Selective Paste Intrusion—Effect of Process-Integrated Welding of Reinforcement Segments on Bond Strength

Integrating Wire Arc Additive Manufacturing (WAAM) into the Selective Paste Intrusion (SPI) process enables the fully additive fabrication of reinforced concrete structures with complex geometries. However, the WAAM process transfers thermal energy into the reinforcement and the surrounding concrete matrix, which may affect the bond between steel and concrete. This study addresses whether the heat introduced by the welding of reinforcement segments impairs the bond between reinforcing steel and SPI-fabricated concrete. To isolate this thermal effect from the geometric process effects covered in a companion study, two groups of 16 mm reinforcing bars embedded in SPI-fabricated concrete were compared: bars whose segments were joined during printing by manual welding at seven joints per bar (process-integrated thermal exposure), and bars with adhesively bonded segments without heat input as the reference. Eight specimens per group from two production series were tested in push-through. A grey-body estimate based on the thermography and an assumed steel emissivity of 0.20 to 0.30 indicates approximate bar-surface temperatures of at least 214 to 277 °C near the joints immediately after welding and of 125 to 162 °C after 2 to 3 min. Despite these temperatures and the repeated heating and cooling cycles during printing, no discernible difference in bond capacity from the adhesively bonded reference group was detected: the welded group reaches a mean maximum bond stress of 20.5 MPa, the adhesively bonded reference 22.4 MPa, and the difference of 1.9 MPa (9%) lies within the scatter of the two groups (coefficients of variation 22% and 21%). With a 95% confidence interval of −7.0 to 3.1 MPa for this difference, a reduction of up to 31% cannot be excluded with this sample. All specimens of both groups failed by bar push-through, with a similar slip at maximum bond stress. Because the weld beads also change the joint geometry, the thermal exposure cannot be fully separated from the joint geometry. Since the procedure investigated here resembles a stud welding process rather than genuine WAAM fabrication, the results indicate that joining reinforcement segments by stud-welding-like processes during printing is a feasible route for integrating reinforcement into SPI with respect to the bond under short-term compressive loading. For genuine WAAM, further investigations are required, since welding occurs far more frequently than in the present procedure because of the smaller layer heights, so that heat accumulates and overlapping thermal cycles can produce even higher sustained temperatures at the steel–concrete interface.