Initial Scope Called for a Partial Repair
Chemical Sulfate Attack Collapsed Roof Prompting a Full Repair
Innovative Formwork Design
Full Structural Liner Installation
Completed

Advanced Pit Within a Pit Liner Extends Service Life

Project Description 

A 1,400-square-foot sulfur pit at a North American refinery required immediate assessment after a partial roof collapse. Following a visual inspection by STRUCTURAL, a full repair was recommended. Because an unscheduled outage creates challenges, however, the owner opted to repair only the collapsed portion and delay the full repair for a scheduled outage.

The initial scope called for repairing the concrete roof slab and pipe penetration, stopping water leaks and creating a vacuum seal on the pit to promote a cleaner environment. This original scope included a four-inch thick concrete overlay, but after it was determined that the existing concrete roof could not support the dead load, an alternate repair strategy was developed to repair the outside walls and pipe penetration, as well as replace some curbs.

Performing the concrete repairs around the pipes was extremely challenging, so an innovative design for the formwork was developed that employed plywood for the base and 4X4s to spread the load. Critical to the success of the project, however, was the selection and installation of a high-end coating system to handle heat and movement, as well as chemical resistance.

While the repair was successful, significant ground water ingress converted the molten sulfur to sulfurous acid — causing additional chemical sulfate attack on the remaining reinforced concrete members. Ultimately, the damage caused the pit roof to collapse and prompted the owner to opt for a full repair. STRUCTURAL crews removed one set of heating coils, four inches of accumulated sulfur cake, and the existing cast-in-place roof. Once the pit had been cleaned, a visual inspection was performed to assess the pit and offer final recommendations for repair.

Due to the deteriorated state of the pit, installing a full structural liner, known as a “pit within a pit,” with new reinforcing steel, waterstops, a shear keyway, and embedded dowels was recommended. Once complete, the new walls measured 14 inches thick with the integration of a new cast-in-place roof onto the full structural liner. Lateral steel shoring and bracing were removed, anchors and reinforcement in beam pockets were installed, high-performance repair mortar was placed and the perimeters of the patched beam pockets were chinked after adequate cure.

The plant was able to remain operational during the repairs with only the sulfur pit offline. The overall repair strategy provided the owner with a long-term solution — ending the cycle of repairing the repair, as well as extending the asset life of the structure.