A 11 ID 2 771S43 MBS

PUBLICATIONS

NBSIR 87-3670

A Review of Autoclaved Aerated

Concrete Products

Robert G. Mathey

Walter J. Rossiter, Jr.

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

March 1988

2

Stimulating America's Progress

1913-1988

U.S. Department of Energy

Office of Conservation and Renewable Energy

1000 Independence Ave., SW

Washington, DC 20585

NBSIR 87-3670

A REVIEW OF AUTOCLAVED AERATED

CONCRETE PRODUCTS

Robert G. Mathey

Walter J. Rossiter, Jr.

Research Information Center

National Bureau of .Standards

Gaithersburg, Maryland 20899

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

March 1988

Prepared for: U.S. Department of Energy

Office of Conservation and Renewable Energy

1000 Independence Ave., SW

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This report is a review of the properties and performance of

autoclaved aerated concrete. The material has a relatively low

thermal conductivity combined with loadbearing capacity for use

in structural and non-structural applications. This feature may

make its use attractive for energy-conserving applications. For

this reason, the review was conducted at the request of the U.S.

Department of Energy.

The review addressed an overview of the manufacturing process,

uses of autoclaved aerated concrete in building constructions,

properties, energy considerations, the availability of code-related

documents, and standards. Uses include block and panel construction

in both loadbearing and non- loadbearing applications for walls,

floors, and roofs. Many properties are reviewed including

density, fire resistance, moisture expansion and shrinkage,

strength, structure, and thermal conductivity. With regard to

energy considerations, the review points out that, for energyefficient applications under severe climatic conditions, the

autoclaved aerated concrete unit alone may be insufficient to

provide the targeted minimum value of thermal resistance. In

these cases, autoclaved aerated concrete has to be used together

with additional thermal insulation. In contrast, autoclaved

aerated concrete alone may satisfy insulation needs in areas

having mild climates.

Key words: aerated; autoclaved; cellular; concrete; insulating

concrete; lightweight; manufacture; properties;

review; standards; uses.

in
TABLE OF CONTENTS

Page

ABSTRACT . . iii

LIST OF FIGURES vi

1 . INTRODUCTION 1

1 . 1 Background 2

1.2 Objectives 6

1 . 3 Scope of the Study 6

2. MANUFACTURE OF AUTOCLAVED AERATED CONCRETE 8

3. USES OF AUTOCLAVED AERATED CONCRETE 12

3.1 Blocks 13

3.2 Panels 14

3.3 Limitations 16

3.4 Design Considerations 17

3.4.1 The European Experience 17

3.4.2 The U.S. Experience 18

3.4.3 Moisture Considerations 19

4. PROPERTIES OF AUTOCLAVED AERATED CONCRETE 22

4.1 Capillarity 23

4.2 Coefficient of Thermal Expansion 23

4.3 Cracking 23

4.4 Creep 24

4.5 Density 25

4.6 Deterioration Resistance 25

4.7 Fire Resistance 27

4.8 Frost Resistance 29

4.9 Moisture Expansion and Shrinkage 31

4.10 Moisture Migration 32

4.11 Permeability 33

4.12 Shaping and Working 3 3

4.13 Specific Heat 34

4.14 Solubility in Water 34

4.15 Sound Absorption 34

4.16 Strength 35

4.16.1 Compressive Strength 36

4.16.2 Tensile Strength 37

4.16.3 Shear and Bond Strength ......... 38

4.16.4 Modulus of Elasticity 40

IV
TABLE OF CONTENTS (Continued)

