Prestressed Concrete Cylinder Pipe (PCCP) organically combines the tensile strength of high-strength steel wire, the compressive strength of Concrete, and the seepage prevention of steel plate, making full and reasonable use of the physical and mechanical properties of each material

FIG. 1 Schematic diagram of PCCP tube structure

The American MP magazine published the experience summary of the Great artificial river project in Libya [3] in the 4th issue of 2005, summarizing the experience and lessons of PCCP construction in the first phase of the project, and improving the corrosion prevention of PCCP pipeline in the second phase of the project. In order to reduce the cost of the project, in accordance with the requirements of Libya, only 30% of PCCP was used as the epoxy coal asphalt protective layer, instead of all cathodic protection. As a result, the pipeline was corroded and burst during operation, resulting in serious losses. The Grand Artificial River Authority is trying to ensure that all installed PCCP pipelines receive cathodic protection, Gerrard said. "At the time of initial installation, some PCCP pipelines are cathodic protection and others are not, depending on the actual environmental conditions," he explained. "To increase the life of the pipeline, The Great Artificial River Authority then decided to install cathodic protection on all sections -- meaning they had to rework and install it on pipes that were already buried. That's what we're doing right now.

In the 5th issue of 2005, American MP magazine introduced that in the soil environment with high chloride concentration, if PCCP is carried by interval water, the wall of PCCP also goes through the alternating cycle of wet and dry during the alternating process of water transfer and shutdown. In this cycle, the chloride in the soil will gather in the outer wall of the pipe. When the chloride in the outer wall of the pipe reaches a certain concentration, it will permeate through the outer cement protective layer and finally interact with the protective film of the steel bar, causing THE corrosion of PCCP [4].

The Pockwock water line uses cathodic protection to prevent these problems. Pockwock Pipeline The diameter of the pipeline is 107cm, mainly for the city of Halfax water supply. By 1985, the first crack occurred after 10 years of operation, followed by at least five more bursts. In 1991, a zinc anode was used for cathodic protection of corroded pipes. The zinc anode is 2m long and weighs 3.6kg. The zinc anode is packed in sulfate/bentonite and placed in a paper tube. Install two zinc anodes at each location and require continuous electrical bonding between pipes. In order to prevent the protection potential is too negative (negative to -975mV), the steel bar hydrogen evolution reaction, so the selection of zinc anode. When the cathodic protection was put into operation, the measured protection current required was very low (less than 200uA/m2). For this reason, some zinc anodes were not connected to the pipeline at the initial stage of operation.

Because of the composition of steel bar and the production process of cold rolling, it has been thought that steel bar is easy to undergo hydrogen evolution reaction. In order to improve the performance of prestressed steel bars, hydrogen evolution tests were performed according to ASTM A227 test method. According to Lewis, many existing PCCP pipes are reinforced with hydrogen-evolution (HDES), and there have been reports of cracks in PCCP pipes near the auxiliary anode ground beds.

In reference 5, the corrosion mechanism of steel in concrete is discussed, and it is pointed out that there are many factors that can change the passivation of aqueous solution in porous concrete. The blunt film is destroyed when chloride ions penetrate the reinforcing bar and reach a critical chloride ion concentration. In wet concrete, local corrosion can occur even when porous concrete absorbs alkaline water. In reinforced concrete structures, the chloride ion concentration is limited to 0.4% of the cement mass, and in prestressed concrete structures, the chloride ion concentration is limited to 0.2% of the cement mass.

Another cause of depassivation is the decrease in the alkalinity of the concrete (i.e., the pH value of the water absorbed). This happens when concrete reacts with atmospheric CO2 to become carbonized. When there is a thick enough concrete covering over the steel members inserted into the structure, especially dense, high-quality concrete with very low voidage, carbonation is not important. If the concrete is of poor quality or if the concrete overlay is thin and thin, carbonation will permeate the reinforcement, eventually making it less blunt. When depassivation occurs by chloride ions or carbonation, there is a risk of corrosion in wet concrete only under conditions of oxygen intrusion. When the ratio of Sc/Sa of cathode surface area to anode surface area is high and the cathode is in a good aerated state, a very high corrosion rate will occur in the anode region.

-- The above text is quoted from Hu Shixin et al. Cathodic Protection of Prestressed Steel Tube Concrete Pipe