3D Printable Concrete Research Articles

3D printable concrete without chemical admixtures: Fresh and hardened properties

AbstractThere are several significant advantages to 3D concrete printing (3DCP) over conventional concreting techniques. Structures are built by printing them one layer at a time. This study aimed to improve the usability and accessibility of this technology to solve on‐site issues, such as those caused by chemical admixture. In order to construct concrete that can be printed in three dimensions, researchers added many chemical admixtures in controlled quantities. This experiment's main goals were to prepare 3D printable concrete (3DPC) without chemical admixture and to evaluate fresh and hardened properties. Slump, flow, extrudability, buildability, open time, and penetrometer tests were carried out for fresh properties. Compressive, flexural, splitting tensile strength, and modulus of elasticity were evaluated for hardened properties. The printed specimens in the longitudinal direction exhibited higher compressive strength than in the downward and lateral directions. Similarly, higher flexural and splitting tensile strength in the longitudinal direction of printed specimens than in the downward and lateral direction. The results of developing 3DPC without chemical admixtures demonstrated good performance in fresh and hardened properties.

Influence of nano-coated micro steel fibers on mechanical and self-healing properties of 3D printable concrete using graphene oxide and polyvinyl alcohol

High ductility cementitious composite is one of the proposed new generations of concrete for use in 3D concrete printing with a large dosage of microfibers. This large volume of fibers negatively affects printed layers’ fresh properties and durability. It also increases the porosity of concrete and is not cost-effective. Accordingly, through an experimental program, this article introduces a novel coating technique to reduce the required quantity of fibers for 3D concrete printing. Polyvinyl alcohol (PVA) is used in the present study to help in coating graphene oxide (GO) nanomaterials on the surface of micro steel fibers. Compressive, direct tensile, and bending tests are considered for the mechanical properties. The self-healing property of the proposed mixture is also checked by applying pre-loading on direct tension and bending samples and followingly putting for the healing period. The results show that the nano-coating method improves the compressive and direct tensile strength of cement mortar so that this technique can be used to reduce the volume percentage of steel fibers to achieve the same mechanical characteristics. Also, promising findings were obtained for the self-healing properties of cementitious composites containing nano-coated fibers.

Improving interlayer bond in 3D printed concrete through induced thermo-hydrokinetics

For 3D printable concrete, lack of interlayer adhesion (IA) is an important consideration. This paper presents an experimental study to investigate a novel method to mitigate lack of IA by enhancing the interlayer bond strength through induced thermo-hydrokinetics. Thermo-hydrokinetics in this study describe the motions of heated fluids and/or the forces which produce or affect such motions. Thermo-hydrokinetics were induced by steaming the interlayer region of a substrate right before placement of a fresh filament layer. The method is applied at 3 different pass times; typical pass time for the arbitrarily selected object for printing, 5 min, and 10 min. A control for each pass time, on which thermo-hydrokinetics were not induced is also obtained. Direct tensile tests are performed to quantify the interlayer bond strength (IBS) of printed specimens. The experimental results show that inducing thermo-hydrokinetics in the interlayer region, even with increasing pass times, is beneficial for the tensile performance of the printed specimen. Steamed tensile specimens showed superior mechanical performance, achieving a 28-day overall mean IBS value of 2.8 MPa, which was a 78 % increase from that of non-steamed tensile specimens. The strength development of the specimens was also quantified, and it was shown that the rate of strength development for the steamed regime specimens was greater than that of the non-steamed regime. Furthermore, scanning electron microscope images provided information into the material in the interlayer region, illustrating that induced thermo-hydrokinetics altered the material in the interlayer region.

