3D printing concrete with recycled coarse aggregates: The influence of pore structure on interlayer adhesion
Introduction
During the past decade, concrete 3D printing has become an emerging technology that has attracted considerable interest in academia and industry [[1], [2], [3]]. To date, numerous studies have been conducted to evaluate the material properties of 3D printed concrete, and some have been applied in the construction field [4].
However, sustainable applications of 3D printed concrete are still unproven [5]. Thus far, although the materials used in 3D printed concrete have been commonly described concrete, most of them are mortar without coarse aggregates; this means that ordinary cement occupies a high content [6] of the material and that the amount of fine aggregate (natural sand) has to be increased to match the high cement percentage [7]. This increase further raises the cost and environmental burden and partially neutralizes the benefits of concrete 3D printing with respect to formwork-free and material-efficient design [8]. Therefore, the application of environmentally friendly 3D printed concrete materials, especially coarse aggregates, is vital for the sustainable development of concrete 3D printing construction [9]. Recycled coarse aggregate (RCA), as the current environmentally friendly recycled material with the largest proportion of construction and demolition wastes, has been widely applied in conventional building construction fields [10,11]. Studies have demonstrated that RCA incorporation can improve the buildability of 3D printed concrete [12]. Applying RCA to prepare 3D printed recycled coarse aggregate concrete (3DPRAC) can not only reduce the amount of cement and natural aggregates (coarse and fine aggregates) and reduce the cost of raw materials but also alleviate the carbon footprint, which has been seen as one of the most promising ways to achieve sustainable development in digital fabrication with concrete [13].
However, the applications of 3DPRAC are still challenging [12,[14], [15], [16]]. On the one hand, RCA incorporation shortens the open time [12], and the workability is difficult to control compared to 3D printed mortar (3DPM) [[17], [18], [19]]. Moreover, this method is substantially limited by the concrete 3D printer [[20], [21], [22], [23], [24]]. For example, incorporating RCAs can substantially increase the particle sizes and plastic viscosities of printed materials [12], leading to the need for larger printheads and pipes to prevent clogging, as well as the requirement for greater motor power and other requirements for printer performance. Until now, compared to 3DPM, few studies have been reported on 3DPRAC [12,13,[25], [26], [27]]. Xiao et al. [13] found that the incorporation of RCA significantly changed the anisotropic compressive strength of 3D printed concrete, but in either direction, the compressive strength was lower than the strength of cast sample. Similar conclusions were obtained by A.V. Rahul et al. [25] and concluded that the critical reduction in strength was attributed to the interface transition zone (ITZ) introduced by RCA incorporation However, Bai et al. [26] suggested that the mechanical properties can be improved by the RCA incorporation because RCA roughened the filament surface and increased the bond area between the filaments. The reason for RCA incorporation on the improvement in 3D printed concrete mechanical properties may be multifaceted from the current point of view.
It is well known that interlayer bond properties are critical to the mechanical and durability performance of 3D printed concrete [28]. Interlayer pore defects, as an unavoidable concomitant product of the concrete 3D printing deposition process, severely degrade the interlayer bond strength, as has been commonly acknowledged in numerous 3DPM reports [[29], [30], [31]]. The number and geometric characteristics of interlayer pores are influenced not only by the filament surface topography [16] but also by the printing time interval [29]. With increasing interlayer printing interval, the degree of hardening increases while the water evaporates from the filament surface, which hinders the deposition fusion between adjacent filaments [32] and reduces the hydration reaction of adjacent filaments [33], causing a significant increase in the number and volume of pores. Eventually, the force properties of the 3D printed concrete interlayer interface are degraded due to changes in the number of pores and their distribution in the interlayer area. From the available reports, diverse factors influence the interlayer bonding performance such as air entrapment [34], material rheology [35], filament surface roughness [16], humidity and degree of hardening [36]; essentially, the overall performance can be broadly attributed to changes in mechanical strength of the interlayer interface by generated pore defects (macroscopic and microscopic levels). Compared to 3DPM, more importantly, uncertainty in deposition-fusion between filaments of 3DPRAC is substantially increased by incorporating coarse aggregates, leading to a considerable rise in porosity between the layers [30] and severely influencing bonding properties. Furthermore, by comprehensively analyzing the pore structures in 3D printed concrete [37], it was found that in addition to porosity, pore structural parameters, such as volume, pore shape, location distribution, and orientation, have an obvious effect on the mechanical properties. However, until now, in-depth analyses of the influences of pore structure on the interlayer bonding properties of 3D printed concrete with RCA have rarely been reported, and this analysis is important to further understand 3DPRAC and its future applications.
This study focuses on the effect of pore structure on the interlayer bond strength of 3DPRAC, aiming to deeply investigate the interlayer bond properties of 3DPRAC and provide a basis for engineering applications. First, the bond strength of 3DPRAC with different RCA replacement ratios under the influence of layer height and printing time interval was analyzed and compared with that of 3DPM. Then, the filament debonding interface and surface topography were characterized via a digital image correlation (DIC) technique. Subsequently, the pore structure parameters (porosity, 3D distribution and geometric characteristics) of the 3DPRAC interior were comprehensively investigated by X-ray computed tomography (X-CT) tests, and the pore structure characteristics of RCA were examined by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) tests. Finally, focusing on the RCA influence on the pore structure of the interlayer interface, the structure and bonding characteristics of the 3DPRAC interlayer interface were analyzed in detail, the correlation between pore geometry and bonding strength was established, and the mechanism of the influence of pore structures on 3DPRAC interlayer bonding properties was revealed.
Section snippets
Materials
P. O 42.5 ordinary Portland cement was used as the main cementitious material (Zhengzhou Jianwen Special Material Technology Co. in Zhengzhou, China). The auxiliary cementitious materials included early strength agents (ESA), fly ash (FA) and silica fume (SF). The admixtures, hydroxypropyl methylcellulose (HPMC), polycarboxylate superplasticizer and PVA fiber, were utilized. The fine aggregate was natural sand with a 3% moisture content and 3 mm maximum particle size. RCA, purchased from
Bonding strength
Fig. 6 compares the variation in bond strength with layer height for 3DPRAC with different replacement ratios. It was found that the bond strength increases and then decreases with increasing layer height in general and does not show a clear correlation with the layer height. At the same layer height, the bond strength decreased with increasing replacement ratio, but even the bond strength of 3DPRAC at a 100% replacement ratio was still higher than that of 3DPM. Further observation showed that
Formation of interlayer interface pores in 3DPRAC
Compared to the 3DPM with a flat and low roughness filament surface, the 3DPRAC filament surface is uneven and porous due to the incorporation of RCA (Fig. 18). This effect, coupled with the high yield stress of the printed material [12], causes the fusion between adjacent filaments to be further hindered during deposition. Therefore, entrapped pores with larger volumes are more prone to form at the 3DPRAC interlayer interface, which is also why extremely high porosity distribution peaks occur
Conclusions
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RCA incorporation substantially increased the filament surface roughness of 3DPRAC, resulting in large-volume pores being more easily introduced into the interlayer interface area during deposition. Compared with the interlayer interface without the printing time interval, the porosity distribution peaks showed abrupt changes in the printing time interval interface area, and the pores were distributed in agglomerates and were better connected. However, no clear correlation was shown between the
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.
Acknowledgements
The authors would like to acknowledge the National Key Research and Development Program of China (2019YFC1907105), National Natural Science Foundation of China (52178251), the Excellent Youth Science Foundation Project of Shaanxi Province (2020JC-46) for financial support. Thank the editor and reviewers very much for their comments and helpful suggestions.
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