Types of Sealant Repairs In Concrete & Concrete Structures
Control joint sealant: control joints, designed to control where cracks appear in concrete placed horizontally or vertically, are themselves are sealed against water,frost,debris and to handle recurrent movement (listed below) using a flexible sealant.
Recurrent movement: Seal and repair cracks in concrete using injection of either epoxy resin or polyurethane where recurrent movement is expected (in my opinion a flexible sealant is what's called for here) when the movement is considered normal, perhaps caused by traffic vibration or temperature swings, and where the movement is not threatening structural damage.
Stop a water leak: Seal and repair cracks in concrete when it is important to quickly stop water leaking through a crack,seal those cracks in concrete using injection of either epoxy resin or polyurethane sealant.
Structural repair: Seal and repair structural cracks: for some situations (usually where an engineer, expert in masonry repair has made an assessment), actual structural repairs are made using injectable epoxy resin and possibly in some (probably low-load) cases by using injectable polyurethane sealant.