Studying the Rammed Earth from the Pingyao Ancient City Walls Using EIS

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The acceleration of freeze–thaw damage on old historic sites is affecting the preservation of rammed earth heritage sites. A recent study published in the International Journal of Architectural Heritage examined this issue by investigating the deterioration mechanisms of rammed earth materials from the UNESCO World Heritage Pingyao Ancient City walls (1). The research, led by Pengju Han of Taiyuan University of Technology, provides new insights into the resilience of raw soil compared to lime-stabilized soil under extreme weathering conditions.

The Pingyao ancient city was founded in the 14^th century during the Han Dynasty (2). The city is known for its architecture, which reflects how Imperial China built and constructed its ancient cities (2).

In their study, the researchers wanted to learn more about how the architecture of the city walls has held up over time. To do so, the researchers simulated long-term environmental stress conditions by subjecting both raw soil and lime-modified (3:7 lime soil) specimens to 15 freeze-thaw cycles (1). They found that raw soil lost 77.9% of its strength, while lime soil retained 67.2% of its strength (1). Through using electrochemical impedance spectroscopy (EIS), the researchers confirmed this difference, with raw soil showing a 20.1% decrease in bulk resistance and significant dielectric degradation, while lime soil exhibited only minor changes due to the formation of calcium carbonate and calcium-silicate-hydrate gels that reinforced its structure (1).

Then, the researchers also used scanning electron microscopy (SEM) to investigate these differences more extensively. They found that raw soil displayed a porous structure and broken interparticle bonds after repeated freeze–thaw cycles, whereas lime soil maintained a denser microstructure, with cracks localized and partially suppressed by cementation products (1). This enhanced durability highlights the effectiveness of lime stabilization in mitigating freeze-thaw damage (1).

This study is important because the applicability of EIS demonstrates its effectiveness as a non-destructive tool for environmental monitoring. The researchers developed an equivalent circuit model that demonstrated a strong correlation (R² > 0.97) between unconfined compressive strength and EIS resistance parameters (1). This provides a promising method for monitoring structural health without causing further harm to fragile cultural heritage sites.

By combining traditional materials with advanced diagnostic techniques, the research establishes a theoretical and technical foundation for the preventive conservation of rammed earth structures.

References

1. Zuo, J.; Bai, X.; Li, D.; et al. Electrochemical Impedance Spectroscopy Investigation and Results Interpretation of Rammed Earth from Pingyao Ancient City Walls (UNESCO World Heritage Site) Under Freeze-Thaw Cycles. Int. J. Arch. Herit. 2025, 1–15. DOI: 10.1080/15583058.2025.2531390
2. UNESCO, Ancient City of Ping Yao. UNESCO.org. Available at: https://whc.unesco.org/en/list/812/ (accessed 2025-09-09).