Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Infra-lightweight concrete

M. Schlaich & M. El ZareefInstitute of Structural Engineering, Technische Universität, Berlin, Germany

ABSTRACT: Fair-faced concrete does not only possess high visual qualities. Monolithic concrete structuresare also particularly durable, and the fact that no plastering or cladding is required leads to cost savings andmakes buildings more sustainable and easier to recycle. However, due to the high thermal conductivity ofnormal concrete, fair-faced concrete without insulation causes prohibitive heating costs in cold countries. Infra-lightweight concrete with a dry bulk density of less than 800 kg/m3 and the corresponding advantageous thermalproperties promises to overcome this problem while maintaining the advantages. At the Technical University inBerlin such infra-lightweight concrete was developed and to prove its practicality a single family house was builtwith it using glass fiber bars as reinforcement. This paper describes this new concrete mix and its properties. Itelaborates on the structural implications when working with infra-lightweight concrete. Design and constructionof the house will also be presented.

1 INTRODUCTION

Monolithic structures of fair-faced concrete not onlyhave a high architectural potential but also are verydurable. Since no plaster and cladding is needed costis saved and recycling is made easier.

Unfortunately, the heat conductivity of normal con-crete (NC) is so high that in cold countries likeGermany monolithic fair-faced concrete buildingshave virtually disappeared. Since the oil crisis of theseventies of the last century it is either necessaryto construct exterior walls as complicated and costlydouble-layer structures with interior insulation that isdifficult to inspect, or one contents himself with fair-faced concrete on one side only and uses conventionalthermal insulation on the other side of the wall.

Therefore, engineers and architects have started try-ing to develop concrete with low thermal conductivityquite a while ago.Already in the 80s of the last century,insulated “foam concrete” with a dry density below1000 kg/m3 was studied as a part of a project fundedby the German Ministry of Investigation and Technol-ogy (BMFT). Weight reduction was achieved by usingpre-mixed protein foams (Widman, H. et al. 1991).Since the only prototype building with this concreteshowed unacceptable cracking due to strong shrinkagedeformations the project was not continued.

Recently, in Switzerland and Germany, somebuildings made of monolithic fair-faced insulatinglightweight concrete have been constructed. Con-crete mixes with densities above 1000 kg/m3 wereused (Faust, T. 2003, Filipaj, P. 2006 & Baus, U.2007). Worthy of mention is a residential house in

Chur, Switzerland, where the architect Patrick Gart-mann used expanded clay and glass as lightweightaggregates to get insulating concrete with heat con-ductivity of λ = 0.32 W/mK and concrete strengthof LC 8/9. Even lighter concrete mixes using onlyexpanded clay are used in shipbuilding and were devel-oped by Professor Christian Thienel (Thienel, K.C.et al. 2007) of the “Universität der Bundeswehr” inMunich. Inspired by the Swiss house and based onthe Munich findings the departments of “Conceptualand Structural design” and “Construction and Build-ing Material Testing”, both belonging to the Instituteof Structural engineering at the Technische Univer-sität in Berlin, started in the summer of 2006 tojointly develop infra-lightweight concrete with verylow thermal conductivity (Schlaich, M. et al. 2007).

Why the term infra-lightweight concrete (ILWC)?The German concrete code DIN 1045-1 defineslightweight, normal and heavy weight concretesaccording to their densities. Lightweight concrete isdefined as concrete with dry densities in the range800–2000 kg/m3. We define particularly lightweightconcrete as that which has dry densities belowthe 800 kg/m3-limit as Infra-Lightweight Concrete,adding the Latin preposition “infra” which means“below”. Rather than the low weight it is the good ther-mal properties resulting from the high air void contentof the concrete that are of interest here.

The concrete mix that was developed at the Tech-nische Universität (TU) Berlin consists only of water,cement, light expanded clay as a lightweight aggre-gate, and an air-entraining agent.The concrete strengthof this mix comes close to that of a lightweight

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Figure 1. Classification of concrete according to theGerman codes.

Figure 2. Finished house with infra-lightweight concretewalls.

concrete LC 8/9. Mixes which yield a closed fair-facedconcrete surface, a dry density of γdry < 800 kg/m3,and a thermal conductivity of λdry,10 < 0.2 W/mK, areconsistently achieved in the concrete lab at TU Berlin.

In Berlin, a recently built single family house withoutside walls made of Infra-Lightweight Concreteproves the practical value of this material. It wasan interesting challenge to adjust typical structuraland insulation details used for normal concrete to theproperties of this material. To reduce the unavoidablecracks due to shrinkage, glass fiber reinforcement barswere used which solve the corrosion problem and inaddition lead to fewer thermal bridges. The experiencegained so far shows that infra-lightweight concreteallows well insulated fair-faced concrete buildings,and that it has the potential to play a role in the futureof building with concrete.

