International Concrete Abstracts Portal

SP102 As a designer you need the wealth of information presented in "Corrosion, Concrete and Chlorides -- Steel Corrosion in Concrete: Causes and Restraints," a compilation of 11 papers covering the corrosion phenomenon. Prompted by the nationwide corrosion problem with concrete, there have been extensive field and laboratory investigations into the specific phenomena that induce corrosion, methods for identifying the magnitude and extent of corrosion in structures, techniques for stabilizing corrosion once it has begun, and design of structures and concretes so that corrosion will not occur. Presenting an in-depth analysis into a variety of aspects dealing with corrosion, this state-of-the-art publication includes such topics as: the marked influence of chloride in causing corrosion, ways for controlling corrosion by using chloride-free accelerators, the reduction of chloride penetration through the use of pozzolanic blast-furnace slag as an admixture in concrete, and the conductive coating aspects of cathodic protection. "Corrosion, Concrete and Chlorides" provides important answers to a complex problem.

Based on a studu of manu structures that have suff eredfrrom damage due to chloride-in duc ed corrosion of reinforced steel, it appears that even the more stringent limits on chloride now being proposed may be too lenient. Further, only determinations of total chloride may provide a basis for rea-sonable assurance against future corrosion.

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).

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.

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.