Through-cracking risk and life-cycle performance of polymer-modified mortars under severe cold service conditions

This study examines how polymer dosage governs the coupled transport–freeze–thaw (F-T)-service-life behavior of mortar, with emphasis on the mechanisms that produce through-thickness cracks at low dosage. Mortars modified with Styrene-Butadiene rubber (SBR), polyacrylate emulsion (PAE), and waterborne polyurethane (PU) (2–8 % by binder) were tested by rapid chloride migration, nitrogen-based intrinsic gas permeability, 200 F-T cycles, LF-NMR, BSE–EDS, TG/DTG, and FTIR. Service-life analysis combined a time-dependent diffusion model anchored at 56 d with the TTRCS model. Low-dosage mixes (≈2 %) developed through-thickness cracks after F–T cycling, whereas mid-to-high dosages (≥6 %) exhibited only hairline surface cracks or minor spalling; under chloride exposure, service life increased from 14.9 years (reference) to approximately 21–35 years. Notably, through-thickness cracking occurred even when intrinsic permeability was as low as 5.596 × 10⁻¹² m² (MSBR2), indicating that bulk compactness alone is insufficient; triggering arises from the coupled effects of interfacial connectivity and water migration. LF-NMR identified a critical pore radius r_c = 4.92 nm. When the ITZ is not effectively narrowed or passivated and the polymer film remains discontinuous, connected pores larger than rc and mobile free water enable closure of the “supply-freezing-pressurization” loop, promoting crack initiation and linkage along the ITZ-capillary network. With increasing dosage, polymer enrichment within the ITZ, a phase-spanning continuous film, reduced Ca/Si overshoot, and a shift from crystalline CH toward ACH collectively lower crack-tip driving forces and arrest cracks at the surface. Practically, low-dosage formulations should be avoided in cold or strongly F-T environments; a 6–8 % dosage window is recommended to balance F-T resistance and transport blocking.