Mitigation of plastic shrinkage cracking with natural fibers - kenaf, abaca, coir, jute and sisal

In this study, a number of different fibers- namely kenaf, jute, abaca, coir and sisal- were investigated as natural alternatives to polypropylene (PP) fibers for reducing plastic shrinkage cracking. The risk of plastic shrinkage cracking of mortars with water-to-cement ratio 0.5 containing either 0.6 or 0.9 kg/m3 of natural fibers was assessed according to the ASTM C1579-21 standard and compared with plain mortars and mortars with PP fibers. The water absorption of the natural fibers was low enough that (at the employed dosages) the effect on the workability and on other fresh properties was small. The natural fibers also had no measurable influence on cement hydration in the examined mortars, as revealed by isothermal calorimetry. The best performance in reducing the width of plastic shrinkage cracks was shown by kenaf and jute fibers at the dosage of 0.6 kg/m3, which outperformed even a higher dosage of PP fibers (0.9 kg/m3). Kenaf fibers in pellets, which are advantageous for dosing and mixing, performed similarly as loose fibers. The distribution of both loose and pelletized kenaf fibers in the mortars was studied by X-ray tomography, showing no substantial difference between the two ways of delivering the fibers.