Fresh and Hardened Properties of Mortars with the Addition of Polypropylene Fibers: Analysis by Factorial Design with Central Points

Abstract This research addresses shrinkage-induced cracking in cement mortars for wall coating applications by investigating the addition of polypropylene (PP) fibers through a two-stage factorial design with central points (2³ and 2²). The objective was to evaluate the influence of PP fiber content (0-0.4 mass %), Portland cement type (II/IP and III), and water-to-cement ratio (W/C = 0.3–0.5) on fresh and hardened properties. In the paste phase, optimal paste (OP) comprised Type III cement, 0.4% PP fiber, and W/C = 0.3. In the mortar phase (OP with W/C adjusted to 0.5 for workability), aggregate ratio (1:1 to 1:3) and granulometry (fine, medium, coarse) were evaluated. ANOVA revealed W/C as the dominant factor. PP fibers (0.4%) significantly affected paste mass loss and dimensional change (drying shrinkage), causing slower drying without compromising mechanical strength. In mortars, shrinkage was primarily governed by aggregate characteristics. All formulations met ASTM R6 (> 3.5 MPa flexural) and P6 (> 8.0 MPa compressive) classifications. Limitations include narrow fiber range (0–0.4%), limited replication (error df = 2 in paste ANOVA), modest mortar flexural model fit (R²=0.502), and absence of a no-fiber mortar control. Durability properties (e.g., freeze-thaw, carbonation) and direct cracking tests were not conducted. Long-term shrinkage was measured only up to 28 days. PP fibers show potential for shrinkage control, but direct validation in mortars requires further study.