PSI - Issue 73

Loran Nermend et al. / Procedia Structural Integrity 73 (2025) 130–137 Author name / Structural Integrity Procedia 00 (2025) 000–000

133

4

slow mixing of dry components for 120 s,

•

• addition of water (C sample) or solution with admixtures (ultrasonicated for 10 min using ultrasonic bath), • high-speed mixing for 540 s. Depending on the use, specimens were either cast or manually transported to an extruder for printing and testing. The specimens were printed using a 3 degrees of freedom (3DoF) Cartesian robot (gantry). The printing process was performed using a special printing head/extrusion system with an auger and a 40 × 10 mm nozzle at a 45° angle. Printed specimens were composed of four layers and were cut into 160 mm length 24 hours after printing. Mixing protocol and specimen preparation process (mixing, curing and cutting) is presented in Sikora et al. (2025).

Table 1. Mix composition [kg/m 3 ]

Sample

Cement

Fly ash

Silica fume Nanosized admixture

Sand

Water

Superplasticizer

Control (C)

580.0 565.5 565.5

166 166 166

83 83 83

-

1300 1300 1300

205 205 205

2.3 2.3 2.3

BG250 BGS250

36.7 49.4

2.3. Methods An electrical resistivity (surface resistivity) test was performed using the Wenner 4-probe resistivity method (Resipod, Proceq) apparatus on prismatic cast 40×40×160 mm specimens. The Wenner 4-probe method works with the principle that current (I) was applied to the saturated surface of concrete using two outermost probes, and the potential difference (V) was measured using two inner probes. Although larger concrete specimens are typically used for evaluation, mortar bars were assessed in this study to enable direct comparison between the different mixtures. Similar approach was applied in the multiple previous studies including works of Lehner et al. (2022) and Liu et al. (2025). The test was performed after 2, 7, 14, 28 and 90 days of specimens immersed in water.

Fig. 3. Specimens after printing process

The elevated temperature test was conducted on prismatic specimens in the dimensions of 40 × 40 × 160 mm using an electric furnace. Prior to testing, specimens were dried for 3 days at 70 °C to remove the moisture from the specimens. Afterwards, the specimens were subjected to elevated temperature of 450°C and 600°C for 2 h with heating rate of 4 °C/min. The compressive strength was tested 24 h after the heating process using using electromechanical universal tester UNIFRAME 250 (CONTROLS S.p.A., Liscate, Italy) with a loading rate of 0.6 MPa/s. Six specimens of each type were tested with mean value taken as a representative. Before conducting the destructive test, a non destructive method based on ultrasonic wave measurements was applied to evaluate the dynamic elastic modulus of