The increasing demand for sustainable construction materials has motivated the use of recycled and lightweight constituents in structural concrete. This study investigates the dynamic response of self-compacting concrete (SCC) modified with recycled polyethylene terephthalate (PET) fibers and expanded polystyrene (EPS) beads and reinforced with glass-fiber-reinforced polymer (GFRP) bars. Four reinforced concrete beams (150 × 200 × 1500 mm) were tested under repeated low-velocity impact using a 37.5 kg drop weight released from 3.5 m. Two beams had solid cross-sections, while two incorporated a rectangular internal hollow. Fresh concrete tests confirmed that the modified SCC maintained adequate flowability and cohesion. The average compressive strength was 22 MPa, and density ranged from 1952 to 2301 kg/m³. The dynamic responses were evaluated in terms of displacement, reaction forces, inertial forces, energy absorption, and crack development. The addition of PET fibers increased stiffness and delayed crack propagation, whereas EPS beads reduced density and altered the failure mechanism from brittle to more ductile. Hollow sections increased displacement and inertial forces due to reduced mass and compression-zone depth. Peak displacement during the second impact increased by 2.8–17.3%, and inertial forces rose by up to 66 kN in hollow beams. The combined use of EPS and PET enhanced energy absorption and improved the impact behavior of SCC beams.
