In 1993 the concrete roof of a digester tank in Bedford, Nova Scotia, collapsed. The most probable reason for the failure was deterioration of the pre-stressed gunite concrete combined with excessive pressure in the tank. The concrete roof was replaced with a "gas holding" steel roof. Over the years the concrete tank, which measured 35 feet in diameter and 22 feet high, had developed vertical and horizontal cracks, so during the remedial work the walls were repaired and waterproofed.
The repair required long term sealing of the cracks in the reinforced concrete wall as well as concrete protection. The designer of the repair, Porter Dillon Ltd. of Halifax, Nova Scotia, selected two materials for the repair: Gem-Crete HDO, a fiber reinforced, micro-silica enhanced mortar, and Cem-Kote Flex ST, a flexible, polymer modified cement. Both products are manufactured by Gemite Products Inc. Mississauga, Ontario.
The Gem-Crete HDO was applied as a primary waterproofing layer. Micro-silica enhanced mortar provides an excellent waterproofing material and the fiber reinforcement gives tensile strength, toughness and substrate crack spanning.
Since it was difficult to determine how much the cracks would move under fully loaded conditions, the Cem-Kote Flex ST was applied as a secondary waterproofing and protective layer. The high level of polymer modification of the product allows for crack spanning and improved chemical resistance when compared with conventional concrete.
After cleaning the tank by sandblasting, the crew covered the vertical and horizontal cracks using a galvanized welded fabric, 16 gauge, 12-inch wide strip with openings 2-inches by 1-inch, mechanically fastened to the concrete.
After the placement of the reinforcing fabric over the cracks, a 1/2-inch thick layer of Gem-Crete HDO was applied to the concrete surface of the tank. The material was mechanically applied using the "wet process" shotcrete method. The following day the surface was thoroughly cleaned with high pressure water. An approximately 1/8-in. thick layer of Cem-Kote Flex ST was rolled on in two coats. The waterproofing, protective system was air-cured for approximately one week before the tank was put back into use.
This restoration and waterproofing work was carried out by Jim Mason Construction Ltd., Waverley, Nova Scotia.
The performance of the repair has been excellent. The interior of the tank was inspected in 1997, three years after the installation, and no leaks or deterioration of the waterproofing system was found. Further inspection in 1999 also revealed no leaks.
