Issue 62

S. S. Ahmad et alii, Frattura ed Integrità Strutturale, 62 (2022) 408-425; DOI: 10.3221/IGF-ESIS.62.28

compared to M2S2P1 and M2S3P3 by 12.9% and 9.3%, respectively, also, by about 6.0% as compared to M2S1. When these mixes were exposed to a high temperature of 400 °C, we can notice that mixes M2S1P2 and M2S3P3 give the highest value of F t, about 4.3MPa; also, F t of mixes M2S1P2 and M2S3P3 were improved about by 30.3% and 22.8%, respectively, as compared to corresponding mixes without PP fibers. On the other hand, when these mixes were exposed to a high temperature of 800 °C, similar behavior was found, representing a severe decrease in the value of F t as compared to the RT value. The results indicated that the Ft for mixes M2S1P2, M2S2P1, and M2S3P3 were decreased by 94.2%, 90.3%, and 50.0%, respectively, at exposure to 800°C compared to RT.

Figure 12: Effect of PP fiber on compressive strength of mixes at different temperatures.

Figure 13: Effect of PP fiber on splitting tensile strength of mixes at different temperatures.

Optimizing the effect of W/(C+SF) ratio, SF, and PP fiber at different temperatures The test results of mixes are optimized in Tab. 4, in which c ompressive strength and splitting tensile strength were obtained from the present experimental work. The optimum possible mix proportion levels, W/(C+SF) ratio, SF content, and PP fiber content were investigated for the F c and F t using the Taguchi method [29-31]. Figs. 14-19 show the best parameters series for the designed mixes at temperatures RT, 400°C, and 800°C.

W/(C+SF) ratio

Mix

Temperature °C

Strength

SF content

PP fibers content

M2S1P1 M1S1P2 M2S3P3 M1S1P3 M1S1P3 M2S1P2

RT 400 800 RT 400 800

Compressive Compressive Compressive Splitting tensile Splitting tensile Splitting tensile

5% 5%

0.106 0.211 0.211 0.317 0.317 0.211

0.31 0.25 0.31 0.25 0.25 0.31

15%

5% 5% 5%

Table 4: Optimal mix design proportions for high performance concrete.

For the W/(C+SF) ratio, 0.25 gave the highest compressive and splitting tensile strength at room temperature and 400 °C because of the fresh state of the mix. It was homogeneous, and there was no excess water. It is well known that there is an inverse relationship between W/(C+SF) ratio and strengths. The results coming from this research agree with the aforementioned fact that for the compressive strengths and splitting, tensile strengths increased with the reduction of the W/(C+SF) ratio, such as [21, 28]. PP fiber of 0.317 gave the optimum mix at room temperature and 400 °C for the

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