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Use of foamed concrete in the structure of passive house foundation slab

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The use of cellular concrete in civil engineering has a long history. It is very popular as a thermal and sound insulation layer of ceiling and roof systems. Structural use is still limited to geotechnical solutions where strength parameters of foamed concrete are comparable to well compacted sand. Thanks to other benefits like self-levelling, self-compaction, high freeze/thaw resistance and biological resistance, foamed concrete gains popularity in such applications as excavation infill, soil stabilisation and lightweight foundations. A foundation slab made of foamed concrete has also another advantage. Very promising thermal insulation makes that solution ideal for thermal saving applications, especially when floor heating is applied. This paper presents the concept of a dwelling-house sandwich foundation slab partly made of foamed concrete. The idea emerged after critical review of existing methods of founding of passive houses with use of structural layer made of non-structural materials (usually foamed polystyrene). In the proposed solution, foamed concrete creates an insulated base slab for a house structure. High drying shrinkage and relatively low tensile strength makes the total sub-stitution of reinforced concrete impossible, but significantly reduces its depth to a thin floor layer. In addition to thermal insulation, the base layer of foamed concrete distributes the wall reactions which makes the proposed solution excellent for weak soils. The described concept is exemplified with realised foundations. Some developed design procedures and recommenda-tions are presented.
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USE OF THE FOAMED CONCRETE IN THE STRUCTURE OF
PASSIVE HOUSE FOUNDATION SLAB
Jacek HULIMKA
Assistant Professor
Silesian University of
Technology
Gliwice, Poland
Jacek.Hulimka@polsl.pl
Rafał KRZYWO ´
N
Assistant Professor
Silesian University of
Technology
Gliwice, Poland
Rafal.Krzywon@polsl.pl
Agnieszka
KNOPPIK–WRÓBEL
MSc., Eng.
Silesian University of
Technology
Gliwice, Poland
Agnieszka@Knoppik.pl
Abstract
The use of cellular concrete in civil engineering has a long history. It is very popular as a
thermal and sound insulation layer of ceiling and roof systems. Structural use is still limited
to geotechnical solutions where strength parameters of foamed concrete are comparable to well
compacted sand. Thanks to other benefits like self-levelling, self-compaction, high freeze/thaw
resistance and biological resistance, foamed concrete gains popularity in such applications as
excavation infill, soil stabilisation and lightweight foundations. A foundation slab made of
foamed concrete has also another advantage. Very promising thermal insulation makes that
solution ideal for thermal saving applications, especially when floor heating is applied.
This paper presents the concept of a dwelling-house sandwich foundation slab partly made of
foamed concrete. The idea emerged after critical review of existing methods of founding of
passive houses with use of structural layer made of non-structural materials (usually foamed
polystyrene). In the proposed solution, foamed concrete creates an insulated base slab for a
house structure. High drying shrinkage and relatively low tensile strength makes the total sub-
stitution of reinforced concrete impossible, but significantly reduces its depth to a thin floor
layer. In addition to thermal insulation, the base layer of foamed concrete distributes the wall
reactions which makes the proposed solution excellent for weak soils. The described concept
is exemplified with realised foundations. Some developed design procedures and recommenda-
tions are presented.
Keywords: Foamed concrete, foundation slab, energy saving
1. Introduction
Cellular concrete was introduced into the construction industry in the middle of the last century.
First applications were limited to non-structural applications like void fillings, roof thermal
insulations and ceiling acoustic damping.
Foamed concrete is one type of cellular concrete, produced in a process of foaming of cement
slurry. This is the most popular, economical and controllable pore-forming process, which re-
sults in creation of the material lighter than conventional concrete. Cement & Concrete Institute
defines foamed concrete as a cementitious material with a minimum of 20% of foam entrained
into the cement mortar [1]. Adequately to the volume of the foam agent, foamed concrete
may be produced in densities between 300kg/m3to 1600kg/m3and compressive strengths
0.5÷15MPa.
2. Potentials of foamed concrete for a structural use
In structural engineering there arises a necessity of application of structural materials which
are light, durable, simple in use, versatile, economic and environmentally sustainable. Foamed
concrete, which was initially used as an insulating material, satisfies all of these requirements
thus has a great potential for structural use, although it may not be a direct substitution for
normal weight concrete.