Effect of coarse aggregate content on the rheological and buildability properties of 3D printable concrete

Additive manufacturing in construction industry is gaining attention among both researchers and industries. The present study deals with the effect of coarse aggregate (CA) content on the performance of 3D printable concrete. The fresh and rheological properties of the designed concrete with 8 mm down sized CA were assessed. The rheological parameters for the material with varying coarse aggregate (CA) volume fraction from 0% to 50% were studied. There is limited literature on the effect of large sized CA on the printability of the concrete. The rheological properties were measured using in-house developed rheometer designed for concrete with CA. The concrete mix with CA content percentage 10%, 20%, 30%, 40% and 50% were compared with control mix (CA content 0%). The static and dynamic yield stress values for 20% and 30% CA content followed decreasing trend and when increased more than threshold limit of 30%, the yield stress values increased. It was observed that the mortar volume ratio, fine aggregate ratio with respect to CA influences the rheological properties and slump flow. Relationship between the slump flow value and static yield stress was fixed for the designed concrete. Extrusion based layer by layer printing of cementitious material without the support of form-work makes the material more prone to failure and qualitative approach to resist the failure of the material while printing is a key aspect in 3D concrete printing and this quality is termed as ‘buildability’. The challenge lies in designing a material which is flowable with minimum dynamic yield stress and increased structuration rate confirming that the material attains the yield strength once the printing is initiated. The concrete mix with flowability 180 mm to 190 mm was having good buildability and smooth extrusion. The buildability of the designed concrete mix was calculated using Kruger’s analytical model in terms of number of layers and validated the model by direct printing method. The printing parameters like printing speed and resting time was fixed for the concrete with 8 mm down sized aggregate. The concrete mix with 40% CA content showed better buildability with optimum static and dynamic yield stress values.

Development of Alkali-Activated 3D Printable Concrete: A Review

The construction world has changed day by day and is becoming more digitalized by introducing new technologies. Three-dimensional concrete printing (3DCP) is one such technology that has automated building process along with several benefits such as reduced material waste, reduced human hazard, and time savings. Traditionally, this technique utilizes cement to construct numerous structures, resulting in a significant carbon footprint and negative environmental impact. There is a need to find alternate solutions to reduce cement consumption. Alkali activation technology has replaced cement completely. The scope of development of alkali-activated 3D printable concrete utilizing agro-industrial byproducts is presented in this study. A review of the fresh and hardened properties of alkali-activated 3D printable concrete was the primary objective. The change in properties of 3D concrete mixes with the variation of additives that influence the ultimate strength parameters is presented. This study explores the curing conditions and in-depth behavior of uses of 3DCP in the construction industry. The environmental benefits over conventional concreting technology are presented. As per previous studies, the optimum mix composition per cubic meter concrete is 600–700 kg/m3 of binder content, 450 kg/m3 of alkali activator solution, and 600–800 kg/m3 of fine aggregate content. This study contributes to the making of 3D printable alkali-activated concrete.

Enhancing mechanical performance of twin-pipe printable concrete using a binary binder system

A twin-pipe pumping (TPP) system has been developed for the set-on-demand purpose of 3D printable concrete, where a static mixer is used to merge two streams (primarily a Portland cement-based mixture and a limestone powder-based mixture) before extrusion. Although the limestone-based suspension works as an accelerator carrier for high mixing efficiency, inert limestone powder would perform as a weak region. To improve the mechanical performance of the final printed element using TPP, the weak limestone-based mixture was replaced by a newly developed calcium sulfoaluminate (CSA) cement-based mixture in this study. Results indicated that the mechanical strength in different loading directions was improved using the PC/CSA system.

Quantification of plastic shrinkage and plastic shrinkage cracking of the 3D printable concretes using 2D digital image correlation

There is great potential for 3D concrete printing to considerably transform the construction industry. An important step toward the transfer of this new technology into practical application is to gain the ability to control and mitigate plastic shrinkage of printed elements as well as related cracking. These phenomena occur at a very early age after the deposition of concrete due to the absence of formwork in the first place. The cracks can negatively influence durability, serviceability, and aesthetics of the printed structures. In this context, it is an important task to quantify plastic shrinkage of printed concrete and to capture the formation and propagation of cracks. The research at hand presents an easy-to-use, inexpensive experimental setup for quantifying the magnitude of plastic shrinkage and an accurate method for assessing plastic shrinkage cracking by means of 2D digital image correlation.

