Achieving heights up to 32 m using flowable concrete
N3 Engine Overhaul Services GmbH & Co. KG is building an overhaul plant for Rolls-Royce engines in Arnstadt near Erfurt in Germany. With effect from 2007, approximately 200 turbofans of the Trent 500, 700 and 900 series will be tested, maintained and overhauled in the facility on an annual basis. These engines power the Airbus A330, A340-500/600 and A380. N3 company is a joint venture between Rolls-Royce plc. and Lufthansa Technik AG.
Next to the new overhaul plant an engine test stand is being built. The construction plans place exceptional demands on the formwork solution and the concrete finish. Contractor Oevermann GmbH & Co. KG and MEVA formwork engineers joined forces to ensure that a concrete finish with extremely low porosity would be achieved.
Blockouts have to be built in with 5 mm tolerance. Plywood cannot cope with this issue. No problem for the all- plastic alkus facing! Thanks to the easy on-site maintenance and repair possibilities the formwork is always in top condition.
Low-porosity finish by all-plastic alkus facing!
The test stand for jet engines comprises four segments. The engines are brought into the testbed through a large, hermetically closed gate. For simulation purposes, air is drawn in through a 23 m high intake funnel. Behind the engines, exhaust fumes are discharged through an exhaust chamber, 11.80 m high and from there through a 32 m high exhaust funnel to the outside. Extreme air pressure and speed demand exceptionally low porosity in the concrete finish. The smallest particles on the wall surface would in effect turn into little missiles as a result of the extreme pressure.
Therefore, the inside walls need to have a hermetically sealed surface corresponding to category 4 of the classification for fair-faced concrete published by the „Deutscher Betonverein“ (German Concrete Association) [edition: August 2004].
Unique for Site-mixed Concrete
Mammut 350 takes concrete pressure of up to 11 m high pours in one go!
Mammut 350 is a sophisticated system with a load capacity of 100 kN/m². Strong enough to build 75 cm thick walls with a pouring rate of 1 m/h - in pours of up to 11 m and a total wall height of 32 m. It is unique for on-site concreting to pour such wall heights with flowable concrete. Thanks to the large-size panels 350/250 a surface of 8.75 m² can be assembled with just one panel.
This represents a great advantage for the Arnstadt jobsite in assembling the formwork for the large wall segments of up to 290 m². Two panels stacked vertically achieve the required 7 m high formwork units with only one horizontal joint, or with two joints when 11 m high. The absolute symmetry of the panels offers a perfect tie hole and joint pattern.
MEVA solution: Flowable concrete poured from only one side!
Huge steel rings with diameters of up to 8 m have to be built into the interior walls to ensure the future ventilation of the test chambers. It is difficult to deaerate the concrete especially at the bottom of such a ring, if it is poured from both sides – as was the original intention.
Here, MEVA developed a unique solution: pouring the flowable concrete around the lower ring segment from just one side. This method ensures that the air locks escape on the other side. Also, this area was compacted with external vibrators. At the base, the steel ring was cut open in three spots in order to control the concrete flow in the closed formwork and to deaerate the concrete. As soon as concrete leaks from these spots, the pouring is complete and the spots can be closed again.
Through this unique solution, voids below the steel ring were prevented. This is of considerable importance for the safety of the testbed as there is sufficient potential for subsequent risks.
A perfect result was achieved through teamwork between the contractor and MEVA engineers, combined with the use of the proven Mammut 350 formwork panels with the alkus all-plastic facing.
The combination of various sensitive factors – flowable concrete, pouring direction, compaction by external vibrators and additional deaeration – meant that continual and thorough on-site consultation was imperative for the success of the project.
The heavily reinforced walls require a suitable concrete and therefore the construction parties developed a recipe. Finally, a CEM III concrete with a low water-cement-ratio was chosen, which had to fulfil the requirements of a concrete consistency of F5 to F6 according to the new DIN standard 1045. As a consequence, flowability was optimized by using a polycarboxylate-ether (PCE).
New calculation method
A calculation of the concrete pressure according to DIN 18218 is no longer possible for a concrete consistency of F5 to F6 as it is used on this project. However, MEVA is a member in the study group Fresh Concrete Pressure of “Flowable Concrete“, which, in 2006, publish-ed a DAfStb-report of the same name (DAfStb = German Committeefor Steel Concrete). In the context of this study, and with the involvement of academics from different universities and experts from other companies, a new calculation meth-od was developed which is based on the Proske-Schuon-Theory. This approach takes into consideration the setting behaviour of concrete, which depends on consistency and temperature.
Comparison of pressure curves based on different model assumptions without compaction by vibrators for a comparatively thin wall.
On the testbed project in Arnstadt, the reliability of this academic formula was verified under on-site conditions. A number of similar building segments were poured under comparable external circumstances. The concrete pressure was gauged during the placing of concrete.
Under these circumstances, the calculation according to DIN 18218 led to a fresh concrete pressure of 40 to 45 kN/m².
For the PCEmodified concrete used here, the rate of pouring was determined to be 1 m/h and the final setting to be completed after 9 hours. According to the Proske-Schuon-Formula this results in a fresh concrete pressure of 100 kN/m². Actual measurements showed pressures between 85 and 100 kN/m².
The time curve of the fresh concrete pressure on the one hand confirms the DAfStb-report, and, on the other hand, it underlines the necessity to revise the so far accepted DIN standard 18218, which in this case would have ended in disaster if applied uncritically.
Static analysis confirms new calculation method
The test stand for jet engines in Arnstadt is the first jobsite where several cycles were poured with identical heights and with the same concrete recipe. This allowed the engineers to analyze the concrete pressure curve. The results are astounding, since the on-site values and those arising from the Proske-Schuon-Formula vary only by +/- 5 %. After 15 pours the Proske-Schuon-Theory is thus validated not only in theory, but in on-site practice also. As a result, the progress report was verified in its first phase.
"Close Teamwork with Oevermann: The only way to achieve success”
Helmut Schuon, Technical Director and Member of Management of MEVA SchalungsSysteme GmbH!
„Had both parties not worked closely together, it would have been impossible to achieve such results. As an example, let us take the heights: We poured up to
11 m using flowable concrete – something that had never been done before. Furthermore, let us consider the low porosity of the concrete: We achieved the same perfect quality – one pour after the other. And it was here that we made another intriguing discovery: We achieved almost the same high quality concrete finish with used formwork – that had been cleaned normally – as with virgin panels of the Mammut 350. This proves that used alkus all-plastic facing achieves almost the same surface without any additional effort. The difference – new or used – had no effect on the alkus facing. It is absolutely non-absorbent. Together with the right concrete recipe the alkus facing accounted for a concrete finish of extremely low porosity – and not just for a mere 1 or 2 pours, but for 15 to 20 completed segments. The building owner N3 Engine Overhaul Services have confirmed they are highly satisfied with this quality. Consequently, all parties come to the closing assessment: A big success story.“
All-plastic facing alkus:
MEVA-guidance on concrete recipe
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