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The obvious advantages, such as the highlevel of environmental compatibility, signi-ficant saving in costs, reusability of the

shielding material and the substantiallyquicker construction method which can beused all through the year, has led to thefirst order which will underline the poten-tial of the advertised sandwich constructionmethod. Forster Bau GmbH is providingthe radiation-protection structure for high-energy therapy for the county hospitals atMhldorf am Inn, Germany.

The order was placed at the end ofNovember 2004 and the radiation pro-

tection building is to be completed in thespring of 2005. Approximately 460 m2

concrete double wall precast elementslabs and 70 m2 precast concrete elementslabs will be used in the building. Thestructure will contain approximately 700tonnes of natural gypsum (calcium sul-phate dihydrate) as the mineral-basedloose filling material. The construction acti-vities on the pilot project will also continueduring the winter at temperatures down to-12C. With this future-oriented construc-

tion method which uses precast concreteelements, the building will be finished aftereight weeks instead of the usual eightmonths it would have taken by using theconventional construction method.

Radiation protectionaccording to the currentstandard of technology

Until now, linear accelerators which deli-ver up to 20 MV x-radiation for medical

purposes have mainly been housed inradiation protection of rooms constructedfrom steel-reinforced concrete or solidconcrete. The walls of these rooms can beas much as 4.00 m thick and have specific

CPI - Concrete Plant International # 1 - February 2005 www.cpi-worldwide.com

Efficient precast concrete construction

saves having to build thick concrete walls

The high-energy therapy unit at Mhldorf am Inn in Germany is the first radiation

protection building in the world to use the sandwich construction method. The ground

plan of the radiation area of this structure shows how the walls, which can be anything

up to 3.10 m thick, have been substituted by two 30 cm thick outer shells consisting

of double-wall slabs filled with a mineral filler

PRECAST CONCRETE ELEMENTS

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Sandwich construction the future for radiation protection buildings

On 14th December 2004, the Federal Ministry for

Economics and Labour awarded the 2004

German Material Efficiency Prize in the

Construction Materials section to Forster BauGmbH. The occasion for this honour, which had

already led to the Bavarian State Prize and the

Professor Adalbert Seifriz Prize being awarded in

the same year for the successful transfer of tech-

nology between science and trade, was the deve-

lopment of an innovative construction methodusing precast concrete elements for the radiation

protection of buildings with radioactive sources.


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gravities ranging from 2,300 kg/m3 to4,500 kg/m3. Only aggregates of high

bulk density are suitable for heavy-aggre-gate concrete to achieve the required har-dened concrete bulk densities of morethan 2,300 kg/m3.

Suitable examples are as follows:

- Baryte (natural barium sulphate) in theform of broken, prepared rock withbulk densities ranging from 4.0 to 4.3kg/dm3

- Iron, as scrap iron, processing wastes,

scrap iron and steel shot with a bulkdensity to 7.8 kg/dm3

- Iron ore in the form of broken, prepa-red rock. Mainly in the form of magne-tite (4.65 to 4,80 kg/dm3); haematite(4.70 to 4.90 kg/dm3) and goethite(3.50 to 3.765 kg/dm3).

The walls and ceilings of conventionalradiation protection structures are linedwith lead in order to reduce the existingradiation dosage (local dosage).

The up to 4 m thick walls made from thisspecial concrete, which is difficult to pro-cess, and the lead linings mean that thecost of construction and dismantling of a

radiation protection shelter is high andranges from e1,200 to 2,400 per m3 pro-

cessed heavy concrete. The costs are alsoaffected by the heavy concrete aggrega-tes which are used in the structure.

Dismantling is particularly expensive.Rope saws have to be used and heavytruck-mounted cranes are required to takeaway the blocks. Because of the heavyconcrete aggregates, preparation of theconcrete blocks is very expensive. Thecrushing machines wear quickly and alarge proportion of the crushed materials

has to be disposed of as special waste,which is also expensive.

The use of sandwichwalls as a pioneeringdevelopment for the future

Because of the well-known cost risks of theconventional construction method, ForsterBau GmbH was on the lookout for a moreeconomical method of construction whichshould also include improved radiation

protection if possible. The aim was toachieve protection even for high-energyaccelerators with particle energies ofseveral 100 MeV. The result of the deve-lopment was a radiation protection build-

ing of precast steel-reinforced sandwichconstruction with a loose filling consisting

of gypsum (calcium sulphate dihydrate,CaSO4x2H2O).

