Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
3 ~ 27~3~
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.
79S
- 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
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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.
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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
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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
7~
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
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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
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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:-
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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.
