Research PaperCorrelation of interlayer properties and rheological behaviors of 3DPC with various printing time intervals
Introduction
Additive manufacturing, also known as three-dimensional (3D) printing, is a technique for fabricating 3D objects based on pre-defined computer models, which originated in the United States in the late 19th century and popularized in the 1980s [1], [2], [3], [4], [5]. In recent years, 3D printing techniques have been successfully applied to biomedicine, aerospace, military, construction industry, etc. However, the 3D printing “ink” for infrastructures are distinct from that used in other fields as cementitious materials have unique rheological properties. Compared with traditional cast-in-the-mold concrete, 3DPC has the advantages of reducing construction waste by 30−60%, decreasing time requirements by 50–70%, saving labor costs by 40–80%, and allowing for higher architectural freedom [6], [7], which has gained widespread interest from industry and academia. However, it still faces numerous challenges. Special fluidity and thixotropy are required [8], the weak interlayer interface and the anisotropic behaviors are concerned [9], [10], reinforcement strategies are still open questions [1], etc. Among them, the interlayer interface, as the weakest link in the structure, affects the mechanical performance and durability of printed elements [11], [12]. Therefore, the study of the weak interlayer interface has become a hot spot.
One of ways to improve the interlayer interface properties is to use a reasonable printing time interval [1], [3]. The printing time interval, as one of critical factors influencing the interface properties, is affected by the geometric shape of printing structures and the printing speed. A more complex shape of printed structure will increase the printing time interval as it takes longer time to finish printing the previous layer [13], [14], [15], [16], and the increase in printing speed will reduce the printing time interval, which may cause other detrimental effects, such as the decrease in interlayer bond strength [17], [18], [19]. Meanwhile, time plays significant role in the cement hydration, thus, the printing time interval may affect not only material behaviors in the hardened state, such as the bond strength and the durability [20], but also the rheological behaviors at early age.
Most studies have found that interlayer bonding decreased with the increase of printing time interval [21]. This can be attributed to two reasons: the first one can be seen as the insufficient interacted bond areas between two layers [12], [22], [23], [24], [25]. More C-S-H was produced with the increase of time interval, and the stiffness of the initial layer was enhanced, resulting in less amount of interacted bond areas between layers. Thus, many unfilled areas formed the macropores between layers. The other reason is believed to be the loss of moisture content on the interlayer surface [13], [17], [24], [26]. The evaporation and drying of water on the interlayer surface during the printing interval would cause insufficient hydration and high porosity, resulting in the decrease of interlayer bond strength. The two reasons also result in a higher porosity at the interface [27].
Nevertheless, scholars found a small number of cases inconsistent with above laws. Sanjayan et al. [28] found that the interlayer bond strength of the specimen printed with 10 min interval was equivalent to that of the specimen printed with a time interval of 30 min, and both were greater than that of the specimen printed with a time interval of 20 min. The explanation for this phenomenon was that the interlayer strength was positively correlated with the surface moisture which experienced the following processes: extrusion of extra water (10 min), evaporation drying (20 min), and surface bleeding (30 min) process. Chen et al. [25] demonstrated that short time intervals (20 s-10 min) had no effect on the bonding properties of the printed material based on the experimental and numerical studies. Since the bottom layer has a low stiffness, the load caused by the weight of the upper layer might rearrange the orientation of the top surface of the bottom layer, increasing the interacted bond area between two layers. As is stated above, many studies have been conducted to investigate the effect of printing time interval on the interlayer bond strength. However, the printing time interval cannot be considered as an independent value, and should be considered together with other process parameters [24].
As noted above, the time interval will affect the porosity of interlayer interface, which determines the transport of ionic species [11], [17], [29]. The durability of printed elements, especially the carbonation resistence and chloride ion resistence, depends on the transportation of ionic species. Therefore, the printing time interval has significant effect on the durability of interlayer interface. As research reported, the increase of the time interval between layers resulted in the deeper and faster chloride attack [30], [31]. It can be inferred that a longer time interval increases the number of interconnected pores at the interface which will further lead to a water uptake through absorption or capillary suction. There are very few researches involving the carbonation resistance of printed elements. Based on a case study, Zhang et al. [32] investigated the durability of large-scale 3D printed cement-based materials and found that 3DPC was more resistant to sulfate attack and carbonation than mold-cast cement-based materials, but had lower resistance to frost damage and chloride ion penetration. Zhang et al. explained that the printed filaments were pressed tightly by the upper large-scale printed components, which left less voids for CO2 to pass through.
