Additive manufacturing is becoming increasingly important in the construction industry. Wire arc additive manufacturing (WAAM) can be integrated into the selective paste intrusion (SPI) to enable the simultaneous printing of reinforced concrete. The bond behavior of a WAAM reinforcement was investigated with pull-out tests and compared to alternative reinforcement types to analyze the stress transfer between the different components. In the first step, the surface of all the reinforcement types was recorded using a laser-based line scan measuring system. This permits the evaluation of the surface parameters, such as the surface roughness R_q, or the related rib area f_R. The WAAM reinforcement showed a bond behavior in the pull-out tests that was comparable to a reinforcing steel bar. Both the bond stresses achieved, and the occurring scatter of the measurement results at the characteristic slip values were almost the same. Even without existing transverse ribs, the WAAM reinforcement reached maximum bond stresses similar to the reinforcing steel. An evaluation of the surface roughness revealed a linear relationship with the maximum bond stress achieved with a logarithmic scaling of R_q. The bond work W_τ, which is a measure of the system stiffness, showed that WAAM reinforcements and reinforcing steel bars have approximately similar behavior.

The Selective Paste Intrusion (SPI) is an additive manufacturing method in which thin layers of aggregates are bond selectively by cement paste only where the structure shall arise. In this way, concrete elements with complex geometries and structures can be produced. To meet the optimum between required layer bonding and sufficient shape accuracy, the rheological properties of the cement paste, i.e., its yield stress and dynamic viscosity, are crucial [1, 2]. The combination of the SPI process and the Wire and Arc Additive Manufacturing (WAAM) process enables the production of free-formed, high-strength reinforced concrete elements, which opens up a wide range of applications. However, the WAAM process generates high temperatures, which affect the rheological properties of the cement paste and thus the printing quality [3, 4]. Therefore, we analyzed the effect of external temperature loads on the rheological performance of cement paste over the entire SPI production period and derived a maximum acceptable temperature load for the combination of SPI and WAAM. The experiments showed decreasing viscosity and increasing yield stress values by stepwise increasing the paste temperature from 20 °C to 60 °C. Between 60 °C and 70 °C, the rheological behavior suddenly changed, and both viscosity and yield stress instantly increased to a multiple of their initial values. In a subsequent numerical simulation of the intrusion behavior of the paste in the particle bed, we could show that the high yield stress and viscosity lead to poor paste penetration and thus insufficient layer bonding, whereas paste temperatures up to 60 °C are not detrimental to the SPI process. Therefore, the results demonstrate that the combination of SPI and WAAM is possible if the WAAM process is adjusted by e.g. cooling strategies, increased distance of the welding point from the particle bed, or increased time intervals between the welding points to avoid paste temperatures exceeding 60 °C.

Integrating Wire Arc Additive Manufacturing (WAAM) into the Selective Paste Intrusion (SPI) process represents a groundbreaking approach for fabricating reinforced concrete structures with complex geometries. This study investigates the bond strength between concrete and WAAM reinforcement under varying temperature conditions to understand the behavior of heated reinforcement bars within fresh concrete and its effect on the related bond strength. By conducting pull-out tests according to RILEM RC6, WAAM reinforcement bars were heated to predefined temperatures of 20 °C (ambient), 60 °C, 80 °C, and 200 °C for 18 min. The results show that while moderate thermal exposure (60 °C and 80 °C) led to a slight reduction in the maximum bond strength, a notable degradation occurred at 200 °C, indicated by a marked decrease in both maximum bond stress and early bond development. These findings provide initial insights into the thermal limitations of WAAM integration within SPI processes. The goal is to address the challenges associated with integrating WAAM into SPI, particularly the adverse effects of high temperatures generated during the welding process on the rheological properties of the cement paste, the penetration behavior of the paste in the particle bed, and ultimately, the mechanical properties of the hardened concrete. This technique allows for producing nearly free-formed reinforcements, thus complementing the advantage of SPI in producing free-formed structures of almost any geometry.