USING SHOTCRETE FOR REPAIR OF CONCRETE STRUCTURES IN A HIGH PURITY OXYGEN ACTIVATED SLUDGE SYSTEM

The County Sanitation Districts of Los Angeles County (Districts) treat 8.8 m3/s (200 mgd) of wastewater using a high-purity oxygen activated sludge (HPOAS) process at its main wastewater treatment plant, the Joint Water Pollution Control Plant (JWPCP) in Carson, California. This secondary treatment process has produced high quality secondary effluent for 16 years. Because the JWPCP HPOAS system utilizes enclosed reactors, carbon dioxide (CO2), a product of oxidation of organic material, builds up in the reactor head spaces. High concentrations of CO2 in solution have resulted in carbonic acid formation. The carbonic acid has caused considerable deterioration to occur in equipment and structures in the secondary treatment process trains. Up to 1.3 cm (0.5 inch) of section loss in some concrete structures has been observed. To repair and replace deteriorated concrete, a pneumatically applied cementitious overlay is being used. Testing of different surface preparations, application techniques, and overlay materials has shown that if the concrete surface is adequately prepared and if a high quality, low water-tocement ratio, silica fume rich cement mix is used for the overlay, the new material will have excellent adhesion to the existing concrete and very low permeability. Accelerated corrosion testing indicated that the cementitious overlays will significantly decrease the rate of concrete deterioration occurring at the JWPCP.