Influence of Aggregates and Resin Content on the Mechanical Behavior of Polymer Concrete and Lightweight Polymer Concrete

Polymer concrete (PC) is valued in infrastructure for its high strength and corrosion resistance, yet how aggregate type governs fracture mechanisms and stress–strain response remains poorly understood. This study investigates the influence of epoxy resin content and aggregate type on the mechanical performance of PC and lightweight polymer concrete (LWPC) prepared from a common epoxy matrix. Compressive, flexural, and tensile strengths were measured for a pure resin matrix, PC (18–30 wt.% resin), and LWPC (33–50 wt.% resin); one-way ANOVA, stress–strain analysis, and scanning electron microscopy (SEM) were employed to assess statistical significance and characterize failure mechanisms. Resin content exerted a non-monotonic and statistically significant effect (p < 0.05) on all three strength properties; the optimum was 20 wt.% for PC and 45 wt.% for LWPC, the higher value reflecting perlite’s elevated resin absorption. PC20 outperformed LWPC45 by 27–56% in absolute strength, yet after density normalization LWPC45 exceeded PC20 by 17% and 23% in specific flexural and tensile strength, respectively. SEM fractography showed that interfacial debonding was the predominant crack path in PC, whereas trans-particle fracture through the porous perlite interior prevailed in LWPC. In both systems, aggregate incorporation eliminated the yielding and post-peak softening of the neat resin, producing a brittle response governed by aggregate-induced constraint. Overall, for material designers weighing PC against LWPC within a single resin system, this study provides a basis for trading off economy, strength, and constructability in material-design decisions.