Keywords

1 Introduction

The rapid development of 3D concrete printing (3DCP) has the potential to greatly reduce labor demand, improve sustainability, reduce construction costs, and effectively overcome the dilemma faced by traditional construction methods  [1,2,3,4]. In recent years, substantial achievements have been made by 3DCP in the field of architecture and civil engineering. One of the challenges is that conventional steel bar reinforcement cannot be directly integrated into the printed concrete. Researchers and engineers have tried different reinforcement methods by applying continuous fiber, shut fiber, microcable, mesh and U-nails etc., to improve the brittleness of 3D printed concrete  [5,6,7,8]. Fiber reinforcement is widely applied for printed concrete due to its effectiveness and ease of operation. Small-sized fibers show less interference with the flexible extrusion characteristics of 3D printing. More importantly, the mechanical properties of fiber-reinforced concrete can meet the structural requirements, such as compressive strength and tensile strain exceeding 100 MPa and 4%, respectively  [9, 10]. Ultra-high performance concrete (UHPC), as a type of fiber-reinforced concrete, can meet these structural requirements  [11, 12] and is currently mainly being used for new structures, reinforcement, and repair of existing infrastructure [13, 14].

The development of 3D printed UHPC (3DP-UHPC) will greatly drive the application of 3D printing technology to structural engineering. UHPC is considered a combination of self-compacting concrete, high-performance concrete, and fiber-reinforced concrete [15]. The good construction performance of UHPC is strong related to the casting procedure, but this advantage is difficult to match with the 3D printing construction of layer by layer stacking. Current research has eliminated the gap between cast and 3D printed UHPC in construction by adding viscosity-modifying admixtures (VMA), such as nano-clay, hydroxypropyl methylcellulose (HPMC), etc.  [16,17,18,19,20]. Similar to traditional UHPC, 3DP-UHPC will be widely used in new and existing structures. For example, the construction of a curvilinear bench with free form and light structure [16]. In addition, more attention is being paid to the dynamic performance. Zhou et al. [20] discussed the performance of 3DP-UHPC based on projectile and explosive impacts tests. Yang et al. [21] carried out split-Hopkinson pressure bar (SHPB) tests and analyzed the strain rate effect of 3DP-UHPC

3DP-UHPC has attracted much attention because of its excellent mechanical properties and ongoing research mainly focuses on the preparation of materials, static, and dynamic mechanical properties, and so on. Given the complexity of the preparation method of 3DP-UHPC and the unknowns of the problems that may be faced in its applications, we systematically reviewed the latest research progress on 3DP-UHPC. Finally, some suggestions are put forward to promote the development of 3DP-UHPC based on the current challenges.

2 Preparation of 3DP-UHPC

2.1 Design of UHPC for 3DCP

The gap between the printing and casting procedures of UHPC mainly depend on rheological properties. 3D printed concrete is a typical yield stress material; that is, its yield stress first increases and then decreases with increasing shear rate, and finally maintains at a certain yield stress platform, as shown in Fig. 1a. The cast UHPC is self-leveling in the static state because its yield stress makes it difficult to maintain its shape. Moreover, cast UHPC is a shear thickening fluid; that is, its yield stress increases rapidly with increasing shear rate, as shown in Fig. 1a. On the other hand, shear thickening fluid means that the shear rate needs to be continuously increased to overcome the yield stress of UHPC. UHPC strips will be narrowed or even interrupted when the yield stress exceeds the maximum shear stress provided by the 3D printer.

Fig. 1
4 images. 1. A graph of shear stress in pascals versus angular rotation in revolutions per minute. 2. A graph of green strength in megapascals versus 3 D procedure in time. 3 and 4. 2 illustrations indicate the extruder, rupture surface, cementitious material, free water, aggregate, and porosity.

a Fluid types of ultra-high performance concrete (UHPC) and 3D printed concrete (3DPC); b printability principle of 3DP-UHPC [22]; c schematic of shear resistance of fresh 3DPC without and with coarse aggregate [4]

