PSI - Issue 64

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

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3.4. Shrinkage cracks Shrinkage cracks were observed in the first and second lab-scale prints for the mixtures without shrinkage reducing admixtures, as shown in Figure 7. The materials set by accelerator admixture were more prone to drying shrinkage cracks. This issue was mitigated in the third lab-scale print of hollow square geometry using a shrinkage reducing admixture with an optimum dosage of 1% of the binder (cement + steel slag).

Fig. 7. Drying shrinkage crack in the first lab-scale print

4. Conclusions This study identified typical issues and possible solutions during the lab-scale 3D printing of ordinary Portland cement-steel slag blended mortars.  Mixture with less open time led to the shortening and breaking of layers in the second half of the first lab-scale print. The same print witnessed over-drying of surface and surface cracking when the optimum accelerator dosage was disturbed due to a change in rheology of mortar during printing. This change in rheology was addressed during the third lab-scale print using compatible superplasticizer of large open time.  The nonlinear rheology shift and demand for accelerator dosage caused inconsistent layer printing and, eventually, the collapse of the printing element in 2 nd lab-scale print. Mixture with large open time also helped avoid the complication of varying the accelerator dosage during printing.  Some cracks due to drying shrinkage were observed due to the large exposure area of printing geometry. The use of shrinkage-reducing admixture helped mitigate such cracks in the third lab-scale print. Acknowledgment The authors would like to acknowledge the Empa Board of the Directorate for financial support. The authors also acknowledge Dr. Frank Winnefeld from the Empa Concrete and Asphalt Laboratory for technical support. The authors are grateful to Dr. Ali Saeedi and the technical staff of the Empa Structural Engineering Laboratory for assisting in laboratory-scale printing. The authors thank Carbicrete, an industrial partner, for providing material and technical support.