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3D printed concrete technology has received a traffic attention in construction industry in recent years, due to the potential advantage: time and labor savings, cost reduction, low environmental impact. In this study, the influence of distribution of aggregate and steel fiber on the mechanical properties of 3 D printed concrete is experimentally investigated. The distribution characteristic of aggregate and steel fiber in 3 D printed concrete are evaluated by using X-ray computed tomography (X-CT) with 2 D image analysis method. The cast concrete is performed as reference. It is found that the aggregate and fiber in cast concrete are relatively evenly distributed and no alignment, while there is an orientation probability of more than 40% for aggregate, and 74% for fiber, respectively, along the printing direction in ± 20° in printed concrete. Meanwhile, using X-CT, the porosity, void sizes, shapes and distributions in printed and cast concrete are also investigated. Furthermore, it is observed that the printed and cast samples comprise of different voids, due to the different construction processes. Finally, the results prove that the performance difference for the printed concrete at different directions possibly depends on the orientation distribution of the aggregate/fiber, as well as the irregular voids, since the aggregates/fibers are orientated by the nozzle along the printing direction. [ABSTRACT FROM AUTHOR]

3D printing technique is an opportunity for the development of architecture industry altered by the emergence of 3D printing concrete ink that shows unconventional characteristics mainly originated from its geometric design. In this paper, a novel 3D printing concrete ink that has good fluidity during movement and satisfying standing behavior at static state due to the structural rebuilding of cement paste advanced by the addition of nano clay (NC) and silica fume (SF) was specially designed to be extruded through a nozzle to print layer-over-layer components for an innovative additive manufacturing process. The buildability, rheological properties (viscosity, yield stress and thixotropy), workability, green strength, open time and hydration heat of the fresh 3D printing concrete were systematically investigated. Results indicated that the buildability of this concrete with a small quantity of NC or SF were increased by 150% and 117%, respectively, and remarkably enhances the thixotropy and green strength. The double-doped NC and SF optimize the buildability. [ABSTRACT FROM AUTHOR]

• Using X-CT combined with MIP is a reliable method for investigating the pore feature in printed concrete interlayer interface. • The bond strength in the middle interface of the printed element is higher than that of the upper and bottom interfaces. • The pores in the bottom interface tend to be interconnected or continuous due to the pressure of upper printed layers. 3D printed concrete is an emerging intelligent construction method, and it has been attracting an exponential attention in this sector. However, one of the major issues in printed concrete is the cold joint between the neighbor layers. In this study, an experimental study on the relation between the number of stacked layers vertically and the interlayer adhesion of 3D printed element was conducted. Splitting method was carried out to test the interlayer bond strength, mercury intrusion porosimetry (MIP) and X-ray computed tomography (X-CT) scanner were used to investigate the pore structure of the interlayer zone. The experimental results indicated that a markable influence of the stacked layers vertically on the interlayer adhesion is found. The interlayer bond strength in the middle interface of the printed element is higher than that of the upper and bottom interfaces, especially the bond strength in the bottom interface is the least. Such result is also reflected in the results of the MIP test: the lower total pore volume and total pore surface area existed in the middle interfaces. The X-CT results showed that a large number of elongated and flat pores present in the interlayer zone, the pores in the bottom interface tend to be interconnected or continuous due to the pressure of upper printed layers. An empirical model was proposed to account for the formation of the irregular pore and the change of bond strengths between the vertical neighbored layers of 3D printed element. [ABSTRACT FROM AUTHOR]

• Using X-CT combined with CsCl enhancing to study the water uptake in 3DPC filament. • Elongated pore can enhance water uptake rate, compared with molded concrete. • X-CT is a reliable method for investigating the pore feature in printed concrete. The water uptake in 3D printed concrete along the printing direction is experimentally studied using X-CT combined with CsCl enhancing, molded concrete is as reference. The pore feature of the printed and molded concrete is analyzed by re-structuring X-CT images. Results show that in printed concrete quicker ingress of water is acquired, compared with molded concrete. This could be related to the elongated pores along the filament in printed concrete, while almost of all the pores in molded concrete are in spherical shape. [ABSTRACT FROM AUTHOR]

• Using X-CT combined with CsCl enhancing to study the water transport in hardened 3DPC. • Steel fiber alignment can enhance water uptake rate, compared with molded concrete. • Steel fiber can not elevate the sorptivity of printed concrete along the filament. In this study, X-CT combined with CsCl enhancing method is used to investigate the effect of steel fiber on the unsaturated flow in printed concrete along the printing direction, and to contrast the molded concrete. Results indicate that this method could be used effectively to monitor the evolution of water ingress into cementitious materials continuously. The addition of steel fiber slightly reduces water absorption rate in printed specimens. Compared with the molded specimen, the fiber alignment enhances the sorptivity of the printed specimen significantly. [ABSTRACT FROM AUTHOR]

• In situ tracking water transport at the interlayer zone of 3D printed concrete using X-CT combined with CsCl enhancing. • The percentages of 0.1 mm-0.6 mm and 1 mm-4 mm voids size are observed to have an opposite effect on the water transport. • Connected or continuous voids induce a preferential water transport along the interlayer zone. • The spherical shape, independent and evenly distributed relatively voids were observed in cast concrete, while the irregular shape and interconnected or even continuous voids in 3DPC were detected. 3D printed concrete is a rising technology recently developed for construction industry. Unlike conventional concrete, since 3D printed elements are built by using a layer-deposition process, the distinct void generated between the printed concrete filaments considerably reduce mechanical strength and durability of 3D printed concrete. Using X-CT technology combined with CsCl enhancement, this study aims at investigating the water transport characteristics at different interlayer zones of 3D printed concrete, cast concrete as the reference. Meanwhile, the void characteristics of different interlayer zones are analyzed to establish quantitatively the relationship between the void distribution and the water transport process. It can be found that those voids at the interlayer zone led to a faster water transport along the printed concrete' interlayer zone than that of cast concrete. In addition, the percentages of 0.1 mm-0.6 mm and 1 mm-4 mm voids size are observed to have an opposite effect on the water. [ABSTRACT FROM AUTHOR]