3D printing is proclaimed to be “the third industrial revolution” [1
], as many believe it has the potential to revolutionize all manufacturing processes. Even though cementitious building materials such as concrete can theoretically be used to create elements of any shape, in today’s concrete construction, simple shapes with a constant cross-section are largely used. This is due to the high cost of freeform formwork on the one hand and the low degree of automation in construction on the other hand. Compared to conventional production, 3D printing techniques hold an advantage for the fully automated production of components of high geometric complexity-even for small quantities. Therefore, 3D printing seems to be ideally suited for construction where piece production and geometric complexity are common [2
]. CNC-based and robot-controlled 3D printing processes in particular offer the opportunity to minimize cost-intensive manual production. Freed from the constraints of conventional manufacturing, the shape of structural elements can be tailored to various requirements and functions. In contrast to traditional construction methods, particularly those used in concrete construction, where a single type of material is poured into formwork, it may also be possible to achieve functional grading (see, for example, [3
]) by creating structural reinforced elements with non-standard geometries and varying the material properties during the printing process. Generally speaking, this would lead to more economic, higher-quality construction processes and more efficient material usage.
For large-scale additive manufacturing in construction, mainly extrusion or shotcrete 3D printing techniques, where the premixed cementitious material is extruded or sprayed in long strands, are applied. In general, two main strategies can be distinguished: (I) the deposition of narrow strands of several centimetres’ width, which are either stitched to create the desired structure or used to build a filigree formwork structure strengthened by an inner structure (compare concrete printing [4
] and contour crafting [11
]) and (II) the deposition of broad strands of several decimetres’ width, which are used to build the whole width of the component in a single pass as a kind of ‘infinite brick’ (compare, e.g., ConPrint3D [15
]). The major advantage of these techniques is the high manufacturing speed for creating large-sized monolithic structures as well as the building size of the produced elements.
High manufacturing speed means the need for a rapid structural build-up of the deposited material, as the stability of the printed strand is based on the self-supporting function of the concrete due to the absence of a supporting formwork. In addition to the inherent stability of the strand, the printed material also has to bear the load of the subsequent layers. To enable the printed layer to bear the load of the next layer without collapsing, a minimum yield stress is necessary [5
]. For this purpose, tmin
defines the point in time when a subsequent layer can be applied to the existing layer [20
], see Figure 1
. Achieving the minimum required yield stress is highly dependent on the mixtures composition and the used admixtures. In particular, the use of set accelerators significantly changes the setting behaviour of the concrete and enables, derived from this, the manufacturing of large building heights.
However, to make deposition techniques mechanically competitive for casting of concrete high interlayer adhesion, e.g., the prevention of cold joints, is required [21
]. Therefore, the roughness of the interface region is relevant, which is predominantly determined by mixture composition (e.g., aggregate size, cement, water–cement-ratio, admixtures). Moreover, time delay between two layers, which often results from the production process itself, is important since limited interlocking occurs after a critical resting time [22
]. For the continuous production of various sized elements, the nozzle will apply the adjacent layer within several seconds up to a few minutes. When production stop occurs, the adjacent layer will be applied several minutes up to a few hours later.
It is important to investigate the effect of yield stress development as well as the effect of different accelerator dosages on the quality of the layer interlocking. For this purpose, a maximum yield stress and, thus, a maximum delay time tmax
need to be defined. This represents an upper limit with regard to the interlayer strength quality, Figure 1
Although the progress in 3D concrete printing has developed enormously [23
], there is still a severe lack of knowledge of the functional interaction of processing technology, concrete rheology and admixture usage. This paper presents for Shotcrete 3D Printing (SC3DP) technology the effect of a range of accelerator dosages (0%, 2%, 4%, 6%) on fresh concrete properties (yield stress evolution) and on interlayer strength.
2. Shotcrete 3D Printing (SC3DP) Technology
SC3DP is a novel additive manufacturing method using an automated wet-mix shotcrete process [24
]. It is assigned to the open space deposition 3D printing technologies and is especially designed for the manufacturing of large-scale components, Figure 2.
