Experimental exploration of digitally fabricated connections for structural concrete

Abstract

[EN] Connections are a persistent challenge for traditional reinforced concrete construction and even more for digital fabrication with concrete (DFC). The latter, on the other hand, opens up new possibilities for producing connections with tailored geometries and surface roughness. Based on the findings of an exploratory, experimental campaign, this paper discusses the design, fabrication and structural testing of digitally fabricated joints using the DFC technology Eggshell to 3D print the formwork for joint surfaces in an additive manufacturing process. The programme includes (i) unreinforced and reinforced construction joints whose joint surface texture was varied to control the roughness and (ii) digitally fabricated dry keyed joints with varying geometry. Both, the construction joints as well as the dry keyed joints, were produced using set-on-demand concrete with a relatively small maximum aggregate size of 0 to 4 mm ¿ i.e., strictly speaking, mortar rather than concrete ¿ as typically applied in DFC. The experimental campaign included thorough instrumentation: a 3D scan of the joint surface before testing and high-resolution digital image correlation (DIC) combined with distributed fibre optical sensing (DFOS) during testing. The deviations from the targeted surface geometry, determined with 3D scans, were below 2 mm, with a standard deviation between 0.4 mm and 0.7 mm for keyed joints. The high-resolution DIC allowed measuring the compressive strains locally at (i) the corrugation of unreinforced or reinforced construction joints and (ii) the key chamfer of dry joints and, hence, studying strain variations over the height of the joints and the efficiency of different keys with varying dimensions. Finally, the shear transfer capacity, evaluated in a push-off test setup, showed that all digitally fabricated joints met or even outperformed the requirements defined in pertinent design codes despite the smaller aggregate size.