Page

4.17 Structure 41

4.18 Temperature Resistance 43

4.19 Thermal Conductivity 44

5. ENERGY CONSIDERATIONS 46

6. STANDARDS AND RELATED DOCUMENTS 50

7. SUMMARY 51

8. ACKNOWLEDGMENTS 53

9. REFERENCES 54

APPENDIX A A-l

v
LIST OF FIGURES

Page

Figure 1. Countries Where Autoclaved Aerated Concrete

is Used, as Described by Bave in 1983 [6] 62

Figure 2. Production of Autoclaved Aerated Concrete

Products 63

Figure 3. Relationship Between the Cube Compressive Strength

and the Density of Autoclaved Aerated Concrete [1] 64

Figure 4. Relationship Between the Compressive Strength and

the Moisture Content of Autoclaved Aerated Concrete

[1] 65

Figure 5. The Micro-Cellular Structure of Autoclaved Aerated

Concrete 65

Figure 6. Example of Distribution of Pore Sizes in Autoclaved

Aerated Concrete [1] 66

Figure 7 . The Relationship Between the Thermal Conductivity

and the Density of Dry Specimens of Autoclaved

Aerated Concrete [1] 66

LIST OF TABLES

Table 1. Estimated 1987 Production of Autoclaved Aerated

Concrete in Some Countries 67

Table 2. Examples of Values of Compressive Strength and

Modulus of Elasticity 67

vi
1. INTRODUCTION

Autoclaved aerated concrete (AAC) is a construction material that

is factory-made and available to the user in blocks and precast

units for walls, floors, and roofs. Blocks for laying in mortar

or glue are produced without any reinforcement [1]^. It has gained

widespread use in many areas of the world including Europe, South

America, the Middle East, and the Far East. In brief, autoclaved

aerated concrete is a porous lightweight concrete whose cellular

structure is generally obtained through an in situ gas-producing

chemical reaction of a sand and cement slurry. It generally

contains no coarse material. Subsequent autoclaving of the

material at high temperature and pressure imparts strength,

dimensional stability, and other properties of the hardened final

product. The autoclaved aerated concrete building material is

obtained as the result of a reaction between the binder containing

calcium oxide and a silica component cured in an autoclave [2],

and a cellular structure producing ingredient. Autoclaved

aerated concrete, is at times, referred to as "aerated concrete,"

"gasbeton," and "autoclaved lightweight concrete (ALC)."

Autoclaved aerated concrete is a type of cellular concrete.

Another type, using preformed foam and cast in place, is commonly

used in the United States for floor and roof fills. The important

properties of autoclaved aerated concrete are present to a large

degree in cellular concretes which have not been autoclaved [3].

-'-Figures in brackets indicate references listed in Section 9.

1
An often reported advantage [4-7] of autoclaved aerated concrete

is the combination of relatively low thermal conductivity, and

loadbearing capacity for use in structural applications. Other

advantages include relatively low density, high strength-toweight ratio, nailability, fire resistance, good thermal resistance,

and high sound insulation value [8,9]. Its low density permits

use of large building units, which is a reported advantage in

pre fabrication.

The thermal conductivity properties offer potential to conserve

energy by reducing building heating and cooling fuel consumption.

The potential for energy conservation through the use of an

alternative material of construction is of interest to the

U.S. Department of Energy (DOE). Consequently, DOE requested the

U.S. National Bureau of Standards ( NBS ) to undertake a study

reviewing the properties and use of autoclaved aerated concrete.

This report presents the results of the study. The study also

provided an occasion to review opportunities for needed research.

1 . 1 Background

Autoclaved aerated concrete was first introduced commercially

over 50 years ago [6,10]. Bessey [4,11,12] has described its

early development and use in some detail. Initial development of

the products including autoclave curing occurred in the 1920s in

Scandanavia, with commercial use commencing in the early 1930s.

However, little growth was experienced in commercial applications

2
until the mid-1940s and the end of World War II. At this time,

its use began to spread in some areas of Europe, in connection

with post war building efforts. In subsequent years, expansion

continued to many regions of the world. For example, the

material was first used in the United Kingdom in about 1950 and

by 1965 nine factories were operating under the direction of four

different companies [4]. Use in the expansion years of the

fifties did not result in U.S. production [13]. In the mid1960s, autoclaved aerated concrete represented a major portion of

all structural materials used in some countries; for example, 20-

40 percent in USSR, Germany, Holland, and Sweden [4]. A 1987

estimate placed the amount of autoclaved aerated concrete used

annually worldwide at about 8.1 x 10® ft® (23 x 10® m®)®. Table

1 provides recent estimates of the annual production of the

material for some countries. In 1987, autoclaved aerated

concrete was produced in 35 countries®.

Figure 1 depicts those countries where autoclaved aerated

concrete was produced as of 1983 [6]. A notable feature in

Figure 1 is the lack of use of autoclaved aerated concrete in

Canada and the United States. Reasons for this lack of use have

received little documentation in the archival literature. Most

of the reasons for lack of use, uncovered during the present

study, are anecdotal, coming from individuals who are familiar

with the production and use of the material. In general, the

®Gunnar Bave, Siporex, personal communication.