Due to the varying nozzle angle from 0° to 180°, full three-dimensional processing are achievable. The resulting geometry of the printed path can be varied by different process parameters such as nozzle distance and path velocity. Unlike the extrusion method, SC3DP uses high kinetic energy for the application of concrete. Specimens manufactured with this technology show an increase of contact surface between the layers, which allows a good interlocking effect [27
]. For this reason, the use of high kinetic energy can lead to the minimization of cold joints [28
]. However, due to the production without supporting formwork, the SC3DP as well as the extrusion process are based on a self-supporting of the printed material. The contrary requirements, which are caused by the pumping process and the required pumpability on the one hand and the subsequent rapid structural build-up as well as pursued interlayer strength on the other hand, necessitate the targeted use of admixtures (e.g., superplasticizer and set accelerator) [18
]. Compressed air is applied in the nozzle to spray the pumped concrete. Moreover, admixtures may be induced in the nozzle, which are homogeneously distributed by the compressed air.
3. Effects on Interlayer Strength
It is commonly known that the bond between two layers in 3D-printing is a prerequisite to avoid weak spots and anisotropy (e.g., [21
]). There are two main factors influencing the interface bond between two concrete layers [33
mechanical effect: mechanical bonding relies solely on the physical attributes of the layers, e.g., micro roughness of substrate, cohesion and friction coefficients of the layers and age of the bottom strand as well as rheological properties of the single layers [31
]. The inability of the layers to interlock lead to a reduced contact area and herewith to a reduced interlayer adhesion.
chemical effect: chemical bonding occurs if the hydration and chemical bonding of particles occurs in between two layers [31
Moreover, it is possible that there are further factors, such as homogeneity, affecting interface bond. However, these factors have not been systematically investigated yet.
Up to now, the effect of interlayer strength in the context of 3D printing has mainly been observed with extrusion-based techniques. Anisotropic behaviour
was observed for mechanical strength of 3D-printed specimen [34
], which is attributed to the oriented interface properties [38
]. In [39
], a flexural strength of a cement-based mortar with a water-to-binder-ratio of 0.42 for layers oriented perpendicular to a loading direction of 10.1 MPa was reported, whereas, for layers with parallel orientation, a reduced flexural strength of 5.3 MPa (approximately 48% strength loss) was determined. The anisotropic effect was found to be more distinct for tensile strength than for compressive strength, which indicates that the increase in interlayer strength could be due to the increase in contact surface, e.g., due to interlocking, of the layers [32
]. The positive effect of an increase in contact area on interlayer strength has also been identified by [40
A reduction of flexural strength with interlayer time
in between the manufacturing of two layers was explained with a higher amount of pores/voids [41
], microcracks at the interface as well as drying and plastic shrinkage [31
]. In the latter, a reduction in flexural strength has been reported for specimen with a water-binder-ratio of 0.42 when the interlayer time in between the layers increased from one minute to ten minutes (up to 45% strength loss) or even one day (up to 85% strength loss), whereas, in [41
], an interlayer strength reduction of 89% for a 10 minutes-interlayer time and 97% for 60 minutes-interlayer time was reported for a mortar with a water-cement-ratio of 0.36 for a printing speed of 3 cm/s. In addition, reference [9
] stated that there is a time window for geopolymer mortars in which the interval time has negligible effect on interlayer strength, whereas, outside of the window, the negative effects are significant. In [42
], it is found that an increase in water–cement ratio
has a positive effect on relative interlayer strength for an interlayer time of two hours. For a specimen with a water–cement ratio of 0.40, the relative interlayer strength was reported to be nearly 100% (compared to monolithic specimen), whereas it dropped to below 60% for a specimen with a water–cement ratio of 0.20. Therefore, extremely localized drying is assumed to be the origin of a drop in bond strength in particular for flowable mortars with low water-to-cement ratios since evaporated liquid is not replaced quickly enough by liquid from the inner part of the layer. Herewith, a weak region with less hydrated cement than in the rest of the layer is prevalent. This could be complemented by the finding that moisture content of the bottom layer has an effect on interlayer strength, where a higher moisture content led to an increase in interlayer strength within the investigated boundary conditions [30
The usage of silica fume in mixture composition
(positive effect) [44
], decrease in aggregate size and aggregate-to-cement ratio (positive effect) [45
] were found to affect interlayer strength. In addition, a high structuration rate of the material is negatively affecting interlayer strength of geopolymer mortars [9
]. In [46
], it is stated in the context of distinct-layer casting of self compacting concrete that the thixotropic behaviour should remain under a certain threshold value to allow subsequent layers to mix. This means that there is a critical timeframe for each mixture composition, which is influenced by factors such as particle packing, cement type [47
], or admixtures [48
], in order to enable a high interlayer bond.
