Evaluating the Efficacy of Hydrophobic Impregnation in Improving Freeze–Thaw Durability of Air-Entrained Concrete Pavement Mixes

Hydrophobic impregnation is a well-known method to reduce water absorption and improve the corrosion resistance of reinforced concrete. However, its effectiveness in enhancing the freeze–thaw resistance of concrete remains unclear. This study aims to evaluate the impact of two hydrophobic impregnation methods, surface impregnation (SIP) and internal impregnation (IIP), on the long-term freeze–thaw (F–T) resistance of concrete. The results demonstrate that the IIP increased the air content of fly ash–based concrete by 20%–93% and reduced the compressive strength by 10%–15%. Both SIP and IIP are effective in resisting water penetration under isothermal conditions but ineffective in preventing pumping during F–T cycles and eventually lead to oversaturation and frost damage. The surface-impregnated concrete shows severe scaling in the long term, while the internally impregnated concrete is damaged both externally and internally soon after reaching critical saturation. The study demonstrates that hydrophobic impregnation does not benefit concrete with good F–T resistance, and maintaining a hydrophilic pore structure is crucial to ensure the free flow of water and the normal functioning of the air void system. The results also suggest that salt frost scaling shares a similar mechanism with regular F–T attacks, both of which depend on the pore structure and the quality of the air void system.