There was one point on which participants of the forum on The Chloride Issue: The New Limits at ACI’s annual convention last March in Denver agreed: We have a lot to learn about the influence of chloride ions in rein-forced concrete. Moderator Arthur L. Walitt, after being introduced by forum chairman Robert Henry, opened the forum by announcing that its purpose was to determine topics on which research is needed. A list of the topics which sur-faced during the forum accompanies this article. The forum, sponsored by ACI Committee 123, Research, opened with prepared presentations by Ted E. Webster, Bernard Erlin, and Richard Gaynor. Excerpts from Websters’s and Gaynor’s presentations appear here, followed by other forum highlights taken from an edited transcript

Usually concrete is an ideal environment for steel. Reinforcing Steel in most concrete structures is not subject to corrosioin. However, when salts (chlorides or sulfates) penetrate concrete and reach steel rebars, corrosion becomes active. Rust takes up a larger volume than the iron from which it is formed, developing pressure as great as 5000 psi within the concrete. This pressure causes cracking and spalling. Ultimately, failure occurs and major repair or replacement is needed. Once salts (from deicing, bleaching, marine environment, foreign aggregate etc.) contaminate concrete, corrosion progresses rapidly. Penetrants, sealants, surface coatings, and membrane barriers are useless in combatting the effects of salts already in concrete. The use of cathodicprotection to control corrosion on reinforcing steel in concrte is relatively reinforcing steel in concrete is relatively new. Although cathodic protection has been employed on pipelines, offshore cathodic protection has bee employed on pipelines, offshore platforms, ship hulls, buried tanks, etc. for more than 40 years, its use on concrete bridge decks was initiated only in the early 70's. Since the development of conductive coatings (1980-82) the effectiveness of cathodic protection has been enhanced.It has become easier to install and is now applicable to many different types of concrete structures (i .e ., docks; harbor facilities marine terminals; bridge substructures such as piers, pier caps, and beams; bulkheads; parking garages; industrial water and waste treatment plants; tunnels coastal buildings, etc. acceptance ofconductive coating cathodic protection continues to grow , new applications develop. This new form of an established technique holds extraordinary promise for large-scale preservation of concrete structures. (SP-102

A method for evaluation of the corrosion potential of chemical admixtures is presented. The method allows the direct measurement of the macrocell corrosion current between two layers of electrically connected reinforcing bars embedded in concrete. By ponding the specimens with chloride-free water, the potential of the chemical admixture to instigate corrosion can be evaluated. By using a chloride-containing ponding solution, in particular a 15% NaCl solution, it may be possible to assess the potential corrosion inhibiting effects of certain chemical admixtures. The test method was used to compare the corrosion activity in reinforced concrete slabs containing a normal dosage rate of calcium chloride, plain concrete and concrete containing two dosage rates of a multicomponent calcium nitrate based non-chloride accelerator. Only the slabs containing calcium chloride exhibited corrosion when ponded with tap water. When subjected to cyclic ponding with the salt solution, both the plain concrete and the concrete slabs containing the two dosage rates of the non-chloride accelerator exhibited corrosion. However, the slab containing the higher dosage rate of the non-chloride accelerator exhibited only 25% of the corrosion activity of the other two slabs. It is speculated that this reduction may be the result of corrosion-inhibiting effects of the non-chloride accelerator when it is added at sufficient rates.

Effects of two non-chloride accelerating agents -- sodium thiocyanate and calcium nitrate -- in time to achieve initial set of two brands of Type I cement were determined at 70 F (21 C) and 40 F (4 C). Results with these two non-chloride accelerators were compared with results with calcium chloride, the conventional accelerator. Tests show:Low or moderate dosages of the two non-chloride accelerators can reduce time to achieve initial set by l-2 hr.- Any one of the three accelerators may be more effective with one ce-ment than with another cement having similar setting characteristics without accelerators In general, all three of the accel-erators are more effective at 40 F than at 70 F.

Corrosion of steel in concrete proceeds at a far greater rate in the presence of chloride ions. Most researchers agree that chloride ions act as an essential part of the corrosion cell by 1) lowering the pH of the concrete pore water in contact with the steel, thereby dissolving the passive oxide film on the steel surface, or 2) penetrating the film to react with and trans-port the metallic iron into the electrolyte. In the latter process, the resulting iron chloride complex ion combines with hydroxyl ions to form ferrous hydroxide in solution and lower the pH. This, in turn, thins out the oxide film and speeds penetration of chloride ions. Eventually the continuing cor-rosive action results in a pit. To initiate corrosion, a threshold concentration of chloride is needed in excess of the amount immobilized by reaction with tricalcium aluminate in the cement. Investigation of the chloride ion content in concrete adjacent to corroding reinforcing steel shows the concentration to be 1.0 to 1.4 lb of chloride ion per cubic yard of concrete (0.59-0.83 kg/m3).