In this paper, the properties of infra-lightweightconcrete and the experience gained with this materialwill be presented. Aspects of conceptual and struc-tural design with infra-lightweight concrete will bediscussed in the last section.

2 MANUFACTURING PROCESS

To develop a new concrete mix is a challenge. In theinitial excitement too many different additives wereused which led to strong aggregate segregation during

Figure 3. Samples of concrete mix.

Table 1. Concrete mix (with dry aggregates).

Weight Volumekg/m3 L/m3

Cement CEM III-A 32.5 330 108Light sand 0/2 200 158Liapor 1/4 25 30Liapor 2/9 170 315Water 165 165Air-entraining agent 2.0 2.0

the initial tests (Fig. 3, photos left). This was mainlydue to large quantities of plasticizer in combinationwith the air entraining agent. The reduction to only afew key components led to a stable mixture. The cutspecimen (Fig. 3, right) shows the equal distributionof expanded clay aggregates that was achieved by themix given in Table 1.

The goal of weight reduction for improving thermalinsulation properties was reached by using approachesthat are usually not taken as they reduce the strengthof the material. However, for the given field of appli-cation the reduced compression strength was stillsufficient because it is still higher than that of masonry.By making the following “mistakes” infra-lightweightconcrete is obtained:

– Light weight aggregates such as expanded clay orfoam glass lead to a high proportion of air poresbut unfortunately to relatively low strength.

– Generous amounts of air-entraining agent. Thelimit is reached when the concrete surface showstoo many pores.

– High water content that does not fully react with thecement, which evaporates over time and reduces thedry density. This also leads to a high workabilityby improving the flow characteristics and reducespores on the surface.

– Low cement content, which not only has a posi-tive effect on the dry bulk density, but also on thetemperature of hydration which reduces early ageshrinkage. Low cement means also saving primaryenergy.

3 MATERIAL AND PHYSICAL PROPERTIES

In order to minimize the “early age shrinkage”, CEMIII-A 32.5 was used in the mixture. CEM I togetherwith the good insulation characteristics of the material

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Concrete age [day]

Cube

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[N

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Figure 4. Development of infra-lightweight concrete com-pression strength.

Figure 5. Splitting tensile strength test.

caused temperatures of hydration of up to 90◦C insidethe specimen. In addition, the use of CEM III leadsto a light grey color, which in the summer reduceswarming of the surface and thus reduces cracks.

Still, infra-lightweight concrete has an average28-day strength of 7 N/mm2, which increases to8 N/mm2 after 56 days (Fig. 4).

The modulus of elasticity reaches Elc = 4000 N/mm2,which is in tune with the typically low valuesfor lightweight concrete (see Elc to DIN 1045-1,Tables 9–10). The experimentally determined valuefor flexural strength is flc,fl = 0.95 N/mm2. The split-ting tensile strength test (Fig. 5) shows a valueof flc,sp = 0.55 N/mm2 (according to DIN 1045-1,Tables 9 & 10 it would be flctm = 0.66 N/mm2). Theresults of the various tests are summarized in Table 2.

Bond stress was determined by pull-out tests (Fig. 6)using bars made of glass-fiber and steel reinforcement,each with diameter d = 12 mm. The maximum bondstress for infra-lightweight concrete with glass-fiberribbed bars reaches 0.87 N/mm2 at 0.703 mm slip. Forconventional steel reinforcement the value increasesslightly to 1.04 N/mm2 at 0.169 mm slip. In both casesconcrete failure occurred in the area of the ribs or

Table 2. Properties of infra-lightweight concrete.

Cube compression strength, 7.00 N/mm2

flck,cube

Flexural strength, flct,fl 0.95 N/mm2

Splitting tensile strength, flct,sp 0.55 N/mm2

Calculated splitting tensilestrength, flctm (DIN 1045-1) 0.66 N/mm2

Modulus of elasticity , Elc 4000 N/mm2

Heat conductivity, λdry,10 0.181 W/(mK)Heat transfer coefficient,U (twall = 50 cm) 0.341 W/(m2K)Fresh density 1.000 g/cm3

Dry density 0.760 g/cm3

Figure 6. Pull-out test.

grooves where the concrete is exposed to high com-pression. The number of ribs in the steel reinforcementbars is about 25% higher than that in the glass-fiberbars. This explains the higher value of bond stress andsmaller slip value of steel reinforcement.

