Mechanical behavior of 3D printed mortar with recycled sand at early ages

One of the technical challenges for the extrusion-based 3D printed concrete is that the requirements of green strength development are much different from ordinary concrete, due to the absence of formwork. This study employed the recycled sand into the mix of 3D printed mortar and explored the green strength development at early ages within 2.5 h after extrusion. 25% and 50% natural sand was replaced by recycled sand which was crushed from waste concrete in the mix. Uniaxial unconfined compressive tests were undertaken to obtain the mechanical properties of the printable mortar, while the failure patterns, vertical load-displacement relationship, lateral deformation and stress-strain behavior were recorded and fully analyzed. It is found that the recycled sand had very limited influence for those immature specimens, while exerted significant effect on the early age behavior for those mature specimens. The addition of recycled sand will change the mortar from plastic material to solid material, especially when the rest time reaches 90 min after the extrusion. It is believed that the high cement paste content and large water absorption of recycled sand in the 3D printed mortar affects the early age behavior. Since the incorporation of recycled sand improves the buildability while reduces the open time of the printed material, it is necessary to reasonably adjust the mix ratio and the amount of recycled sand in practical printing. (C) 2020 Elsevier Ltd. All rights reserved.