Selective Paste Intrusion: Integration of reinforcement by WAAM — Concept and current research with special attention to cooling strategies

Selective Paste Intrusion (SPI) is an additive manufacturing (AM) process in which thin layers of aggregates are selectively bonded by cement paste only where the structure is to be produced. In this way, concrete elements with complex geometries and structures can be created. Reinforcement is required to increase the flexural strength of the concrete elements and, thus, enable their applicability in practice. Integrating the reinforcement is a difficult task, particularly in the case of SPI due to the layer-wise printing method. Especially with respect to possible complex structures, the production of the reinforcement needs to be adapted to SPI, thereby offering a high degree of freedom. One concept for a reinforcement integration is combining the two additive manufacturing processes SPI and Wire and Arc Additive Manufacturing (WAAM). However, since the two processes serve different fields of application, their compatibility is not necessarily given. Ongoing investigations show that the temperatures caused by WAAM adversely affect both, the cement paste rheology required for sufficient paste penetration into the particle bed and the overall concrete strength. This paper provides an overview of ongoing research focusing on different cooling strategies and their effects on the compressive strength of SPI-printed concrete parts. The studied strategies are increasing the distance between the printing nozzles and the particle bed for higher convectional cooling, and passive cooling by utilizing dry water particles. Temperatures up to 206 °C were measured at a distance of 40 mm to the welding zone, which indicates the need for active cooling. Dry water showed adequate cooling properties and can be deployed to create a free-flowing, water-storing bulk material, but has a negative impact on the compressive and flexural strength of SPI-printed specimens.