3
lack of use is attributed to factors associated with production

costs and to tradition in the use of conventional building

materials. For example, the costs of building a plant to

manufacture hollow core concrete blocks may be 20 to 25 percent

of the large initial investment to construct an autoclaved

aerated concrete plant. In 1968, Bessey [12], writing in his

paper on the worldwide development of autoclaved aerated concrete,

indicated that the spread of the use of the material into the

United States and Canada was marginal. He described the reasons

for lack of growth as follows:

"The factors involved are many. Except in dense urban

areas, timber still plays a large part in the construction

industry in these countries. The concrete block

industry is well organized and efficient, and the

lightweight-aggregate block is highly competitive in

cost and is generally of good quality. An aerated

concrete plant must be of good size and have a market

which will utilize it at nearly full capacity in order

to be economic, for the capital costs and fixed

overheads together constitute a high proportion of

costs .

Bessey [11] also indicated that general factors affecting global

development of autoclaved aerated concrete were partly climatic,

partly economic, and partly psychological. Specific factors

which he has discussed on this point include: the local building

regulations or codes affecting building construction, availability

4
of raw materials, climatic conditions, fuel and energy requirements,

and costs due to production, transportation, erection and

maintenance [4,12].

It is interesting to note Bessey [12] indicated that, at the time

he was writing (1968), two factories in Canada and one in the

U.S. had achieved some minor degree of success. He did not state

where these factories were located. The plants referred to by

Bessey are not in operation today (Figure 1).

During the period 1955 to 1972 an autoclaved aerated concrete

factory operated in Montreal, Canada. Some 54 x 10^ ft^

(15 x 10^ m^ ) of its reinforced products were imported and used on

a variety of building projects in the United States^.

Renewed interest in autoclaved aerated concrete use has recently

occurred. For example, a news item indicated that a 42,000 ft^

(3900 m^) facility was completed in 1986 in Florida [14]. The

building had autoclaved aerated concrete precast wall and roof

panels which were imported from Sweden. The columns and beams

consisted of conventional, precast, prestressed concrete. It was

reported that the autoclaved aerated concrete components were

imported at a relatively high shipping cost. In contrast, labor

costs were reportedly saved due to the relative ease and reduction

^Samuel Aroni, University of California at Los Angeles,

personal communication.

5
of time associated with the installation of the lightweight

autoclaved aerated concrete precast planks. The product was not

covered by the building code and therefore a special permit was

needed before the material could be used in Orange County,

Florida [ 14 ]

.

1 . 2 Objectives

The objectives of this report are:

(1) to describe the process used in the manufacture of

autoclaved aerated concrete,

(2) to present an overview on the use and properties of

autoclaved aerated concrete products, and

( 3 ) to provide a listing of standards and related documents

developed for autoclaved aerated concrete.

1 . 3 Scope of the Study

The present study was limited to reviewing existing information.

No laboratory or field tests were conducted. The major sources

of information were: (1) the archival chemical and engineering

literature; (2) worldwide building research organizations in

Canada and Europe (including England, Denmark, France, Norway, and

Sweden); (3) discussions with researchers knowledgeable with

autoclaved aerated concrete technology; and (4) literature from

and discussions with representatives of manufacturers of autoclaved

aerated concrete products.

6
As a construction material which has achieved worldwide use, the

body of technical information on autoclaved aerated concrete

including research reports, standards, design criteria, codes of

practice, and related documents is extensive. A bibliography of

archival literature having 492 citations was published in 1983

[15]. The bibliography is included in a Proceedings of an

International Symposium entitled "Autoclaved Aerated Concrete,

Moisture and Properties" [16]. In 1954, Valore [13,17] gave a

review of methods of preparation and physical properties of

moist-cured and high-pressure steam-cured cellular concretes,

ranging in density from 10 - 100 lbm/ft^ (160 - 1600 kg/m^). The

review covered three decades beginning in the mid-1920s.

It is noted that at least one process for aerating and autoclaving

a lightweight aggregate structural concrete has been described in

the literature [18]. This lightweight aggregate concrete has a

higher density and greater strength than autoclaved aerated

concrete. The scope of the present study precluded a review of

this material and other lightweight aggregate concretes.

7
2. MANUFACTURE OF AUTOCLAVED AERATED CONCRETE

The manufacture of autoclaved aerated concrete products has been

described in the archival literature [1,19-24] and in manufacturers'

publications. Although the processes used by different manufacturers

are proprietary and have differences between them, the overall

procedures of each are considered comparable. The manufacture of

autoclaved aerated concrete products is carried out in factories

where quality control measures can be implemented regarding

materials and handling, as well as the individual steps involved

in the overall process.