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The methods for modifying UHPC to achieve printability are the chemical hydration accelerated hardening method [20] (CM) and the physical flocculation method (PM)  [16, 17]. The CM matches the hydration rate of UHPC with the printing rate by adding materials that change the cement hydration rate. For example, three levels of fast, medium, and slow hydration rates of UHPC can be represented by the curves ①–③ in Fig. 1b. These different rates can be achieved by reducing the accelerator dosage, such as sulfoaluminate cement. The curves ①–③ can match the printing rate of slow, medium, and fast, respectively. However, the matching gap between the hydration rate and printing rate of UHPC usually leads to a short open time of CM. It is easy to cause large deformation and rough surface dry cracking or even collapse and fracture due to insufficient hydration and too rapid printing. PM matches the printing rate by adding VMAs, such as silica fume (SF), nano-clay, and HPMC, to make thixotropic structures in the UHPC before hydration structure formation. For example, based on curves ①–③ in Fig. 1b, the addition of the same VMA gets curves ④–⑥. The addition of VMA makes the material printable earlier. The thixotropic structure will produce obvious deformation when the accumulated weight of the UHPC exceeds its yield stress, affecting the forming accuracy and even causing collapse. Therefore, PM is limited to a certain range of printing rates to allow the hydration rate to follow up.

The potential third printability control method, namely the framing effect provided by raw materials, needs attention because of the current development trend of mixing large-size aggregate into UHPC and the existence of steel fibers. In our previous study on the printability of large-size aggregate, we found that construction deformation and strength were derived from the bonding force of cementitious materials and the aggregate biting force [4], as shown in Fig. 1c. Therefore, the contribution of the framing effect to the improvement of yield stress cannot be ignored.

Designing UHPC for 3DCP will be based on the above principles. Specifically, the purpose of the preparations is to improve the early yield strength of UHPC, reduce its fluidity and improve its shape retention ability by adding a regulator or VMA alone or adding both. Limited by the lack of quantitative preparation theory, the specific dosage can only be obtained by testing. It should be noted that determination of these doses is significantly related to the printing equipment and the selected printing process, which reduces the repeatability of the material mix to a certain extent.

2.2 Manufacturing of 3DP-UHPC

Table 1 lists the raw materials and mix proportions of 3DP-UHPC in the existing literature. Zhou et al. [20] also adopted CM to promote the hydration rate to achieve matching with the printing process, specifically by adding slag. Arunothayan et al.  [16, 17] used PM to achieve printability of UHPC, by adding HPMC and nano-clay. It cannot be ignored that the proportion of SF is relatively large, accounting for ≈30% of the cementitious materials. Excessive SF also contributed to the printability of UHPC. To avoid the defects of the short open time of CM and the low unit construction rate of PM, our group proposed a method of combining CM and PM to achieve printable UHPC[19]. Specifically, sulfoaluminate cement was used to replace the 10% mass fraction of Portland cement to accelerate the hydration rate to preliminarily match the printing rate. Then, the thixotropic material Nano clay and HPMC were added to supplement the insufficient yield stress of UHPC to realize the continuous and stable printing. Here, the mass ratio of the accelerator composed of sulfoaluminate to the flocculant composed of nano-clay and HPMC was ≈11:1. The mix proportions of Yang et al. [18] are the most popularized, because the raw material composition does not different from the cast UHPC base except for the addition of an appropriate amount of nano-calcium carbonate, and its mechanical properties are also the highest, which may be related to the use of a water reducing agent.

Table 1 Mix proportions of 3DP-UHPC (kg/m3)
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The microstructure of UHPC mixed with sulfoaluminate cement, sulfoaluminate cement and VMA was observed by scanning electron microscopy. The preparation time of the two samples was between the initial and final setting. Figure 2 shows the obvious differences in the microstructure of UHPC before and after adding VMA. The microstructure of UHPC without VMA is mainly C-S–H gel with 3D network structures (Fig. 2a); that is, 3DP-UHPC prepared by CM. The microstructure of UHPC doped with sulfoaluminate cement and VMA is mainly flocculent, formed by the adsorption of hydration products by VMA. A similar flocculent microstructure after adsorption was reported by Zhang et al. [23].

Fig. 2
2 scanning electron microscopy images. 1. It indicates S F and hydrating structures C-S-H gel. 2. It indicates C H, S F, and flocculation structures nanoclay and H P M C.