During a test of a simple infra-lightweight beamwith a single point load at mid span at the TU Berlin,failure occurred in a rather unusual manner. The upperglass-fiber bars in the compression zone could notcarry the required bond stress, and were simply pushedout. Spalling occurred at the support face of the beamat the end of compression reinforcement (see Figure 7at the top left part of the photo).

Particularly striking are the comparatively high val-ues of shrinkage and creep and the low modulus ofelasticity, which need to be considered during design.Initial tests show that the end shrinkage value reaches0.9 mm/m. However, 70% of this value is reached in the

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Figure 7. Infra-lightweight concrete beam with pushed-outupper reinforcement layer.

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Figure 8. Shrinkage of infra-lightweight concrete.

first 3 weeks (Fig. 8). After that it appears that the con-crete behaves like normal concrete. The influence ofthe dimensions and the humidity of the test specimenon the shrinkage must be further investigated.

The first creep test also shows that the initial highincrease in deformation slows down significantly afterabout 50 days. The curves are shown in Figure 9 forsustained loads equivalent to 30% and 50% of thecharacteristic compressive strength. The samples wereloaded after 28 days.

With great excitement the results of the heat conduc-tivity test were expected. The heat transfer resistanceexperiment for sample sizes of 50 × 50 × 5 cm wasconducted in the laboratory of the Material Test-ing Agency (MPA) in Berlin and resulted in a valueof heat conductivity of λdry,10 = 0.181 W/mK. Theheat conductivity depends on the mean tempera-ture of the samples. This relationship is shown inFigure 10.

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Figure 9. Creep of infra-lightweight concrete.

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Figure 10. Heat conductivity.

The theoretical heat transfer coefficient (U-valuewith completely dry samples) for a wall thickness ofS = 0.5 m can be calculated as:

where: 1/αi = 0.13 W/m2K and 1/αa = 0.04 W/m2Kare the respective inner and outer heat transfer resis-tance values according to DIN 4108 Part 4.

Last but not least a CIF-test was done, in which thecapillary sucking and weathering due to freeze-thawcycles were investigated. Twenty eight such cycles,according to DIN CEN/TS 12390 part 9 resulted ina weathering loss of only 340 g/m2. The high contentof the air voids in the infra-lightweight concrete has abeneficial effect. Surface damage or spalling in severewinter conditions is not a threat with this material.

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Heat layerCold layer

Sample

(5x50x50cm)

Infra-lightweight

concrete

Sensors

PC Adapter

Figure 11. Layout of heat-transfer resistance test.

4 HANDLING AND CONSTRUCTION

The reproduction of laboratory test results on site is notan easy task. However, the mixture proved to be sur-prisingly stable.The concrete was ordered with slightlysmaller slump test width than needed and on site thedesired value was obtained by adding small amountsof water to the mix (Fig. 12 above).

Achieving good surface quality was particularlydifficult. Different formwork types and different form-work release compounds were tested. For the buildingdescribed here, simple and new concrete planningtables without any release agent were used for eachcast. A sample wall was cast in order to investigateissues such as the influence of climatic conditions onsite and the time the mix remained in the transportvehicles.

Since pumping of light-weight concrete withexpanded clay concrete is considered to be difficult,for this project the concrete was cast using a concretebucket. To minimize the height of fall, an appro-priate tube was attached to the bucket. The 50 cmthick exterior walls were cast story-wise in layers ofapproximately 50 cm. A conventional concrete vibra-tor placed at distances equal to five times its diameterwas used. No clear optimum for vibration time and dis-tances could be identified. Mix stability and surfacequality were surprisingly independent of the variationof these parameters. Only very long vibration timesled to desegregation. To eliminate potentially segre-gated material, some extra concrete was poured andthe upper few centimeters of the concrete wall wereskimmed after the end of vibrating.

Stripping of the forms took place after at least7 days. For concrete curing, the walls were coveredwith plastic foil. Fortunately, all concreting took placein the mild winter 2006/2007, so that hydration heatwas cooled but at the same temperature rarely reachedthe freezing point.

Figure 12. Slump-test and casting with tube.

To reduce the formation of pores on the surfacea concrete consistency with a w/c-value = 0.5 anda width of slump test of about 60 cm was chosen.Naturally, such a semi-liquid concrete can easily becompacted. Still, for the first-time application of infra-lightweight concrete rather porous surface areas couldnot be avoided altogether. What also happened wasthat cement glue stuck to the formwork, which at suchlocations created a rough surface. All pores and roughzones were later touched up with a “soft” mortar madeof cement CEM III-A and an expanded glass aggregateand later sealed with a transparent silan based water-proofing system. This rather simple treatment led toan interesting, lively, but smooth concrete surface.

