32.
The rotatiny wheel 31 drives a shaft-encoder 33 which generates 150 pulses/rev. Around the rim of the wheel is a tyre 34 of water absorbent foam plastics material.
A sintered ~ilver/silver chloride half-cell 3 Ifor ,example such as is des~ribed in British Patent Application Publication No~ 2,169~,410 ) 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 3B 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 haviny a clear plastics cover.
Holes are drilled through the shaft such that the electrolyte hithin the reservoir chamber 40 can flow into 2~9~
g 1 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 illed through an orifice ~not shown) in the shaft of the device, which is then ~ealed 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 i5 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 12. Power and signal cables for the buffer amplifier and shaft-encoder are terminated within the handle by a 6-pin 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 eliminatinq errors in the distance measurement which would arise due to the shaft rota~ing 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 2;~ 796 used overhead, or up and down verti~al walls.
There is pr~vided a + 5 volt power so~rce for the buffer amplifier, and a ~ 5 volt s~urce f~r the shaft-encoder 33 connected to the device via the plug 45. When rotated, the 5 shaft-encoder 33 produces tw~ pulse trains 90~ out of phase wi~h each other. lf the rDtational ~;ense of the wheel is reversed one ~aveform rever~e~ through 180~ !
relati~Te ~o the other. ~ignal ~onditionin3 ~lectroni~
can be u~ed to combi~e the ~wo pulse trains in a Isuitable 10 manner to refiult in a pul e ~rain o~ pulse~/rev t4 x 15~, ~he number Df pul6es/rev ~f ~each waveform) ~n~
directional sen~e ~lgnal. The ~lectronic~ ~can ~e .designed ~o produce thi~ pul~e train for only ~ne direction ~ rotation ~f the ~heel. If rctate~ in the 15 reverse ~irecti~n no pul~e ~rain would then ~e prod~ced.
~he ~irectional.~ense may however, be Iever6ed ~y ~
~wi~c~. With the shaft-encoder and a~ocisted circuitry ~roducing ~0 pulse~rev~ lof the wheel ,and ~ith a wheel circumference of 600 mm, 1 pul6e i6 produced for every 20 millimetre travelled. The circuitry can include ~ - 10 counter to reduce the number ~f pul~e6 to 60/rev ~hus ~iving an alternative output of 1 ~ul~e/cm. ~nalogue circuitry may also be included to prDvide a . ow-pafis a filter of lO~z pass band and 40dB 2Ittenuation at SOE~z.
25 The signal from the Ag/AgCl half-cell 35 i~ fed to the ~uffer amplifier 7 ~he output of which i6 ~ed to a potential mea~uring unit lnot ~hown) which i6 ~160 conne~ted, via a cable tnot fihown), to the reinforcing ~ar in the concrete tas in ~ig. 4 ) ~
Data output from the potential wheel is recorded using a chart recorder (which uses a stepper-motor, driven ~y a train of pulses, to drive the paper chart) with a remote drive input. ~rhe train of pulsec from the shaft-encoder and associated circuitry is fed into the 35 remote drive input so that the chart paper is effectively driven forward in relation to the rotation of the wheel.
~sing a chart recorder that steps the paper forward at a2~%7~6 1O.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.
10It 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 will 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.
25The results of tests made using one embodiment of the device are given below:-~2~7~;

ExamPle I
An area A of a concrete block 8' was ponded in seawater to artifically produce an efect similar to that produced by corrosion due to reinforcing bars. The block is shown in Figure 8a, with d~sion 1 being 350 mm and the ponded area represent-ing a circle in the centre of $he block of approximately 75 mm diameter.
Pigures 8b to 8e illustrate the resulting traces along lines i to iv in ~igure 8a. It will ~e n~ted that, although the active portion of the c~rrosion A was small relative to the area of the block 8~the scan located the active portion A
successfully.
_ ample II
A site test was carried out on a concrete 'pier' wall 8" in Figure 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 m~st negative potentials were recorded at the base of the concrete pier wall ~ground -level).
~5 Example III
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 Figures lOa to lOd: Pigure lOe shows conventional 'spot' data (converted from Cu/CuS04 to Ag~AgCl).
Figures lOa to lOd show a very localised anode on the beam. This anode i5 not indicated with such clarit~ `o~ conventional spot data in Figure lOe.

