In the midst of several large projects aimed at upgrading and expanding the Missouri River Wastewater Treatment Plant, CCTV inspection of primary clarifier effluent channels revealed severe microbial induced corrosion (MIC). This caused many areas of rebar exposure, delaminated concrete and failed joints.
Structural rehabilitation of the failing concrete pipe and channel were top priority, but the most important goal was “to ensure the channels got restored back to their structural and hydraulic requirements,” according to Civil Engineer Jon Riede.
Project owners chose centrifugally-cast concrete pipe (CCCP) as the preferred rehabilitation method, as it appeared to offer the best combination of structural repair, MIC inhibition and cost-effectiveness. For permanent MIC inhibition, an MIC-resistant admixture, ConShield, was incorporated in all new concrete.
On site work took place from Jan. 16, 2018, to May 14, 2019. Although this time of year is considered dry season in Omaha, dewatering was a constant challenge and Ace crews had to monitor weather closely. At several times, teams evacuated to avoid flooding due to storm events. Access to failing pipes was limited, and staging areas were small, so a purpose-built, compact trailer for pumps and mixers was assembled for the CentriPipe process.
To compensate for cold weather, concrete material was mixed and warmed on site just prior to pumping. The channels were relined with thin, structurally sound layers, which only minimally affected pipe capacity.
The new, structurally sound pipe cast within the old channels by the CCCP process adheres to the original substrate and is usually quite thin—0.5 to 1.5 in. for this project. Original channels walls were approximately 12 in. thick. Even layer thickness was verified and assured by the use of depth pins, visual inspection, and photographic documentation.
City staff operated gates and used some bypass pumping to keep effluent channels in service during the entire project. The rehabilitation has held up with no incident and is considered to be successful.