5 STRUCTURAL DETAILS

The large shrinkage and creep values of infra-lightweight concrete require a structural design thatleads to as little restraining forces as possible.The non-reinforced samples in the laboratory showed that aftera short time few but large shrinkage cracks occurred.

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Figure 13. Maximum crack width after one year.

In the exterior walls of the house, glass-fiber rein-forcement bars (inside and outside d = 8 mm spacedat a = 15 cm horizontally and vertically) were used,which led to good crack distribution. So far, all crackwidths remain well below 0.1 mm, and those cracks areeasily bridged by the silane based hydrophobic agentthat was applied to the external surfaces: when it rains,no cracks begin to show.

The low modulus of elasticity, in conjunction withstrong creep and shrinkage must be carefully con-sidered when relative deformations are analyzed.“Soft” external walls of infra-lightweight concretemay shorten considerably more than a “hard” buildingcore from normal concrete.

Due to the low strength of the material the wallsmust be treated like masonry walls. A normal concreteceiling slab cannot be rigidly fixed to a wall of infra-lightweight concrete! The different structural detailsof the house were developed accordingly.

The deck slab-to-wall connection in Figure 14shows that additional insulation is needed because theceiling slab reduces the thickness of the insulatingwall. The edge of the slab was insulated with foamglass panels as well as at the mullion transom façade.Such panels were inserted in the wall to reduce thenegative effect of thermal bridges (Fig. 15).

For reasons of expediency and contrary to whatis shown in Figure 19 – for the flat roof a slab ofnormal concrete (C20/25) with insulation on top wasbuilt. Only the fascia at the cantilevering front faceof the roof is made of infra-lightweight concrete. It isconnected via steel stirrups to the roof slab. Externallouvers as sun and light protectors are fixed under thecantilever.

The small bathroom windows in the north façadeare placed flush into the voids of the infra-lightweightconcrete walls. Together with interior wood panels,they provide sufficient insulation.

Figure 14. Connection of deck slab to wall ofinfra-lightweight concrete with vertical glass-fibre bars.

Figure 15. Wall-facade joint (horizontal cross-section).

Figure 16. Connection Roof-Fascia.

6 PROSPECTS

Infra-lightweight concrete does not have enoughstrength to be used for load bearing deck slabs. A pos-sible layout of a house with low energy consumption

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Figure 17. Window north façade.

Figure 18. North façade during construction.

could be as shown in figure 19: Infra-lightweight con-crete for the exterior walls, lightweight concrete withgood insulation properties for the roof-slab, and nor-mal concrete with good heat storage capacity for theinterior ceilings and walls.

Of course, after only one year of research with infra-lightweight concrete many questions need yet to beanswered. Issues such as structural detailing, dura-bility, and long-term behavior as well as questionsregarding the reinforcement of infra-lightweight con-crete are the subject of a PhD thesis presently underpreparation at the Institute of Structural Engineeringof the TU Berlin. The subject of the study is also to

Figure 19. Layout of fair-faced concrete house with wallsof infra-lightweight concrete.

find out what maximum strength can be reached withinfra-lightweight concrete without losing its favorableinsulation properties. On the other hand the question ofwhat minimum value of λ can be reached with normallightweight concrete, to be used for load-bearing roof-slabs, is investigated. Our studies show the limits ofinfra-lightweight concrete, but also its great potentialfor building with fair-faced concrete.

7 THE TEAM

Mix for Infra-lightweight concrete:The authors with Paul Osselmann and Karsten

Schubert (TU Berlin) and Maik Dostmann (Liapor).

Structure:Architect: Clemens Bonnen, Amanda Schlaich,BerlinStructural engineer: Mike Schlaich, Lars Werner,BerlinConstruction company: Kasimir Bau, BerlinConcrete factory: Lichtner Beton, BerlinGlass-fibre reinforcement: Schöck, Baden BadenProof engineer: Hartmut Kalleja, Berlin

REFERENCES

Baus, U. 2007. Sichtbeton, DVA.Faust, T. 2003. Leichtbeton im konstruktiven Ingenieurbau,

Ernst & Sohn.Filipaj, P. 2006.Architektonisches Potential von Dämmbeton,

vdf, Zürich.Schlaich, M., Hillemeier, B. & Schubert, K. 2007. Infrale-

ichtbeton – Potenzial für den Sichtbetonbau, Proceedings51. BetonTage, Ulm, Germany.

Thienel, K.C. & Peck, M. 2007. Die Renaissance leichterBetone in der Architektur. DETAIL Heft 5, S. 522–534.

Widman, H. & Enoekl, V. 1991. Foam Concrete-Propertiesand Production, BFT, Heft 6.

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