In addition to this, the process of applying the layer
has been identified to affect interlayer strength. In [34
], it is stated that there is no significant effect on interlayer strength of 3D printed concrete when the distance from the nozzle to layer is changed. By contrast, [49
] found an increase of the nozzle height to have a negative effect on interlayer strength of geopolymers mortars. This is supported by [9
], who measured the same correlation and found additionally the mixture composition as an influencing factor when varying the nozzle distance. Reference [41
] investigated the effect of printing speed on interlayer strength. They measured a strength reduction of approximately 50% when increasing the printing speed from 1.7 cm/s to 3 cm/s and explained the effect with an increase in voids and a decrease in surface roughness of the contact area in between the layers. In [28
], the interlayer adhesion is compared for extrusion- and SC3DP-technique. For the same material, the mechanical strength (expressed in flexural and compressive strength) of the SC3DP-specimen is higher than that of extruded specimen. This is assumed to be due to higher induced kinetic energy, which leads to a reduced air void content (expressed in a higher density) and herewith higher strength.
The findings regarding the effects on interlayer strength are not always consistent—compare [23
]. In addition, there are nearly no data available on systems with significant structural build-up as it is prevalent in SC3DP-technology. The development of fundamental theories is necessary to explain the observed effects regardless of a specific mixture composition. Moreover, coherent testing procedures regarding the production of the specimen (e.g., geometry of specimen, loading direction) need to be established to make results comparable in the future.
6. Conclusions and Outlook
This contribution focusses on the effect of accelerator dosage and interlayer time on fresh concrete properties and interlayer strength in Shotcrete 3D Printing (SC3DP). In the experimental investigations, the accelerator dosage is varied from 0% to 6%. A severe increase in yield stress after deposition and in yield stress evolution over time is observed when adding accelerator. This enables a higher vertical building rate, which is highly relevant for practical applications. By using a 2% accelerator, the deformation in the lowest layer is reduced significantly from 2.7 mm to 0.8 mm compared to the unaccelerated material for an applied stress of 30 kPa. By using a 6% accelerator, the deformation of the lowest layer is even reduced to 0.3 mm. By calculating the deformation modulus Ed, the effect of accelerator dosage on the mechanical properties of fresh concrete could be quantified. For material with 6% accelerator compared to a 0% accelerator, the deformation modulus is about 14 times higher.
An increase in accelerator dosage leads to an increase in yield stress evolution. A clear correlation of yield stress and resulting interlayer strengths is found. In order to quantify and explain this effect, mechanical performance tests as well as additional µCT-images of the interlayer zones are carried out. Since various time spans may occur before the application of a subsequent layer, four interlayer times (0, 2, 5 and 30 min) are examined. Based on the computed tomography results, an interface tortuosity, which is a parameter describing the interlocking of the layer’s interface, could be determined for each setup. Independent from the accelerator dosage, the largest decrease in interlayer strength is observed for interlayer times between 0 and 5 min. For longer interlayer times (5 to 30 min), no further significant reduction of interlayer strength is measured. With regard to a practical application, an interlayer time of less than 2 min is recommended, which needs to be considered e.g., for path planning in order to avoid a significant loss in interlayer strength. By including the analysis of µCT-results, a correlation between decreasing interlayer strength and decreasing interlayer tortuosity is noticed. Thus, it can be stated that, in SC3DP, the interlayer strength is significantly influenced by interlayer tortuosity. Varying interlayer tortuosity is deduced to the initial yield stress of the underlying layer to which a subsequent layer is applied. Interlayer tortuosity is higher and herewith a higher interlayer quality is generated, when short interlayer times are prevalent.
Finally, to quantify the overall mechanical performance of SC3DP elements, a model is developed depending on compressive strength and interface tortuosity. These findings can be used to improve interlayer strength in the future, e.g., by purposefully manipulating the interlayer roughness by process parameters. In this particular field, further studies are envisaged.