Investigation of the remaining service life of the ‘egg cups’ at sludge digestion tanks in Hengelo

The sludge digestion tanks of The Energy Factory in Hengelo are a sight to behold. The three egg-shaped tanks are visible from afar. Under contract to the Vechtstromen Water Authority, Witteveen+Bos investigated the remaining service life and structural safety of the concrete at the bottom of two of the tanks.

An inspection involving non-destructive testing methods was used to detect the location of the rebar. In addition to this, cores were drilled and later analysed in a laboratory. Simultaneous with these activities, a brief literature study was conducted into the effect on the concrete of the sludge in the tanks.

However, the most interesting part of this contract involved studying the photos taken during the construction of the tanks in 1974. The substructure, also referred to as the egg cup, was the main object of attention during the investigation. The superstructure’s shell is constructed of segments, prestressed in two directions, on top of the substructure. The knowledge of the since-retired Witteveen+Bos engineers involved in the construction in 1974 and the old photos proved to be a valuable resource in creating an image of the construction of the egg cups.

On the basis of the investigation, Witteveen+Bos provided advice about the structural safety of the tanks and a proposal was submitted concerning the maintenance intervals of the sludge digestion tanks.