PSI - Issue 67

E.D. Reis et al. / Procedia Structural Integrity 67 (2025) 39–46 Reis et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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3. Results and Discussions Table 1 presents, in this order, the mean values, the outcomes of Tukey's means contrast test (5% significance), and the coefficients of variation (CV) for the assessed properties — compressive strength (f c ), tensile strength (f ct ), porosity (P), and ultrasonic pulse velocity (UPV) — considering the three CNT contents (0%, 0.05%, and 0.10%). All the p-values of the Anderson-Darling normality surpassed 0.05, validating the Tukey test results.

Table 1. Test results for the evaluated properties (f c , f ct , P, and UPV). Sample f c (MPa) f ct (MPa) P (%)

UPV (km/h) 4.103 b (2.11) 4.278 a (0.77)

C0

30.43 b (4.34) 35.67 a (3.63) 37.45 a (4.82)

2.19 b (9.24) 2.89 a (12.72) 2.95 a (12.92)

7.63 a (2.33) 6.69 b (4.96) 8.13 a (6.29)

C0.05 C0.10

4.188 b (0.71) Note: For a given property, means followed by the same letter do not differ statistically according to the Tukey test (p < 0.05).

The study results show that adding CNTs to the concrete mix statistically alters its properties. The reference sample C0 (without CNTs) showed expected values for f c (30.43 MPa), f ct (2.19 MPa), P (7.63%), and UPV (4.103 km/h), considering the dosage and mixing method used in the manufacture of the concrete in this study. With the addition of 0.05% CNTs (sample C0.05), there was an increase of 17.2% in f c , 32.0% in f ct , and 4.3% in UPV, while P was reduced by 12.3%. These improvements were confirmed as statistically significant by the Tukey test, showing the effectiveness of CNTs in strengthening the concrete matrix. On the other hand, the sample with 0.10% CNTs (C0.10) showed even greater increases in f c (23.1%) and f ct (34.7%), along with a modest 2.1% increase in UPV. However, this sample also showed a 6.6% increase in P. The Tukey test allows for the inference that the increases in strength were positively significant, but that of P was undesirable since greater porosity can compromise the durability of concrete by allowing aggressive agents to penetrate more easily. The UPV, conversely, was equivalent to the reference sample. These results confirm the findings in the literature that CNTs enhance the f c and f ct by promoting cement hydration and serving as bridges for stress transfer across cracks and voids, as well as improving the durability of concrete by occupying pores and voids, thereby decreasing P and elevating UPV (Xu et al., 2015). It should also be noted that excess CNTs can lead to agglomeration, creating larger pores and adversely impacting these outcomes (Jung et al., 2020), which could account for the observed rise in P when using 0.10% CNT. Moreover, given the minimal amounts of CNTs, their impact on the concrete was undetectable through SEM images. Thus, before analyzing the causes behind the enhancement or degradation of the properties examined, in light of employing CNTs pre-dispersed in cement particles within an isopropanol medium, it is worth contrasting these variations with findings reported in the current literature, as shown in Table 2 to Table 5.

Table 2. Comparison of compressive strength (f c ) variation with literature. Reference CNT content f c (MPa)

Variation

Reference

CNT-concrete

(%)

Asil and Ranjbar (2022) a Abinayaa et al. (2014)

0.150% (PF) 0.045% (NS) 0.100% (PF) 0.200% (AS) 0.100% (AS) 0.100% (AS) 0.030% (NS) 0.100% (PF)

32.8 38.2 30.4 50.0 45.4 47.5 29.0 36.4

43.3 49.2 37.5 61.5 55.6 57.5 35.0 42.3

+32.0 +28.7 +23.1 +23.0 +22.5 +21.1 +20.7

Current study Irshidat (2021)

Hawreen et al. (2019) Carriço et al. (2018) Hassan et al. (2019)

Song et al. (2017) +16.2 Note: a These authors used lightweight geopolymer concrete; PF — powder form; AS — aqueous suspension; NS — not specified.