The properties of foamed concrete depend on its composition (type of binder, proportions of
ingredients) and methods of formation and curing (resulting microstructrure). The pore struc-
ture has major influence on such parameters as strength, permeability, diffusivity, shrinkage and
creep, hence its characterisation is highly important.
2.1 Strength parameters
Compressive strength is a common performance measure for concretes. A hypothetical com-
pressive behavior (stress–strain relationship) of a cellular material is presented in Figure 1 [2].
Figure 1: Stress–strain curve of a cellular material [2]
In conventional applications of foamed concrete as a filling material there is no need of high
strengths. Therefore it is believed that foamed concrete, which typically consists of cement,
filler, water and foam, is a low-strength non-durable material with no possibility of structural
applications where high strengths are desired.
Nevertheless, the use of fine fillers, such as simply fine sand [3] or pozzolanic materials such as
pulverized fuel ash, slate waste or silica fume instead of or in addition to cement/sand [4, 5, 6]
provides promising increase in strength (up to 25MPa and above). Obviously, higher strength
corresponds to higher density: compressive strength increases linearly with density [5]. Cement
& Concrete Institute in [1] states that 1600kg/m3is the minimum density at which typical
foamed concrete could be used in structural elements while Jones & McCarthy in [4] suggest
that the value can be lowered to 1400kg/m3for the foamed concrete with pozzolanas.
A brittle mode of failure is observed in compressed foamed concrete samples. Coupled with
relatively low values of compressive strength, it is believed to be the result of unconfined con-
ditions of tests leading to early crack initiation [2]. Confined conditions reduce the occurrence
and propagation of cracks. It was not possible in practice, however, to achieve the stress-strain
relationship as presented in Figure 1. The elements usually fail in elastic stage, not exhibiting
plastic properties due to expected microstructure damage and consequent densification.
In comparison to aggregate concretes, in which there is a slight increase in strength after 28
days, a further gain in strength is observed in fly-ash concrete after 56 days [4] or even 91 days
[3]. Therefore, foamed concrete elements require comparatively much longer period of curing.
The value of tensile strength of foamed concrete is typically 10 ÷15% of the compressive
strength while the flexural strength is about 25% [5]. Application of pozzolanic materials de-
creases the values of tensile strength and elastic modulus [4]. This is worrisome because these
parameters are low enough in typical foamed concrete itself. Therefore, foamed concrete of a
given grade exhibits worse characteristics for flexural applications than normal weight concrete
of the same grade, particularly in case of deflections.
However, low tensile strength and elastic modulus are inherent properties of concrete dealt with
application of reinforcement. The use of polypropylene fibres is a very popular reinforcement
method [4], but it was also proposed to reinforce foamed concrete elements with vinyl fibres [6]
or with more sophisticated nanodispersive reinforcement in a form of carbon nanotubes [7].
Moreover, a further improvement of strength parameters can be achieved by lowering the wa-
ter/cement ratio of fresh foamed concrete mix. To sustain its beneficial workability, some foam-
compatible plasticizers need to be applied [4].
To increase the strength properties of foamed concrete along with improvement of its weight
and workability, the foam can be modified by introduction of specially designed types of foam
agents or addition of styrofoam [6, 8]. The choice of admixtures must assure stability of the
foam and proper formation of the bubbles in the final material.
2.2 Early-age properties
The thermal–moisture effects of hardening process must be considered as one of the most im-
portant problems of all cement-based materials, including foamed concrete. The process is
similar as in typical concrete, in which the hydration heat and drying shrinkage lead to cracking
of the element.
Nevertheless, in foamed concrete, characterised with significant level of porosity, the rate of
water migration from hardening concrete is faster than in conventional concrete. Therefore,
drying shrinkage in foamed concrete poses much greater risk. A change of foamed concrete
composition may be considered to reduce the threat of negative thermal–moisture effects which
are the greatest when only cement is used as a binder [5]. Application of pozzolanic admixtures
allows to decrease the hydration heat, thus lowers the potential thermal shock, and retards water
desorption which results in decreased shrinkage [4]. The use of fine aggregate (sand) also gives
promising results in this matter.
