Decatur Dam

Western’s Springfield branch, working for Global Structures, recently completed concrete repairs at the Decatur Dam. The scope of work involved the removal and replacement of deteriorated concrete on both the upstream and downstream sides of the dam using pre-placed aggregate repair mortar. Global Structures performed the upstream repairs which required certified divers to perform the repairs under water. The backside (downstream) side of the dam was de-watered to allow for repairs. The constant pumping of water and monitoring was completed by Global Infrastructures.

Saw-cutting, demolition, cleaning of reinforcing steel, installation of supplemental steel, surface preparation and clean up are daily tasks the Western crew performed. The intricate portion of this project was the use of pre-placed aggregate concrete. The surface repair is standard, but the installation of the pre-placed aggregate is different than regular concrete repair. Several steps are required in order for the installation of the material to work.

After the demolition, cleaning of the steel and installation of the supplemental steel and surface prep is complete, wooden forms are installed. The forms are installed in 15” tall increments, starting at the bottom of the repair opening. Prior to fastening the forms, a bead of sealant is applied around the perimeter of the repair opening. This acts as a gasket so that the material cannot leak out when it is placed. Once the 15” wide form is attached, supplemental anchors are installed for additional stability. Placement of the grout requires maintaining a pressure of five PSI inside the forms. Therefore, forms must be designed to limit deflections to meet ACI criteria for surface profile.

The form is then filled with thoroughly cleaned course aggregate (i.e., 3/4” up to 2”). Once the first form is filled, a bead of sealant is applied to the top of the previous form and the bottom of a new form and around the repair opening. This cycle continues until the end of the repair opening is reached. Due to the locations of the dam repairs and their configurations, all of the aggregate was placed into the forms manually with five gallon pails.

Once the forms were installed and secure, 1 1/2” inlets with gate valves were installed every five to six feet on center. The forms were then flushed with water to provide a saturated surface dry (SSD) condition. The Western crew them pumped BASF Master Builders Masterflow 928 non-shrink grout into the forms through the inlets with a 2,000 PSI grout pump.

The process begins at the bottom and the grout is pumped until it comes out the next inlet. The pump is then shut off and the gate valve is closed. The grout is then pumped into the next adjacent inlet. This process is continued until the entire form is filled.

Once the form was completely full, the crew continued to pump grout into the last inlet to pressurize. The internal pressure is maintained in order to force the grout into the pores and micro-cracks in the existing concrete surface. This repair method provides superior bonding. The last gate is closed to keep it under pressure until the material cures. The forms stayed in place for seven days and were then removed.