Issue 78

C. S. Karadumpa et alii, Fracture and Structural Integrity, 78 (2026) 146-167; DOI: 10.3221/IGF-ESIS.78.09

MPa at 7 days (+196.8% vs control value of 0.63 MPa, nearly tripling early-age capacity), 3.23 MPa at 28 days (+31.8% vs 2.45 MPa), and 4.10 MPa at 56 days (+28.9% vs 3.18 MPa), maintaining substantial superiority throughout as shown in Figs. 5 and 6. This reflects CNT's primary reinforcement mechanism—their high aspect ratio (>1000) and extraordinary tensile strength (50-200 GPa) enable effective nano-bridging across microcracks and weak zones, with individual CNTs spanning pore spaces to physically restrain crack opening and propagation. GO 0.03% showed more modest gains: 1.59 MPa at 7 days (+152.4%), 2.67 MPa at 28 days (+9.0%), highlighting fundamental mechanistic differences—GO primarily improves matrix density benefiting compressive strength, while CNTs provide fibrillar bridging elements that are particularly effective in enhancing tensile properties. Higher CNT dosages exhibited reduced effectiveness despite greater content: CNT 0.2% reached 2.95 MPa (+20.4%) and CNT 0.3% reached 3.01 MPa (+22.9%) at 28 days, both inferior to CNT 0.1% (3.23 MPa), suggesting that proper dispersion outweighs quantity—as agglomerated bundles may be less effective in bridging cracks and may introduce localized stress concentrations. The hybrid formulation achieved 2.72 MPa at 28 days (+11.0%), an intermediate value but closer to GO 0.03% (2.67 MPa) than CNT 0.1% (3.23 MPa). This behavior may indicate interactions between GO and CNTs that reduce the effectiveness of CNT-mediated crack-bridging mechanisms. CNT 0.1%'s persistent tensile advantage at extended age is particularly valuable for durability, as higher tensile strength reduces cracking under service loads, shrinkage, and thermal stresses— cracks being primary pathways for chlorides and sulfates causing long-term degradation.

7 Days (MPa)

Mix ID

Mix Description

% vs Control 28 Days (MPa) % vs Control 56 Days (MPa) % vs Control

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Mix 7

Control (0%)

0.63 0.90 1.59 0.86 1.87 1.46 1.75

—

2.45 2.54 2.67 2.07 3.23 2.95 3.01

—

3.18 3.28 3.31 3.21 4.10 3.29 3.50

—

GO 0.01% GO 0.03% GO 0.05% CNT 0.1% CNT 0.2% CNT 0.3%

+42.9% +152.4% +36.5% +196.8% +131.7% +177.8%

+3.7% +9.0% − 15.5% +31.8% +20.4% +22.9%

+3.1% +4.1% +0.9% +28.9% +3.5% +10.1%

Hybrid (GO 0.03% + CNT 0.1%)

1.51

+139.7%

2.72

+11.0%

3.35

+3.6%

Mix 8

Table 7: Split tensile strength test results.

Figure 5: Split tensile strength comparison with dosage of GO.

Figure 6: Split tensile strength comparison with dosage of CNT.

Flexural strength Flexural strength exhibited the most dramatic enhancement among all mechanical properties. The experimental tested specimen is shown in Fig. 7. The control mix achieved 7.5 MPa at 28 days, while optimal nano-modified formulations exceeded 15 MPa representing an improvement of more than 100%: GO 0.03% reached 15.6 MPa (+108.0%) and CNT 0.1% achieved 15.8 MPa (+110.7%) as shown in Tab. 8 — essentially doubling control strength through synergistic mechanisms including improved tensile capacity in the tension zone, crack deflection and bridging, enhanced C-S-H bonding, and improved aggregate-paste interfacial bonding.

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