Conventionally reinforced concrete tabletop project profile

Sept. 13, 2006


Structure: Conventionally reinforced concrete tabletop vessel support structure – massive concrete frame that is pile supported

Location: Midwest United States

Age: 30 years

Materials of Construction: Standard 60-ksi reinforcing steel bars with 3,000-psi concrete

Process Environment: Multiple vessels (vertical drums) are filled with 1,000-degree Fahrenheit residual petroleum products. The hot petroleum is cooled, quenched, and subsequently cut out of the drum and conveyed into an adjacent subsurface pit.

Loading Conditions: Each drum fluctuates at 12-hour intervals from dead load (~200,000 pounds) at ambient temperature to loaded condition (~1,000,000 pounds) at 1,000 degrees Fahrenheit and then back to the dead-load state where the process begins again.

Owner’s Concern: Upgrading the process for efficiency and maximizing the process unit production by installing new larger drums during a planned outage (T/A). However, obvious examples of embedded reinforcing steel corrosion and significant concrete material section losses in association with increased drum loadings cast doubt as to the long-term capability of the structure to support the new imposed loads.

Repair Approach: Contracting with a design-build concrete repair firm, an evaluative approach was selected as the path forward with the owner/designer/contractor active participants in the repair process. The evaluation discovered the physical characteristics of the concrete materials and embedded reinforcing steel to be in accordance with original construction documents. However, the existing chemical properties of the concrete were poor with a high water/cement ratio (i.e., relatively porous), carbonation present to the depth of the embedded reinforcement and heavily laden with chloride ion (several times the ACI threshold amount). Additionally, original construction defects in the form of localized voiding (i.e., honeycomb) and a lack of protective concrete cover (1/4 inch instead of the specified two and a half inches) at some locations had accelerated deterioration.

A structural analysis of the massive structure revealed the original conventionally reinforced concrete construction was marginally overstressed (less than five percent) with the new imposed loads but found to be acceptable in its original as-built condition. Unfortunately, the deteriorated condition placed the structure at risk with the new imposed loads and the structure would require repairs prior to drum replacement.

Repair Strategy: Based on the information developed during the course of the evaluation, the owner selected a repair-in-kind approach to take place while the process was on-line. Strengthening was not required; however, at a minimum, the original concrete cross section and reinforcing steel integrity had to be reestablished. The on-line approach intent was to keep civil contracting work outside the T/A due to the large amount of interdisciplinary tradesmen associated with process modifications, operating within the same workspace during the outage. Work would be performed around the operating process. The environment within the structure becomes hazardous during cutting operations, and all contracting personnel were evacuated during the approximate three-hour cutting sequence. The reinforced concrete structure supports multiple vessels, so effectively only five hours per shift were available for work activities and the schedule changed daily. In addition, scaffolding, equipment and debris needed to be cleared out of the process operators’ way during cutting activities, and the concrete substrate work surfaces required protection due to potential hydrocarbon contamination created by incidental contact with the process stream (carbon-rich tailings).

Repair Program: Repair activities initiated by convening a series of jobsite preplanning meetings involving the owner’s operating personnel and contractor supervisory personnel to walk through the repair process step-by-step, identifying areas of obstruction and areas requiring protection or shielding. The meetings also helped identify opportunities to be creative in equipment placement and utility relocation. The team developed safety execution and contingency action plans during these meetings as well. Work platforms, air, water and electrical lines as well as debris handling chutes supported above and out of the way of the process area were key to the project’s success.

The repair program was performed under hot-work permit conditions and included the use of standard demolition techniques (small pneumatic jackhammers with water misters) forming square sawcut rectilinear repair cavities with follow-up surface preparation using high-pressure (greater than 10,000 psi) water-blasting. QA/QC of prepared surfaces required insitu, uniaxial bond pull-off test results exceeding 200 psi, prior to installation of repair materials. Corroding reinforcing steel bars were examined upon exposure and augmented when reduction in cross section exceeded 20 percent. Passive cathodic protection, in the form of embedded anodes, were installed within repair cavities to arrest the halo corrosion effect along repair perimeters in heavily laden chloride-contaminated parent concrete substrates. Repair details included the use of mechanical anchors, prefabricated fire-retardant panelized formwork, non-corrosive formwork assemblies and ties. All formwork joints were caulked, creating mortar-tight seams. Ball-type valves were attached to the formwork along with form vibrators, and the repair material placement proceeded in accordance with form and pump repair techniques. Subsequent to curing, the forms were removed, repair surfaces ground smooth and a thick curing compound applied to exposed, newly repaired concrete surfaces.

Lessons Learned: The repair progressed over a period of several months, working around variable process schedules, temperature extremes, unscheduled plant events and unscheduled owner-generated downtime. Significant uncompensated downtime for entire work crews can greatly affect the profitability of a difficult repair project, and a clear path of communication must exist between the owner and contractor as well as a verifiable paperwork audit trail in order to recover costs that could be interpreted as the cost of doing business. This project involved a multi-year warranty period, requiring periodic inspection by qualified representatives from the project participants. There are costs associated with this work; however, the contractor has an opportunity to revisit the owner, review the work, repair small anomalies but also point out the successes of their work product in-service.

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