Project Description

The Sulfur Pit is located in the Middle East and was designed and built in the late 1970s. During the past 10 years, the owner reported several visual signs of deterioration including concrete cracking, exposed and corroding reinforcing steel and spalls on the surface of the interior walls and roof slab. Several restoration attempts were attempted in recent times on the walls, including shotcreting, but none were successful in providing a long term remedial solution to the ongoing deterioration.

Condition Assessment

Structural was contracted by the owner to perform a Condition Assessment survey to evaluate the current condition of the unit and assess the extent of deterioration through a detailed investigation protocol which utilizes STRUCTURAL TECHNOLOGIES’ past knowledge and experience with Sulfur Pits to come up with a program which aims to detect and gauge some of the defects specific to Sulfur pits.

The investigation included:

• Ground Penetrating Radar (GPR) scanning to establish the structure of the embedded reinforced steel;
• Acoustic hammer sounding to determine zones of hollow or delaminated concrete;
• Impact echo testing to determine the condition of the concrete;
• Concrete core and powder samples were collected to determine compressive strength and levels of contamination.

The roof slab was deemed to be in “poor to very poor” condition which necessitated temporary shoring to avoid possible collapse or caving in of the roof which might pose serious safety concerns.

Repair Process

The original scope of work included removal and replacement of the sulfur pit roof slab along with installation of a durability liner (150mm) on all perimeter walls and interior columns.

During the demolition work, several on-site observations were revealed that served as roadblocks in the way of performing the original scope. These findings included a completely corroded rear mat of rebar in the perimeter walls, in addition to the presence of a 50mm void extending to about 600mm from the top of the wall. This meant that the repair program had to be re-evaluated to determine its feasibility against these findings. STRUCTURAL TECHNOLOGIES determined that the original repair was no longer possible due to the absence of the rear rebar in the walls which should have been tied into the new roof. The client was reluctant to accept such recommendations since it meant that the shutdown duration was to be significantly extended.

The concept of constructing a pit within a pit was proposed where new structural walls would be constructed to support the pipe racks on top of the Sulfur Pit roof and at the same time, provide sufficient reinforcement to tie into the new roof.

The new scope and engineering design were approved. Dowel holes were first drilled into the pit floor slab to enable doweling-in of the wall reinforcement. Then, the wall reinforcement was laid out and the wall was formed. The wall was cast using Form and Pour methodology in accordance with ACI RAP-4. Following the removal of wall formwork, slab decking was reinstalled and the roof rebar was laid out for the roof casting. Existing penetrations were laid out to ensure block-outs were made before the roof pour. The roof was then poured in five (5) phases according to a predetermined plan. Finally, after roof curing, the shoring was removed and the 150mm durability liner was constructed for the interior columns whose condition did not require major repairs.

Challenges

Change of scope from a durability liner to a full structural liner that takes into consideration a pipe rack support system, and the formulation of a suitable repair program in a timely manner.
Several roof penetrations and vents had to be moved to accommodate the new structural liner along with the corbels which obstructed some of the penetrations at their existing locations. This was done with the co-ordination of the client’s operations and process engineers to establish an optimal location for all the penetrations.
Extremely high temperatures during the day meant that all concrete pours had to be done during the night; the cementitious T-strata material had to be cooled down to enable the material to be poured at an appropriate temperature. Ice was used to cool the water to around 2C to help bring down the temperature of the mix.