Aging-Induced Variations in the Slant-Shear Behavior of Cement-Emulsified Asphalt Mortar: Insights from Experimental Tests and Statistical Modeling

Cement-emulsified asphalt mortar (CEAM) is a critical component of high-speed slab tracks, providing both vibration cushioning and resistance against shear. The long-term performance of CEAM is strongly affected by asphalt aging, yet the slant-shear behavior of aged mixtures, which is directly linked to braking, acceleration, and curved-track loading, has not been fully characterized. This study investigates the shear stress-strain response of aged and unaged CEAM using inclined-plane tests at 45° and 60°, considering three temperatures (5, 25, and 50 °C), three loading rates (0.5, 5, and 50 mm/min), and three asphalt-to-cement ratios (A/C = 0.5, 0.7, and 0.9). To isolate the effect of aging, two dimensionless indices, the shear-strength ratio (SSR i ) and fracture-energy ratio (FER i ), were defined and analyzed using response-surface models (RSM). The results show that mixtures with higher asphalt content can partly mitigate aging-induced reductions in strength and energy, whereas binder-lean mixtures exhibit pronounced losses, especially at low temperature. Increasing temperature reduced absolute shear strength and fracture energy in both aged and unaged specimens, but the aged material displayed comparatively greater thermal stability. Faster loading consistently increased both strength and energy, and this rate sensitivity was largely unaffected by aging. Response surface modeling confirmed that SSR i and FER i increased with asphalt content and temperature; ratios below unity were found mainly at 5 °C and low A/C, while values between 1.0 and 1.5 were observed under warmer conditions, indicating partial compensation of aging effects. These findings provide a quantitative basis for performance-oriented mix design of CEAM and highlight strategies to improve durability, such as optimizing asphalt content for different climates, strengthening cold-region sections with cement or protective measures, and ensuring sufficient shear reserve through structural design. • Aging protocol reveals slant-shear softening in CEAM with fine resolution. • Degradation quantified by SSR and FER indices with response surfaces showing trends. • High asphalt content (A/C ≥ 0.8) mitigates aging-induced losses effectively. • Low-asphalt mixes at 5 °C yield poor results with SSR and FER values <1. • Response-surface model enables mix-design maps for durable CEAM pavements.