Due to the high thixotropic requirement of 3DPC, its rheological behavior is time-dependent. Thus, the printing time interval also affects the rheological behavior of 3DPC. The rheological behaviors further influence the properties of printed elements in hardened state [1]. However, scholars paid more attention to how to meet the competing rheological requirements [33]. Generally, the additives were induced to meet the special requirements of printed material. For instance, Manu et al. [34] studied the rheological properties of printed material with various aggregate-to-binder ratios(a/b), and observed that the plastic viscosity, yield stress and storage modulus were increased with the increasing of a/b. The rheological behavior of a 3D printed fly ash-based geopolymer in the presence of slag and silica fume was investigated by Guo et al. [35], the results showed that an increase in slag and silica fume caused the thixotropic property, plastic viscosity and yield stress to increase first and then decreased. However, the researches about the variation of rheological behaviors(i.e., yield stress, plastic viscosity, and fluidity) with the different time interval were few, and the study of the correlation between these parameters had not been reported.
Above all, the bond strength, the durability, and the rheological behavior of the interlayer interface are all affected by the printing time interval. Meanwhile, these properties are also related to each other. For example, a high yield stress and plastic viscosity will produce a high bond strength, and a low bond strength is related to a low durability of interface due to the high porosities. Therefore, the correlation among them should be further studied. As a statistical method for measuring correlations between factors based on their trends, GRA method can be used to seek the link in an unknown system based on the limited data [36], [37]. Thus, GRA was used to discuss the correlation among them in this study.
To explore the correlation among the properties of 3D printed interlayer interface, this paper investigated the effect of the printing time interval(i.e., 1, 2, 3, 5, 10, 20, 30, 60 min) on the interface properties of 3DPC, such as rheological behavior, shear bond strength, resistance to chloride-ion and CO2. Based on the above results, a quantitative relationship between printing time interval and shear bond strength was established. Additionally, gray relational analysis was used to explore the correlation between shear bond strength and other properties including rheological behavior, surface humidity, the durability. Finally, the interface bonding properties were observed by back-scattered scanning electron microscopy (BSEM) and the GRA was also used to explore the correlation among the porosity, the shear bond strength and the durability.
Section snippets
Raw materials
Portland Cement (PC) complying with the Chinese Standard GB175-2007 [38], and Sulphoaluminate Cement (SAC) conforming to the Chinese Standard GB20472-2006 [39], were used as the binder in this study. SAC was also employed as an accelerator for 3DPC to increase the structural build-up of slurry. Their chemical compositions were shown in Table 1. The sand with a maximum particle size of 1.2 mm was added in the 3DPC, the sand had an apparent density of 2.60 g/cm3 and fineness modulus of 2.1.
Dynamic yield stress and plastic viscosity
The evolution of dynamic yield stress and plastic viscosity with time interval is represented in Fig. 5. Based on this, it becomes clear that both rheological parameters of the paste gradually increase with higher time intervals. The dynamic yield stress was 1.73 Pa and the plastic viscosity reached 0.097 Pa s when the time interval was 1 min, while those of the paste reached 6.36 Pa and 0.15 Pa s when the time interval was half an hour. The dynamic yield stress rose by 268% when the time
Conclusions
To study the correlations among the properties of 3D printed interlayer interface, this paper conducted a series of experimental studies such as rheological test, oblique shear strength test, chloride penetration test, and carbonization test. The following conclusions can be drawn based on the above results:
- (1)
The interlayer shear bond strength can be expressed as a function of the printing time interval and has approximately a negative exponent relation with the printing time interval. Based on
CRediT authorship contribution statement
Yanqun Xu: Conceptualization, Formal analysis, Writing - original and revised draft, Mehodology. Qiang Yuan: Conceptualization, Supervision, Resources. Zemin Li: Methodology. Caijun Shi: Supervision. Qihong Wu: Visualization, Investigation. Yanlin Huang: Data curation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial supports by the National Natural Science Foundation of China (Nos. 51778629 and 51922109), the Innovation-Driven Project of Central South University (No. 2020CX011) and the Fundamental Research Funds for the Central Universities of Central South University (No. 2021zzts0232) are greatly appreciated.
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