A characteristic feature of calcium sulphatedihydrate is that it contains chemicallybonded water. Crystalline hydrates suchas CaSO4xH2O contain waters of crystal-lisation in stoichiometric amounts. Thebound waters of crystallisation, which re-present approximately 20% of the totalweight of the gypsum, makes the protonsavailable during shielding while calcium,

because it has an atomic number of 20,can absorb gamma radiation. For this rea-son, calcium is substantially better forshielding purposes than silicon, which isthe main component of the concrete andonly has an atomic number of 14.

Instead of the metres thick radiation pro-tection concrete walls, which are heavilyreinforced in order to limit the crack width,the sandwich method uses a twin-shellSandwich structure. This consists of thin

concrete-filled concrete double wall slabswith limited widths with a loosely compac-ted filling of gypsum (natural gypsum orREA gypsum) in between.


The double-wall slabs of the inner shell during assembly


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Gypsum filler betweenthe sandwich walls providesa high level of radiationshielding

Assessments by various independent

experts have produced the followingresult: even with a maximum proton ener-gy of 250 MeV, the protective shelldesign of the sandwich structure deve-loped by Forster Bau with approximately0.3 m thick outer shells and the gypsumfilling inside showed the same or, in somecases, an even better radiation shieldingeffect. Besides this, the usual 5 cm thicksteel plates, which are usually fitted to theinner surface of the wall in order to wea-ken the local dosage rate, are no longer

necessary.

The layout of the gypsum filling in thesandwich structure is determined using theMonte Carlo Simulation of radiation trans-port in non-homogenous media with theaid of a special program developed byProfessor Reinhold G. Mller, Head of theDepartment of Radiation Physics at theUniversity of Erlangen, Germany. The re-sults of the first simulation have shown thatthe use of gypsum for radiation shielding

is both an effective and economical solu-tion. The shielding effect of the sandwichwall can also be further improved withaggregates according to requirements

(type of radiation, power, energy andavailable surface area etc.) and the geo-metrical wall thickness can be reduced.This novel sandwich structure can beimplemented with all kinds of walls, cei-lings and floor slabs, and the whole struc-

ture can be made liquid tight.

Patent protection for thenew method

Patent rights have been applied for natio-nally and internationally. The author ofthis article had already started an initia-tive for the development of the sandwichstructure filled with loose dry gypsum in2001 in a patent application for an extre-mely economical school building consist-

ing of a sandwich structure which couldbe disassembled and reassembled. Theexternal walls of this system building aremade from precast element double wallslabs filled with foamed and granulatedwaste glass and the internal walls arefilled with dry pit gravel.

Since it was possible to supply all the cer-tificates for fire protection, sound insula-tion and earthquake loads at the time thepilot project was set up, Forster bau

GmbH was able to design an extremelycost effective radiation protection buildingusing a loose mineral filling when the cus-tomer asked for it.

Advantages of theinnovative sandwichconstruction method

The sandwich construction method used inthe new generation of radiation protec-

tion buildings replaces the metre-thick rein-forced radiation-protection walls with alight twin-shell structure. Since the con-crete double-wall slabs are prefabricatedat the factory, they have almost crack-freesurfaces which effectively block the pene-tration of radiation. Likewise, becausethere is no heat of hydration to set up atemperature gradient across the thicknessof the concrete, unlike solid concrete,there are virtually no problems fromcracks due to shrinkage. The user is there-

fore provided with a crack-free, uniformradiation protection structure.

Placement of the mineral wall filling canbe continuously monitored on site. Thedesired wall area can also be custom madeand subjected to a system of quality assur-ance inspections. The previous problems,such as the demixing of heavy concretesduring processing or crack formation dueto the heat of hydration, are considerablyreduced.

Construction can continue all-year rounddue to the novel use of prefabricated thinconcrete double-wall slabs as lost wall


The double wall slabs of the

outer shell. Continuous slots

have been provided in the

outer shell for projecting floor

discs.


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shuttering for radiation protection build-ings and staff costs are also reducedduring construction.

When this construction method has beenconsidered scientifically tested and techni-

cally established, the enormous savingspotential for the investor and user will beconfirmed in black and white. The mainfactors which are responsible for achiev-ing these savings are the extremely cheapmaterial and the pronounced decreasein construction time as a result of usingprefabricated elements and fillers whichare placed dry. By using loose REAgypsum, it is possible to reuse the sortedmaterials during construction and afterdismantling. Other advantages are the

extremely high level of environmentalcompatibility and the saving of availableresources. This can produce other costsavings for the source of finance (healthinsurance company in Germany) of onco-logical radiation treatments.