Scanning electron microscopy images of a hydrating structure: C–S–H gel; b flocculation structure formed by nano-clay and hydroxypropyl methylcellulose (HPMC)

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2.3 Mechanical Anisotropy of 3DP-UHPC

The compression and flexural properties of 3DP-UHPC in the existing literature were statistically analyzed. The average compressive strength of all 3DP-UHPC samples at 28 days was in the range of 120–160 MPa. The flexural strength of specimens was between 30 and 50 MPa. The printing procedure had\s a favorable effect on the orientation of fibers; that is, the orientation of steel fibers was parallel to the printing direction. Figure 3a, b show computed tomography scans of single or interwoven printed 3DP-UHPC samples. The consistency between fiber orientation and printing direction is obvious. The fiber orientation and stress direction should be designed to be consistent to reach optimal application of UHPC materials [24]. For example, the flexural strength of 3DP-UHPC specimens nearly doubled that of cast UHPC specimens [22], as shown in Fig. 3c. The flexural and tensile properties of 3DP-UHPC will be either ductile or brittle in different directions due to fiber orientation [24]. 3DP-UHPC has obvious mechanical anisotropy and generally has weaker mechanical properties along the interstrip or interlayer directions, as shown in Fig. 3d. Therefore, it is necessary to optimize the printing path to fully realize the toughening effect of fiber orientation and achieve the excellent bearing performance of 3DP-UHPC structures [18].

Fig. 3
4 images. 1. A scan indicates fiber, nozzle, and printing direction. 2. A scan indicates printing directions and the first, second, third, and fourth layers. 3. A graph of flexural strength in megapascals versus displacement in millimeters. 4. A vertical bar graph plots M O R in megapascals.

Computed tomography scans of a single printed and b interwoven printed 3D printed ultra-high performance concrete (3DP-UHPC) sample; c flexural strength [22] and d the modulus of rupture (MOR) [24] of 3DP-UHPC. CS, cast sample; PS, printed sample

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3 Applications of 3DP-UHPC

3.1 Large-Scale Special Components

3DP-UHPC with its ultra-high mechanical properties is expected to alleviate the contradiction that 3D printed concrete structures cannot be directly used in engineering under steel-free reinforcement conditions. Some specially shaped structures with independent bearing capacity have been constructed, although no large-scale application of 3DP-UHPC has been found. There are two construction paths for 3DP-UHPC in new structures; one is the construction using 3DP-UHPC alone, such as a curvilinear bench [16] or hollow column [17], as shown in Fig. 4a, b; another is to use 3DP-UHPC to construct permanent templates, such as the specially shaped columns constructed by our group, shown in Fig. 4c. Attention should be paid to the construction of steel-free reinforcement concrete structures with 3DP-UHPC, namely UHPC-reinforced concrete (URC). This has obvious advantages in maintaining the flexibility of 3D printing and reducing the time and labor consumption of rebar implantation.

Fig. 4
3 photographs. 1. A curvilinear bench with a vertical height of 680 millimeters and a length of 1650 millimeters. 2. 4 photographs of a hollow column with a height of 600 millimeters and a diameter of 300 millimeters. 3. A specially shaped column with a height of 1500 millimeters.

a Curvilinear bench [16]; b hollow column [17]; c variable diameter hollow column

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3.2 As the Reinforcement Materials

The in-process reinforcing method (IRM) was inspired by reinforced concrete materials under the technical constraints of steel bar implantation. As a type of IRM, the dual 3D printing procedure for URC has proved to be a potential and effective reinforcement method [19]. This is because UHPC and concrete are both cement-based materials, which means that there are advantages in the printing path following and interface bonding performance [25]. The principle of an extrusion system for dual 3D concrete printing is that UHPC and 3DPC are fused at the nozzle to complete synchronous printing, as shown in Fig. 5a. Figure 5b shows the three-direction profile of the sample printed by this technique. It can be seen that the 3DP-UHPC is similar to the rebar arrangement in concrete. Another advantage of this technique is that the concrete wrapping UHPC blocks air and water to protect steel fibers from corrosion. In addition, it can be predicted that this method will maximize the utilization rate of UHPC to achieve structural strengthening and toughening, such as printing along the stress line of the bridge or in the required position based on the results of topological optimization.

