Long-term Pull-out behavior of coir fiber from cementitious material with statistical and microstructural analysis

In this study, the long-term pullout behavior and microstructure of the embedded coir fibers (a bundle of multi-monofilament fibers) from cementitious matrix were investigated. The experimental results were analyzed through statistical tests (i.e., Shapiro-Wilk test, Kolmogorov-Smirnov test, and Analysis of variance with Tukey's honestly significant difference test) to address the reliability of the analyses and conclusions. The duration of fiber embedment in the matrix (i.e., 1, 2, 4, 8, 12, and 24 weeks), and the addition of fly ash (i.e., weight replacement ratio to cement for 0%, 10%, and 20%) were considered as the test parameters. The results show that the fibers with larger diameters tended to have tensile failure rather than pullout failure during the test. When the fibers had larger embedment length in the cementitious material, the pullout load-displacement curve changed from slip-hardening to slip-softening pattern. The highest fiber pullout strength was recorded when tested at 24-week. However, the highest fiber pullout energy was not recorded at 24-week but at 2-week. It can be concluded that with a longer embedment duration, the interfacial bond between fiber and matrix will increase but the coir fiber itself will be embrittled. This is due to the penetration of mortar into fiber surface along with a longer embedded duration, which was observed under scanning electron microscope. The pozzolanic reaction of fly ash did not improve the fiber-matrix interfacial bond strength. To the contrary, the pullout energy of the samples with 20% fly ash at week 1 was significantly lower than that with 10% fly ash or without fly ash. This was due to the unreacted spherical fly ash particles on the fibers surface that acted as lubricant during pullout.