A compendious review on lack-of-fusion in digital concrete fabrication

Abstract

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.

Introduction

Additive manufacturing technologies, also generally known as 3D printing, have significantly influenced how objects are created [1]. Objects with complex geometry can be manufactured in the comfort of one’s house, using a variety of materials and colors [2], [3], at competitive manufacturing rates [4], [5]. Several technical industries are embracing this technology to improve component quality and reduce fabrication time, including the automotive and aerospace industries [6], [7] as well as the biomedical industry [8]. Biomimetics have also recently been explored using 3D printing technology [9]. In contrast to the wealth of advantages that additive manufacturing presents, one of the main challenges is lack-of-fusion (LOF), which essentially refers to a weak interlayer bond between deposited filament layers resulting in reduced mechanical properties [10]. This is a well-known drawback of the Fused Deposition Modeling (FDM) method [11], [12], but is also common in other additive manufacturing methods, including Selective Laser Sintering (SLS) [13], Selective Laser Melting (SLM) [14], [15] and Powder Bed Fusion (PBF) [16], [17].

Additive manufacturing technologies are also rapidly being introduced in the construction industry as of late, mainly to reduce construction time and waste [18], whilst augmenting architectural freedom in design [19], [20]. The popular method-of-choice is currently extrusion-based printing [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], which is similar to FDM in the sense that layers are added onto each other except that no heat is required to facilitate pumping and placement of concrete. Novel developments in concrete printing methods include shotcrete 3D printing [31], [32], [33], [34], which essentially entails spraying concrete in layers instead of extruding filaments, injection 3D concrete printing [35] and particle-bed printing [36], [37]. Current notable 3D printed concrete applications include the fabrication of topologically optimized post-tensioned bridges [38], [39] and slabs [40], [41], bespoke façade elements [42], [43], affordable housing [44], [45], [46], custom landscape elements [47], [48] and renewable energy technologies [49]. Momentous developments in steel 3D printing are also made, with the first 3D printed steel bridge fabricated by MX3D [50], [51]. Further to niche 3D printed concrete (3DPC) applications, several 3DPC-specific materials have been developed, including engineered, strain hardening cementitious composites (ECC/SHCC) [52], [53], lightweight coarse clay aggregates with maximum particle size of up to 10 mm [54], lightweight foam concrete [55], nanoparticle-included concrete [56], [57], geopolymer or alkali activated materials [58], [59], [60], fiber-reinforced materials [61], [62], and styrene butadiene rubber (SBR)-modified concrete [63]. Material characterization processes, i.e. to determine strength evolution and suitability for printing applications, include inline quantification [64], slump flow and mini-slump tests [65], [66], [67], rheology [68], [69], [70], [71], [72], [73], [74], [75], [76] and green or fresh-state mechanical tests [77], [78]. Based on these tests, characterization-specific buildability models have been derived (i.e. mechanistic [79], [80], [81], [82], [83], [84], rheology [85], [86], [87] and rheo-mechanics [88], [89]) to predict the number of stacked layers attainable before failure occurs. Some of the latest models defined in literature include a shape retention model [90] that determines the largest feasible filament cross-sectional size that will not yield plastically under self-weight and a 3DPC design model [91] that determines the optimum print parameters to successfully construct the entire specified object in the least amount of time, i.e. at the fastest vertical build rate achievable. Pertinent reinforcing strategies have also been developed to improve the ductility and safety of 3DPC structures [92], [93], [94], [95].

Similar to printing steel composites and plastic, extrusion [96] and spray-based [34] concrete printing also experience LOF between successively deposited filament layers. It is important to note that, unlike in conventional FDM methods where LOF is largely thermo-induced, LOF between concrete filaments is due to either weak hygro-chemo interaction between layers [97] or poor mechanical surface bonding between layers [98] or both, depending on the pass time between successive layer depositions [99]. Whether batch mixing, or continuous mixing is used for 3DPC may influence the LOF. In the former, the required volume of concrete is mixed and transported to the printer before commencing the 3D print. The time-dependence of fresh concrete properties imply that there is a change in its behavior from the start to end of 3D printing the concrete batch. In continuous mixing, the freshly mixed concrete is fed to the printer in a pseudo-continuous mixing-printing process, whereby age difference between printed concrete is near zero throughout the 3DPC process. Batch mixing may exacerbate LOF, due to aging of the concrete before being printed. As a consequence, printed concrete is typically classified as an orthotropic material [100]. The weak interlayer strength compared to the stronger bulk intralayer strength causes premature failure of the concrete, typically increasing the variability associated with hardened state mechanical properties of 3DPC [101]. This could be detrimental for the mass-scale adoption of 3D printing in industry. Complications such as accounting for orthotropy and the weakest interlayer region in structural design, increased maintenance due to the likelihood of cracks forming in the typically thin-walled 3DPC structures and reinforcement to attain the required level of structural integrity contribute toward a less cost-competitive technology. Much literature is available on the effects of LOF on mechanical properties of 3DPC; however, limited literature is available on possible solutions to this problem. This paper critically investigates LOF in digital concrete fabrication, reporting on mechanisms responsible for LOF, interlayer bond strength characterization methods, detailed discussion on the consequences of LOF and lastly methods that have been implemented to improve fusion between concrete filament layers.