2.3 Physical, chemical and biological resistance
Foamed concrete, being a porous material, exhibits significant sorption characteristics [5]. Wa-
ter vapour diffusion can be observed even in a dry state. While in a direct contact with water,
moisture transfer occurs by absorption of water and transmission by capillarity (sorptivity). The
sorption characteristics are the best in cement–sand foamed concretes with a large foam content
and get worse when density increases or when pozzolanic materials are added [9].
The material has excellent frost resistance in a dry state [3]. Nevertheless, increased amount of
water in pores is reported to intensify freeze/thaw reactions. At very high degrees of saturation
the material becomes brittle and undergoes complete failure [5].
Foamed concrete, especially with a pozzolanic admixture, possesses poor carbonation resis-
tance with high rates of carbonation [4], so it must be carefully protected in the environments
where carbonation-induced corrosion may occur. Application of carbon steel for reinforcement
should be avoided. On the other hand, foamed concrete – as a cementitious material – has very
good biological resistance.
Foamed concrete exhibits good fire-resistance properties and is an incombustible material [5].
In comparison to normal weight concrete, foamed concrete has better fire-proof properties and
is less prone to strength loss in fire, especially at lower densities. This is because it is a relatively
homogeneous material with low thermal conductivity and diffusivity.
2.4 Thermal and acoustic insulation
Foamed concrete provides a high level of sound and thermal insulation, mainly thanks to its
density and high porosity [1, 2, 4, 5, 8, 10]. The amount, size and distribution of pores has a
crucial meaning.
Nevertheless, the side effect of its high porosity are corresponding sorption characteristics,
which have negative influence on the thermal resistance [9]. Thus, it is essential to minimise
the contact of a foamed-concrete element with water.
2.5 Transportation and installation
Foamed concrete has high workability. It is a free-flowing and self-compacting material, so
it does not require compacting, vibrating or levelling [1, 8]. Therefore, application of foamed
concrete is beneficial for productivity and comfort at erection stage.
Thanks to the production technology of foamed concrete, i.e. in-situ foam entrainment, the
volume of the material is limited, so it is efficient in transportation and placement [3, 8].
3. Application in foundation slabs
The main advantage of foamed concrete is its light weight which ensures economy in the design
and execution of supporting structures, including foundations. Additionally, it provides a high
degree of thermal insulation, making foamed concrete a perfect material for use in a passive
houses design.
Introduction of foamed concrete as a replacement of compacted soil in a base layer for a foun-
dation has a series of advantages. The material has the strength properties at least as good as
well-compacted soil. It can be easily placed (poured) and does not settle, so no compaction is
required. Its light weight ensures limitation of loads imposed to the subsoil along with providing
uniform distribution of reactions from the supported structure. No lateral pressure is exerted.
Foamed concrete is currently used for light weight foundations in densities below 800kg/m3.
However, there are applications of higher-density foamed concretes where there is higher de-
mand on strength, such as in road works as a compensation bed or under flooring in industrial
buildings [8]. Promising results of these applications may be a good basis for use of foamed
concrete as a support for heavier building structures.
The proposed foundation slab is a sandwich solution with a foamed concrete base layer and a
reinforced concrete structural layer. In a standard solution, as presented in Figure 2, under the
load-bearing walls the continuous 40cm x 20cm wall sleeper ribs of reinforced concrete are
applied in a form similar to conventional strip foundations. The grid is joint with a diaphragm
in a form of a 10-cm thick reinforced concrete slab into a monolithic structure. The voids are
filled with hard extruded-polystyrene panels.
Figure 2: Foundation slab for good geotechnical conditions
The dimensions of the reinforced concrete ribs are designed in a way that assures a desired
load-bearing capacity of the element itself and limitation of stresses exerted by the supported
structure on the foamed concrete bed and then on the subsoil.
If hydro–geological conditions on a site are poor, modification of the slab can be applied with a
plain concrete ring continuous foundation at the outer perimeter of the foundation slab. Figure 3
shows the ring foundation when the minimum foundation depth (0.5 m below terrain level)
is satisfactory. Figure 4 presents the ring foundation for a slab on heaving soils, where the
foundation base has to be located below the freezing depth (min. 0.8 m below terrain level).
