Are fibers sensitive to mixing?

The postcracking behavior of fiber-reinforced concrete (FRC) is mostly influenced by the type of fiber, the amount of fibers, fiber orientation, and the concrete strength. Manufacturers of fibers propose a minimum mixing duration after fibers are added to concrete to obtain an acceptable dispersion of fibers. What about the maximum mixing time? Fiber properties (tensile strength, length, surface characteristics, shape, and density) are normally specified by the fiber producer. However, the properties of hardened FRC are influenced by the properties of fibers after mixing. An important question is whether the properties of fibers change due to an occasionally long mixing time. Can surface characteristics, length, or tensile strength of fibers change during mixing and can this affect the properties of FRC? In an extensive experimental study, two types of macro polymer fibers, two types of steel fibers (without coating or with brass coating), two basalt fibers (with different lengths), and two braids of hemp fibers (without protection and with linseed oil protection) were tested. The test results indicate possible deterioration for some of the fibers in concrete for too long mixing times. Possible modes of deterioration are shown.