Designing polypropylene fiber reinforcement for 3D concrete printing: Rheology control, thixotropic recovery and mechanical performance

This study examines the influence of polypropylene (PP) fiber length and dosage on the rheological and mechanical performance of cementitious mortars and their validation in 3D printed concrete (3DPC). Mortars reinforced with 6 mm and 12 mm PP fibers at dosages between 0.10 % and 0.35 % were tested for yield stress, viscosity, thixotropy and strength development. Fiber addition significantly altered rheology, with static yield stress increasing by up to 101 % for 12 mm fibers and 22 % for 6 mm fibers. Dynamic yield stress showed similar trends, with long fibers providing over 60 % enhancement compared with approximately 20 % for short fibers. Distinct functions were identified: 6 mm fibers accelerated structuration, while 12 mm fibers improved re-flocculation and viscosity recovery, reaching developments of + 117 % and + 133 %. Mechanical tests confirmed that 12 mm fibers enhanced compressive strength by 17 % at 7 days and sustained 10 % gains at 28 days, whereas 6 mm fibers primarily improved flexural strength by 12 % at their optimum content. Normalization analysis indicated optimal dosage ranges of 0.20–0.25 % for 6 mm fibers and 0.15–0.20 % for 12 mm fibers, with the latter providing a more balanced performance. Validation through 3D printing confirmed the effectiveness of the optimized 6 mm mixture, where one-layer Z-direction specimens achieving 6.6 MPa flexural strength at 28 days compared with 2.7 MPa for cast specimens. These results highlight the importance of PP fiber tailoring in balancing printability and mechanical reliability for extrusion-based additive manufacturing of concrete. • Rheological properties enhanced by 0.2 % PP fiber at both 6 mm and 12 mm lengths. • 12 mm fibers increased dynamic yield stress by over 60 % at optimal dosage. • 6 mm fibers achieved 20 % yield stress development with wider processing range. • One-layer Z-direction specimens achieving 2.4 times flexural strength increase at 28 days compared with cast specimens. • Normalized analysis confirmed 0.2 % as optimal dosage for pumpability and buildability.