Figure 3: Foundation slab for poor geotechnical conditions. Standard version
A further distinction has to be made between the standard version (Figure 4) and the so-called
“thermo” version (Figure 3). The main differences are the thermal insulation properties of the
two solutions. It must be noted that application of the standard version requires relatively good
Figure 4: Foundation slab for poor geotechnical conditions. Thermo version
geotechnical parameters of the subsoil as the loads from the foundation are transmitted directly
onto the subsoil; foamed concrete plays only the insulating role.
4. Numerical model
Behaviour of a foundation slab depends mainly on its deformability and stiffness of the subsoil.
These two parameters were chosen as variables in the FEM analysis.
A scheme of the modelled structure with a detailed arrangement of the FEM mesh is shown in
Figure 5. The model was created using cuboidal finite elements. As a support the Winkler sub-
soil with linear elements was used. Possibility of ground separation under the foundation was
ensured by excluding supporting bars after the occurrence of tensile stresses during calculation
process.
Figure 5: Scheme of the finite element model
All analyses were performed with use of the MAFEM 3D program created and developed in the
Department of Structural Engineering of the Silesian University of Technology. The program,
based on the Willam Warnke material model modified by Majewski [11], allows of the elasto–
plastic analysis of cohesive–frictional materials.
A large group of models with different depths of the foamed concrete layer and elasticity of the
subsoil Winkler bars was analysed. Concrete and steel material characteristics as well as geom-
etry were constant for all models. The assumed values for normal weight concrete were taken
as: uniaxial compressive strength 30MPa, uniaxial tensile strength 2.7MPa, initial modulus of
elasticity 22GPa, Poisson’s ratio 0.166, ultimate strain at uniaxial compression 0.0022.
Foamed concrete parameters were taken on the basis of the data disclosed by the producer: com-
pressive strength was taken as 0.8MPa, tensile strength 0.21 MPa, initial modulus of elasticity
1.0GPa, Poisson’s ratio 0.166 and ultimate strain at compression 0.02.
Stiffness parameter of the subsoil, defined by the kcoefficient, varied between 0.5 and 50MN/m2.
4.1 Results of the analysis
The aim of the analyses was to find the conditions of the work of the foamed concrete slabs
loaded with the reaction of the sleeper ribs. Possible failure models were defined.
Figure 6: Distribution of σxstresses characteristic for bending of the foamed concrete layer
Figure 6 presents an exemplary σxstress distribution diagram characteristic for bending of the
foamed concrete layer. Dashed lines represent the boundaries of the zone where the stresses
are almost constant. The lines can be simultaneously considered as the sections where bending
condition should be controlled.
Figure 7: Punching surface in the foamed concrete slab
Figure 7 presents the direction vectors of the main tensile stresses. The line perpendicular to
these vectors (denoted with a white line) defines the punching surface. As it is shown, the
inclination angle of this surface is equal to approximately 60. Such a result complies with the
results obtained in [10].
Figure 8 and Figure 9 show the passive earth pressure distributions for various subsoil stiffness
parameters and various depths of the foamed concrete slab. It can be noticed that the range of
the pressure zone widens and becomes more uniform along with the weakening of the subsoil
(Figure 8). A similar effect can be observed as the depth of the slab increases (Figure 9). The
maximum pressure occurs under the rib and reaches the greatest values for the stiffest subsoil
and the thinnest slab.
Figure 8: Passive earth pressure distribution depending on the subsoil stiffness
Figure 9: Passive earth pressure distribution depending on the foamed concrete slab thickness
5. Design model
Wide applications need simple solutions. This is the reason why a simple design model was
created on the basis of the achieved FEM results. The model is presented in this section.
There are five possible failure situations:
1. compressive failure of foamed concrete under the rib of the RC foundation slab,
2. flexural failure in the bottom zone of the foamed concrete layer,
3. punching shear failure of the foamed concrete layer,
4. separation of the edge of the foamed concrete layer due to combined flexural and punch-
ing effects,
5. depletion of bearing capacity of the subsoil with no damages in the foundation sandwich.
The last failure situation is a geotechnical problem and will not be discussed in the paper.
