Epoxy and polyurethane injection are often mentioned in the same breath because the installation method looks similar — drill ports, pump resin in under pressure — but the two resins are built for different jobs, and choosing between them depends entirely on what the crack needs to do afterward.

Epoxy resin cures rigid and extremely strong, effectively welding the two faces of a crack back together and restoring a meaningful share of the concrete's original structural capacity. It's the right choice for structural cracks — ones caused by loading, settlement, or design issues — where the goal is to stop the crack from moving and getting worse, not just to keep water out. Because epoxy is rigid, though, it isn't designed to accommodate ongoing movement; if the crack keeps flexing, a rigid epoxy repair can itself crack again.

Polyurethane resin cures flexible, so it can't restore structural strength the way epoxy does, but it excels at exactly the job epoxy struggles with: sealing a crack that will continue to move slightly with thermal or moisture-driven expansion, while remaining watertight. For basement leaks, tank cracks, and most non-structural water-ingress problems, that flexibility is precisely what's needed.

The practical rule of thumb: if the question is 'will this crack get worse structurally,' bring in an engineer and consider epoxy. If the question is 'water is coming through and I need it to stop, and the structure itself is sound,' polyurethane injection is almost always the faster, more appropriate, and more cost-effective repair.