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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]

• A new avenue to prepare rubber aggregate with excellent interfacial adhesion and compatibility was developed by combining ultrasonic mechanochemistry and a modified slurry coating. • The interface between rubber particles and cement matrix was improved and the mechanical properties were vastly enhanced. • The modification of rubber powder significantly improved the abrasion resistance of crumb rubber concrete. • The abrasion mechanism of CRC was systematically analyzed and the abrasion mechanism models of the rigid and semi-rigid body were established. • The interface between rubber particles and cement matrix played an important role in controlling the abrasion resistance of CRC. The abrasion resistance of crumb rubber concrete (CRC) has always been a research hotspot. However, the large reduction in the mechanical properties of CRC limits the improvement of its abrasion resistance. In this study, a new method of modifying waste rubber powder (WRP) combining mechanochemical action, ultrasonic energy accumulation and the coating effect of the modified slurry was proposed. The mechanical properties of CRC with rubber volume fractions of 0%, 15%, 25% and 35% were determined. The abrasion resistance of CRC was studied by the ball bearing and ball milling methods. The abrasion mechanism of CRC was analysed, and abrasion mechanism models of the rigid and semirigid bodies were established. The results showed that the compressive and flexural strengths of modified CRC (MCRC) were increased by 74.2% and 45.7% respectively, compared with CRC. The abrasion resistance of concrete first increases and then decreases with increasing rubber content. Compared to the control concrete, the abrasion resistance of CRC and MCRC was increased by 14.9% and 24.8%, respectively. The mass loss rates of CRC and MCRC were 29.78% and 24.98% respectively, under the same volume replacement. The enhancement of abrasion resistance of the CRC is mainly due to the absorption of wear energy by rubber. The rubber-cement matrix (R-CM) interface played an important role in controlling the abrasion resistance of CRC. [ABSTRACT FROM AUTHOR]

• Effect of saturation degree on water absorption of concrete was investigated. • Effect of volume fraction and particle size of coarse aggregates on water absorption were studied. • A model for predicting the rate of water absorption of concrete was established. Better understanding of water transport in concrete is of great significance to assess the durability and predict service life. In this study, effects of water to cement ratio (W/C), volume fraction of coarse aggregate (V a) and particle size of coarse aggregate on water absorption of concrete were investigated. Besides, more attention was paid to the effect of saturation degree (θ) on water absorption of concrete. The results showed that the process of water absorption can be divided into two stages: rapid absorption and steady absorption. The rate of water absorption increases with the increasing W/C of concrete. An increase of V a reduces the water absorption of concrete; however, the particle size of coarse aggregate has no effect on the water absorption of unsaturated concrete. The initial rate of water absorption decreases with the saturation degree of concrete increasing. According to experiment results, a model for predicting the initial rate of water absorption of concrete with any saturation degree is established. From that, the initial rate of water absorption of concrete with any saturation degree can be calculated. [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]

Apparent gas-permeability (k a) of concrete highly depends on pore-structure and medium. However, the effects of gas-medium on permeability have yet been investigated in depth. In this work, concrete with various water-to-cement ratios and fly-ash/slag dosages is prepared for k a testing. Various gas-mediums (CO 2 and O 2) are applied for analyzing their effects on the k a. Meanwhile, pore-structure characteristic, inner gas-pressure, and gas-absorption in concrete are observed for exploring permeation mechanism. The results illustrate that the narrowing pore-scale effect and surface-diffusion contribution derived from gas-absorption of CO 2 and O 2 both have minor impacts on the k a. The k a of O 2 (k O2) is higher than that of CO 2 (k CO2) and their relative permeability (k O2 / k CO2) ranges from 1.18 to 2.29. These deviations are mainly caused by significantly different Knudson diffusion occurring in the area near the concrete outlet-gas side (approximately 5%). • Gas permeability of concrete is measured by various osmotic-mediums (CO 2 and O 2). • Inner gas-pressure distribution of concrete is obtained under steady-state flow. • The apparent gas-permeability of concrete tested by O 2 is higher than that by CO 2. • Gas absorption of CO 2 and O 2 has minor impacts on apparent gas-permeability. • The deviation of apparent permeability is mainly caused by Knudson diffusion. [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]

