Elements fabricated by extrusion-based 3D concrete printing (3DCP) display anisotropic mechanical properties when subject to loading conditions orientated in principle cartesian axis directions. ...Various studies show that the mechanical characteristics of 3DCP components are lower than their mould-cast counterparts, reportedly due to the existence of weaker, more porous, interfacial joints in-between filament layers. To promote the widespread adoption of 3DCP, the elastic response and strength parameters of 3DCP elements must be fully comprehended. Numerous attempts have been made to improve the mechanical properties of 3DCP using fibre reinforcement. At the root of all mechanical properties are the microstructural morphology and constituent characteristics of the mixture. It has been shown that the 3DCP process influences the ensuing microstructural morphology, and it is known that constituent type, constituent proportion, porosity content and void topology affect a porous medium's bulk strength and elasticity. Therefore, the correlation between such observations warrants additional investigation concerning the influence of porosity on the anisotropic mechanical properties of 3DCP components. In this research, the effects of porosity metrics such as total defect content; 3D void topology (shape, size, and orientation); pore spatial, size and compactness distributions; and interconnectivity are quantitively investigated for a fibre-reinforced printable concrete (FRPC) mixture via X-ray computed tomography and related to the elasticity, compressive strength and observed fracture patterns of mould-cast and concrete printed specimens consisting of the same fibre reinforced printable concrete matrix. The experimental findings indicate higher porosity, on average and at interlayer locations, in 3DCP samples as well as alterations in the spatial and topological attributes of voids in 3DCP samples. Furthermore, it is shown that both reductions in the elastic modulus and compressive capacity can be physically linked to the porosity metrics detected in 3DCP elements. Elasticity is dominated by porosity content and compressive capacity by the interrelation between porosity content, loading direction relative to the interlayer orientation, stress concentrations at void boundaries, and the increased deformability of the composite configuration. From the consideration and interpretation of these aspects, the anisotropic mechanical response of 3DCP samples is elucidated, and potential remedies are proposed.
Poisson’s ratio, defining the lateral to longitudinal strain of a material under uniaxial load, is an extensively used material property in engineering analysis and design. For conventionally cast ...concrete, an isotropic static Poisson ratio typically ranges between 0.15 to 0.25. However, no ratio has been established for 3D printed concrete, and is currently widely assumed to be 0.2 and isotropic in computational modelling applications. This layer-wise additive manufacturing technology is notorious for yielding orthotropic mechanical properties due to the presence of weak interlayer regions at the structural level and elongated oblate voids at the material level. This study therefore aims to characterise the static Poisson ratio of printed concrete. Specimens were prepared from a printed element and uniaxially tested both parallel and perpendicular to the printing direction. Digital image correlation technology was employed to facilitate the capturing of specimen strains, followed by micro-computed tomography scans to determine void topography. The results indicate larger Poisson ratios apply for 3D printed concrete compared to its cast counterpart; up to 17 and 33% increases were obtained when printed specimens were tested perpendicular and parallel to the printing direction, respectively. This orthotropic behaviour is ascribed to the oblate voids present in the printed specimens.
•Rthix, the re-flocculation rate, is up to one order of magnitude larger than Athix.•Addition of an optimum quantity nano-silica increases thixotropy.•Optimum superplasticizer dosage improves ...thixotropy behaviour.•Nano-silica addition improves material buildability for 3D concrete printing.
This paper presents a novel rheological thixotropy model that specifically appertains to the characterisation of materials that are suitable for 3D printing of concrete (3DPC). The model accounts for both physical and chemical influences on a material’s microstructure, denoted by Rthix (re-flocculation) and Athix (structuration) respectively. Rheological analyses are performed on a reference material with varying superplasticizer (SP) and nano-silica (nS) dosages in order to determine their effects on the aforementioned parameters. Specific focus is placed on the re-flocculation thixotropy mechanism. The advantages of adding nanoparticles to concrete for 3DPC is practically validated by printing circular hollow columns until failure occurs. The result is supported by the thixotropy model, which is applied to the materials that are used for the 3DPC tests. It is concluded that, for this study, Rthix is a better measure of thixotropy behaviour that is suitable for 3DPC than Athix.