Fig. 5
A diagram and 3 photographs. 1. It indicates the air compressor, sealing bearing, bearing balls, pump, extrusion motor, printing direction, 3-D P-U H P C, and 3-D P-C. 2. They display orthogonal cross-sections of reinforced concrete specimens that indicate 3 D P-U H P C and 3 D P-C.

a Schematic of extrusion system for dual 3D concrete printing; b three orthogonal cross-sections of 3DP-UHPC reinforced concrete specimen [19]

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3.3 As Protection Against Explosion and Impact

3DP-UHPC application has been extended to anti-explosion and anti-impact structures due to its good energy absorption capacity. The effectiveness of 3DP-UHPC has been demonstrated by experimental studies on explosion resistance and impact resistance. Yang et al. [21] tested the dynamic compression performance of 3DP-UHPC. Zhou et al. [20] explored the unique behavior of 3DP-UHPC subjected to projectile and explosive impacts and revised an empirical formula for calculating the crater depth of 3DPC. The preparation process is shown in Fig. 6(a). Our recent work explored the contact explosion performance of 3DP-UHPC. In that work, 3DP-UHPC with different thicknesses was used to strengthen ordinary concrete. The preparation process is shown in Fig. 6(b). It is worth noting that there is no rebar inside the slabs of the test group. We found that PURN8, the thickness of 3DP-UHPC replacing ordinary concrete was 53.3%, was marginally improved to that of reinforced concrete.

Fig. 6
2 photographs. 1. Seven photographs display the technology for preparing 3-D printed targets, from the printed crack resistance layer to the casted plain U H P C mortar. 2. A photograph of printed odd and even layers, casting concrete, and finished P U R N.

a Technology for preparing 3D printed target [20]; b construction process of test slabs. UHPC, ultra-high performance concrete

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4 Challenges of 3DP-UHPC

Despite the increasing amount of research published thus far on 3DP-UHPC, many challenges and research barriers require further innovative exploration.

4.1 Standardization of Materials

The most fundamental challenge is standardization of 3DP-UHPC. In the preparation process of 3DP-UHPC, our proposed method that combines the CM and PM had qualitative mixing ratio designs based on experience. Following a certain rule of mix ratio design will be more conducive to standardization of 3DP-UHPC and improvement of material properties. The mixing ratio design of 3DP-UHPC has been completed, but regulating the fluidity of UHPC brings a series of challenges to the ultra-high performance of the material, such as mechanical and durability weakening problems due to the density of the material and the bonding performance between the fiber and the matrix. In addition, steel fibers cannot solve the strain-softening behavior after peak stress, and it is difficult to achieve large tensile strains of materials. Exploratory studies have found that mixing steel fibers and polyethylene fibers can effectively improve this problem [10]. Therefore, an integrated 3DP-UHPC mix ratio design method combining comprehensive fluid performance regulation, fiber–matrix interface, and synergistic toughening of multiple fibers can contribute to optimization of material properties, and further research on this material is required, such as durability and mechanical properties under different strain rate conditions, etc.

4.2 Flexible Design of Structures

It is very important to manufacture 3DPC structures considering both safety and economy in structural applications. 3DP-UHPC has good mechanical toughness under different strain rate conditions and will be widely used in the construction of new structures and the reinforcement of existing military and civil buildings. Reinforcing plain concrete where it is needed provides a specific way to achieve the construction of 3DPC structures. 3DP-UHPC is regarded as a disruptive method for construction due to the advantages of 3D printing and its economics. Initial exploration has demonstrated the feasibility of 3DP-UHPC to partially replace or even completely replace steel reinforcement for concrete structures, but its application in larger-scale structures requires refined design, such as combining prestressing methods and topological optimization methods, design, etc. The reinforcement of existing buildings with 3DP-UHPC is also a very important structural application scenario. The coordinated deformation of interfaces of 3DP-UHPC and other materials under different working conditions will require special attention, such as the shrinkage deformation performance and load-deformation performance of the two materials. The structural response of 3DP-UHPC under strain rate loading will also be a very interesting research area.

5 Summary and Outlook

Combined with the reviewed research, we have provided a specific understanding of the research progress of 3DP-UHPC, which will be of significance to promote the application of 3DCP.

  • Integrated material design combining fluid modification, fiber–matrix interface enhancement, and hybrid fiber toughening can be used to improve the mix design of 3DP-UHPC at the theoretical level, such as quantifying the impact of VMAs on the printability of UHPC, improving fiber–matrix cohesion through nanomaterials, and finding critical fiber lengths, etc.

  • Further exploration of the mechanical properties and durability of materials under different strain rates and establish corresponding constitutive models to provide a theoretical basis for structural design.

  • Further exploration of the application of 3DP-UHPC in newly constructed 3DPC structures or the reinforcement of existing buildings and explore the structural response laws under different strain rate loads to realize the safety and economy of 3D printed structures.