Decoupled pre-freeze properties and earlier freeze–thaw deterioration in RDP–defoamer-modified joint-filling mortar

Redispersible polymer powder (RDP) and defoamers are widely used to tune the workability and hardened response of cementitious mortars, but favorable pre-freeze indicators do not necessarily translate into freeze–thaw durability. This study examined a polypropylene-fiber-reinforced joint-filling mortar for prefabricated pavement bases using reference (REF), RDP-modified (RDP), defoamer-modified (DF), and combined RDP–defoamer (RDP–DF) formulations, with a commercial grout as an external benchmark. The primary comparison was between RDP and RDP–DF: both mixtures contained 280 kg m -3 mixing water and differed by the incorporation of 0.15% defoamer by binder mass. Pre-freeze mechanical, transport, and volume-stability responses were characterized, followed by longitudinal freeze–thaw tracking using mass loss and relative dynamic elastic modulus (RDEM) and by residual mechanical evaluation. Relative to RDP, RDP–DF reduced water absorption and 28-day drying shrinkage by 46.0% and 50.0%, while secant modulus increased by approximately 23%. Flexural and compressive strengths decreased by 28.8% and 25.4%, and permeability did not improve. After 15 freeze–thaw cycles, mass loss increased from approximately 0% to 5.48% and RDEM decreased from 83.94% to 64.59%. The median first observed cycle at which mass loss reached or exceeded 5% decreased from 50 to 15 cycles. In an independent 15-cycle batch, post-freeze–thaw compressive strength was 23.8 MPa for RDP and 17.2 MPa for RDP–DF, a difference of −27.7%. For similar RDP-modified joint-filling mortars, the results support combining pre-freeze properties with deterioration trajectories and residual mechanical capacity when assessing freeze–thaw durability.