Round is „bio“

Circular formwork faster than conventional wallform shuttering

Renewable raw materials are among the most valuable domestic resources with very favourable future perspectives that made renewable energy become a real issue aside from energy out of recycled and waste materials. Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material and release valuable products (i.e. fertilizers) as well as biogas in a very large quantity.
Biogas is then used for producing energy that in turn is supplied to domestic end users at firm and more convenient prices in accordance with the new European Laws on Renewable Energies (EEG). What is the focus for building contractors and investors?

Kerpen

For hundreds of years biogenic fuels have been available in the Lower Rhine bay in form of wood as well as lignite and hard coal. However, a reorganization process involving also local renewable raw materials took place as coal mining came to an end. The number of biogas plants increases every year.
At present, Kerpen biogas plant is being expanded with the addition of an ancillary fermenter consisting of a large 12.5 m high round tank (32.0 m inside diameter and 35 cm wall thickness). The natural gas (so called biogas) produced in this plant is processed and fed into the regional gas supply network.

Shell construction

The plant management assigned Bruno Klein construction company (Jünkerath) the task of accomplishing the shell work between June and August 2010. The company which has been mainly active in bridge construction and civil engineering services has extensive experience in water treatment and biogas facilities and sees its own future in this prospering market.  

The footing was established 3.5 m under the ground upper level on load-bearing Rhine gravel.
 
268 m³ C35/45 cement and 25 t reinforcement steel were used for the foundation slab that was designed with a 50 cm large protruding strip. The circular formwork was finally erected on it.

Circular formwork – easy and fast installation

The 12.5 m high walls were no customary job, not even for this renowned company which decided to rely on PASCHAL edge circular formwork with trapezoidal girders (TTR) to solve the difficult task. After numerous consultations with the manufacturer, PASCHAL dealer (HSB, D-Ensdorf) and the building contractors, it finally became clear that the formwork was to be achieved by using 9.75 m high preassembled segments, adapted to the given radius and raised by another 3 m directly on the site. Jürgen Mueller, local site supervisor was positively surprised by this method and stated: “Everything was done so fast and trouble-free, just as for a conventional wall formwork!” As the TTR formwork segments are even larger than certain wall formwork panels, the timing per square meter of complete assembly was even shorter. Assembling the segments on the site was a very simple task. 

The job was conducted stepwise, whereas each phase consisted of a 16.76 m wide and 12.5 m high section. “The overall interior surface of the wall formwork was lined with a protective plastic film of polypropylene,” explains Grad.-Eng. Rudolf Huth, project and site supervisor. “This film provides concrete surface protection against aggressive gases released during the fermentation process”. The job planning included six phases, whereby the scaffolding was completely installed against the segments. An additional phase on the inner side of the tank consisted of a single-sided forerun formwork part. After hardening of the concrete, the formwork sets have been moved over the entire height (30 m2) starting from the outer face. Intermediate cleaning of the perpendicularly standing segment sets was easy to accomplish from the scaffolding erected in front. This resulted into shorter working times so that a complete step could be finished in one week’s time.  

Fast and safe: another crucial issue of this project and the large curved wall faces was also the number of tying points. Only 4 ties are typically required for a typical 2.4 x 3.0 m large TTR formwork segment for a permissible fresh concrete load of 60 kN/m². This results into a very limited number of only 0.56 tying points per square meter. The number of ties is further reduced if tie-rod guides on top are used, which is possible without reduction of the permissible fresh concrete load thanks to the rugged trapezoid profiles. This translates to better quality of the concrete surface and significant cost savings.

Even after six work steps, the formwork showed a consistent radius and did not require any further adaptation. “As this kind of formwork is quite heavy, you need a powerful crane to handle it,” explains Jürgen Mueller. Having had this equipment with us, it all worked out very well especially because we also got “excellent technical service from PASCHAL consultants and supervisors.”

Large props are used to back-up the 12.5 m high TTR Circular Formwork by PASCHAL. Working platforms were installed at 4 levels for increased safety and effectiveness. Also a heavy-duty crane was set up in the middle of the construction site in order to handle the large formwork units and reduce the overall work time.

“Every week, we had to handle about 400 m2 polygonal scaffolding modules, 400 m2 circular formwork, 200 m2 concrete protection bands, joint seaming, related fittings, about 200 m2 wall reinforcement, shuttering and concreting and all this in a very confined space, explains Mr. Huth, chief manager on the site. “As walls were erected by means of segments, these tasks could not be performed at the same time nor overlap each other. It was all done step by step in one progressive sequence.”

76 m³ C35/45 concrete with 0.16 grain size were used for each work step along with 12 tons prefab reinforced steel elements and 16 mm concrete reinforcing steel.

Safe tank construction at lesser cost for the contracting authority

The stepwise method for both formwork and concreting in conjunction with the use of formwork segments as high as the given structure, provides significant saving of the costs for climbing scaffolds. 
Additionally, this stepwise modular work method has the clear advantage of requiring fewer formwork segments thus reducing rental and transportation costs for an overall better economy of the site. This method was designed to enhance compliance with the applicable rules and regulations on tank construction.

Time saving: On the left the two sided formwork part, on the right the forerun single sided formwork part. This method saves half of the normal working time.    

Reference Standards for tank construction:

• WU Richtlinie (R7, L21):  Minimum wall thickness for class A buildings: 24cm
• DIN EN 206-1/DIN 1045-2, Tab.4: Concrete covering with tie rods  diameter up to 25 mm and exposure class XC4 > 4 cm
• WU Richtlinie (R7, L21), Tafel 5.2: When concreting uneven surfaces concrete covering must be increased by the difference of levels and at least  2 cm.
• DIN EN 206-1/ DIN 1045-2: C35,/45 concrete must be used for reinforced elevated tank/liquid manure plants.
• DBV recommendation „design gap distance, white tank “: A-class construction joints must be taken into account (gap distance in m) = h (white tank)/2*d (wall thickness);  i.e. 12.00/(2*0.35) m = 17.14 m!!!

TTR – ADJUSTABLE CIRCULAR FORMWORK WITH TRAPEZOIDAL GIRDERS

For this project, the adjustable circular formwork with trapezoidal girders (TTR) was implemented.
The TTR formwork features a 21 mm thick, 15-layers shell of Finnish birch plywood suitable for nailing. This is the only system capable of handling curving radii of up to 2.5 meters where most of the other formwork plywood of the same thickness would break or start warping. Thanks to the so called “two-girders segments” (i.e. a segment with two longitudinal girders), this system can be adapted to radii of even up to 1.0 m with an 18 mm plywood. This ensures formwork stability, absolute roundness and exact measures regardless of the repeated shifting. The formwork can then accurately match the circumference of the entire structure and without any adaptation work on the site. A square meter of finished formwork can withstand up to 60 kN fresh concrete loads, although it only features 0.28 tying points per square meter. Thanks to these benefits, this technology best fits demanding applications as for technical buildings and high-quality architectural works as well as round tank constructions as in this case. Fast shifting and consequently, a small number of tying points, accurate matching of the structure roundness without further adaptation and special additional fastening are just the main reasons that make this system suitable for any job where simplicity, safety, short times and efficiency are a priority. The small number of tying points also remarkably improves the quality of the concrete surface. 

Outlook  

In the meantime, the shell construction has been successfully completed in compliance with the work schedule and all equipment has been installed.