Modification of the Environment

The Remedies and their Limitations 

Insulative Remedies 

The insulative methods currently employed to isolate reinforced concrete structures from the corrosive environment include surface coatings and membranes, polymer impregnation and 

overlays of polymer concrete, low slump concrete, or latex moditied concrete. These methods are suitable only when the surfaces of the concrete structure are exposed for 

treatment.  Ideally, these b'miers would prevent continued intrusion of harmful contaminants and the availability of oxygen or moisture to sustain the corrosion reactions. 

However, in existing  structures active corrosion is already underway and harmful species have contaminated the concrete.  Insulative methods used after active corrosion is 

experienced do not stop corrosion but may mitigate the effects of the corrosion processes. However, if insulative methods are used without initially decontaminating the concrete, 

sufficient amounts of the contaminants, oxygen, and moisture may be entrapped such that corrosion progresses until structural integrity is threatened.  In many cases, they can 

successfully extend the useful life of a structure. 

Methods available for rendering the environment less corrosive include the removal or elimination of harmful constituents from the electrolyte.  These constituents could be in one form of chemicals such as water and chlorides, gases such as oxygen or hydrogen sulphide, or electrical currents. 

Water can often be eliminated by facilitating drainage away from rather than through a structure.  Chlorides can be eliminated by a process known as electro-chemical chloride removal. The removal is accomplished electrochemically, by using a suitable electrolyte, an ion exchange resin, and a noble-anode. The reinforcing steel is the cathode (positively charged) in this electronically circuit. The negatively charged chloride ion (Cl) is attracted to the positively charged anode where it is trapped by the exchange resin.  In this test, up to 90 percent of the chlorides present in the concrete above the top mat of reinforcement is removed. Disadvantages are that the method is expensive, time consuming, and requires the application of high direct current voltage which generates heat (around 200° F or 90'C) which, in turn, can induce cracking of the concoct.  In addition, the permeability of the concrete will be increased. 

Harmful gases such as oxygen and hydrogen sulphide can be stripped by chemical process from the electrolyte, thus making it less corrosive.  This method is applicable predominantly   for structures exposed in aqueous solution. 

Corrosion of steel in concrete can be caused by environmental factors other than chemical constituent such as chlorides, moisture, oxygen. Stray electrical currents can result in corrosion by electrolysis, i.e. cathodic interference. In corrosion by electrolysis, direct current strays from an exterior source and is collected by the steel in a reinforced concrete structure. In as much as the collected current must return to its source to complete the electrochemical circuit, the current is discharged from the structure at some locations. At the point of current discharge from the structure to the electrolyte, metal loss is experienced. The most cornmeal sources of stray currents include cathodic protection systems, electrified railways. and clectroplating plants. This type of corrosion is most commonly manifested in grounded structures, i.e. those in contact with the earth. A method of mitigating this type of corrosion, implemented for many years in the hurried pipe industry, is the installation of resistance bonds. In resistance bonding, the structure being affected is electrically connected through a resistor to the source of the interference currents. In this manner, the current returns to its source via metallic path such than no metal loss from the affected structure occurs.  Another method uses Paganini anodes to drain the collected current. Collected current is passed on to the electrolyte and back to its source from the surface of the anode which corrodes, rather than the structure. 

Active Control of Electron Flow 

The method of providing the highly negative steel potentials required for  immunity is referred to as cathodic protection. In cathodically protecting a structure, a favorable electrochemical circuit is established by installing an external electrode in the electrolyte and passing current (conventional) from that electrode through the electrolyte to the structure to he protected.  This current polarizes the potential of the cathodic surfaces (relatively positive) on the steel to that of the anode (more negative) surfaces. When this is accomplished. there is no current flow between the formerly anodic and cathodic surfaces and corrosion is arrested. This represents a balanced or equilibrium condition. In normal practice, sufficient current is passed to the surfaces so that the formerly anodic areas will be receiving current from the electrolyte and their potential will he shifted to the more negative direction. 

There are two ways in which the protective electrochemical circuit can be established. One method uses an electrode made of a metal or alloy more negative than the structure to be protected. For example, ~f the structure to be protected is constructed of steel, either magnesium. zinc, or aluminium may be coupled with the structure.  In as much as a protective galvanic cell is set up between the steel and the alloy selected, this method is known as the "galvanic anode method of cathodic protection". Also Since the galvanic anodes pass current to the electrolyte, they corrode or sacrifice themselves to protect the structure. Hence, magnesium, zinc, and aluminium are termed sacrificial anodes.  Sacrificial anodes corrode at relatively high rates.  Corrosion rates for magnesium, zinc, and aluminium are of the order of 17,26 and  12 Ib per amp year, respectively. 

The other way in which the favourable electrochermcal circuity can be established is by introduce  electrical current from an external source. Because an outside source of current is used. this nettled  IS tern led "impressed current cathodic protection". This method also requires :he installation of an external electrode in the electrolyte with the structure to be protected However, since the current flow is not dependent on the favourable potential difference between the electrode and the structure to be propelled, more noble materials can be selected for the socie. These materials include high-silicon cast iron, graphite, anti even more noble materials such as platinized-titanium or plagiarized-  niobium. These letdowns corrode or are consumed very slowly, less than per ,amp year. 

There are two ways in which cathodic protection can be achieved. A comparison the respective advantages and disadvantages of the system are shown in Table. The initial application of cathodic protectionist to bridge decks was in 1974 and other applications have subsequently  made with encouraging results. 

Comparison of Electrochemical Circuit Characteristics

The cathodic protection of reinforced concrete structures is thus proven technology and the problems  being currently encountered deal with criteria of protection, design and inspection of the installation. In  buried structures or structures exposed in water or in soils, low resistance electrochemical circuits can normally be established. However, on other structures such as bridge decks, a highly conductive overlay consisting of a coke breeze asphalt mixture or closely spaced anodes to reduce the circuit resistance and lo promote uniform distribution of current to all reinforcement is required.