Rebound and orientation of fibers in wet sprayed concrete applications

Abstract

Spraying of concrete is a well-established and economical alternative to conventional casting techniques. It is widely applied in repair/reinforcement of building elements, rock consolidation and for the construction of temporary or permanent tunnel linings. Sprayed concrete (shotcrete) may be reinforced with fibers, steel rods or steel meshes to increase its mechanical performance under bending. In wet spraying, the concrete is premixed with water and then sprayed by means of compressed air. The fibers are added to the fresh concrete and then sprayed together with it. Because of their high elastic modulus and tensile strength often steel fibers are used for such applications. Recently, macro-synthetic plastic fibers have been proofed to be a suitable non-corroding alternative.The mechanical performance of fiber reinforced shotcrete is mainly influenced by the amount, the distribution and the orientation of the fibers, which are parameters that are influenced by the special application technique. Because of the relatively high impact velocity, the concrete and the fibers do not adhere completely onto the treated surface and generally a large quantity of rebound is observed.The rebound behavior of macro-synthetic polymer fibers and of steel fibers was studied in field tests as well as in laboratory experiments. Concrete was sprayed on well-defined artificial stone reliefs and the fiber as well as the material rebound was collected and analyzed. On the example of macro-synthetic fibers, the influence of different fiber parameters on the rebound behavior was studied. Single fiber shooting experiments and direct observations by means of a high speed camera revealed their impact behavior and hence the causes for fiber rebound.The fiber orientation in sprayed concrete was analyzed by means of X-ray computer tomography. Special morphological filtering allowed the analysis of the properties of plastic fibers despite their low density. A preferential orientation of these fibers along the sprayed surface was observed.