Highlights • The various regular- and irregular-shaped aggregates are constructed using a pixel-based method. • The effects of aggregate characteristics on elastic and diffusive properties of concrete are investigated. • The aggregate characteristics apart from the aggregate type, play a slight role in the elastic properties of concrete. • The aggregates characteristics have a significant influence on the diffusive properties of concrete. Abstract Concrete is a complex three-phase composite material at a mesoscale, thus its overall elastic and diffusive properties strongly depend on that of each component. However, the effects of aggregate characteristics are little considered, especially the aggregate shape effect. In this study, a set of numerical methods is employed to evaluate the effects of aggregate characteristics on the elastic and diffusive properties of concrete. Firstly, a concrete representative volume element (RVE) is constructed using a pixel-based method by packing aggregates surrounding a soft interlayer as ITZ in a homogeneous mortar matrix. The different aggregate characteristics including morphology, fineness and type, are respectively considered. Subsequently, a set of the finite element-based and finite difference-based numerical method that is designed for the pixel-based concrete structure, is applied to predict the elastic modulus and the diffusion coefficient of concrete. The simulated results indicate that aggregate characteristics apart from aggregate type play a negligible role in the elastic modulus of concrete, while the effects of aggregate characteristics on the diffusion coefficient of concrete are pronounced. The aggregates with less regularity or coarser size can decrease the diffusion coefficient of concrete. Furthermore, the property of ITZ is another pronounced factor for the elastic and diffusive properties of concrete in addition to aggregate area fraction. The weaker ITZ can significantly decrease the stiffness and increase diffusivity of concrete. The modelling results are expected to guide the design of concrete materials from the perspective of aggregate characteristics. [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]

• An extrusion-based 3D printer for concrete with the largest aggregate size of 20 mm was introduced. • The range of ratios of cement and aggregate of printable concrete with coarse aggregate was recommended. • The direction-dependent mechanical properties of 3D printed concrete were investigated. • The voids distribution and the microstructure of interlayer area in 3D printed concrete were studied through X-CT and SEM. Concrete with coarse aggregate in the 3D concrete printing (3DCP) has broad prospects for high strength, low cost and shrinkage. In this study, an extrusion-based 3D printer was designed and utilized to print concrete with the largest aggregate size of 20 mm. Printable concrete with coarse aggregate was designed by different volume ratios of cement to aggregate (C/A). Then the effect of C/A on the printability and direction-dependent mechanical performance was investigated. X-ray micro-computed tomography (X-CT) was used to detect and analyze the voids distribution characteristics of printed specimens. Scanning electron microscope (SEM) investigations of microstructure at the interlayer area were conducted. Results indicated that the initial flowability of printable concrete should be within 178–200 mm, and the recommended printable C/A was within 0.35–0.60. The decrease of C/A improved the maximum printing height and mechanical performance but weakened the shape-stability of the multi-layer structure. X-CT results indicated that reducing excess slurry content caused by the decline of C/A decreased the compactness of the printed structure. The compressive strength and flexural strength of 3D printed specimens showed a direction-dependent characteristic, mainly related to the non-uniform distribution of voids revealed by X-CT. SEM images revealed the "micro-bridging" morphology in the interlayer area and proved that there were carbonation and structural weakening problems at the surface and surrounding of this area. [ABSTRACT FROM AUTHOR]

• Gas permeability of concrete was measured considering on coarse aggregate settlement. • Concrete permeability was tested using various gas pressures and confining pressures. • Permeability curve can be observed with four zones following gas-pressure increasing. • Steady zone with 15.0–30.0 bar gas pressures was recommended for permeability test. • Confining pressure of 50 bar was suggested for concrete gas-permeability measurement. Permeability is an important characteristic associated with the durability of cementitious materials. Gas permeability testing could overcome some shortcomings of traditional methods but its key parameters have so far not been identified for long periods, reducing comparability of test results. In this study, coarse-aggregate settlement in concrete was taken into consideration by dividing concrete cores into smaller segments and the gas-permeability of concrete specimens was measured by the modified CemBureau method. Apparent gas-permeability (K a) values of the segments were determined under multiple inlet-gas pressures (1.0 bar–40.0 bar) and various confining-pressures (30 bar–90 bar), which then were applied to calculate the intrinsic permeability (K int) values and Klinkenberg coefficient (β k) to obtain optimal parameters of inlet-gas pressure and confining-pressure. Meanwhile, X-ray computed tomography (X-ray CT), mercury intrusion porosimetry method (MIP) and specific gravity method (SG) were used for auxiliary calibration of test parameters. The results showed that four zones with different inlet-gas pressures were observed for apparent gas-permeability: unstable zone, metastable zone, stable zone, and anamorphic zone. Combined with gas-permeability, pore characteristics were employed for auxiliary calibration of parameter optimization. In sum, 15.0 bar–30.0 bar inlet-gas pressure and 50 bar confining-pressure were determined as the best parameters for concrete gas-permeability testing. [ABSTRACT FROM AUTHOR]