Because the structure had a dense rebar con‑ figuration, proper placement and vibration of the concrete for the 35' tall walls was critical. The contractor wanted to make sure that no concrete drop was greater than eight feet, so standard pour door panels were incorporated into the outside gang at six foot intervals.

The structural design called for four access holes in the wall. This required the outside forms to allow penetration by a three foot diameter stainless steel manway. To accom‑ modate this element, the 40' wide gangs were composed of three 12' wide Flex‑Form panels and one 4' wide job-built section attached with filler angles. This area utilized Flex‑Form rolled ribs to create the correct curve.

The top six foot section of the inside surface of the digester tank required a special PVC lin‑ ing to protect the concrete from deterioration caused by exposure to hydrogen sulfide gas (a.k.a. sewer gas). The lining is designed to be temporarily attached to concrete formwork and then left in place when the forms are removed. The lining has a smooth face and a back with T-shaped fins that lock the lining into the concrete. The liner is time-consuming to patch, and the contractor wanted to avoid the expense of patching tie holes in this area. To eliminate tie holes through the liner, the gangform design included a top “dry” tie and a “wet” tie 6'-6" below the top edge, just under the lining.