Production is a continuous process which may be envisioned as

having four stages, as shown in Figure 2. The four stages are

depicted as: (1) raw materials, (2) molding and cutting, (3)

autoclaving, and (4) stocking and shipping.

Raw Materials . The major raw materials are, in many

processes, sand and Portland cement. In place of sand,

other siliceous materials such as sandstone, shale ash, fly

ash, and mixtures are often used. For example, fly ash has

been substituted for ground sand to the extent of 100 percent 4 . Lime or cement/ lime mixtures are at times used instead of

Portland cement. The raw materials are used as fine powders

which are normally obtained through grinding. Grinding may

be done either through a wet or dry procedure.

4 Poul Nerenst, H + H Industri, personal communication.

8
Accurately proportioned quantities of the raw materials form

the mix. In many cases, a slurry of the siliceous component

is made in heated water whose temperature is controlled.

The slurry is placed in a mixer into which the cement and/or

lime are added. Admixtures such as surfactants and compounds

for controlling the rate of cure may also be added.

Generation of the cellular structure is normally done

through the addition of aluminum powder as the mixing

procedure is ending. The aluminum powder in the alkaline

medium (due to the cement or lime) produces hydrogen gas

which, in turn, generates the cells. The addition of the

aluminum powder is controlled so that gassing does not begin

during mixing, but during the molding stage. For example,

one control technique uses aluminum powder surfaced with a

layer to delay the reaction with the alkalies. Other methods

for producing the cellular structure are the introduction of

a preformed foam into the mix, or the incorporation of air

by whipping [ 1 ]

.

Molding and Cutting . The mixture (from stage 1) is put into

steel molds where the production of hydrogen begins and the

mass expands, creating the aerated cellular structure. The

expanded mass is allowed to set to a soft cake under controlled

temperature and humidity conditions. The reaction time for

expansion and setting is 30 minutes to a few hours (about 1

9
to 4) depending upon the raw materials. After setting, the

cake is cut into the dimensions of the final product (e.g.,

block or panel units) using highly strung wires (piano

wires) or multiple circular saws.

If the final product is to be used for structural applications,

steel reinforcement is placed in the mold before casting the

autoclaved aerated concrete. The factory procedure allows

accurate placement of the reinforcement in the molds so that

the set cake may be cut into the intended dimensions without

damage. The steel reinforcement is protected from corrosion,

normally by coating [25]. For example, one process uses a

coating based on Portland cement, latex, and finely ground

sand. Another process uses a bituminous compound.

Autoclaving . The third stage of the process is the autoclaving

whereby the aerated concrete cures to its final strength and

dimensions. Autoclaving changes the physical-chemical

nature of the hydrated cement materials and produces a

material with lower drying shrinkage [26]. The conditions

for autoclaving vary somewhat for different proprietary

products. In general, the temperature is about 356°F

(180°C) at a pressure of about 10 - 12 atmospheres (1.0 -

1.2 MPa). The time varies depending on the size and density

of the products, and may range from about 10 to 20 hours.

Storage of the uncured cake, even through moist and warm,

10
for more than 2 or 3 days prior to autoclaving may be

detrimental [27],

Stocking and Shipping . After the cured units are removed

from the autoclave, quality assurance tests are normally

conducted. Also, if desired, additional shaping through

milling or related means may be carried out and a surface

finish may also be applied. After autoclaving and cooling

the units are ready for use without storage. If not needed

immediately after production, the units are stored at the

factory site and then delivered to the job site for

installation.

11
3. USES OF AUTOCLAVED AERATED CONCRETE

The factory-made autoclaved aerated concrete units delivered to

the job site are of different types and have many uses. The

types of units most commonly produced are blocks and precast

panels for floors, walls, and roofs. Less often, complete modular

room units are produced in the factory from wall, floor, and roof

panels. The blocks and panels are used in both loadbearing and

non-loadbearing constructions for both residential and

commercial/industrial buildings. As a consequence, autoclaved

aerated units are found in single and multi-story constructions

oriented both horizontally and vertically. Larger units are

reinforced with steel bars or welded mats to resist damage due to

superimposed loads and from transportation and handling [8]. It

also has been used as an extra layer of insulation for other

components of the building envelope [28].

