PSI - Issue 70

R. Mohanraj et al. / Procedia Structural Integrity 70 (2025) 82–88

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estimate, results must be interpreted carefully and ideally supplemented with other techniques for comprehensive assessment, especially for mixes incorporating nano-additives. 4. Results and Discussion 4.1. Ultrasonic Pulse Velocity Test

Table 1. UPVT SiO 2 Beam SiO 2

7 DAYS (m/s)

14 DAYS (m/s)

28 DAYS (m/s)

1% 2% 3% 4% 5%

3416 4290 4420 5010 5220

3605 4310 4530 5090 5300

3540 4480 4600 5130 5340

Table 2. UPVT SiO 2 Cube SiO 2

7 DAYS

14 DAYS

28 DAYS

1% 2% 3% 4% 5%

4416 4290 5420 5310 5420

4505 4320 5530 5390 5500

4540 4380 5590 5430 5540

Higher Ultrasonic Pulse Velocity (UPV) values indicate a denser and more homogeneous concrete composition with fewer voids (Table 1 and Table 2). The research identified a consistent pattern of increasing pulse velocity in both beams and cubes as the percentage of SiO₂ nanoparticles increased, reaching its peak at 5% SiO₂, which suggests improved microstructure and lower porosity due to the inclusion of nanoparticles. Specifically, the velocity in beams at 28 days exhibited an impressive 56% increase, growing from 3416 m/s at 1% S iO₂ to 5340 m/s at 5%. For cubes, the best performance at 28 days was seen with SiO₂ content between 3% and 5%, attaining a peak velocity of 5590 m/s at 3%, along with similarly elevated values around 5540 m/s at both 4% and 5%. As anticipated, extending the curing duration from 7 to 28 days typically resulted in higher velocities, illustrated by the 5% SiO₂ beams, which rose from 5220 m/s to 5340 m/s. The lower performance of the 2% SiO₂ cubes at 7 days (4290 m/s) compared to those with 1% SiO₂ (4416 m/s) was an unexpected discovery. This could be because of different testing procedures or uneven nanoparticle dispersion. The 3% SiO₂ cube performed better than higher percentages, indicating that this could be the ideal dosage for cubes. Implications: According to the study's assertions of increased durability and decreased permeability, nano- SiO₂ improves the density and ho mogeneity of concrete. Beams and cubes may have different ideal SiO₂ percentages (for example, 5% for beams versus 3% for cubes), undersco ring the necessity of formulations tailored to the particular application. 4.2. Rebound Hammer Test Rebound hammer tests, which link increased compressive strength and surface hardness with higher rebound values (generally ranging from 10 to 31), revealed that the consistent addition of 3% SiO₂ produced elevated rebound numbers (between 19 and 31), with the highest recorded value reaching 31. On the other hand, the 5% SiO₂ concentration exhibited notable fluctuations in rebound results (ranging from 12 to 28), which could be attributed to the agglomeration of nanoparticles at this increased level. Table 3 depicts that t he samples containing 4% SiO₂ also demonstrated inconsistency, with outliers such as Sample 4 showing a rebound of 10 and Sample 5 reaching 29, indicating possible localized defects or improper mixing. Although lower SiO₂ concentrations (1 -2%) provided slight improvements compared to the control mixtures, the rebound hammer outcomes supported the findings from the Ultrasonic Pulse Velocity Test (UPVT), confirming that the most reliable enhancement in surface strength was