5.1 Compressive failure of foamed concrete
This failure situation is possible for extremely advantageous geotechnical conditions when the
foamed concrete layer is founded in hard soils, like rocks or well-graded sands. The local
compressive pressure caused by the rib of the foundation slab should be checked according to
the condition:
Qr
brib ff cd ,(1)
where:
Qr continuous load transferred from the sleeper rib of the RC slab (per meter of the
rib’s length),
brib width of the sleeper rib,
ff cd compressive strength of foamed concrete.
5.2 Flexural failure of foamed concrete
This failure may appear as a consequence of typical bending of the foundation slab caused by
the passive ground pressure. The impact width (the area where the passive earth pressure will
appear) depends on the thickness of the FC layer, the angle of the load transfer and soil de-
formability. On the basis of the FEM analyses described in section 4., the following expression
to calculate the impact width may be proposed:
rimp =4.7·h0.8
f cs
4
k,(2)
where:
hf cs depth of the foamed concrete slab (given in [m]),
k stiffness parameter of the subsoil (given in [ MN/m]).
The original curvilinear shape of the subsoil stress block may be substituted by a bilinear (trape-
zoid) one shown in Figure 10. A fold point is situated under the fourth part of the rib’s width.
Such an assumption results from the considerations presented in subsection 4.1 where it was
proved that the stress change attenuates at this point. Considering vertical force equilibrium,
the maximum value of the passive earth pressure could be found as:
σmax =Qr
rimp +0.75brib
.(3)
Figure 10: Substitute passive earth pressure block
The maximum bending moment can be determined with the cantilever model presented in Fig-
ure 11, in which a substitute bilinear passive earth pressure block is assumed. The value of the
maximum bending moment at the section of the fold point is equal to:
Mmax =σmax ·(rimp +0.25brib )2
6.(4)
Figure 11: Cantilever model for bending of the foamed concrete slab
The load-bearing condition requires that the stress resulting from the bending moment exerted
on the slab of given geometry cannot exceed in any cross-section the material’s tensile strength:
Mmax
Wff ctd ,(5)
where:
ff ctd tensile strength at flexure of foamed concrete.
The comparison was made between the results obtained in the FEM analysis and with the pro-
posed design model. The results are presented in a form of diagrams in Figure 12 and Figure 13.
The line in the diagrams represents the ideal correlation. It can be noticed that the results are
close to the line; this proves that the proposed design model is precise to a satisfactory extent.
Figure 12: Comparison of results of FE analysis and proposed model. Impact width
Figure 13: Comparison of results of FE analysis and proposed model. Passive earth pressure
5.3 Punching shear failure of foamed concrete layer
The FEM analysis has shown that the angle of stress distribution is close to 60(Figure 7). The
design model for punching is presented in Figure 14. As it is shown, reduced punching force
neglecting passive earth pressure within the zone of a direct load transfer must be introduced.
Figure 14: Design model for punching shear in the foamed concrete slab
5.4 Edge separation of the rib with the end of the foamed concrete layer
Effective protection for that failure situation is provided by proper joining of the sleeper rib to
the top floor. It may be realised by anchoring the floor reinforcing bars inside the rib. Due to
that requirement it is also recommended to cast the end rib together with the floor.
6. Summary
The presented model was developed to meet the satisfy of the building market for insulated
sandwich foundation slabs design instruments. By now, on the basis of the resented technology,
a group of single-storey houses has been erected in the southern Poland and Slovakia. This is
the experimental polygon for evolution of the concept of the foamed concrete sandwich slab
to more demanding structural solutions. Additionally, in the nearest future, a full scale tests of
a sleeper rib on the foamed concrete slab are planned to confirm correctness of the presented
FEM and design model.
7. References
[1] CEMENT & CONCRETE INSTITUTE. Foamed concrete. http://www.cnci.org.za/, 2010.
[2] ABDUL RAHMAN M. Z. A., ZAIDI A. M. A. and RAHMAN I. A. Analysis of com-
parison between unconfined and confined condition of foamed concrete under uni-axial
compressive load. American Journal of Engineering and Applied Sciences, 3(1):68–72,
2010.
[3] COX L. and VAN DIJK S. Foam concrete: a different kind of mix. Concrete, 36(2):54–55,
2002.
[4] JONES M. R. and MCCARTHY A. Preliminary views on the potential of foamed concrete
as a structural material. Magazine of Concrete Research, 57(1):21–31, 2005.
