The effect of accelerator dosage on fresh concrete properties and on interlayer strength in shotcrete 3D printing

Recently, the progress in 3D concrete printing has developed enormously. However, for the techniques available, there is still a severe lack of knowledge of the functional interaction of processing technology, concrete rheology and admixture usage. For shotcrete 3D printing technology, we present the effect of accelerator dosages (0%, 2%, 4% and 6%) on fresh concrete properties and on interlayer strength. Therefore, early yield stress development up to 90 min is measured with penetration resistance measurements. Deformation of layers under loading is investigated with digital image correlation and a mechanical testing machine. One point in time (10 min after deposition) is examined to quantify vertical buildability of elements depending on the accelerator dosage. Four different interlayer times (0, 2, 5 and 30 min), which occur for the production of small and large elements as well as due to delay during production, are investigated mechanically as well as quantitatively with computed tomography regarding the formation of cold joints. With increased accelerator dosage, an instantaneous increase in early age yield stress and yield stress evolution was observed. An increase in interlayer time leads to a reduced strength. This is mainly attributed to the observed reduced mechanical interlocking effect of the strands. Finally, a model to describe interlayer quality is presented. In the end, advantages as well as limitations of the findings are discussed.