CA2127607A1 - Realkalization and dechlorination of concrete by surface mounted electrochemical means - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for realkalising and dechlorinating corroded areas of reinforcement steel in concrete and masonry electrochemical means, characterised by an electrode (anode) composed of alkaline salts upon which is superimposed electrically conductive material. When the current is switched on, alkalinity is generated at the cathode (steel bar), by the production of hydroxyl ions, while chloride ions at the cathode (steel bar), being electrical charge carriers, also migrate toward the anode under the influence of the current flow.

Description

21276~7 REALKALIZATION A~ DECHLORINAl`ION
OF CO~lCRE;'rE 8Y SURFACE MOUN'rED
ELECTROCHEMICAL MEANS
' D E S C R I P T I O N
o _ield of the Inventlon T~e present i~vention i~ concerned w~ th ~he treatmen~ of af~ected concrete in the vlci~lty of st~l reinf~rcem~nt by electrochemlcal mQans. Fo~ ~he better understanding o~ the .5 invention herein described, the expr~s~ion "affected concre~e"
means, where the context so requ~re~, concrets or masonry whloh ha~ been sub~ected to carbonatlon ~tt~ck or aggr~sslve ~on ~enetration.

~ ~ around Steel corro~on proble~ occur as th~ result of lt~ tendency to revert to lts lower energy oxe State when i~ is e~posed ~0 mols~ure and oxygen. During this proce~s it de~elops an oxlde sur~ace scale o~ ~UStA A ~11 amount o surfaoe r~3tlng wlll not g~nerally impair the tenslle ~t~ngth, whereas loc~ ed ~lttln~ corro~ion, if ~evere, will have that effect.
In alkaline solutlons, ~teel corro~on m~y be prevented by th~
formatlon of a th~n pessive oxide film. As th~ reQult ~f the presence of cement, the envlr~nment wi~hin concrete ls u~ually gO highly alkallne. Cement when hyd~ated prod~ces an alkallne pore solution wlth a high p~ value which may exceed 13. H~h pH means very small quantitles o hydrogen ion~, whlch, in t~rn, mean~
large quantities o hydroxyl ion~. Such high alkalinlty promote~
~; i the f~rmatlon of a pas~i~e oxide film which prevents fur~her 4S metal dissolutlon.

... . ... .. . ... . . ..

2127~7 Steel remalns pass~e a~ thes~ h~g~ pH-g, the llm~tQd oxldation required to retaln a passive ~llm ha~lng no practical affect on th~ steel or its surro~nding concrete. However, this passivity ~i-will b~ lmpaired by a reduc~ion o~ the p~ to below 11~5, or by ~he prese~e o~ aggressi~e ions such as c~loride ion~ the passive oxide hr~akQ down, allowing metal lons to m~gra~
thro~gh the defect~ve lattice of iron oxide, fres~ oxide depo3its at th~ metal interface will be manifest as rust~ When steel start~ to corrode, the process is accelerated by th~
hydrolysis o~ the corroSion product~ which results ln a ~urther red~ction in the pH a~ t~e site~ of active metal d~s~olution.
Thu3 the corro3:~0n attack on re~nfo~cing steel car~ cont~nue within affected concrete. ~;

Ru~t has a greater volume than th~ ~eel ~rom whlch ~t is derlved. ~hi~ incr~as~ in volume w~ll caus~ tsns~l~ stresses, whlch i allowed to go unchecked, wlll lead to disruption and spalllng o~ the Concre~e with attendan~ r~s~s to t~ pu~lic, and ~oss~bl~ seriou~ de~erioratlon in the strength of the s~uc~ure.
.
Passlv~ film b~eakdown malnly occurs in two way~, ~l~he~ from oarbonatlon, or ~rom the ingreQ~ of solùbl~ chlori~e ion~
Carbonatlon occurs by ~he ing~ess t~rough permeabl~ and cracked conCrete of ca~bon dioxlde, which, when comb~ned with water, attaoks th~ hydratlon ~roduct~ of cement thereby depressin~ the ~ :~
pH o~ the cement matrix at the relnforcement level to a value bolow ~ 9 ' :' . .' .
Chl~ride lons are highly moblle, and ~he ~echanic6 o~ chlorlde induced corrosion are highly complex. In sufflclent quant~tle~
chlorlde ion~ a~ part~oularly pron~ to caus~ corro~ion b~cause ~ .

