Issue 68

K. W. Nindhita et alii, Frattura ed Integrità Strutturale, 68 (2024) 140-158; DOI: 10.3221/IGF-ESIS.68.09

maximum displacement with the deflection at the first yield [40]. The ductility formula is shown in Equation (3), where μ is ductility, Δ u is maximum displacement (mm), and Δ y is yield displacement (mm). Figure 16 shows the ductility values of self healing concrete specimens with a corrosion rate of 20%.

 

u





(3)

y

Figure 16: Concrete ductility test results.

Figure 16 shows that adding 0.1% bacterial variation causes an increase in the ductility value in concrete with a difference of 2.22 compared to normal concrete (0% variation). However, adding 0.6% bacterial variation to concrete resulted in a decreased ductility value compared to adding 0.1% bacterial variation with a decrease difference of 2.75. Meanwhile, adding 1.5% bacterial variation resulted in the smallest ductility value among all bacterial variations. It showed a decreasing trend after reaching its peak at the addition of 0.1% bacterial variation. This is caused by calcite precipitation or the formation of lime cells that fill the voids in the concrete. Thus, the concrete becomes denser and more compact [41]. However, from the study's results, it was found that adding more than 0.1% of bacterial variations resulted in decreased ductility due to the encapsulation nature of bacteria, which easily expands and shrinks when exposed to water. Thus, the most optimal level of encapsulated Bacillus subtilis bacteria as a self-healing agent is 0.1%. Concrete stiffness test results Stiffness is the ratio between the maximum load and the displacement in concrete [42]. Concrete stiffness can be calculated by comparing concrete's maximum force and maximum deflection [43]. The stiffness formula is shown in Equation (4), where K is stiffness (kN/mm), F is maximum force [44], and δ is maximum displacement (mm). The stiffness value of self healing concrete with 20% corrosion can be seen in Figure 17. F K   (4) Figure 17 shows the results of the stiffness values, where adding a bacterial variation of 0.1% increases the concrete stiffness value of 0.85 kN/mm when compared to 0% bacterial variation concrete (normal concrete). Meanwhile, adding 0.6% bacterial variation decreased the stiffness value compared to 0.1% bacterial variation concrete, and the stiffness value decreased when the concrete was given an additional 1.5% bacterial variation. This is caused by bacterial precipitation that accumulates and fills the concrete voids after cracks occur so that the concrete becomes denser and can withstand greater loads with small deflections [24]. However, the addition of bacteria to concrete reached an optimal point of 0.1% due to the effect of the encapsulated bacteria swelling when exposed to water. Thus, it also affects the cavity's size if the bacteria's encapsulation increases in the concrete.

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