Using heavy concrete with MagnaDense, instead of using rock anchoring, meant a shorter construction time and lower cost.
The carpark level in Täby Kommunhus (city hall), Sweden, is below the groundwater level. As a consequence the groundwater exerts an uplift force. This force would lift the structure if it were not heavy enough or anchored in the bedrock. The conventional solution to this problem is rock anchoring. In this case, the construction engineers and contractor chose an innovative but unproven solution using heavy concrete. Their choice means a shorter construction time and lower cost.
Peter Hniopek, the Engineering Project Manager at Sweco, explains, “Since the foundation is below the groundwater level, the groundwater exerts an upward pressure on the entire structure. The building’s upper office level has a wider span than the two car park levels. As a result there is not enough dead weight on two column lines on the car park floor. At high groundwater level, the foundation pad will lift along these two lines. To counteract this, the concrete in the column footings has been replaced with the right volume of heavy concrete.”
An alternative to the norm
The normal procedure in a similar case is to use rock anchoring. Something which the contractor had proposed at the outset. However, this would have been complicated by the fact that wells for ground-source cooling had been drilled under the pad. It was then that the company realised that heavy concrete gave the desired dead weight without excessive concrete volume. Furthermore, this is a faster construction method, meaning the agreed delivery time could be met. Then this alternative was opted.
Like casting a large boat
Daniel Kedland, Skanska’s production manager for the foundation work, explains. “The foundation pad and walls are cast below the groundwater level. It’s like casting a large boat. We decided that by casting two ‘loaves’ under the foundation we could create sufficient dead weight to counteract the upward force of the groundwater.”
Achieving a higher weight with the same volume required a high-density aggregate for the concrete. LKAB Minerals markets MagnaDense, a natural iron ore product. We adapted this product not only for this type of use but also as loose ballast in structures. MagnaDense has a specific gravity that can exceed 5 mt/m³. Furthermore it is available in 0–2 mm, 0–8 mm and 2–20 mm fractions, making it ideal to produce heavy concrete.
Flowing like normal concrete
Daniel Kedland, production manager at Skanska, added, “I had never worked with heavy concrete before. I was concerned that the concrete would be very stiff. Moreover, I thought it would be difficult to get a good slump flow and we would have to work it a lot. But it was like normal concrete. It worked very well.”
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