# Overview of the Development of 3D-Printing Concrete: A Review
## 3 Performance Requirements of 3D-printing Concrete Materials
### Mechanical Properties of 3D-Printing Concrete  
During the construction of 3D-printing concrete, slurry is piled up layer by layer. Slurry should not only support its own weight, but also be able to withstand the weight of the slurry accumulated on the upper layer. Thus, the slurry needs certain mechanical properties, strong early strength, and higher late strength, otherwise it will collapse and deform due to insufficient strength. Compressive strength and flexural strength are used to characterize the mechanical properties of 3D printed concrete, which can be improved by adding mineral admixtures and optimizing the gradation. [PERSON] et al. [40] used ordinary cement, fly ash, silica fume, fine sand, glass fiber and other materials to develop new 3D-printing materials. The 28 d compressive strength is in the range of 36 to 57 MPa, and the flexural strength is about 10 MPa. [PERSON] et al. [33] developed a high-performance fiber concrete suitable for 3D-printing by reducing the water-binder ratio and optimizing the material gradation method, which has high compressive strength (about 100 MPa) and flexural strength (11 MPa). [PERSON] et al. [26] found that glass, basalt, and carbon fiber can effectively improve the bending strength of concrete. [PERSON] et al. [29] studied the 3D-printing calcium aluminate cement water-based adhesive and found that it can effectively enhance the strength of the design structure, and studied the impact of saturation and other compressive strength, at the saturation level of 170% When, the maximum compressive strength of 3 DP cementitious material is 8.26 MPa, as shown in Figure 8.  
Figure 7: (a) Retarder dosage versus open time. (b) Superplasticiser dosage versus open time. Reprinted with permission from ref. [16]. Copyright 2012, RILEM. Fiber reinforcement is also an effective means to improve mechanical properties such as compressive strength and flexural strength, but too high fiber content will reduce fluidity, increase the risk of nozzle clogging, and reduce bonding strength. [PERSON] et al. [41] studied the fiber reinforcement effect of different fibers, as shown in Figure 9. The mechanical test of the printed sample showed that the bending strength was increased by 180%. [PERSON] [33] and [PERSON] [40] etc. respectively selected 12 mm/8 mm (length/diameter) polypropylene fiber and 0.5% alkali-resistant glass fiber (6 mm) to improve the surface properties of the sample. [PERSON] et al. [26] found that the compressive strength of glass fiber and basalt fiber sample with 1% volume content is only 13 MPa, while the compressive strength of carbon fiber sample with the same content can reach 30 MPa. 3D-printing concrete has high requirements for compressive strength as conventional concrete, the interlayer bond strength is also a necessary condition for maintaining structural stability. Studies showed the 3D print concrete component is easily destroyed at the interlayer bonding when the load is received [42]. The weakness of the 3D-printing  
Figure 8: Compressive strength of 3 DP cubic samples with lithium carbonate. Reprinted with permission from ref. [29]. Copyright 2017 Elsevier Ltd.  
Figure 9: Flexural strength of fiber-reinforced samples printed in x and y direction. Reprinted with permission from ref. [41]. Copyright 2014 Elsevier B.V.  
concrete structure is an inter-interlayer adhesion, and the interlayer bonding performance significantly affects structural mechanical properties. Interlayer print interval is a key factor affecting bond strength [33]. [PERSON] et al. [43] found that the methods that increasing the interval, print speed and printhead height can reduce the interlayer bond strength of 3D-printing concrete structure. [PERSON] et al. [44] observed that the interlayer bond strength decreases over time intervals. When the interval time interval is 1 min, the bond strength is reduced by 50%, and when the interval time interval is 1d, the bond strength is reduced by 90%. Adding a mortar between the interlayer can effectively improve the interlayer bond strength.