We will be using shotcrete to repair a concrete box culvert that has some minor spalling. Do we need to apply a bonding agent before applying the shotcrete? How should we prepare the surface?

No bonding agent is required. A key to a successful repair is proper surface preparation. The surface receiving the shotcrete must have the deteriorated material completely removed, be thoroughly cleaned, and in a saturated surface-dry condition (SSD) at the time of shotcrete appli­cation. Another key item is proper curing and protection following shotcreting. Details can be found in the Task Force 37 Report “Guide Specification for Shotcrete Repair of Highway Bridges.” The document is available from the American Associ­ation of State Highway and Transportation Officials (AASHTO), Washington, DC.