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~ he resu~ts ~btained from this test indicated the following advantages over conventional techniques:
(a) the continuous monitoring al~ng the len~th of the beam has much greater resolution than the spot reading system;
(b) it is possible to ~iss a ~ery negative reading using the spot technique if the area is very localised; and (c) it is possible to survey an area in ~bout one tenth of the time taken using a conventi~nal spot reading techni~ue.
Figure 13 shows a further embodiment of the present invention, in which the absorbent member comprises a ~5 "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 rest.ricts 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 (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A device for determining corrosion of a reinforcing member in concrete, comprising:
a water-absorbent means disposed to be in substantially continuous contact with the surface of the concrete as the device is moved along the surface of the concrete, an electrolytic half-cell including a) container means for retaining an ionic fluid for the half-cell;
b) means for supplying a potential to the container; and c) an output element having a portion in frictional contact with the water-absorbent means for providing an ionic fluid path from the container means to the water-absorbent means and comprising a member for permitting a restricted flow of said ionic fluid to flow from said container means to said water-absorbent means to prevent excess application of said ionic fluid to the surface of said concrete, the restricted flow being sufficient to provide a conductive path from the device to the surface of the concrete, whereby the half-cell forms with the concrete and reinforcing member a full cell; and measuring means for measuring the output potential of said full cell thereby to determine corrosion of the reinforcing member.

2. A device as claimed in claim 1, in which the output element comprises a porous stylus member.

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

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

5. A device as claimed in claim 1 further comprising a rotatable member having a periphery and being supported for rotation, with the absorbent means provided around the periphery of the rotatable member.

6. A device as claimed in claim 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 claims 1 or 2 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.

8. A device for determining corrosion of a reinforcing member in concrete, comprising:
an absorbent member able to be wetted with water, and disposable to be in substantially continuous contact with the concrete as the device is moved along the surface of the concrete;
cell means providing an electrolytic half cell and including an ionic fluid reservoir;
a porous stylus member for substantially restricting the flow of fluid from said fluid reservoir to said absorbent member to prevent excess application of said fluid to the concrete, yet allowing flow of fluid sufficient to provide an electrical conduction path between the device and the concrete, thus 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 the corrosion of said reinforcing member.

9. A device as claimed in claim 8, in which the half cell comprises silver/silver chloride.

10. A device as claimed in claims 1 or 2, which further comprises:

means for providing an output indicative of distance travelled by the device along the surface of the concrete;
means for providing an output indicative of said potential; and means for displaying said outputs.

11. A method of determining corrosion of a reinforcing member in concrete, the method comprising the steps of:
providing an electrolytic half cell containing ionic fluid and having an output element providing a restricted flow path for ionic fluid from the half cell;
providing a water absorbent member at the periphery of a rotatable structure such that the member is maintained in contact with the surface of the concrete during rotation of said rotatable structure;
interposing said member between the output element and the concrete to provide a conductive path from the half cell to the concrete, so that a full cell is formed comprising said half cell, the water-absorbent member and said concrete;
moving said water-absorbent member along and in contact with the surface of the concrete; and measuring the potential of the full cell so formed.

12. A method as claimed in claim 11, which comprises measuring the angular displacement of the rotatable structure; and recording said potential and angular displacement.

13. Apparatus for determining corrosion of a reinforcing member in concrete, the apparatus comprising:
an electrolytic half cell containing ionic fluid and having an output element providing a restricted flow path for ionic fluid from the half cell;
a water-absorbent member provided at the periphery of a rotatable structure such that the member is maintained in contact with the surface of the concrete during rotation of said rotatable structure and interposed, in use, to provide a conductive path from the half cell to the concrete, so that a full cell is formed comprising said half cell, the water-absorbent member and said concrete;
means for moving said water-absorbent member along and in contact with the surface of the concrete;
means for measuring the potential of the full cell so formed;
means coupled to the rotatable structure to measure its angular displacement; and means for recording said potential and angular displacement.