Effects of Polyoxymethylene fibers on toughness of high performance mortar and the pull-out performance with matrix strength

The incompatibility between strength and toughness is one of the primary challenges for hardened concrete, especially at higher strength level. Fiber toughening is a commonly employed approach to effectively mitigate this shortage. In this paper, a new type of polyoxymethylene (POM) fiber, was used to fully replace steel fiber to prepare high performance mortar. The flowability, mechanical strength, bending toughness and fiber distribution were studied with different fiber length (8 mm and 12 mm) and fiber content (1 %, 2 %, 3 %), and the single fiber pull-out properties with different matrix strength were investigated. The results revealed that POM fiber reduced the mortar flowability, but enhanced both flexural and compressive strength, which reached 26.97 and 107.40 MPa, with increase of 56.3 % and 25.8 % than the plain sample, respectively. Longer fiber and higher fiber content contributed to the strength development, but a high dosage of long fiber caused distribution problems and resulted in strength regress. However, the hybrid uses of 8 mm and 12 mm fiber showed the best distribution and strength performance. The damage mode of hardened mortar in four-point bending was transformed from brittle fracture to ability of certain load-bearing capacity after first cracking with the addition of POM fibers, and showed typical strain hardening when fiber dosage reached 3 %. The first crack load was changed slightly with fiber length and content, but the post-crack load peak was significantly increased with fiber content. Due to the hydrophilic surface, the interfacial bonding was stronger between POM fiber and matrix, the debonding strength was increased with matrix strength and embedded depth, and the fiber surface was cut to generated debris to obstruct the sliding pathway during sliding process, resulted to a higher energy consumption. However, too high matrix strength resulted to the excessed debonding force, caused the shorter sliding process. From our results, the MC matrix shown the largest energy consumption on fiber pull-out process.