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The tensile behaviour of engineered cementitious composites (ECC) is highly dependent on their microstructure characteristics. To date, the strain-hardening behaviour of printed ECC in relation to its microstructure is not yet fully understood. This study presents a systematic investigation on the macroscopic mechanical properties of normal and printed ECC with various polyethylene (PE) fibre lengths (6 and 12 mm) in relation to their microstructural features in terms of pore structure characteristics, fibre orientation and fibre dispersion through a series of mechanical tests and X-ray computed tomography (CT) and backscattered electron (BSE) image acquisition, processing and analysis. Results indicate that it is desirable to use block specimens for mould-casting fabrication as contrast to printed ECC samples. The printed ECC containing 1.5 vol% 6 mm and 0.5 vol% 12 mm PE fibres by extrusion-based 3D printing exhibits unique tensile ductility of over 5% and average crack width of less than 100 μm. Regarding pore structure, normal ECC has a higher probability of large pores (over 1 mm3) than printed ECC, which would increase the risk of damage localization and lead to a significant variation in tensile properties. Besides, normal ECC with thickness of 30 mm and printed ECC possess a similar fist cracking strength as indicated by similar pore size and fracture toughness. Compared to normal ECC, printed ECC has a more uniform dispersion of PE fibres, the orientation of which is more perpendicular to the loading direction, resulting in a higher average tensile strength and strain capacity than normal ECC. [ABSTRACT FROM AUTHOR]

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]