Root Concavity

Root concavity occurs when you don't put enough material into the root while welding or pull the weld too fast. The result is a too thin, concave underside of the weld that just isn't properly fused through. The thing is critical because the weld is weakest there. You quickly get notches and the risk of cracks, especially when the weld is later loaded. You can counteract by welding more calmly and with a bit more material so that the root is properly filled. If you already have the concavity, the only thing that helps is rewelding from the other side, if possible, or you grind out the spot and set a new root.