o~ tt~ir ability to damage the passlvity of the oxide f ~ lm . ~ .
Indeed, lt ha~ been ~ound ~hat even ve~y hlgh pH concrete ~annot .
provide corroslon protectior~ to ~tee~ exposed fo~ long p~xlod~
to chloride-bearing environmen~.

c~nc~pec3 2 ':.,: ' ', ."

,. . 2i27fia7 At ambient te~p~ra~ureg, th~ prOCQs~ of corro~ion i~
predominantly electrochemical. An oxidation re~ction at th~
anode is balanced by the xeduction reactlon at the cathod~, and is accompanied by the flo~ of electron~ between the anode and the cathode. Consequently, an eff$cacious counte~-measur~ i some form o~ electro-chemical intervention.

One meth~d preven~lng iron dlssol~tion is th~ treatment known a~ ca~hodic protection. The principle is th~t a contLn~lng .0 u~lmpeded curr~t flowing a~ a requir~d ~trength f~om th~ new anode to the re~nforcemen~ will overcome the natural corrosion ~urrent, a~d pr~viou~ly anodlc areaY of th~ steel become cathodic.

L5 One ben~ficial side-effect o* cathodic protection is that ~hl~ride io~s mig~a~i~ away from the æte~l t~ward~ the anode.
Another beneficla~ effect i~ thst oxyge~ is converted i~to hydroxyl ions, rai~i~g the pH at the concr~te to steel interface.
One of the ~irst atteMpts to utllise the bene~icial affec~ ~f chloride re~oval, too~ place ln November 1976, when Mr. ~.E.
Slater using a calclum hydroxide ~olutlon electrolyte, ~ucces~fully removed ohloride ion3 f~om s-t~l reinforced co~crete. However, a r~latively high voltage and current ~as necessar~, and the apparatu~ could only b~ u~ed on a horizontal ~lat surface.

Another exa~ple o ~ method ~o~ the removal of chlor~des from steel reinforced concrete i.~ di~lo~ed and clalmed in European Pat~nt No~ ~ 200 428, whe~e low~ v~ltage and curr~nt l~vels ar~
involved, and the temporary external anode net 1~ contalned wlthin electrolytia material whic~ i~ applied by means Of a spray. This treat~en~ may be preceded and ~ollowed by sandblasting; the second occa~ion belng in preparatlon for the ! application of repa~r material~ to the treated concrete.

I:oncrl~p.c3 276~7 A ~et~od for the realkaligation o~ a zone of carbonated steel ln concret~ i~ dlsclosed and clai~ed in European Patent O 264 421. Low ~olte~g8 electr~c currents are ~assed between a ~empo~ry ~nod~ wlthin a sultable ele~trolytlo ~ediu~, such a~
a pa~le~-machQ poultlc~, containing alkallne salt~, ana, th~
~te~l exposed w~thin th~ car~onated concrete to be-treated, ~th each having oppo~ite polari~y, whereby alkaline hydroxyl lons migrate to the reinforce~ent in th~ carbonated zon~.

O sot~ methodcirequlre lengthy and extensive preparatlon, and, areno~ wholly sympathetlc ~o the envlronment, becau~e of the high ~onc~tratlon a~d ~o~umulatlon of waste material~ resulting from the ~ro~es~, in particular, th~ w~stef~l degeneration of the eleotroly~ic me~ium~
SUMMARY O~F T~E INV~NTION
One object of the present lnvention ~g to provide a method whereby a gQl compound incorporating alkallne metal ~alt~
(cons~ltuan~s o~ whi~h m~ght ~e any one or more of the element~
O ~odium calclum, lithium, or potas~i~m~, in solution with a permeable material upon which an electrode i~ s~perimpo~ed a~
an integral part for the ~rpose of generating alkalinity at the oat~odic areas of reinorcement ~teel.