3D printable concrete: Mixture design and laboratory test methods

Experimental results of the mix design and fresh properties of a proposed concrete for 3D printing are presented in this paper. Layer-by-layer sample components were built by the designed concrete and extruded through a nozzle. The two most critical rheological properties of the designed mix are extrudability and buildability as they are both highly influenced by mix proportion and the presence of superplasticizers. Therefore, this study focused on mix proportion design approaches and laboratory test methods, which generally allowed access to the fresh concrete’s printable properties. Because they are efficient and convenient in construction, materials including cement, fly ash, natural sand, and superplasticizer were selected for mixing proportion analysis. Slump-flow tests, V-funnel tests, buildability tests, and extrudability tests were also employed to access the printability of the proposed mixture in this study. This first analysis on the mixture design approach and laboratory test methods to evaluate the printable properties of 3D-printable concrete can be considered a reference and orientation to further develop 3D printing technology in Vietnam.

Influence of test protocol on determining the rheological properties of cement pastes mixtures for concrete 3D printing

The rheology of cement paste plays a decisive role in the buildability, extrudability, and pumpability of 3D printable concrete. In the context of the same, this research examines the effect of the test protocol on the rheological properties of cement paste. The experimental studies are conducted on cementitious paste with a w/c ratio of 0.30. Three testing protocols of triangular, trapezoidal, and step up – step down procedures are evaluated under shear rates of 50 and 100 s-1. The yield stress and plastic viscosity are further predicted with Hershel-Bulkley, Modified Bingham, and Bingham models. It is found that the yield stress, flow, and consistency index are highly dependent on the selection of rheology protocol. The yield stress increases with the rise of the shear rate. The hysteresis area is maximum for a step up - step down protocol followed by a triangle and trapezoidal protocols. The hysteresis area at a shear rate of 100 s−1 is 24.36%, 14.64%, and 68.12%, higher than tests conducted at a shear rate of 50 s−1 for test protocols of the triangle, trapezoidal, and step up - step down, respectively. The repeatability and reproducibility of prediction using the Hershel-Buckley model are better compared to other models.

Effect of varying shear rates at different resting times on the rheology of 3D printable concrete

3D printing offers a paradigm shift in the construction industry. The material undergoes different stages like mixing, pumping, extruding, and printing with disparate shearing patterns and the effect of shear rates must be brought into consideration. The rheological properties of 3D printable concrete with varying shear rates at different resting times were studied. The designed mix has an ultra-high compressive strength of 110 MPa. Different hystresis curves with shear rates of 0–25 s−1 (P1) and 0–50 s−1 (P2) are studied. The tests were conducted at 10 min intervals up to 30 min resting time. The experimental down flow curves are further analyzed using the Bingham model, the modified Bingham model, and the Herschel-Bulkley model. It is observed that the rheological properties of 3D printable concrete are highly dependent on the resting time, and the applied shear rates and demand judicious selection of rheological models to predict yield stress and plastic viscosity. The comparison between experimental and analytical models indicates better predictability with the modified Bingham and Herschel- Bulkley model. The yield stress values for the modified Bingham model were found to be varying from 95 Pa to 178 Pa and Herschel-Bulkley model from 130 Pa to 177 Pa for P1 and P2, respectively.

Rheology of 3D printable concrete prepared by secondary mixing of ready-mix concrete

The requirements for pumpability, extrudability, and buildability of 3D printable concrete are very high, and it is hard for ready-mix concrete to meet these requirements. In this study, a novel method, namely secondary mixing was performed to adjust the ready-mix concrete with high fluidity to a printable state by adding an appropriate amount of viscosity modifying agent before printing. The effects of hydroxy propyl methyl cellulose (HPMC) and standing time on the rheology evolution of 3D printable ready-mix concrete (3DPRC) before/after secondary mixing were comparatively investigated. In addition, recycled coarse aggregates (RCA) with a maximum size of 10 mm were applied to prepare 3DPRC. The results showed that adding HPMC during secondary mixing on-site facilitated the buildability of 3DPRC by increasing its static yield stress. The standing time exhibited a great effect on the improvement of dynamic yield stress. Meanwhile, the RCA increased the dynamic yield stress during the standing period and the plastic viscosity during secondary mixing. These effects can contribute to the printability of ready-mix concrete.

