Experimental Study of Drying Shrinkage of Microfiber Reinforced Concrete

For increased durability of concrete pavements and floors, it is necessary to control crack nucleation and limit their width and propagation. The addition of fibers to the cement matrix is an effective method to control shrinkage cracks. The effectiveness of fiber reinforcement in mitigating cracking can be assessed using methodologies recommended by standard codes. An experimental study was conducted to investigate the efficiency of adding polyamide microfibers (nylon 6.6) and alkali-resistant glass microfibers in preventing cracks in conventional concrete at early ages. Both types of microfibers were individually tested in dosages of 0.60 and 1.20 kg/m3 of concrete volume. A comprehensive testing program was implemented to evaluate these mixtures by measuring compressive strength, flexural tensile strength, and restricted drying shrinkage. The nucleation of cracks, their opening, and propagation were also analyzed with the help of a digital microscope. As a result, it was found that the concretes with the addition of microfibers reached a cracking age similar to that of plain concrete, with a high or moderate-high cracking potential. However, plain concrete showed more significant and continuous crack openings along its extension, ranging from 0.13 to 0.18 mm, with propagation throughout the radial thickness. Consequently, the fragility and susceptibility of plain concrete to crack at early ages are emphasized. The cracking pattern in the composites reinforced with microfibers differed from the reference sample, presenting smaller cracks. For the addition of 1.20 kg/m3 of alkali-resistant glass microfibers, the cracks showed very small openings (0.05-0.08 mm), which is called hairline cracks. Furthermore, it was also found that the addition of microfibers could limit the propagation of cracks in the radial direction, even at dosages of 0.6 kg/m3. It was observed that only the results of the restricted drying shrinkage tests would not be sufficient to evaluate the contribution of the addition of microfibers in controlling cracking. The analyzed images and the measured openings were fundamental to complement this evaluation.