Cathodic Protection as a Corrosion Control Alternative

1 CONCORR, Inc. 19931By William T. Scannell and Ali SohanghpurwalaCorrosion of reinforcing steel in con-crete is a widespread and enormouslycostly problem in all parts of theUnited States. Numerous concrete structuresincluding bridge decks and substructures,parking garages, balconies and others aredeteriorating as a result of reinforcing steelcorrosion. Virtually any reinforced concretestructure is susceptible to the ravages of cor-rosion if subjected to the right Corrosion process that takes place inconcrete is electrochemical in nature, verysimilar to a battery. Electrochemical corro-sion is Corrosion which is accompanied bya flow of electrons between Cathodic andanodic areas on a metal surface.

2 In concretethe electro-chemical Corrosion reactions aremost often triggered when three factors chloride, oxygen and moisture meet at thereinforcing steel surface. A sort of naturalbattery develops within the reinforced con-crete structure, generating a low-level inter-nal electrical current. The points where thiscurrent leaves the metal surface and entersthe concrete electrolyte are called current leaving the concrete and return-ing to the steel does so at the cathodes. Cor-rosion or oxidation (rust) occurs only at Corrosion of reinforcing steel oc-curs, the rust products occupy more volumethan the original steel, causing tension forcesin the concrete.

3 Since concrete is relativelyweak in tension, cracks soon develop asshown in Figure 1, exposing the steel to evenmore chlorides, oxygen and moisture andthe Corrosion process accelerates. As corro-sion continues, delaminations separationswithin the concrete and parallel to the sur-face of the concrete occur. Delaminationsare usually located at, or near, the level ofreinforcing steel. Eventually concretechunks break away or spall signs of Corrosion -induced dam-age on many types of reinforced concretestructures are becoming more and moreprevalent. In many parts of the country onecan hardly drive across a bridge or enter aparking garage that doesn t have some de-gree of Corrosion rate of concrete deterioration at anygiven time is dependent on many factors in-cluding Corrosion rate, reinforcing steel con-centration, concrete properties, cover and theenvironment, to name a few.

4 Once corrosionhas begun there is one thing for certain itwill only get worse and it will do so at anever-increasing rate. Ultimately, if corrosionis allowed to continue, structural integritycan be compromised due to loss of sectionof the reinforcing steel and/or loss of bondbetween the steel and the concrete, and re-placement may be the only order to mitigate or Control a corro-sion problem (provide low future mainte-nance and long term Protection ) specific in-formation is needed for any given , proven technology and scien-tific methods are available to evaluate cor-rosion of reinforcing steel (and other em-bedded metals)

5 And associated damage onreinforced concrete structures. These tech-niques are designed to determine the extentof damage, define the Corrosion state of steelin undamaged areas, evaluate the cause, orcauses, of Corrosion , and determine the po-tential for the steel to corrode in the futureresulting in further damage. It is only afterthis information is obtained through a de-tailed Corrosion condition evaluation that asuitable repair and Protection specificationcan be developed for a Corrosion -plaguedstructure. It is important to point out that con-crete itself can deteriorate regardless of thecondition of embedded reinforcement.

6 Ex-amples of this include freeze/thaw damageand alkali-silica reactions. Various concretetests are therefore often conducted as partof an overall there are similarities betweencorrosion of conventionally reinforced con-crete structures and pre-tensioned or post-tensioned structures, the majority of this ar-ticle applys to conventionally reinforcedconcrete structures only, particularly withCathodic Protection as aCorrosion Control AlternativeFigure 1. Schematic of concrete deterioration due to Corrosion . (Diagram: Report 12/88)2 CONCORR, Inc. 1993respect to the applicability of Cathodic ProtectionWhat is Cathodic Protection ?

7 Simply put, Cathodic Protection (CP) is a widely usedand effective method of Corrosion people, engineers included, think ca-thodic Protection is some kind of believe CP is so complicated andexpensive that it has no practical use in theconcrete rehabilitation industry. Then thereare those who say CP doesn t work or that itis unreliable in the long term. The facts, how-ever, show that CP is not so complicated, isoften the most cost-effective course of ac-tion, has practical application on reinforcedconcrete structures, and that it most defi-nitely works.

8 Of course, performance of CPsystems, like all other Corrosion protectionsystems, is directly dependent on soundspecifications, proper installation, and moni-toring and maintenance. With CP, one can-not simply install it and forget it. Good longterm performance of all CP systems requiresgood monitoring and maintenance proce-dures, a reason why CP is sometimes dis-counted as an Alternative Protection some investigators and specifiers ap-parently don t recognize is that there is nosuch thing as a Corrosion Protection systemwhich does not require periodic inspectionand maintenance.

9 In fact, some corrosionprotection systems actually require periodicreplacement or decades, Cathodic Protection has beensuccessfully used to protect pipelines, shiphulls, off shore oil platforms, heat exchang-ers, underground tanks, and many other fa-cilities exposed to a corrosive , its first application to steel in con-crete was only in 1973, but we ve come along way since then. Cathodic Protection ofsteel in concrete is simply a means of fight-ing fire with fire, or in this case, electricitywith electricity. The Corrosion process gen-erates electric currents. Cathodic protectionsupplies a source of external current to coun-teract the Corrosion current.

10 Hence, corro-sion stops, or at least is greatly millions of dollars of research inthe areas of Corrosion of steel in concreteand Corrosion mitigation, ca-thodic Protection evolved asthe only technique which couldpositively arrest Corrosion ofsteel in existing concrete struc-tures. In fact, some time agothe Federal Highway Admin-istration (FHWA) acknowl-edged that Cathodic protectionwas the only rehabilitationtechnique that had proven tostop Corrosion in salt-contami-nated bridge decks regardlessof the chloride content in theconcrete. It should be noted,however, that CP is not alwaysneeded nor is it applicable onevery structure.