The impact of slag swelling on building structures demonstrated through long-term measurement analysis: a case study lesson

This paper presents a case study of structural damage resulting from the swelling of zinc slag used as a subbase layer beneath a commercial building. Although the material had been approved for construction, geotechnical, chemical, mineralogical and long-term geodetic investigations revealed that internal chemical reactions within the slag led to gradual volume expansion. Over ten years after construction, these changes resulted in significant floor uplift and structural issues, including wall cracking and floor deformation. The difference in floor level exceeds 100 mm, and the annual swelling during the last measurement period was more than 8 mm/year. The formation of secondary minerals (gypsum and ettringite) was identified as a key indicator of slag deterioration. Mineralogical assessments showed volumetric increases ranging from 25 to 51%, with higher swelling observed in areas with greater slag depths. The study highlights the limitations of current slag qualification methods and underscores the need for more rigorous testing methods to predict slag behaviour in field conditions. The findings offer valuable insights into construction practices involving industrial by-products and highlight the need for stricter standards to prevent future structural failures.