Sediment-Based Bricks with Waste Flax Fibers: Mechanical Behavior Analysis and a Possible Way of Optimization—Part II

A preliminary investigation demonstrated the feasibility of manufacturing raw earth bricks from river sediments, which can be randomly reinforced using short flax fibers. The main findings concern the moisture content of the sediment–water–fiber mixtures—which is close to the maximum moisture content obtained in the standard Proctor test, i.e., 22% ± 3%—and the ambient air-drying time—which ranges from 13 to 14 days for bricks without fibers, but is reduced by 2 to 3 days when flax fibers are incorporated. All bricks (4 cm × 4 cm × 16 cm) were manufactured in the laboratory at the sediment optimal moisture content of 22% and compacted using normal Proctor energy. The short-fiber content, ranging from 2 cm to 4 cm, varied from 0.1 wt% to 0.5 wt%. The dry bulk densities obtained are close to 1.27 g/cm3 (10% below the optimal density obtained in the standard Proctor test), and none of the bricks showed any damage or shrinkage exceeding 1.25%. The incorporation of short flax fibers modifies and improves mechanical behavior. This article focuses on the analysis of mechanical behavior through flexural and compression tests and examines how to optimize fiber content or fiber length. The influences of moisture content, fiber content, and fiber length on flexural and compressive strengths were investigated. All force–displacement curves were thoroughly analyzed. Different mechanical parameters (the maximum force or stress and strain at failure, flexural stiffness, and modulus of deformation) were derived based on the physical characteristics and summarized in tables. These parameters were then discussed. Results regarding flexural strength showed a clear increase following the incorporation of flax fibers, regardless of their length or the amount used. As for unconfined compressive stresses, the effect of fiber incorporation is less significant. The effect can be more beneficial if the water content is increased by 2 to 3%. This has been demonstrated for fibers with a length of 4 cm. Mechanical strengths show that they depend more on the choice of water content and its control during production, even in the laboratory, than on the sediment preparation process (i.e., manually or knife-mill crushing) and on the method of compaction. A simplified toughness index was proposed and used to determine an optimal fiber content for each fiber length. As the fiber length increases from 2 cm to 4 cm, the optimum fiber content tends to decrease. The simplified toughness index shows that, for a length of 2 cm, the sediment–fiber mixture is fairly easy to achieve, and an increase in dosage is possible. For lengths of 3 and 4 cm, an optimum of 0.4 wt.% seems to be reached, but for a length of 4 cm, toughness indices vary, even if the optimum seems to reach 0.4% wt.%. At high fiber contents, such as 0.4 wt.%, the mixture poses a problem of homogeneity and constitutes a fiber content limit for the earth brick dimensions used.