The advantages of the innovation canbasically be summarised under the follow-ing five headings.

(1) Cheaper method of erecting radia-tion protection buildings

The amount of concrete and concrete rein-forcing steel used for the wall thicknesseswhich are designed for 20 MeV is

decreased significantly. The filler material,gypsum or REA gypsum, is significantlycheaper than steel-reinforced concrete.The waste product, REA gypsum, is verycheap and available in various forms allover the country. Concrete-reinforcingsteel, which has almost doubled in pricesince the beginning of 2004, is not nee-ded for a large part of the structure and,as a scarce material, can be used morewisely in the economic cycle.

(2) Shorter construction period

Due to the extensive use of precast ele-ments and filling materials which are atleast earth-dry, the carcass structure driesout more quickly. This shortens the timebetween the start of construction and com-missioning by months and significantlyreduces the intermediate financing costs.Conventional methods often require an

allowance of nine to twelve months for thecarcass to dry out ready for use.

(3) Variable layout due to different fill-ing materials

Different minerals can be used as fillingmaterials depending on the shieldingrequirements and type of radiation used.For example, nature gypsum CaSO4x2H2O,REA gypsum or even limestone CaCO3 oranorthite Ca(Al Si2O8) is suitable for useas a filler material in radiation protectionconstruction for medical acceleratorsystems producing x-rays up to 20 MeV.

With high-energy radiation from particleaccelerators (proton accelerators etc.),

both the gamma radiation and fast neu-trons must be effectively shielded. Whilenormal concrete contains silicon as themain component with an atomic numberof 14, calcium with an atomic number of20 has a better shielding effect againstgamma radiation. For this reason, the gyp-sum filled sandwich structure is more effec-tive than a concrete structure.


The section shows that Precast slabs Nos. 4 +6+7 +1 are part of the inner shell. Slabs 14 +15 +23 envelop the structure as the

outer shell. Slabs 26 +27 function as border elements for height differences in the area of the overspill and load-bearing suspender

beams of the ceiling at the same time


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Since different layers containing differentmaterials can be combined, the designpossibilities are numerous. The shieldingeffect of the sandwich wall can be improvedor the geometrical wall thickness reducedwith other suitable aggregates or other

sandwich layers depending on the re-quirements (type of radiation, power,energy and available floor space). A fillercan also be replaced with an alternativematerial at a later date.

(4) Cheaper demolition and dismantlingof radiation protection structures

Once the period of economic use of theradiation protection building has elapsed,the majority of thin steel-reinforced concrete

elements of the wall skins can be easilypulverised with the usual demolition tool.The mineral filler is removed from the cavi-ties using conventional loading equipmentand stored sorted according to type.Here, the enormous economic differencebetween this and the dismantling of con-ventional radiation protection structuresbecomes obvious.Until now, the metre-thick walls had to becut to pieces using rope saws, removedusing heavy and expensive lifting equip-

ment and then the blocks, which are den-sely packed with steel and heavy concreteaggregates, broken up. All of this expenseis eliminated with the sandwich method.

(5) Less environmental pollution

After exposure to radiation up to 20 MeV,the filling material consisting of gypsum,REA gypsum or lime rock can be regardedas non-polluted. Particularly because ofthe high purity of chemically pure REA

gypsum, the long-lived radiation activitiesfrom elements with higher atomic numbersproduced in the high energy ranges areonly small.

Thus, the gypsum can be used again afterthe technical life of the investment is over.This saves the high disposal costs andavoids pollution of the environment by theinestimable problems of hazardous waste.

Research programme with

the University of Erlangen,Germany

In a research project, Forster Bau GmbHtogether with the Department for Radia-

tion Physics at the Institute of MedicalPhysics is examining a special medicalapplication for the sandwich method inthe hospital and out-patients departmentof Erlangen University Hospital, Nurem-berg, Germany. The objective is to deter-

mine the nuclear physical and structuralphysical properties of gypsum or REAgypsum and structural elements made fromgypsum in radiation protection for medici-nal accelerator systems, in this instance,initially for operation with x-ray radiationup to 20 MeV.