Simplified assessment for one-part 3D-printable geopolymer concrete based on slump and slump flow measurements

A combination of three-dimensional (3D) printing and one-part geopolymer concrete is an innovative and sustainable construction method to prevent environmental degradation. To perform well in a 3D printing application, one-part geopolymer concrete requires rheological specifications that will generate concrete that is printable. However, measuring rheological properties and interpreting the rheological results is time-consuming. It needs to have a standardized field-friendly method to assess the printability of one-part geopolymer concrete before printing. This study introduces a simplified way of predicting the printability of one-part geopolymer concrete for 3D printing with the use of a slump and slump flow diagram. Nineteen (19) mixtures of one-part geopolymer concrete with various water/binder ratios and different content of sodium tetraborate pentahydrate (borax) added were studied. The slump and the corresponding slump flow of these concrete mixtures were measured. Then, the actual printings were carried on to assess the printability of these mixtures. The open time of concrete was also determined. After 3 days of curing, the compressive strength of the concrete was evaluated. The investigation indicated that the mixtures with a slump in a range of between 15 mm and 30 mm accompanied by a slump flow of between 210 mm and 240 mm were found to be well-suited for 3D printing. Furthermore, the results figured out that to achieve a desirable open time for successful 3D printing, the borax content of 2–4% should be added while the appropriate water/binder ratio of higher than 0.34 should be used.

Phase change material infused recycled brick aggregate in 3D printed concrete

In this paper the effects of the addition of a paraffin phase change material on the strength and printability of 3D printed concrete are studied. Phase change materials are latent heat storing materials, which garner and release large amounts of energy as they change phase. The addition of phase change materials to concrete produces a composite material with maximised latent and sensible heat storage capacity. Used in buildings, this composite material has the ability to minimise unwanted heat transfer across the building envelope.An existing mix design (RBA-3DPC), in which 64% of the natural aggregate in a 3D printable concrete (3DPC) had been replaced with recycled brick aggregate, is adjusted by adding phase change material to the pores of the recycled brick aggregate by vacuum impregnation, creating PCM-3DPC. Rheological characterisation tests are performed on reference mix designs (3DPC and RBA-3DPC) and the PCM-3DPC mix design, and used in a buildability model to validate the number of printable layers. Mechanical characterisation tests including cube strength tests, direct tensile tests and uniaxial compressive tests are performed on cast and printed specimens of the mix designs. There is no existing research on the effects of the combined addition of recycled brick aggregate and phase change material in 3D printed concrete.It is concluded that the PCM-3DPC has the highest number of printable layers predicted by the model and realised by a cylindrical column print and overall, PCM-3DPC has greater strength compared to RBA-3DPC, and lower strength compared to 3DPC. The PCM-3DPC exceeds the RBA-3DPC interlayer tensile strength by 6%, intralayer compressive strength by 43% and interlayer compressive strength by 9%, and subceeds the 3DPC interlayer tensile strength by 15% and interlayer compressive strength by 13%.

Development of Test Methods to Evaluate the Printability of Concrete Materials for Additive Manufacturing.

This study proposes test methods for assessing the printability of concrete materials for Additive Manufacturing. The printability of concrete is divided into three main aspects: flowability, setting time, and buildability. These properties are considered to monitor the critical quality of 3DCP and to ensure a successful print. Flowability is evaluated through a rheometer test, where the evolution of shear yield strength is monitored at a constant rate (rpm), similar to the printer setup. Flowability limits were set based on the user-defined maximum thickness of a printed layer and the onset of gaps/cracks during printing. Setting time is evaluated through an ultrasonic wave pulse velocity test (UPV), where the first inflection point of the evolution of the UPV graph corresponds to the setting time of the concrete specimen. The results from this continuous non-destructive test were found to correlate with the results from the discrete destructive ASTM C-191 test for measuring setting time with a maximum difference of 5% between both sets of values. Lastly, buildability was evaluated through the measurement of the early-age compressive strength of concrete, and a correlation with the UPV results obtained a predictive model that can be used in real-time to non-destructively assess the material buildability. This predictive model had a maximum percentage difference of 13% with the measured values. The outcome of this study is a set of tests to evaluate the properties of 3D printable concrete (3DP) material and provide a basis for a framework to benchmark and design materials for additive manufacturing.

