**3.1 Useful properties of natural fibers in building materials**

This section includes the discussion on the physical and mechanical characteristics of natural fibers which are important for their recycling in construction materials. Water absorption of fibers has a significant influence on the strength of building composites as swelling and shrinkage of fibers with their interaction with water introduce cracks in building composites. There is also a competition between the water required for hydration in the case of cementitious matrices reinforced with natural fibers and the water absorption-desorption of the incorporated fibers. Natural fibers have a low density which makes them suitable additives in lightweight building composites. Geometrical characteristics of fibers include length, diameter, surface roughness and cross-section. Increasing fibers length has a positive impact on the tensile strength of building composites however their compressive strength decreases with long fibers. Diameter of technical fiber increases with increasing elementary fibers which increases the tensile strength of technical fibers. Alignment of elementary fibers at different microfibers angles is also important for tensile strength of fibers. Higher tensile strength of fibers increases the tensile strength and toughness of composite materials. Surface roughness of fibers is essential for bonding between matrix and fibers in composites. The pull-out strength of the fibers shows how the interfacial bonding of the fibers with the matrix behaves. Failure and sliding of fibers at different loads and depths change the resistance and failure mechanism of building composites. Life and performance of fibers decrease with time. This is rapid in an alkaline environment associated with concrete structures. Treatment of fibers improves the shelf life and resistance.

Natural plant fibers are used or recycled with or without treatment. Whatever their future use, they are subject to biochemical and physical, morphological and mechanical characterisations. The biochemical characterization concerns the evaluation of the quantities of cellulose, hemicellulose, pectin, lignin, impurities and water. The physical parameters investigated include density, dimensional aspect, geometry, microfibrillar angle and water absorption capacity. The analysis of the morphology of the fibers focuses on the microstructure, the dimensional variations (diameter, length and section). The mechanical parameters sought are most often related to the traction of the fibers (isolated or anchored in a matrix). In this section, only a few procedures (treatment) or characterization tests intended for the use or recycling of fibers in construction materials are mentioned.
