In which of the following situations should you consider re-welding?

Re-welding should be considered when the concavity of the weld exceeds acceptable limits. Concavity refers to the inward curvature of the weld bead, which can significantly affect the structural integrity and performance of the joint. If the concavity is beyond specified parameters, it may compromise the strength and quality of the weld, making it necessary to re-weld in order to achieve the required standards and ensure safety.

In contrast, while a shiny appearance in a weld could indicate a potential issue with the weld's formation, it does not directly necessitate re-welding unless associated with other defects. Similarly, a dirty weld area is more about poor preparation than the weld itself, but it should be cleaned before welding rather than requiring re-welding. Lastly, welding on a vertical surface presents different challenges but does not inherently imply a need to re-weld unless other defects are present.