On the pilot project high-energy therapyMhldorf am Inn, the theses in regard tothe shielding properties of gypsum or REAgypsum in the form of a loose filling mate-

rial in a sandwich structure consisting ofconcrete double wall slabs are beingscientifically examined and verified. Thisresearch project is being used as a scien-tific confirmatory test for the general useof loose, compacted gypsum for structuralradiation protection in medicine.

The project should provide users of thisinnovative construction method with guide-lines for the thickness of the shieldingstructure which can be used to enable the

wall thicknesses to be calculated easilyeven during the planning phase. Germanindustrial standard DIN 6847/2 [Me-dical electron accelerator systems Part 2:Radiation protection rules for the building]will be amended as part of this work.

Economical prospects

The economical potential, scope of appli-cation and other applications of the resultsin other areas show that the economical

prospects for the Sandwich are exten-sive. These include applications such asthe protective shell of atomic reactors orapplications in the high-energy sector aswell as the medical application. The follow-ing applications look particularly promis-ing.

Medical accelerators up to 20 MV

In Germany alone, 400 accelerators arecurrently being used in radiation therapy.

There is a need for approximately 5 moreof the usual linear accelerators per 1 mil-lion citizens and approximately 1 devicewith heavy particles (protons and carbonions) per 10 million citizens. In the Euro-

pean market, there is a demand for ap-proximately 3,000 more radiation de-vices with linear accelerators. Becausethe radiation protection law has recentlybeen tightened up, there is also a need forexisting radiation protection systems to be

replaced.

Commercial radiation equipment

Another area of application is commercialradiation equipment such as sterilisationsystems or non-destructive materialstesting systems in industrial companiesand research facilities.

Protective installation for intermediateatomic storage or disposal atomic storage

facilities

Discussions with power station operatorsare currently under way on how to set upintermediate storage facilities cost effec-tively. Suggestions have already beensubmitted.

Radiation protection upgrading of exist-ing atomic reactors and nuclear powerstations

The provision of older nuclear reactorswith additional sandwich protection againstaircraft crashes and exposure to veryserious fires from kerosene is being consi-dered and now seems to be technicallyfeasible. With the Chernobyl reactor, theprotective shell which was added after theaccident is already beginning to disinte-grate. The cause of this is the high level ofradiation. With the sandwich system, aprotective shell could be erected whichcould be refilled, besides which, the shield-

ing effect of a gypsum filling is significant-ly superior to that of a concrete jacket.

Structural radiation protection in thehigh-energy range up to GeV

The Association of Heavy Iron Research(GSI) at Darmstadt in Germany have askedForster Bau GmbH to include the sand-wich system in the plans for the new inter-national research centre FAIR with a totalinvestment of around 675 million euro. In

the new FAIR facility, physicists want tounravel the mystery of how the masses ofparticles develop by making very densenuclear matter.



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The state of the matter which existed for a short time, fractions ofa second after the Big Bang during the emergence of the universe,will also be examined more closely by producing a quark gluonplasma. The GSI is planning an experiment for this purposecalled CBM (Condensed Baryonic Matter) in a joint internationalcollaborative project. This will involve directing a stream of heavyions on to matter. During this process, there will be an enormousamount of radiation released which must be shielded in accor-dance with the radiation protection regulations. The radiation pro-

tection structure (which conventionally consists of concrete wallsup to 21 m thick) could be replaced with a sandwich system. Theadvantages of erecting the planned building using the newmethod in comparison to conventional radiation protection build-ings are currently being investigated.

If the research produces the desired results, which is assumed tobe the case by several radiation protection officers, the economi-cal risks will be zero.

Impetus for industry and the environment

Discussions about the disadvantages for a German site are usual-ly focused on the personnel costs. The effect of the material costson the competitiveness of the company should also be discussedat the same time. With its Sandwich from Ingolstadt, Forster BauGmbH has demonstrated how much impetus a construction com-pany can give to the market, industry and environment at relative-ly little expense.

www.cpi-worldwide.com CPI - Concrete Plant International # 1 - February 2005


Further information:

Forster Bau GmbHMercystr. 5, 85051 Ingolstadt, GERMANYT +49 841 97367-0, F +49 841 [email protected], www.forster-bau.de

Dipl.-Ing. Jan Forster

born 1951

Studies of Civil and Struc-

tural Engineering at Munich

Technical University

Diploma in 1977, worked several years in an office for

structural planning; later own office for structural calculation,

engineering, and design.

Since 1996, technical CEO of Forster Bau GmbH.

Since 1998, also CEO of Forster System-Verbau

Engineering GmbH. Holds several patents.

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