Enhancing mechanical properties of 3D printed cementitious composites utilizing hybrid recycled PP and PET fibers

The integration of fibers offers a means to fabricate intricate, load-bearing architectural configurations that were previously challenging to achieve with conventional 3D printed concrete. This study investigates the incorporation of recycled plastic fibers (RPFs) derived from polyethylene terephthalate (PET) and polypropylene (PP) to enhance the mechanical properties of 3D printed cementitious composites (3DPCC). Two distinct fiber lengths, 6 mm and 12 mm, were utilized for each fiber type across various fiber volume fractions ranging from 0.3 % to 1.5 % of the mortar volume. The results show that the optimal volume fractions are 1 % for 6 mm fibers and 0.7 % for 12 mm fibers. The findings indicate that hybrid combinations of PP and PET fibers achieve superior mechanical characteristics compared to the use of individual fiber types. The assessment of fresh mortar properties included fluidity, buildability, and extrudability, along with the evaluation of compressive and flexural strength as indicators of mechanical properties. Additionally, this study assessed the influence of a hybrid fiber mix comprising 40 % PP and 60 % PET of the 6 mm fiber length volume fraction, which exhibited the highest average compressive (20.6 %) and flexural strength (44.9 %) in the Z direction for 3DPCC, surpassing the performance of individual PP and PET fibers at the same volume fraction across all the volume ratios. When 12 mm fibers were utilized, the compressive and flexural strengths in the Z direction increased by 20.8 % and 46.7 %, respectively, for a mix of 20 % PP and 80 % PET of the total volume fraction. Concerning buildability, the control sample achieved a maximum of 29 layers, whereas the addition of 1.5 % PET fibers at 12 mm enabled the structure to reach 48 layers.