Encasement of Pre-Placed Reinforcement in Injection 3D Concrete Printing: the Effect of Rheology and Process Parameters

Abstract

Injection 3D Concrete Printing (I3DCP), where material is robotically injected into a carrier liquid and remains stable has been successful at producing complex concrete structures. I3DCP is capable of overcoming the directional limitations faced by other additive fabrication methods. However, this technology has been limited to producing compression-only structures, as its thin concrete strands are incapable of withstanding significant tensile loads. A potential solution is the introduction of reinforcement into concrete structures. This study focuses on the injection of a fine grain concrete into a carrier liquid with defined rheological properties and the capability to encase reinforcement bars, which are spatially fixed inside the carrier liquid, during the printing process. The effect of material- und process related parameters on the encasement quality are studied. The rheological parameters of the carrier liquid are varied by solid volume fraction and the addition of viscosity modifying admixtures. The shape of the nozzle (flat/U-Shape), the nozzle traverse speed (ranging from 20 mm/s to 60 mm/s) and the distance from nozzle to rebar (ranging from 5 mm to 15 mm) are systematically studied. The quality of the encasement is evaluated by image analysis. Selected probes are mechanically tested in pull-out-tests. It is observed that with increasing yield stress of the carrier liquid the reinforcement is less encapsulated. This effect can be counteracted by changing the nozzle shape and/or print speed. Finally, the potential and limitations of using reinforcement bars in I3DCP are discussed.