Effect of Aggregate Gradation on the Properties of 3D Printed Recycled Coarse Aggregate Concrete

A simplex centroid design method was employed to design the gradation of recycled coarse aggregate. The bulk density was measured while the specific surface area and average excess paste thickness were calculated with different gradations. The fluidity, dynamic yield stress, static yield stress, printed width, printed inclination, compressive strength and ultrasonic wave velocity of 3D printed recycled aggregate concrete (3DPRAC) were further studied. The experimental results demonstrate that, with the increase of small-sized aggregate (4.75–7 mm) content, the bulk density initially increases and then decreases, and the specific surface area gradually increases. The average excess paste thickness fluctuates with both bulk density and specific surface area. The workability of 3DPRAC is closely related to the average excess paste thickness. With an increase in average paste thickness, there is a gradual decrease in dynamic yield stress, static yield stress and printed inclination, accompanied by an increase in fluidity and printed width. The mechanical performance of 3DPRAC closely correlates with the bulk density. With an increase in the bulk density, there is an increase in the ultrasonic wave velocity, accompanied by a slight increase in the compressive strength and a significant decrease in the anisotropic coefficient. Furthermore, an index for buildability failure of 3DPRAC based on the average excess paste thickness is proposed.