The Effect of Timing Polymer Fiber Addition on the Compressive Strength of Adobe Bricks: Towards an Approach Compatible with Sustainable Architecture

In the context of advancing sustainable local building materials, this study evaluates the mechanical impact of polypropylene (PP) fiber reinforcement on adobe bricks, specifically addressing the novel variable of fiber addition timing relative to the traditional 45-day biological fermentation process. Two experimental scenarios were investigated: fiber addition before fermentation and fiber addition after fermentation. In the pre-fermentation scenario, the unreinforced control specimen achieved the highest mean compressive strength (1.92 MPa), followed by reduced values of 1.66 MPa (0.25%), 1.60 MPa (0.50%), and 1.55 MPa (1.00%). In the post-fermentation scenario, the control recorded 1.81 MPa, while the PP-reinforced mixtures reached 1.73 MPa (0.25%), 1.65 MPa (0.50%), and 1.77 MPa (1.00%). Across both stages, PP fibers consistently decreased in strength due to weak bonding at the fiber–soil interface, as their hydrophobic nature disrupts the fermentation-derived biopolymer network formed by straw decomposition. Overall, this study highlights the limitations of synthetic fiber reinforcement within biologically stabilized adobe and contributes to the ongoing development of sustainable earthen construction systems.