PSI - Issue 10

M. Papachristoforou et al. / Procedia Structural Integrity 10 (2018) 155–162 M. Papachristoforou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

161

7

0 20 40 60 80 100

Cement 500kg/m³

0 10 20 30 40 50

40.24

38.83

27.34

50% river sand + 50% limestone Compressive strength (MPa)

100% river sand 100% limestone

0.3

0.4

0.5

0.6

0.7

Compressive strength (MPa)

Water/Binder ratio

Fig. 8. Compressive strength versus Water/Binder ratio.

Fig. 9. Compressive strength for different aggregates.

80

Binder 500kg/m³

0 10 20 30 40 50 60

47.97

60

32.95

31.86

40

20

0

Cement Compressive strength (MPa)

4500 Compressive strength (MPa) 5500

6500

7500

Cement + FA Cement + LFS

UPV(m/s)

Fig. 10. Compressive strength for different binders.

Fig. 11. Compressive strength versus UPV.

workability levels, printing them with a printing system with screw extruder and setting criteria for accepting them or not as printable, printability windows were obtained. The criteria that were applied of either accepting or not the mix ture as printable were proven adequate. Experimental data showed that flow table test was more consistent than the other methods and printability window for the printing system used was found between flow table expansion values of 18 and 24 cm. Time after mixing for moving from the upper limit to the lower was also measured and was highly depended by the type of aggregates used. A maximum of 30 minutes was obtained without using any retarder additives. Electric power consumption of the motor that rotates the screw extruder was considered as a parameter of measuring real-time workability of the mixture, making it possible to modify it on time in real scale applications by adding chemi cal additives during printing. Very strong correlation was observed with expansion values from the flow table test, however it should be noted that the threshold values of power obtained by the experimental data refer only to the specific equipment and materials used. Nevertheless, after calibration, the method can be applied to other printing equipment and materials. Regarding the type of aggregates, an adequate number of river sand and combination of river sand and limestone based concrete mixtures were able to be printed successfully. On the other hand, most of limestone based mixtures were considered as not printable. Additionally, the limestone mixtures required higher amount of water and super plasticizer in order to achieve the same level of workability with the other mixtures, leading to lower values of com pressive strength. Higher strength levels (maximum 70 MPa) were obtained for the combination of river sand and limestone. The use of alternative cementitious materials such as fly ash and ladle furnace slag as a replacement of cement (20wt.%) results to average reduction of compressive strength by 30% and density by 10%, compared to mixtures with