.5 Another object ~f the inventlon ls to pro~ide a method whereby ahlorid~ ion~ are remo~ed ~rom the ~oncrete.

The lnstant process is ~ uniqu~ and flexi~le ~iystem. It minimise~ expend~tures by ~nabllng the electrode to be ready-,a fo~med at an~ approp~iate locatlon, convenient for the sita. In this context, the electrode may be recycled a~ many t~mes as are neces~axy and effective, without the nead for disassembly, or, the necessity for lncreme~s of alkallne ~alts to be applied ~'or abso~ption, but simply by ensuring lts placement ln an ~5 environment capable of preserving a mois~ ~ondltlon p~ior to ra-fixing ln a new loca~ion I~ obserues health ~nd s~fety, and envi~onmental considexatlons by avoidlng the use of chemicals ~er~po~3 4 :' ` ` 2~27~07 . ..:
in co~centrations that would otherwlse b~ ~azardou~ In use, thQ
s~stem ls much ~leaner, requiring no heavy doslng o~ the electrode either to k~ep it wet, or, for ~h~ removal of waste.
The s~stem ls less noi~y, ra~se~ less dust, and leaves less wa~te, than other known system~. At the co~clusion of the treatment there are no vis~ble corrosion stains of the ~tructure or the elec~od~. A further advant~ge of the inst~nt process ls ;~
th~t it is comp~tibl~ with st~te of the art c~ncrete ~echnology and t~chniques. ~:
.0 These and many other attendant ~dvantages of the inven~ion will become apparent as the inventlon becomes better unders~ood by reference to the following deta~led description when considered in conJunction-with the accompanyi~g drawings.
.5 BRIEF DES~RIPTION OF THE DRAw}NGs Preferred embodiments of th~ pre~ent ~nven~ion will be de~crlbed with reference ~o the a~company~ng drawin~s w~ereln:
FIG 1. is a p~spect~e view sh~wing th~ basic structure ~0 of the electrode ln section.
FIG 2. ls a ~chematic d~agram show~ng the electxod~ ln sltu and in connection with re~no~cement ~n affected con~r~te.
FIG 3. ~g a gra~hlcal represent~tlon indicati~g cuxrent den~ity and drive voltage plot~ed a a functlon o~ tlme ln ~5 the application of another aspect of the i~vention.
FIG 4. is a graph~cal r~pr~sentation indicating curre~
density ~lott~3d as a functi~n o~ time ln the applicatlon o ~ne aspeot of the lnvention.
F~G 5. ls a graph~ a~l r~pres~ntation indlcatlng ~H
tr~atment progresses and at its conclus~on DETAILED DESCRIP~ION OF T~E PREFERR~D ~MBOD~MENTS
Re~erring now to F~g 1, the invention in its most general aspects comprlses an ele~trode ~anode) o~ elther a pre-~ormed pe~me~ble synthe~ic sponge-llke materlal, lmpregnated with an admixture o~ alkaline salt~ in solu~ion, to wh~ch has been added a setting agent to form a gel, and upon whlch is ~uper~mpo~ed concrepec3 5 -- 2276~
a~ elactrlcally ~ond~cti~e ma~erial ~uch ag a conductive palnt or metal me~h.

P~eparation of th~ ~lectrod~ ~anode) co~mences wlth th~
appllcation of the conductive material to one surfac~ o~ pre-formed permeable synthetlc spong~-like or rigid material.

A p~mary a~ode, such as ~ car~on fibre tape of sufflcient dimension to ~acilitat~ the re~uired density o~ e~ectric curren~
.0 to pas~ ov~r the surf~ce o the ~p~i~ary) ~l~ctrod~ i~ then superimpos~d to complete the elect~ode.

