Apparatus for forming inverts in manhole floors between angularly-related pipeline sections includes a set of a plurality of wedge-shaped forming elements each having a molding surface of corresponding compound longitudinal and transverse arcuate curvature, the forming elements being of respectively varying longitudinally arcuate extents and being adapted for longitudinal end-to-end joinder for producing a variety of angular curvatures. The forming apparatus is set-up in the manhole between the pipeline sections with the molding surface downwardly facing and concrete is poured about the apparatus, the concrete thereby setting in conformity with the apparatus' molding surface to create a curvilinear invert between the pipeline sections.