Mechanical Performance of an Injectable Wax–EPS Composite for Non-structural Void Filling

The mechanical performance of a wax–expanded polystyrene (Wax–EPS) composite was investigated as an injectable filler for retaining-wall and sub-slab voids. The composite was prepared by blending paraffin wax with expanded polystyrene beads at volumetric ratios of 80/20, 60/40, and 40/60 (wax/EPS). Tensile bond strength was evaluated using cement–composite–cement sandwich briquet specimens tested under displacement control, while compressive behavior was examined using cubic specimens. Tensile bond tests on composite–cement sandwich specimens yielded average strengths of 0.93 ± 0.08 MPa for cement mortar, 0.88 ± 0.13 MPa for the 80/20 Wax–EPS mixture, 0.78 ± 0.12 MPa for 60/40, and 0.74 ± 0.13 MPa for 40/60. The decreasing trend with increasing EPS content confirms the weak interfacial adhesion of EPS with cement. Compression tests showed that the Wax–EPS composite exhibited complete viscoelastic recovery without fracture up to 50 kN, demonstrating its potential as a lightweight, reusable filler for non-structural void repair in concrete elements. The results confirm that the Wax–EPS composite provides adequate mechanical stability for use in non-structural void-filling applications, combining low density (0.85–0.51 × 103 kg/m3) with recoverable deformation and strong compatibility with concrete substrates.