Issue 71

K. Federowicz et alii, Fracture and Structural Integrity, 71 (2025) 91-107; DOI: 10.3221/IGF-ESIS.71.08

Figure 13: Compressive strength results of: a) recycled fines mixtures, b) biochar mixtures.

Fig. 13 presents the mean compressive strength results (3 specimens for each test) of the mortars. The data are also presented in Tab. 4, along with the standard deviation and coefficient of variation. The compressive strength results correlate with the calorimetric studies. At low levels of cement replacement with recycled fines, an increase in performance was observed in all test periods. Beyond a critical level, further cement replacement with RF led to a deterioration in strength. Replacing 1.25 vol.% and 2.5 vol.% accelerated the strength gain, especially within the first 24 hours, with the differences being significantly smaller after seven days. In the case of using biochar, the effect of cement replacement was less pronounced, especially in the first 24 hours, where no significant differences were observed. The most considerable decrease in compressive strength was noted for the BC1000 mixture, which was 15%. In later periods, the differences were even more minor, although BC125 and BC250 showed a slight increase in strength compared to the REF. After seven days, BC250 gained just under 8%, while BC1000 lost 10% of compressive strength.

1 day StD [MPa]

3 days StD [MPa]

7 days StD [MPa]

f cm [MPa] 31.81 38.64 41.10 38.12 30.49 33.24 35.53 32.17 27.82

CoV [%]

f cm [MPa] 45.25 47.40 48.95 44.52 37.70 44.29 46.11 42.67 37.63

CoV [%]

f cm [MPa] 56.65 61.35 63.47 57.68 52.61 59.33 60.99 54.10 50.88

CoV [%]

Ref.

1.22 1.69 1.55 2.17 1.31 1.38 1.41 2.24 1.27

3.7 4.4 3.8 5.7 4.3 4.2 4.0 7.0 4.6

1.80 2.13 4.13 1.59 1.12 0.73 1.17 1.20 2.17

4.0 4.5 8.4 3.6 3.0 1.7 2.5 2.8 5.8

2.59 2.80 2.63 2.04 2.09 1.35 1.15 2.69 2.33

4.6 4.6 4.1 3.5 4.0 2.3 1.9 5.0 4.6

RF125 RF250 RF500 RF1000 BC125 BC250 BC500 BC1000

Table 4: Mean compressive strength with standard deviation and coefficient of variation.

Microstructure Microscopic images of 3D-printed concrete samples and pristine aggregates are presented in Fig. 14. In the reference 3D printed concrete samples (images a and d), empty cavities formed by the air bubbles or from the loose aggregate can be noticed. Similar cavities were seen in the 3D-printed concrete supplemented with recycled fine aggregates. Compared to the reference sample, the amount of cavities in recycled fine slightly increased. The image of recycled fine aggregates is presented in Fig. 14c. Since recycled fine aggregates have a size below 125 mm and a color similar to river sand used in samples, it is difficult to identify recycled aggregates in the 3D-printed concrete.

102