Back to article listing

Strange creature spitting out concrete


Liisa Joensuu/Tmi Magic Words

Spring brought a one-tentacled creature to the construction site: it turns around when ordered, and spits out concrete. The creature is easily controlled by its keeper, but it can hardly remain a secret. The hydraulic boom that rises high above the site fence reveals the ongoing concrete castings.

The one-tentacled creature requires food regularly, and without the load of concrete delivered by a concrete truck it will flat out refuse to perform any tricks. When the truck backs up to the pump and starts delivering its eight cubic metres worth of nutrition, the creature drinks greedily and obeys its keeper on the slightest hint of the remote control.

The site alien will bow and stretch in any odd direction when asked to. At its best, the creature will use its hydraulic boom to spit out concrete 52 metres from its core, the pump truck. The footings and basement walls are currently being cast.

The hose needs a holder, because one stroke can spill out dozens of litres of concrete. The user of the concrete pump stands in the background, controlling the movements of the boom and the speed of the concrete with the remote control unit he is holding. To the left, the finaliser of the mixture is waiting for his turn.

Vibras shake out any air bubbles

Three people are needed for completing the casting. The finaliser’s task is to sink an electronic rod, called vibra, into the mixture to shake it and condense it.

The vibra is sunk into the fresh mixture a couple of times. The casting is only made a small layer at a time, and each layer is condensed to the previous ones by vibrating it.

Making and condensing the casting in small batches makes it more uniform in quality, and ensures a sturdy structure. Finishing up one batch of wall can easily take the whole day, because the speed of progress may be only 40 cm per day. The footing, or the foundation of the building, is being made here.

Carpenter Peter Volt is an experienced vibra user. Experience is necessary if the tool is to shake the concrete, rather than its user. Strength is also needed: the average vibra weighs approximately 20 kilos, although their sizes vary. The rod is metallic, and its diameter is 65 millimetres. The user must also be able to move and hold the rubber shaft, which is some ten metres long.

Spring is the perfect time for concrete casting. The weather is not too cold for the hydraulic parts, and the sun is so hot as to dry up the concrete too quickly. When the concrete pump is ready for the day, it folds its tentacle up.

Walls can be finished faster with large moulds

The first large moulds for casting the basement walls were erected on the sides of the pit around mid-April.

The pre-constructed moulds make casting significantly faster, because they enable completing an 11 metres long and six metres high wall section at a time.

‘We only use five large moulds, because the same ones can be reused. They are delivered in pieces of 2.5×3 metres, and assembled here. When one wall is dry, the moulds can be moved with a crane to another spot,’ says Kyösti Kontio, general foreman from E. M. Pekkinen Oy.

The moulding is done in two parts: the outside mould is set up and the steel reinforcement which takes support from it is prepared first, and then the inside mould is installed. Concrete is cast between the moulds to surround the reinforcements. 33 square metres of concrete is used for one wall section. The moulds lean on bedrock or grooved wall.

‘We build the walls so that every other section is cast at a time. The empty sections are then cast later. There will be a total of 22 sections of wall in the pit. Once the entire basement is walled, the moulds are dismantled and delivered back to the company that rents them,’ explains site engineer Risto Sell.

Part of the library is attached to the bedrock using bolts that are 15 metres long. The nuts are tightened using hydraulic jacks, and concrete is pumped into the hole. A concrete footing is then built on top.

The frame of the library is supported by a giant steel arch. Approximately one third of the footing at the north end of the arch is now ready. A beam or a similar structure is then installed to the structure to support the end of the arch.

The planned construction of the eventual central tunnel has been considered in the design of the southern end of the site. A staircase will be built to the area inside the grooved wall to allow an emergency exit in the future.

When the requirements of the central tunnel are considered and the staircase built at this stage, the central tunnel can later on be built underneath the library without disturbing its operations.

Approximately 30 people are currently employed by the library construction, two thirds of whom are subcontractors for E. M. Pekkinen.

Share the article