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This study investigated 3D concrete printing of permanent formwork for concrete column construction. The effect of different hydroxypropyl methyl cellulose (HPMC) contents (0, 0.0003 and 0.0006 by mass of binder) and the water-to-binder (W/B) ratios (0.27, 0.29 and 0.31) on the rheological properties, structural build-up and mechanical performance were studied using several mixtures for manufacture of the permanent formworks. The results showed that the mixture with the HPMC = 0.0006 and the W/B = 0.27 showed the maximum static yield stress, largest thixotropy and maximum green strength, and thereby selected as the optimum mixture. The plastic failure of the optimum mixture was also predicted using a thixotropy model and was compared with the experimental results. Subsequently, three concrete columns with different longitudinal steel reinforcement ratios (0.0%, 1.9% and 2.5%) were constructed using the printed concrete as the permanent formwork and tested in compression. Good bonding was observed at the interface of the cast-in-place concrete and the printed concrete permanent formwork. In addition, it was observed that the initial stiffness, the maximum bearing capacity and the corresponding longitudinal displacement of the concrete columns increased, as the longitudinal reinforcement ratio increased. The counterpart concrete columns using the conventional formworks were also constructed and tested for comparison. In comparison, the concrete columns made using the printed concrete as the permanent formwork obtained a higher stiffness and bearing capacity than the counterpart conventional concrete columns. The reasons for the differences are explained. [ABSTRACT FROM AUTHOR]

• A novel mix proportion was developed to produce ECC samples by 3D concrete printing. • The printed ECC beam presented a ductile failure mode under four-point bending. • The practical application feasibility of printed ECC beam was implemented. Engineered cementitious composites (ECC) is emerging as promising candidate cementitious materials for 3D concrete printing (3DCP). This paper experimentally investigated the bending behaviors of 3D printed ECC beams with different geometric configuration. A novel mix design for ECC with ultra-high molecular weight polyethylene (PE) fiber was developed and the PE-ECC beams were casted by two methods, i.e., mold-cast and extrusion-based 3D printing. The four-point bending tests were conducted for plain ECC beams, reinforced concrete (RC) beams as well as cast-in-situ unreinforced ECC beam. The influences of different parameters including fabrication methods, print angles and superposition types were particularly investigated. The anisotropic mechanical properties under bending of printed beams and bending capacity of cast in place reinforced concrete beams were also investigated. Also, the strain distribution of longitudinal steel reinforcement in reinforced concrete as well as printed ECC beam were studied. The results indicated that the printed ECC beam presented a ductile failure mode accompanied with strain hardening behaviors. Also, the ECC beams exposed to z direction loading showed better mechanical anisotropic properties when compared with those exposed to x direction. [ABSTRACT FROM AUTHOR]

The current 3D concrete printing (3DCP) technology is limited by the reinforcing methods. Conventional steel reinforcement is hard to be incorporated in the 3DCP process. To overcome this limitation, this study aims to develop 3D-printable engineered cementitious composites (ECCs) exhibiting ultra-high tensile strain capacity of more than 8%, which can be used for digital construction of 'self-reinforced' concrete structures, reducing the reliance on the conventional steel reinforcement. Different volume fractions of polyethylene fibers (1%, 1.5% and 2%) were used to reinforce the ECC matrix. The fresh properties (including the workability, rheological properties and buildability) and the hardened properties (including the compressive and flexural strengths, and the uniaxial tensile performance), along with the microstructure of the developed 3D printable ECCs were experimentally investigated. Conventionally mold-cast ECCs were also prepared and tested for comparison purposes. The results showed that the developed 3D printable PE-ECCs exhibited strong strain-hardening behavior with the tensile strength and tensile strain capacity of up to 5.7 MPa and 11.4%, respectively. In addition, the results showed that the 3D printed PE-ECCs exhibited superior tensile performance to the conventionally mold-cast PE-ECC counterparts. This finding is in good agreement with the results of the microstructural analysis. Unlabelled Image • A novel 3D-printable (3DP-ECCs) with ultra-high tensile ductility of more than 8% was developed for digital construction. • The 3DP-ECCs exhibited uniaxial tensile strength and tensile strain capacity of up to 5.7 MPa and 11.4%, respectively. • The 3DP-ECCs exhibited superior tensile performance to the conventionally mold-cast ECC counterparts. • The microstructural analysis explained the underlying reasons for the observed superior tensile performance of the 3DP-ECCs. [ABSTRACT FROM AUTHOR]