A large copper mine and refinery in the western United States had a dilemma. Their cell house, which contains over 1,500 cells, each holding more than 20,000 gallons of electrolyte, had experienced severe corrosion and structural degradation of the support columns for the tanks. These columns support the cells in their solvent extraction and electrowinning process. This process entails immersion of a stainless steel cathode or “starter plate” into the electrolyte. Pure copper is deposited onto the starter plate during this 10-day digestion process. The collected copper is then further refined at a separate location. Over time, highly acidic leakage from the cells had corroded the support columns to the point that their ability to adequately withstand the imposed load was in doubt. Additionally, the refinery desired to upgrade the facility’s ability to withstand seismic activity.
The leakage, primarily copper sulfate and 25% sulfuric acid at a pH of 1.0 or less, corroded not only the concrete but more significantly the No. 8 reinforcement bar (rebar) encased in the concrete. Corrosion of the rebar resulted in an increase of internal pressure due to expansion of the corrosion products, therefore putting the concrete in high tensile stress. The direct effect of this stress was cracking and spalling of the concrete. Figure 1 shows a typical degradated column requiring restoration.
The original construction of the columns used No. 8 rebar spaced 6-inches on center vertically and 18-inches on center horizontally. The refinery’s standard repair procedure was to remove corrosion products from the concrete and steel and then to top them with a polymer-modified portland-cement mortar. This standard repair method requires two (2) to three (3) days per column, and although temporarily affective, did not meet the company’s desire for a long-term solution. They decided upon a new approach using a polymer concrete (PC), which is a bisphenol A based-epoxy. This material is designed for maximum flowability, mechanical properties and chemical resistance. The PC repair system utilizes the polymer concrete for encapsulation, chemical protection, mechanical support and resistance to physical abuse. Figures 2 illustrates the method by which the stainless steel rebar was attached to the columns after surface-preparation. Stainless steel rebar was imbedded into the concrete floor using an epoxy mortar. Channels were saw-cut vertically in the concrete column. These channels provided a recess into which the rebar was bent and then secured into place with the epoxy mortar. Grouting of the rebar with this high strength epoxy mortar also served to provide tensile stress relief. By lowering stress relief, corrosion rates are reduced.
To further ensure structural integrity and to upgrade seismic capabilities, the company chose to use fiberglass reinforcement (FRP) strips and wraps under the PC. The strips were installed vertically on the columns and a fiberglass fabric was wrapped around the columns horizontally. The columns were formed and the polymer concrete was poured into place completely encapsulating the columns, the rebar and the FRP. This method required two (2) days per column. To date, 75 columns have been repaired using this method. Figures 3 and 4 show the forming and pouring of the PC. Figure 5 shows the PC after the form has been removed and the FRP that was applied to the columns.