Counteracting steel corrosion on the outside face of twenty-four separate 24-ft. diameter x 4.5 ft. tall reinforced fan stack “rings” required partial depth concrete repair. Application of an epoxy coating on the concrete interior eliminated the vapor transmission and stopped the corrosion cell. Access to the inside of the fan stack necessitated professionally-engineered steel supports and planking. Safety in the power plant was extremely important. Both personal safety and all tie-off requirements were met successfully. Coating surface preparation was completed using 20,000-psi water wands. This project included partial removal of joint grout and joint waterproofing, as well as 2400 sq. ft. of partial-depth concrete spall repair. Chipping hammers (weighing 15 lbs. each) were used for bulk concrete removal. Dry sandblasting, with the appropriate respirator equipment, was used for surface preparation. Dry mix shotcrete was employed as the repair material. The average repair thickness was 4 inches. Accurate scheduling on this particular project was critical. Over a four-week period, the plant was shut down while the numerous repairs inside the stacks were completed. These included: 1,980 lineal feet of joint repair; 2,100 lineal feet of epoxy injection; 13,500 sq. ft. of surface preparation; and an epoxy coating. Concrete repairs were completed after operation start-up. Subsequent testing proved the effectiveness of our corrosion reduction measures.
Field Problems Encountered
- The discovery of a previously unidentified crack changed the site condition. Epoxy injection was added to the scope of the work and other detailing was altered to offset increased costs.
- We alleviated concerns that arose prior to the letting of the contract regarding concrete outgassing by taking water vapor transmission tests before applying the coating.
- Concerns regarding both concrete substrate temperatures and their subsequent effects on concrete-to-concrete bonding in partial-depth patches were addressed by tensile bond pull-off testing of the concrete repairs.