They are the first two of a total of four towers that will shortly support a steel construction of the ‘landscraper’. This is the building that not only acts as an office building, but also as a noise barrier between the A10 motorway and the buildings of Tripolis Park that are presently being refurbished. At the moment, the contractor is starting work on the two other cores, which are also built with the help of slip forming. At the end of October, there will consequently be four of these concrete behemoths.
A type of mould
‘It could have been done even faster’, says De Koning, work planner at G&S Bouw. ‘At locations where you receive a permit to continue working 24/7, the towers would have been ready in half of the time – in nine days. Because this is in the heart of Zuidas, we have a permit to work 12 hours consecutively. Nonetheless, the construction with the help of slip forming is very fast.’ De Koning walks to the slipform construction, a large, artisan wooden frame that does not necessarily have a highly dynamic appearance. ‘But it does work really well’, says De Koning. ‘You can actually best compare this slip forming with a mould in which you pour concrete from the top whilst you place the reinforcement in the formwork.’ Two years ago, the core of the EMA office was built in the same way. The method takes place according to a fixed process. ‘The first step is to place a concrete subsoil on the foundation, 2 metres below street level, for the tower to be built. You then link the first metres of reinforcement of the tower to the reinforcement of the foundation. On top of this – and in between – you built the slipform construction that slips upwards by means of a hydraulic system.’
Step by step
The process of pouring the concrete and placing the reinforcement in the slipform construction is very precise. ‘It is a step by step process where the placing of the reinforcement always needs to be ahead of the concrete, which is poured into the slipform construction from the top’, continues De Koning. ‘And the concrete constantly needs to be checked on the correct composition, because the hardening process can be affected by the temperature and the weather conditions.’ It is also very important that the dimensions and the ‘mould’ of the slipform construction are correct. This is calculated well in advance by a whole team of people. ‘It should actually already have been done before you start. For instance, in the construction we also created recesses for the lift entrances and exits and future wiring of installations. If you do not think about that beforehand, then you will have a real problem once the concrete has been poured.’
That it is a special technique was proven by the fact that a team from Austria was hired. ‘It is specialist work that requires focus; if you fail to pay attention for a split second, the slipform construction could, for instance, get stuck in the concrete’, emphasises De Koning. There are usually several specialists in and on the slipform construction who, on the spot, work on the placing of the reinforcement, monitor the recesses, and check if the reinforcement has all been placed properly. All whilst the concrete is poured in the thereto-designated openings in the slipform construction. Meanwhile, the hydraulics do their thing, and keep pushing the slipform construction ever higher, at a speed of approximately 25 cm per hour. ‘You must work very meticulously when pouring the concrete: the concrete layers must have hardened just enough to be able to support a new layer.’
Fast and efficient
G&S Bouw exclusively has two active concrete lorries whilst work is carried out on the slipform construction. De Koning: ‘This is the only way to guarantee a good process during which you always have enough concrete to your disposal.’ During the construction of the towers, a tight schedule is followed. ‘The builders from Austria were only hired for a certain period. You must have exactly the right material available in advance, so that they can do their work.’ De Koning is pleased with the cooperation and the two 12-storey towers that have now been completed. ‘Of course there are more roads that lead to Rome, but this is definitively fast and efficient. Let us start with the next two.’