CONCRETE PIPE Chlorides The most significant aggressive action of chlorides is corrosion of steel in reinforced concrete. Most problems occur as damage to bridge decks resulting from use of de-icing chemicals. Maintenance problems have also been encountered with reinforced concrete seawater structures, such as piling and piers, because of chloride-induced corrosion of the reinforcement. Portland cement concrete protects embedded steel against corrosion under conditions that would be highly corrosive to bare steel. The protection is an electro-chemical phenomenon in which the high alkalinity of concrete, normally a pH around 12, passivates the steel. This passivating influence will normally remain effective as long as the pH remains above 10, even in the presence of free oxygen. The chloride ion has the ability to disrupt this protective mechanism. Research has established there is a critical chloride ion concentration at the concrete-steel interface beyond which corrosion will occur, and that oxygen must also be present to support corrosion. The greater the chloride ion concentration beyond the critical level, assuming oxygen availability, the more rapidly corrosion will progress. Seawater has approximately 20,000 parts per million of chloride. Many concrete pipe installations are completely immersed in seawater and are performing satisfactorily after many years. This is primarily due to low oxygen solubility in high chloride waters plus the extremely low diffusion rate of oxygen through the saturated concrete cover. It is necessary to evaluate all the installation conditions to determine the probability of chloride induced corrosion. Concentration effects which generally occur along with enhanced oxygen availability can produce the critical chloride ion concentration at the steel-concrete interface that is needed to produce corrosion. These effects will more readily occur under the following conditions; low quality concrete of high permeability and porosity, cracks, and the inclusion of calcium chloride in the concrete mix. A number of conditions can reduce the severity of chloride attack. Increased concrete cover will normally extend service life but will not prevent eventual corrosion under severe exposure conditions. High quality concrete with low permeability, and the absence of cracks and voids, will also extend the life of the pipe under severe exposure conditions but will not prevent eventual corrosion if the mechanism of chloride build-up continues. Under extreme exposure conditions, the use of barrier type protective coatings is the most effective alternative. A very small percentage of concrete pipe is installed under conditions for which serious chloride build-up is probable. The American Concrete Pipe Association has not received any reports or evidence of chloride induced corrosion damage to reinforced concrete pipe. PAGE 52 LAFARGE PIPE eMANUAL