Mechanical Performance of Commercially Available Premix UHPC-Based 3D Printable Concrete.

Several recent studies have attempted to formulate printable cementitious materials to meet the printing requirements, but these materials are designed to work with specific printing equipment and printing configurations. This paper aims to systematically develop and perform characterization of a commercially available ultra-high-performance concrete-class material (UHPC) modified to be printable. Four percentages of superplasticizer were used (100%, 94%, 88%, 82%) to adjust the UHPC mixture for 3D-printing requirements. A superplasticizer amount of 88% was considered adequate to meet the requirements. Several fresh and hardened properties of UHPC were measured experimentally: shape-retention ability and green strength were investigated in fresh state, and compressive and flexural strength were evaluated in three loading directions to evaluate the anisotropic effects. Furthermore, the strength of the interlayer bond was investigated. The UHPC developed in this study met the criteria for extrudability, buildability, and shape retention to ensure printability. In comparison with mold-cast UHPC, printed UHPC exhibited superior flexural performance (15–18%), but reduced compressive strength (32–56%). Finally, the results demonstrated that a commercially available UHPC-class material can be used for 3DCP, which possesses all necessary properties, both fresh and hardened.

Shape stability of 3D printable concrete with river and manufactured sand characterized by squeeze flow

Without formwork support, shape stability is one of the most critical properties of 3D printable concrete. This study evaluated the influence of the substitution of river sand by manufactured sand (0%, 15%, 25%, 35%, 50%, 75%, and 100% by volume) on the shape stability of freshly printed concrete. Shape stability was measured by squeeze flow tests with two different loading modes including an increasing force mode and a constant force mode. In the increasing force squeeze flow test, the elastic and plastic behaviors of mixtures were analyzed, while in the constant force squeeze flow test, modified yield stress was derived, with the consideration of the strength correction factor. Furthermore, two series of square wall layouts were printed and the failure patterns (either plastic collapse or elastic buckling) were well predicted by the squeeze flow. In addition, results demonstrated that the substitution of river sand by manufactured sand enhanced the shape stability of the 3D printable concrete, which closely related to the packing fraction of aggregate.

Hydration re-initiation of borated CSA systems with a two-stage mixing process: An application in extrusion-based concrete 3D printing

Enhanced structural build-up and strength development are crucial for various construction processes, especially in extrusion-based concrete 3D printing. However, controlling the stiffening process could be quite challenging as there are conflicting requirements during the pumping and deposition phase. We present a study to enhance the structural build-up of a calcium sulfoaluminate (CSA) cement-based system using a two-stage mixing method. A borated CSA cement mixture having a long open time (> two hours) was intermixed near the nozzle/print head of the 3D printer with a mixture of limestone and calcium hydroxide having an indefinite open time. The hydration and the early-age mechanical behaviour of the different combinations of these two mixtures were investigated by isothermal calorimetry, XRD analysis, unconfined compression tests, and ultrasonic pulse wave velocity tests. The results indicate that in the presence of calcium hydroxide, the borated phase (ulexite) covering the ye'elimite and being responsible for the long open time, rapidly destabilizes and the hydration reaction re-initiates. The hydration progresses rapidly, resulting in enhanced pore segmentation and a rapid increase in mechanical properties (compressive strength and elastic modulus), significantly faster than what is reported for conventional 3D printable concrete. Finally, a 1.5 m tall column was printed with this two-stage mixing method in a time period of <10 min – demonstrating the feasibility of the developed mixture for 3D printing applications.

