A waterstop assembly for use as embedded components in at least a pair of adjacent concrete slabs, defining elongated first and second waterstop panels including a main body sheet to which is mounted by coextrusion or insertion to both of its opposite top and bottom ends a soft expansible flange made from hydrophilic thermoplastic material. Each hydrophilic soft flange has such an expansion capability as to provide strong radially outward pressure applied by the soft flange against the adjacent concrete slabs, thus providing watertight interconnection therebetween. A welding block is taken in sandwich between overlapping transitional end portions of the first and second waterstop panels. Integral ridge and groove connectors interconnect the welding block with the waterstop panels. Resistance wires connected at one end to an electric power source fuse the welding block and the first and second waterstop panels, wherein a watertight joint is formed therebetween. An upright steel post is mounted spacedly parallel to a corresponding one of first and second waterstop panels, and a transverse anchor arm transversely spacedly retaining waterstop panel in upright operational condition parallel to the steel post before and during first concrete pour.