The University of Nantes has developed a 3D printing technique (BatiPrint3D TM) dedicated to the construction of the walls of a house. This innovative on site construction technique is based on the deposition of two layers of expansive foam used as a formwork for a third concrete layer. It allows to build at the same time the structure and the insulation. In this process, the polyurethane (PU) foam acts as a formwork whereas PU has weak mechanical properties. The filling with concrete can yield high deformations and even failure of the formwork. The aim is to present the experimental device that allowed to test different filling procedures and to determine the one that yields low deformations of the formwork in accordance with the maximum value given by the standards of construction. The results show that spacers are needed and that the height of concrete poured must not exceed 40cm.