Shear strength at the interface of old and new concrete: Foaming agent content and freeze-thaw cycles impact

The object of research is the interfacial shear strength between old and new concrete under the coupled effects of foaming agent content, mix proportion parameters, and repeated freeze-thaw cycles, with the aim of identifying critical thresholds for durable repair systems in cold climates. Method. A total of 540 specimens were prepared using three cement grades (C300, C350, C400), three water-to-cement ratios (0.4, 0.45, 0.5), and five foaming agent dosages (0−0.45 wt% of cement). After curing periods of 3, 7, 14, 21, and 28 days, the samples were subjected to 300 freeze-thaw cycles (from -18°C to 4 °C, 4‑hour cycle, following ASTM C666 Procedure A), and interfacial shear strength was measured and compared with non‑cycled controls. Results. Increasing foaming agent content continuously reduced shear strength under standard conditions (average drop of 7.1−10.1% at 0.45% dosage), but under freeze-thaw exposure it significantly enhanced frost resistance, with strength gains up to 571% at 0.45% dosage. A critical dosage threshold of 0.25−0.35 wt% was identified, beyond which further improvements diminished. Higher cement grade (C400) and longer curing (28 days) notably reduced frost-induced degradation, while W/C = 0.45 exhibited the highest sensitivity to foaming agent content. The most severe strength loss occurred at early curing ages (3−7 days), whereas 28‑day cured aerated mixtures retained over 80% of their initial shear strength after 300 cycles. These findings provide quantitative design parameters for optimizing lightweight foam concrete mixtures that balance structural durability with thermal insulation performance in freeze-thaw environments.