Tacoma Digester No. 3

In 1986 three concrete digesters were built as part of the City of Tacoma’s Central Treatment Plant.  The conventionally reinforced digester walls were prestressed horizontally using external Howlett Post-Tensioning Systems around the perimeter of the digester core walls.  The Howlett system included uncoated 1-1/8” diameter steel rods spaced between 6” and 1’-6” on center.

Over time, the uncoated system, wrapped around the digesters, began to corrode causing the bars to fail on Digester No. 3.  In 2002, the City of Tacoma sought to upgrade the tank by removing the failing Howlett bar system and installing a new external post-tensioning system.  VSL encapsulated strand post-tensioning was chosen to provide longer lasting protection and strengthening.  This new system included 0.6” dia. 270 ksi strand encapsulated with corrosion protectant grease and a watertight extruded 60 mil thick HDPE sheathing.  The anchorages were galvanized and injected with grease to provide further long-term durability.

STRUCTURAL TECHNOLOGIES provided the structural design of the digester wall upgrade which included a seismic analysis for the UBC Zone 3 location.  Crews also supplied and installed the new external post-tensioning system.  The system was designed and installed to avoid the large number of pipe penetrations and man-ways of the existing digesters.  Once the strand system was in place, external shotcrete was applied over the entire digester walls for additional protection of the strengthening system.