PSI - Issue 64

Zohaib Hassan et al. / Procedia Structural Integrity 64 (2024) 1184–1191 Hassan Z. et. al / Structural Integrity Procedia 00 (2024) 000 – 000

1187

4

precisely 8 minutes after the end of mixing. The accelerator dosage was kept constant at 21 ml/L optimized at the start of the printing for the whole print.

Fig. 2. Schematic diagram of 3D concrete printing setup

2nd lab-scale print: This printing was performed immediately after the end of the 1st lab-scale print using the same mix design for the same geometry. However, the only variable was the accelerator admixture dosage used during the printing. This was to maintain the flow with the changing rheology of the fresh mixture. 3rd lab-scale print: This printing was performed with the same mixture design using different superplasticizers, yielding ample open time as shown in Figure 3 for a hollow square geometry (outer side = 330 mm, height = 300 mm). The purpose of square geometry used for the third print was to extract prism specimens for mechanical strength testing. Printing was started with delay, 30 minutes after mixing, with the constant accelerator dosage of 27 ml/L optimized at the start of the printing. Typical issues in 3D concrete printing of ordinary Portland cement-steel slag blended mortar were evaluated in first and second lab-scale prints. The third lab-scale print was performed to evaluate the efficiency of the identified solutions. Table 4: Printing process parameters

Layer width (mm)

Layer Height (mm)

Printing Speed (mm/sec)

Accelerator dosage at the start of print (ml/L)

Description

1st Lab-scale print

30

8

43

21

2nd Lab-scale print

30

8

43

Variable (21-6)

3rd Lab-scale print

35

8

60

27

3. Defects and remedies 3.1. Layer shortening and breaking

Figures 4 and 5 show the printing of a hollow cylindrical geometry in two different trials. Shortening and breaking of the layers, predominantly the second half of first lab-scale print in shown in Figure 4. This issue arises if the material rheology is not constant or starts changing over time. Figure 3 shows the time-dependent flow of the same mix design used in the current study with different superplasticizers adopted from Hassan et al., 2024. Superplasticizer Type-1 yields less flow as time passes, with a sharp decrease after 20 minutes of mixing. In comparison, superplasticizer Type-2 continues to offer an adequate flow of more than 80% even after 2 hours. Hence, is the observed issue can be attributed to the short open time associated with the suitability of the