The prep~red permeable materia- ~s ~hen lmmesse~, ~pside down, i~ the prepared gel solu~ion, and compress~d to expel all alr .5 before the solu~lon ls allowed to cool and set A~ indicated abov~, th~ electrode ~anode) ln it~ pr~ferre~ for~
will be rea~y-~onned, p~ior to it`s plRcemen~ in ~ltu for th~
c~mmenceme~t of t~e treat~ent to affQctQd concrete as set ou~
!O in this description.
: .'' . ' ':
In some in~tance~ to faallitate to secura fixlng of the electrode to the af~ected concrete to be treated, it may be ~xpedient to fix the conductive material to a ~ig~d board ~5 materlal, such a~ ply~od, which ls used to ~old th~ electrod~
~gain~t th~ ~ffected concrete.

~efore fixing th~ electrode to t~ ea of the affected concrete~ it may be pr~ferable to prepare lt's surac~ by applylng an alkaline gel, such as calclum carbonate, to reduce acid generation and improve electrlcal contact.

As regards Fig. 2, th~ negative terminal of the power 6upply i~
conn~ct~d to the ~teel in the affect~d conorete. The poeitive terminal is connected to the elec~ro~e located on the surface of the ~tructure. When the current is swi~ched on, electrochemical r~actions take plaoe on ~he surface of the concr~p~c3 6 2~276a7 electrode, (anode) and at the affec~ed Concret~ to steel interface (cathode). ~ the steel bar (cathod~ ^ neg~tive -termlnal), the passage o~ current converts water to hydrogen ~a~ :
and hydroxyl ions, or oxygen plug wa~er to hydroxyl ions.

Simultaneously, the oxidat~on process converts wat~r into oxy~en ~as and hydroyen at t~e suxface of the elect~od~ (anod~ -positive terminal), and ions migrate between the alectrode and the steeL through the elactrode material and th~ affectsd concrete. ~he generation of hydroxyl ~on~ results in an increase ln alkallnlty at the cathode.

Posltive ion~ whlch move toward the st~il under the influence of the curr~n~; have the i~econdar~ a~fect, o~ securin~ the hydrox~l ions whi~ih produce th~ alkalini~y ~ithin t~e affected oncrete surrounding the steel~

The increa~e in alkal~nity can be calculated rom the magnltude o~ the current passing, the length o~ time for which it ls applied, and the rate ~t which hydroxyl ions ar~ pull~d away f rom the st~iel b~r ~nder ths lnfl~ence of ths c~r~ent flow.
Altern~l~ely, the rate at whlch pos~tive lons ar~ tran~ported into the affected conarete may be used ~o calc~late the minlmum inc~ea~e in overall hydsoxyl ion concentration.
~esldes the bsneficial effect o~ an increase in alkallnlty, c~loride lon~ w~ic~ are al~o charge carriers, m~gratz ~oward~
th~ anod~ undsr th~ lnflue~c~ of th~ l~posed ~l~t~la f~eld.
Chlorides will migrat~ towa~ds the a~ode if ~ey lie in th~ipath of the current, and are not bound as insolu~le complexe~.
Conversely, chlorldes not lying in the ~ur~ent path, wlll not be ~emoved by this technique. Th~ emphasi~e~ the need for an ade~uate fea~ibility survey to be efflclently carrled out~

Thus the ob~ ect of ~he invention is to co~trol an~ utillse electro-chemical principlei to achieve res~dual alkalinity ln the affected concrete and to remove chloride from the affected concrspec~ 7 ` ` 2127~0~1 -, : .
concrete. .