In concrete, early age pore water evaporation results in volumetric shrinkage that, if restrained, can cause plastic shrinkage cracking (PSC). 3D printed concrete (3DPC) is vulnerable to PSC due to a ...lack of formwork, minimal bleeding water, low aggregate to binder ratio and high quantities of fines in the mixture. A novel experimental method was developed to determine the PSC risk, evaluating the efficacy of PSC prevention measures and understanding the behaviour of PSC in early age 3DPC. This study evaluated the free shrinkage of 3DPC specimens as well as identified and systematically introduced sources of restraint to induce PSC. The free shrinkage results showed a rate of strain gain and peak strain significantly higher than commonly found in ordinary concrete. Severe cracking was observed when the shrinkage was restrained under a moderate evaporation rate. Cracks formed within the first 2 h after printing, earlier than in ordinary concrete. The proposed method was employed to study the fundamental behaviour of crack formation and propagation in 3DPC. The unique filament interlayer plane has a notable effect on the transfer of shrinkage deformation in the specimen. Differential horizontal deformation in consecutive layers and consequential interlayer slip was observed when the shrinkage was restrained. It is reasonable to conclude that interlayer slip caused by early age shrinkage has the potential to reduce the long-term interlayer bond strength and the durability of 3DPC.
While interest in 3D printing of concrete (3DCP) and structures has been growing, a major obstacle for implementation of 3DP construction method is the need for steel reinforcement and the challenges ...this presents to the 3DP process. Engineered Cementitious Composites (ECC), also known as Strain-hardening Cement-based Composites (SHCC), hold promise to attain structural integrity, durability, reliability and robustness without steel reinforcement. This article surveys the state of the art on 3DP research with ECC and suggests needed research to direct future development. Research in Asia, Europe and the United States has demonstrated printability and buildability of 3DP-ECC that exhibits characteristic tensile ductility of cast ECC. Nonetheless, a number of outstanding research areas are identified, including those associated with more sustainable mix-design, rheology control, microstructure, filament/filament interface weakness, and long-term durability. Resolution of these challenges will better position the research community to addressing full scale construction, print speed, and print quality.
Providing additional water to the hydrating cementitious particles is essential to achieve the optimal mechanical performance of the low w/b concrete mixes preferred for 3D printing. This study ...incorporates superabsorbent polymers (SAP) and additional water in 3D printed concrete (3DPC) to promote the hydration process through delayed internal water release. The study shows that a retentive SAP modifies the rheological development by absorbing the pore fluid for a short period after printing. The absorption-induced stiffening increases the thixotropy and buildability by 49% and 25%, respectively. A retentive SAP increases the flexural strength and interlayer adhesion by 19% and 10%, respectively. This is due to the internal water release that promotes hydration. Evaporation of the interlayer moisture during the pass time has the opposite effect— evaporation reduces the interlayer adhesion. Based on this assumption, an analytical model is proposed. The model accounts for the pass time, bleeding, and the environmental evaporation rate to estimate the surface moisture and predict the lack of interlayer adhesion. In this study, the model accurately (RMSE = 2.5%) predicted an interlayer adhesion reduction from 30% to 50%. The interlayer adhesion results of other studies could also be predicted.
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•An analytical model for interlayer adhesion of 3DPC is proposed based on the amount of surface moisture on the interlayer.•For this mix, the model accurately (RMSE=2.5%) predicts the reduction in interlayer adhesion from 30% to 50%.•Internal curing with superabsorbent polymers increases the interlayer adhesion by 10% and flexural strength by 19%.•SAPs also improve the initial thixotropy by 49% and buildability by 25% compared to the reference.
Lack-of-fusion (LOF) in digital concrete fabrication is one of the paramount technical issues that must be elucidated before mass industry implementation of this emerging technology. This review ...paper initially accentuates the ramifications of poor interlayer bonding, which include impaired mechanical and thermo-mechanical performance in the hardened concrete state, durability issues pertaining to corrosion of reinforcing steel and the complexities associated with numerical modeling of 3D printed structures. Focus is then placed on the mechanisms that induce LOF, which constitute an intricate combination of mechanical interaction, chemical bonding and intermolecular forces between filaments. Comprehension of these mechanisms is required to develop appropriate solutions to this problem. The paper presents current interlayer bond strength (IBS) characterization tests and their associated (dis)advantages. Lastly, solutions employed in literature to reduce LOF are presented to give readers a holistic perspective of the current state-of-the-art surrounding LOF in 3D concrete printing.
