Issue 50

M. Papachristoforou et alii, Frattura ed Integrità Strutturale, 50 (2019) 526-536; DOI: 10.3221/IGF-ESIS.50.44 531 mixing, however printable mixtures were observed after 15 and even after 30 minutes. Comparison of binder quantity with the workability parameters tested shows that when binder content increases, values of expansion and yield stress are increased. Power consumption however is decreased and this can be attributed to the fact that higher quantity of cement renders to lower quantity of aggregate in a given volume of concrete and consequently, less friction induced by the aggregates in the moving parts of the screw extruder. Mixtures with FA seem to lose more rapidly workability compared to mixtures with cement or cement and LFS as binder. No clear results were observed regarding the type of aggregates. CL1 FL1 F1 LL1 L1 Expansion 0' 21 22 16.5 15.5 20.5 15' 16.5 17,5 - - 17 30' - 15,5 - - - Yield stress (Pa) 0' 120 90 970 730 120 15' 1170 410 - - 1170 30' - 610 - - - Power consumption of screw extruder motor (W) 0' 732 680 741 812 694 15' 773 740 - - 790 30' - 784 - - - Table 5 : Workability results for mixtures with 500 kg/m³ binder. CL2 FL2 F2 LL2 L2 Expansion 0' 30 28 29 22 23,5 15' 26 25 24 16,5 21 30' 23.5 18 19 15 18.5 Yield stress (Pa) 0' 90 0 0 30 90 15' 260 730 690 700 260 30' 440 1020 980 730 440 Power consumption of screw extruder motor (W) 0' 615 605 605 651 643 15' 631 633 647 679 669 30' 647 684 665 698 683 Table 6 : Workability results for mixtures with 830 kg/m³ binder. In Fig.6, mixture F2 with river sand as aggregate and part of binder FA, can be seen 0 (left) and 15 minutes (right) after mixing. The mixture on the left, even though it fulfills criteria 1,2 and 3, does not meet criteria 4 regarding buildability, since height ratio of 1st layer versus 5th is 0.3, so it is characterized as not printable. On the other hand, the right specimen fulfills all 4 criteria and is characterized as printable. Regarding hardened concrete properties, density of concretes with 500 and 830 kg/m³ binder was 2100 and 2225 kg/m³, respectively. Compressive and flexural strength of mixtures is given in Figs.(7,8). Mixture with no SCMs as binder reached a compressive strength of 70 MPa. Substituting 30% of cement with SCMs reduces compressive strength to 55 MPa and flexural strength from 12.1 to 8.7 MPa. Similar strength development was observed for both FA and LFS mixtures. Slightly lower strength was measured only in mixtures with FA, when limestone filler substituted 50% of natural sand. As expected, higher binder content showed increased strength. UPV, a non destructive method to estimate strength, was also measured on specimens with the same moisture conditions and correlated with experimental results of compressive strength (Fig.9).