In one example, an elec~ro~e 1~ prepar~d contaln~ng 2~ by w~ight agar powdo~, or, aarr~ge~n~n, a gener~c term for a g~l ag~n~
j pre~ared from ~ea we~d, ~nd ~odium bicarbonats solution 8t ~
concen~ration of 1 molar or ~aturat~on, whic~ever i~ the 1esser~ ::
A current density of lA/m2 i9 applied.

In another ~xampl~ a proprieta~y gel agent such a-~ "Ge11ing System SRM" may b~ used ln ~h~ preparatio~ of ~he electro~e, to whlch a current denslty o~ lAmp~m2 is applled.

Fi~ 3 glves thQ current and driv~ voltage as a func~ion of time.

It w111 be noted ~hat the drive voltage may r~s~ to 30 v~1ts t~
maintaln lAmpJm2 curren~

In another ~ampl~, an electrode again Containing 2% by w~lght agar powder, or, carrageen~n, but w1t~ 0.~ Molar :
O potassium chloride solution is prepar~d, to wh~ch a currQnt den~ity o lAmpfm2 ~ 9 app1ied.

Fig. 4 shows t~e current den~ity as a functlon o~ time, with a ~ ;
drive voltage 1~mited to a ma~imum of 18 Volts. :
S .,.
It s~ould be no~ed that ln both examples, no water ls added to the electrod~ (anode), after aommenclng th~ application of the current. ~efore -nd ~ter current appllcation th~ p~ o~ th~
electrode i~ determined u~ing ~ un~verYal ~ndicator. Initlally, . .
0 a pH of 6 was i~dicated priox ~o the application of the current.
A~ter compl~tlon of ~he ourrent application, the affected conc~ete adjacen~ to the steel may be ~ested for p~ changes, using either a universal indicator or ~henolphthale~n, when a ri~e in excess of 12pH will be lndicated.
, .. :
F~g 5. shows in contour the ~ gradients at th~ elect~ode, and :~
interface of steel to affected concrete. :.

co~crn~e3 B

2~7fi~7 WhiIs preferred embodlment~ of the ln~entlon ~ave ~sen shown and descrlbed ln detail, it would be readily ~nderstood and appreclated tha~ numerous o~issions changes and additlons may be made without depart~ng f~om the splrit and scope o~ the S present invention as deined ln th~ following clalms.

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c~ rcp~c3 9

Claims (11)

1. A method for treating affected reinforced concrete or masonry, which comprises an anode comprising permeable synthetic material impregnated with a gel containing alkaline metal salts selected from the group consisting of one or more of the ions sodium, calcium, lithium or potassium in solution, with electrically conductive material superimposed, whereby an electric current is passed between the said anode and the steel reinforcement, and whereby alkalinity is generated at the cathodic areas of steel reinforcement, and whereby chloride ions migrate away from cathodic areas of steel reinforcement toward theanode.

2. A method according to claim 1 wherein chloride ions are extracted electrochemically under the influence of electric current flow from the cathodicareas of steel reinforcement.

3. An anode for treating affected reinforced concrete or masonry comprising permeable synthetic material impregnated with a gel containing alkaline metal salts, selected from the group consisting of one or more of the ions sodium, calcium, lithium, or potassium.

4. An anode according to claim 3 characterised as being ready-formed prior to it's placement in situ for the commencement of treatment to affected concrete.

5. An anode according to claim 3 to which electrically conductive material is applied to the upper surface of permeable synthetic sponge-like material to forminto an electrically durable layer.

6. An anode according to claim 5 to which electrically conductive coating material is applied to the upper surface of permeable material adapted to incorporate a layer of material capable of securely fixing the said anode to theaffected concrete to be treated.

7. An anode according to claim 5, characterised as being capable of being recycled and reconstituted.

8. An anode according to claim 6, characterised as being capable of being recycled and reconstituted.

9. An anode according to claim 3 of sufficient dimension to facilitate an electric current of required density to pass over the surface of it.

10. An anode according to claim 9, characterised as being capable of being recycled and reconstituted.

11. An anode according to claim 3, characterised as being capable of being recycled and reconstituted.