Effects of Fiber Length, Content, and Surface Treatment on the Compressive and Flexural Response of Date Palm Fiber-Reinforced Concrete

Date palm fiber provides a local approach to utilizing agricultural residues in cement-based materials, yet reported strength changes vary with fiber geometry, dosage, moisture condition, and treatment. Evidence remains limited for direct comparisons of untreated, alkali-treated, and hornified fibers across the same length–content matrix. This study examines one fiber-free C40 concrete and 27 fiber-reinforced conditions comprising three fiber volume fractions (1%, 2%, and 3%); three fiber lengths (10, 20, and 30 mm); and three fiber states (untreated, 1% NaOH-treated, and hornified through ten wet–dry cycles). Compressive strength is measured at 7, 14, and 28 days, while flexural strength is measured at 28 days. All conditions exhibit increased compressive strength with age. Concrete containing untreated 3%–30 mm fibers exhibits the highest 28-day compressive strength of 40.8 MPa, which is 8.5% above the reference value of 37.6 MPa. Concrete containing untreated 2%–20 mm fibers exhibits the highest flexural strength of 3.6 MPa, which is 12.5% above the reference value of 3.2 MPa. The untreated 2%–30 mm condition is the only condition that exceeds the reference in both tests, with gains of 4.0% and 3.1%, respectively. Alkali treatment and hornification reduce 28-day compressive strength relative to every corresponding untreated condition, with mean reductions of 17.9% and 18.0%, respectively. Their mean flexural strengths are also lower than those of the untreated group. The findings show that the measured mechanical response varies with the combination of fiber length, nominal content, and fiber state, and that neither treatment method provides an overall strength advantage over untreated fibers under the tested conditions. Untreated fibers produce the leading compression–flexure combinations, highlighting the importance of selecting fiber length and content based on the target mechanical response. All differences and rankings represent descriptive comparisons of the reported mean strengths.