Criticality of microstructural evolution at an early age on the buildability of an accelerated 3D printable concrete

The buildability of a 3D printed concrete structure is influenced by the layer compression during printing, which in turn is governed by the rheological and early age mechanical properties of the mix. The study aims at understanding the effect of alkali-free aluminium sulphate-based accelerator on the microstructural evolution, which affects the macrostructural behaviour governing buildability of the structure. The effect of accelerator on hydration, phase composition, location and arrangement of ettringite, and porosity evolution of two paste systems with and without viscosity modifying admixture and superplasticizer is observed at initial hours. Further, fresh properties, yield stress evolution, and mechanical strength at early and later ages of the printable mix is evaluated. The squeeze flow test, which simulates the physical process of printing, is performed to understand the rheological characteristics of the mix under compressive loading in the initial hours. It is concluded that the precipitation of ettringite in large quantities at the initial hours controls the rheological and mechanical behaviour of the mix. The total ettringite formed increases for accelerated paste system at 30 min compared to the paste without accelerator. The ettringite is located at surfaces of clinker and interstitial spaces, with the crystals interlocked physically with each other. The squeeze flow test further suggests that the flow hardening increases tremendously on increasing the accelerator dosage due to an increase in solid volume fraction of ettringite and the resultant physical interlocking. Finally, a theoretical model is proposed considering the significance of ettringite interlocking and microstructure densification under compression on the plastic flow hardening for accelerated mixes.

A two-phase design strategy based on the composite of mortar and coarse aggregate for 3D printable concrete with coarse aggregate

In this investigation, a two-phase design strategy based on the composite of mortar phase and coarse aggregate phase is proposed for designing 3D printable concrete (3DPC) with coarse aggregate. Coussot model and Krieger-Dougherty model were utilized to establish a quantitative relationship between the rheological property of 3DPC and the volume fraction of coarse aggregate, aiming to design 3DPC with coarse aggregate based on the mix proportion of printable mortar and thus facilitate the mixture design. It is found that the plastic viscosity of 3DPC with coarse aggregate is excessively high when the volume fraction of coarse aggregate reaches to a certain value, which is the main reason restricting the printability of 3DPC with coarse aggregate. The printability of 3DPC with coarse aggregate cannot be solely evaluated by yield stress, the plastic viscosity should be taken into consideration together with yield stress for evaluation as well.

The incorporation of recycled brick aggregate in 3D printed concrete

In this paper a more sustainable 3D printable concrete is developed by the replacement of natural aggregate in a concrete mix with recycled brick aggregate (RBA). An existing 3D printable concrete mix is used as reference and adjusted by replacing 64% of its natural aggregate with RBA. The potential for sustainability in the building sector by automation as well as recycling of construction and demolition waste is therefore addressed. To date no research exists on the use of recycled brick aggregate in 3D printed concrete, which has particular relevance in developing countries, where construction with bricks remains a popular option.Two different types of RBA from local demolition projects are considered in three mix designs containing Type 1 RBA, Type 2 RBA and a combination of Type 1 and Type 2 RBA. To arrive at the three mix designs, the water absorption and porosity of each RBA type are first tested to determine the estimated additional water requirement in each concrete mix. The ASTM mini-slump cone test is used as a replacement of a full rheological characterisation to determine the achievement of acceptable printability.After the fresh-state characterisation, the mix design containing the more porous Type 2 RBA is chosen for mechanical characterisation based on availability of materials and relevance of highly porous recycled brick aggregate in future studies. Mechanical characterisation tests are performed on the chosen RBA mix, as well as the reference 3D printable mix for comparison. The mechanical characterisation tests include compressive cube strength tests, direct tensile tests on 3D printed specimens, and uniaxial compressive tests on 3D printed specimens. The results of the mechanical characterisation tests show that the replacement of 64% of the natural aggregate with RBA in the mix reduces the compressive cube strength by 25%, the interlayer tensile 3DPC strength by 20% and the compressive 3DPC strength by 14% and 20% in Direction 1 and Direction 3 respectively.