Steel Mill Slag At Your Construction Site?

If you have a critical structure that is supported on or above steel mill slag, be aware that the structure may experience significant differential movement problems for years after completion of construction. Great care should be taken if steel mill slag is being considered for use as structural fill for construction projects.

Slag is a waste product of metal production. Originally molten, congealed slag is visibly similar to lava. The composition and characteristics of slag vary with the type of metal being produced. Slag produced by steel mills is relatively abundant in industrial areas of the world and is often exported from steel production sites for use as fill material.

Steel mill slag produced from electric furnaces or open hearths typically contains one-third to one-half non-hydrated lime (primarily CaO with some MgO). The lime is added as a fluxing agent during steel production. Lime can absorb water through hydration for many years after the slag has been created. This hydration process can produce volume expansions of 10 percent or more. Moisture absorption and slag expansion can occur even for slag fills located above the water table and beneath buildings. Soil moisture in the vapor phase is sufficient to result in lime hydration over a period of months to decades.

The use of steel mill slag as engineered fill for support of structures can result in significant difficulties related to post-construction expansion of the slag fill. Common problems include 1) lifting of floor slabs, 2) differential movement between slab-supported partition walls and structural walls that are supported on deeper foundations, 3) pavement buckling and floor lifting over utility trenches that penetrate slag, and 4) utility line breakage or leakage.

Steel mill slag often can be used safely for non-structural fills such as roadway embankments, landscaping, aggregate for asphalt concrete pavements, etc. (The chemical content of the slag should be evaluated along with its physical characteristics.) However, problems can arise when steel mill slag is present beneath structures that are intolerant of differential ground movements.

For more information, please contact Jim Miller at 425-861-6000.

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