When a flow control and pressure release valve in an anaerobic filter failed in January 1989 at the Highland Creek Treatment Plant in East Toronto, Canada, the resulting buildup of fluid and methane gas pressure caused extensive cracking in the plant's domed concrete roof. As an alternative to replacing the cracked dome, engineers decided to restore it using a structural bonding process that involves injecting epoxy adhesives to permanently rebond cracked, delaminated concrete. After the epoxy is injected, it cures to a solid with a high bond strength, restoring structural strength and transferring stresses across the bonded crack.
The first phase of the project began with workers placing entry ports of .25-inch polyethylene tubing along the cracks, spacing them 7 inches apart. Two-component epoxy paste adhesives, applied to crack surfaces, served as temporary dams to seal the cracks for epoxy injection. To inject the two-component epoxies used for this phase of the project, workers used a two-component injection pump equipped with a hand-held mixing head and an injection nozzle. The second phase of the project involved microcrack sealing, a process that relies on penetration of the repair material into cracks by gravity and capillary action.