Super dense concrete has solved a tricky basement challenge in Sweden.
Designers of deep basements extending below the local water table have always to consider hydrostatic pressures, in particular the vertical forces generated by the basements’ inherent buoyancy.
These can generate enough uplift to threaten the overall stability of the building above. If enough of the deadweight of the building is carried by the basement structure, this risk can be nullified - but that was not the case at the new seven storey city hall in Täby, Sweden.
Heavily fissured granite had to be blasted and sawn to create space for a two storey basement car park measuring 53m by 36m in plan. The lower storey is entirely below normal groundwater level. Uplift, however, is not cancelled out by the building’s deadweight, as main contractor Sweco engineering project manager Peter Hniopek explains.
“The office above has a wider span than the car park levels, which means there isn’t enough deadweight on two column lines on the lower floor.
“Without special measures, at high groundwater levels, the foundation pad would lift along these two lines.”
Originally Sweco had proposed to solve the problem with rock anchors, but the need to drill wells for ground-source cooling below the basement would have complicated installation and probably added significant extra costs.
Luckily, a member of the Sweco project team had previous experience of heavyweight concrete. The alternative of adding high density ballast to the basement looked increasingly attractive, the more it was evaluated.
“We realised that heavy concrete would give us the extra deadweight needed without excessive volume,” says Hniopek. “Furthermore, it would be much quicker than rock anchoring, meaning we could meet our agreed delivery time.”
The laws of physics also endow high density concretes with another crucial advantage in what is effectively an underwater environment. Archimedes’ Principle states that a body immersed in water will be subject to an uplift equal in weight to the volume of water displaced.
Thus a cubic metre of concrete will lose around 1t of deadweight when immersed.
Sweco was aiming for a density of 3,700kg/m3, which in effect meant that the high density concrete would have an effective deadweight of 2,700kg/m3 when immersed, more than twice that of normal concrete in a similar situation.
This was based on the choice of magnetite (iron ore) as aggregate. LKAB Minerals could supply magnetite in zero to 2mm, zero to 8mm and 2mm to 20mm fractions from its mine at Kiruna in Sweden, understood to be the largest underground iron mine in the world. A density of 3,700kg/m3 was easily achievable and compressive strengths equivalent to normal mixes would have been no problem (see box).
However, on the Täby city hall project, compressive strength was not an issue and was not specified. The normal concrete basement slab was designed to take all loading.
Basement subcontractor Skanska production manager Daniel Kedland says that constructing the basement is “like casting a large boat.” He adds: “We calculated that by casting two ‘loaves’ under the base slab we could create the deadweight we needed.” (See diagram).
These “loaves” measure 3m wide by 550mm deep, and extend across the 53m length of the basement. The concrete arrived ready mixed and could be placed with the truckmixers’ chutes. Kedland admits he had some concerns before placing began.
“I had never worked with heavy concrete before. I was worried that it would be very stiff and difficult to compact. But it was like normal concrete, and there were no real problems.”
Overall the verdict on the super dense concrete was very positive. It was much easier to work with than some feared, and was quicker and simpler than the rock anchoring alternative. And it was significantly cheaper as well.
Heavyweight concrete’s history
Heavyweight concretes first appeared on the large scale in the 1950s and 1960s, paralleling the growth of the nuclear power industries in the UK and United States.
Their much greater density made them a realistic option for radiation shielding, reducing the thickness and volume of concrete needed to contain deadly ionizing gamma rays.
Later the booming offshore oil industry also took advantage of the higher densities to ballast pipelines and seabed installations. Most major hospitals now have radiotherapy treatment centres, where heavyweight concrete is usually the radiation shielding solution. It has also found uses in counterweights on cranes.
Ultimately, concrete density is determined by aggregate density. Options include barites or barytes (barium sulphate), the iron ore magnetite, iron shot, or even lead shot. Barites concretes
have a density of around 3,500kg/m3, 45% greater than normal concrete, and are non-magnetic. With magnetite a density of 4,000 kg/m3 is possible, more than 60% higher than normal.
Should even higher densities be required, iron shot concretes can offer 5,900kg/m3, while lead shot aggregate can yield a density of 8,900kg/m3. Such concretes are very expensive and difficult to handle.
All heavyweight concretes suffer from similar drawbacks. Truckmixers and skips can only carry a significantly smaller volume than normal, and formwork pressures will be higher.
Heavyweight concretes can be successfully pumped, but wear and tear on pumps and mixers will be greater. And more energy is usually needed to achieve full compaction unless plasticising admixtures are used, so poker vibrators have to be inserted at closer centres.
Compressive strengths equivalent to normal density concretes are readily achievable.
A typical C25/30 mix with a wet density of 3,900kg/m3 using magnetite aggregates would have a cement content of 290kg/m3 and a water/cement ratio of 0.55. A plasticiser would be included.
Swedish magnetite aggregates in a range of sizes are available in the UK from LKAB Minerals. Barites (barytes) aggregates are produced from mines in Scotland and to a lesser extent in England, although the primary use of UK barytes is to increase the density of drilling fluids for oil and gas exploration. Barytes production and hence the availability of barytes aggregates for concrete is influenced directly by the fortunes of the oil and gas industries.
Durability of magnetite and barytes concretes is similar to that of mixes using other types of natural aggregates. Durability and compressive strength can be enhanced by the inclusion of condensed silica fume (microsilica) and modern high range water reducing admixtures (superplasticisers).