[5] NARAYANAN N. and RAMAMURTHY K. Structure and properties of aerated concrete:
a review. Cement & Concrete Composites, (22):321–329, 2000.
[6] BYUN K. J., SONG H. W., PARK S. S. and SONG Y. C. Development of structural
lightweight foamed concrete using polymer foam agent. ICPIC-98, 1998.
[7] YAKOVLEV G., KERIENE J., GAILIUS A. and GIRNIENE I. Cement based foam
concrete reinforced by carbon nanotubes. Materials Science, 12(2):147–151, 2006.
[8] WAHLMAN T. Foam concrete in soil. Light and stable. http://aercrete.se/, 2010.
[9] KUNHANANDAN NAMBIAR E. K. and RAMAMURTHY K. Sorption characteristics
of foam concrete. Cement and Concrete Research, (37):1341–1347, 2007.
[10] SCHERFEL W. and ZAVACKY J. Kompozitny penobeton GeoPBG D5 ako alterna-
tivne riesenie podkladnej roznasacej vrstvy pod zakladove dosky i primyselne podlahy.
http://sircontec.com/, 2010.
[11] MAJEWSKI S. Mechanika betonu konstrukcyjnego w uje¸ciu spre¸˙
zysto–plastycznym.
Monografia. Wydawnictwo Politechniki ´
Sl ˛askiej, 2003.
... The aim is an alternative replacement of gravel bed of the most popular thickness of 200 mm and lean concrete base of 100÷200 mm thickness with a single layer of foamed concrete with thickness providing required subgrade thermo-technical and strength parameters. The proposed foundation slab [7], [8] is a sandwich solution with a foamed concrete base layer and a reinforced concrete structural layer (Fig. 2). The load-bearing walls are supported on sleeper ribs of reinforced concrete in a form similar to conventional strip foundations. ...
... As can be expected, maximum passive earth pressure mainly depends on its elasticity and the stiffness of FC slab [7]. The range of the pressure zone widens and becomes more uniform when the subsoil is more pliant or when the depth of the slab increases. ...
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Knowledge of sorption characteristics like absorption and sorptivity of building materials are of importance as they affect the durability and other properties. In the case of lightweight concrete like aerated and foam concrete, the moisture movement behaviour becomes more complex as such concretes contain much larger volume of air voids. This paper is aimed at investigating the sorption-related properties of foam concrete as affected by its composition and pore structure. Water absorption by complete immersion and sorptivity are measured for various mixes with different fly ash replacement levels for sand and different foam volume. It was found that sorption values are lower than the corresponding base mix (without foam) and the values reduce with increase in foam volume. It is also found that sorption depends on the filler type, density and pore structure and also on permeation mechanisms.
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Low self-weight (800 to 1600 kg/m3), high workability (flowing and self-compacting) and excellent thermal insulating properties (< 0·50 W/mK) make foamed concrete attractive for many construction applications. Indeed, it is now a well-established material in void filling and highway reinstatement uses. However, its wider use in structural applications has been inhibited by its technical and engineering unfamiliarity and a perceived difficulty of achieving sufficiently high strength (say > 25 N/mm2). This paper describes a laboratory study of the development of foamed concrete, utilising two types of fly ash, with the potential for use in structural applications. ‘Fine’ fly ash (i.e. to BS EN 450) was used to partially replace Portland cement and a ‘coarse’ fly ash (i.e. to BS 3892-2) to replace sand fine aggregate. In addition, the potential of polypropylene fibres in foamed concrete to enhance plasticity and tensile strength was examined. The key early age, engineering and durability properties were measured and these data show that foamed concrete is indeed viable for structural uses. To further demonstrate the concept, the results of full-scale pilot tests on conventionally reinforced foamed and normal weight concrete beams are also reported and their performance compared with the BS 8110 and Eurocode 2 requirements for serviceability. However, the characteristics of foamed concrete, particularly high drying shrinkage strain and relatively low tensile strength and stiffness performance, mean that straight substitution for normal weight concrete is not possible and more innovative structural forms will need to be developed.
Structure and properties of aerated concrete: a review
NARAYANAN N. and RAMAMURTHY K. Structure and properties of aerated concrete: a review. Cement & Concrete Composites, (22):321–329, 2000.