Bar-wrapped pipe (C303)
Bar-Wrapped Pipe (BWP) is the simplified term for AWWA C303 Bar-Wrapped Steel Cylinder Concrete Pressure Pipe. BWP is comprised of a welded steel cylinder that serves as a watertight membrane and works together with steel reinforcing bars wrapped under tension around the cylinder to provide strength. An internal concrete lining and external mortar coating provide corrosion protection to the steel components. BWP is produced in diameters of 10-inches to 72-inches, and standard lengths of 24 feet to 40 feet. BWP is typically used where pressures are 300 PSI or less.
The design for bar-wrapped concrete cylinder pipe was developed in 1942, and it was produced in large quantities as early as the 1950s. The AWWA standard for the pipe was issued in 1970 as AWWA C303-70 Standard for Reinforced Concrete Water Pipe-Steel Cylinder Type, Pretensioned. More recently the term “pretensioned” has been replaced with the more commonly used term “bar-wrapped.”
BWP vs. PCCP
While BWP appears similar to PCCP in cross section, their design and materials are significantly different. PCCP is a concrete pipe that remains under compression because of the prestressing wires, whereas in BWP, the cylinder plays a much larger role in the structural integrity of the pipe.
BWP is essentially designed as a steel pipe with mild steel used to manufacture the steel cylinder and steel bars. PCCP utilizes mild steel for the cylinder, but high strength steel is utilized for the wire which is wrapped under high tension. As a result, the bar in BWP and wire in PCCP respond differently to environmental conditions that facilitate corrosion.
The high strength steel wire in PCCP is smaller in diameter and wrapped under higher tension, therefore corrosion makes it quite vulnerable to breakage. The mild steel bars in BWP are thicker in diameter and wrapped under less tension, therefore corrosion takes significantly longer to lead to breakage.
Main forms of failure in bar-wrapped pipe
BWP utilizes a cement-mortar coating placed over the steel cylinder and bars to provide protection to the pipe. The coating places the steel in an alkaline environment that prohibits corrosion. If the coating deteriorates or is damaged due to improper handling, operations, aggressive environments, etc., the alkaline environment will be compromised which may lead to corrosion of the steel cylinder and bars. As these elements corrode, the steel cylinder and bars will pit, thereby reducing the effective area of steel. If corrosion continues to progress, the pipe will likely develop a small, non-visible leak, which will grow with time. Eventually a large visible leak or a pipe failure will occur.
It should be noted that there may or may not be broken bars with deteriorated BWP. Deterioration can begin on the bars or on the cylinder. Also, since the structural elements of BWP consist of mild steel, a brittle (sudden) failure is unlikely. Rupture can occur, however it usually takes place after an extended period of deterioration, usually preceded by leakage. This has anecdotally been contradicted in field observations, where reported sudden failures have occurred, however in these cases, it is highly likely that the pipe was previously leaking for an extended period of time, but the leak was undetected.