Structure: Nine-inch-thick slab-on-grade secondary containment area with protective lining system for protection against 98-percent sulfuric acid
Location: Western United States
Age: 20 years
Materials of Construction: Standard 60-ksi reinforcing steel bars with 3,750-psi concrete. Jointing included construction, contraction and isolation with slab panel joint layout of 1.5 (long side) to 1.0 (short side). Construction joints were keyed with integral waterstops. Contraction joints were preplanned with dowels inserted during casting and sawcut subsequent to finishing. Isolation joints incorporated waterstops and were drilled, doweled and single-side greased with expansion-board.
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Process Environment: The basement-level slab-on-grade (SOG) exists below a series of rectangular acid tanks containing 98-percent concentration sulfuric acid. Due to product immersion, the process stream overflows the acid tank’s freeboard and spills onto the SOG, below. The SOG is sloped toward sumps, and the collected acid is pumped from the basement-level back into the first-level acid tanks for recycling. Ambient temperatures exist at the basement level; however, the 98-percent sulfuric acid is processed at 120 degrees Fahrenheit.
Loading Conditions: Primarily, loads experienced by the SOG include pedestrian foot traffic and occasional light motor vehicles.
Owner’s Concern: A recent protective lining restoration (less than six months old) of the SOG was in a generalized state of failure. The owner spent a significant amount of time, effort and money on the failed program, and the contractor who performed the work would not honor the implied warrantee conditions of the contract. The contractor asserted that the concrete substrate was contaminated with acid and that the owner was responsible for surface preparation and for providing an uncontaminated concrete substrate prior to protective lining installation.
Repair Approach: A team was assembled to discover the root-cause failure mechanisms of the new protective lining. Results confirmed the substrate was in fact contaminated down one inch into the existing concrete substrate. After the area was shot-blasted, it appeared to be sound and clean and ready for application of any type of protective lining. However, once collected concrete specimens were tested in an analytical laboratory, the low pH and acidic residue left within the concrete’s pores made a strong case for removal and replacement of contaminated concrete materials prior to installation of the protective lining.
The approach to this project was complicated by the fact that the failed protective lining was installed during a planned process unit outage (T/A), and any further restoration programs would need to be performed while the process unit was on-line. Due to the sheer size of the SOG covering several acres, innovative ideas and out-of-the-box solutions had to be considered regardless of whether the solutions had ever been employed in an industrial environment. Equipment and techniques used in the commercial, public and transportation sectors were evaluated with roadway demolition techniques as well as parking structure repair strategies viewed as available and appropriate for the subject structure.
Repair Strategy: To develop a successful protective lining prototype, it was critical that the depth of contamination was accurately determined and a pilot program be instituted for evaluation. Essentially, a trial area was selected and a full-scale mock-up installed of the proposed protective lining system. The system’s performance was then evaluated over time as the mock-up was installed using the same equipment, manpower, supervision and materials as would be expected during full project production.
Repair Program: Following the successful service of the mock-up for a minimum period of six months, the owner selected the mock-up means and methods for implementation. Dripping and intermittent deluges of 98-percent sulfuric acid were a primary concern not only to the work product but to the safety of contracting personnel as well. Overhead acid-resistant plastic sheeting was installed, creating a working envelope and barrier containment against process acid contact. Demolition and removal of contaminated concrete materials required production techniques similar to those employed in highway work, yet the work had to clear low-overhead concrete beams. A low-profile, roadway-milling machine with carbide-tipped teeth easily ground through the contaminated concrete to a depth of one and a half inches.
The areas were then high-pressure (greater than 10,000 psi) water-blasted, and a high-early strength bonded concrete overlay was installed on top of prepared surfaces, redeveloping SOG design cross-sections and contours (i.e., drainage slope). The overlay reestablished the original jointing layout to reduce the potential of unplanned reflective cracking in the protective lining system. The quick-setting, rapid-outgassing concrete was ready for protective lining application within three days of overlay installation. Once the protective lining was installed, curing of the chemically resistant polymers occurred within another three days. Then the protective lining could be placed into service and the containment sheeting removed and disposed of in an environmentally responsible manner.
Lessons Learned: Truly aggressive worksite conditions require well-planned and considered measures. Qualitative as well as quantitative means and methods are required to ensure success in a repeatable manner. Understanding the root-cause deterioration mechanisms sometimes requires wading through data, as in the case of the original failed protective lining, to separate facts from conjecture. These facts are important to the decision-making process, as is verification of means and methods of construction through pilot test areas and mock-ups.