•An analytical filament shape retention model is derived and proposed for digital concrete fabrication.•Only rheology material properties are required as input for the model.•The model predicts the ...maximum stable filament layer height whereby no plastic yielding occurs under self-weight.•A finite element model verifies the accuracy of the analytical model.
The shape retention capability of an extruded filament is of cardinal importance for quality 3D printed concrete elements. Not only is it a prerequisite for surface aesthetics, but it also contributes towards buildability. Optimisation of filament layer height allows for construction time and cost saving possibilities. This research develops the theoretical framework for an analytical shape retention model that predicts the maximum stable filament layer height at which no plastic yielding occurs, based only on the rheology of a material. The Mohr-Coulomb failure criterion is employed and the model simplified by conservatively negating the effect of interparticle friction. A model is also developed that determines whether sufficient friction is present to induce confinement within a filament layer. An experimental verification process via filament extrusion confirms the applicability of the model. A 6.7% difference in stable filament layer height is obtained by comparison with a finite element analysis, proffering as numerical verification for the model.
Concrete structures additively manufactured by extrusion-based 3D concrete printing are reportedly orthotropic in mechanical behavior and exhibit relative weakness in interfacial regions. ...Microstructure, including porosity content, 3D porosity distribution and pore morphology presents a physical basis for these phenomena. Here, a first and comprehensive microstructural investigation is reported, using X-ray computed tomography to visualize and quantify porosity, pore sizes, shapes and distributions in extrusion-based 3D printed concrete. 3D printed plastic molds are used to sample specimens from freshly 3D printed concrete filaments, for minimum disturbance. As reference, similar specimens of the exact same concrete mix, but cast without compaction, instead of being 3D printed are included in the study. A fixed dimeter of 20 mm, but varying height is used to include a single filament layer (10 mm), two layers (20 mm) and four layers (40 mm). Both typical horizontal interfaces in multi-layer elements, and vertical interfaces between multilaterally deposited filaments are studied. Whilst a single 3D printable concrete mix are considered, print variables of pass time (0–60 min with 15 min intervals) and print speed (80, 100 and 120 mm/s) are considered to investigate their potential alteration of the microstructure. Findings are significant, indicating tri-axial spheroid shaped air voids present in printed specimens, elongated and flat in the print direction, compared to mostly spherical voids in cast specimens. This prompts for more research to be conducted into the effect of stress concentrations at micro-cracks or voids in 3D printed concrete, which especially impacts mechanical behavior. Furthermore, it is found that vertical and horizontal interlayers comprise of similar porosity, and that it is inaccurate to qualify the homogeneity of typically fissile 3D printed concrete elements based solely on superficial cross-sectional photographic evidence from saw-cut samples.
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In the fresh state, conventional lightweight foam concrete (LWFC) has low yield stress which challenges shape retention and buildability in digital construction. Several literatures attempt to ...address the rheological performance of 3D printable lightweight foam concrete (3DP-LWFC) in essence. This research presents a comprehensive rheological characterisation with controlled shear rate tests and flow curve tests over different foam volume fractions and densities of 700, 1000 and 1400 kg/m3. LWFC is appropriately adapted for extrusion-based 3D printing in the experimental program by incorporating a small amount of nanopowder (replacing 2% of cement mass) for increased yield shear stress, and calcium sulfoaluminate cement replacing 10% of cement mass for improved thixotropy in the fresh state. Accordingly, this raises the yield stress to 347–812 Pa for 700–1400 kg/m3 LWFC compared to static yield stress below 100 Pa of conventional LWFC, improves thixotropic performance in terms of the rate of reflocculation (Rthix 0.21–3.15 Pa/s) and rate of structuration (Athix 0.06–1.02 Pa/s), viscosity (2.5–3.4 Pa⋅s), and elastic shear modulus evolution. Foam volume is found to significantly influence the rheological properties. To analyse the constructability, shape retention and buildability are investigated, resulting in up to 15 deposited filament layers to be reached in a buildability test. Lastly, a practical example is presented whereby a façade element is printed with 3DP-LWFC at a wet density less than 1000 kg/m3, yielding a lightweight element with buoyant characteristics that further expands the application potential of 3D concrete printing.
•A comprehensive light weight foam concrete comparative study of rheology and 3D printability is presented.•3D printable foam concrete is adapted and validated to be constructable at reasonable building rates.•A digitally fabricated and parametrically designed buoyant LWFC façade element is presented.