Blocks and panel units are manufactured to very close dimensional

tolerances [29]. For example, the French Standard (NF P 14-306)

for autoclaved aerated concrete blocks for walls requires that

the dimensional tolerance of the length be ± 0.1 in. (± 3 mm) for

a minimum block length of 30 in. (760 mm). Close dimensional

tolerence is important with regard to modular construction to

assure that individual units fit together properly. Consequently,

the sizes of the factory-produced units are generally based on a

modular system of standard products. One reference points out

that the modular construction concept provides for simple and

12
fast construction, comparatively low costs, and good architectural

possibilities [30]. Any deviation from the standard sizes of

units could lead to extra costs. As an example of construction

efficiency, Nettleton [31] has indicated that a team of eight

persons erected, on a prepared foundation, all wall, floor, and

roof units for a block of 12 flats in 18 working days. The units

were basically 2 ft (0.6 m) wide and of varying lengths up to 16

ft (5m).

Construction time is shortened and building costs reduced by

increasing the size of panels [32]. The use of large panels

results in a reduction in the number of joints and less manual

labor is involved on site.

3 . 1 Blocks

A general application of autoclaved aerated concrete has been in

the form of masonry block. Blocks are available in standard

sizes and generally do not contain reinforcement [21]. They have

been widely used for side cladding and as "in filling" for

external walls, as well as for external and internal loadbearing

walls in low and multi-story construction. In early constructions,

blocks were laid using mortar joints. In later uses, construction

techniques expanded to include use of thin glue joints (0.08 -

0.1 in. or 2 - 3 mm) in place of mortar joints [33]. This technique

was possible because of the dimensional precision of the blocks.

13
Since autoclaved aerated concrete blocks are of relatively low

weight, they are often somewhat larger than concrete masonry

units manufactured in the United States. Due to the cutting

process during manufacture, the blocks may have an architectural

surface (often referred to as a wire cut or scratched surface)

[34]. This surface allows the use of the blocks without a

decorative rendering or parging. External surfaces of autoclaved

aerated concrete should be protected from exposure to water.

3 . 2 Panels

As previously mentioned, autoclaved aerated concrete panels are

manufactured in various sizes and are used in vertical and

horizontal applications in building construction. Vertical wall

panels are available in "story-height” increments, and have been

used in loadbearing applications up to three or four stories high

[1,10]. Wall panels are also laid up horizontally. These panels

normally do not carry any load other than their own dead weight.

Another common use of autoclaved aerated concrete is for roof and

floor slabs. These panels have span lengths of 20 - 25 ft (6 -

7.6 m) [10]. During erection, the slabs are commonly placed, direct

from a truck, on loadbearing walls or a structural frame without

use of a mortar bed. Bituminous roofing membranes have been

applied directly to assembled roof panels [1,34]. Regarding this

practice, it has been reported that the autoclaved aerated

concrete panels fit closely together and serve as thermal

14
insulation [34]. The design of the roof should consider movement

between adjacent panels to prevent splitting of the roofing

membrane [1,35]. In the conduct of the study, reports addressing

the performance of roofing membranes applied directly to autoclaved

aerated concrete panels were not found in the literature.

Autoclaved aerated concrete panels are also accepted in several

countries for basement walls below ground level [1]. For this

application, precautions must be taken to allow moisture present

during manufacturing to dry from the walls. One method involves

the application of impermeable protection on the outside of the

wall below ground level.

In concluding the remarks on autoclaved aerated concrete in panel

construction, it is pointed out that this practice in the 1960s

involved the use of large-sized panels. For example, in 1965,

Bave [36] described European panels having widths up to 5 ft

(1.5 m) and lengths up to 26 ft (8m). With thickness up to 10

in. (25 cm), large panels might weigh about 2 tons (1800 kg). In

Russia, where considerable autoclaved aerated concrete is used

for both industrial and residential buildings, the panels were

even larger, with spans up to 40 ft (12 m) [37]. However, as far

back as the late 60s, it was recognized that excessively large

panels could have a serious drawback, with cracking sometimes

occurring during manufacture or in-service [32]. Recently, in

1980, Bave [38] noted that the trend of using large panels, as

15
occurred of the sixties seems to be broken, at least in the

countries in Western Europe. The demand for large panels

decreased due to architectural preferences and cost considerations.

At present, the use of large panels is mainly in industrial

constructions

.

3 . 3 Limitations

Although autoclaved aerated concrete has been a long-accepted

construction material, the literature is not without reference to

limitations in its use. For example, used in modular units, any

deviation from standard sizes could lead to extra costs [30].

Some limitations have been reported by Tietz [39], including:

the material chips easily; the face tends to get damaged easily,

e.g., from scaffolding; dowels are required between units; the

external face is hygroscopic and needs a protective coating which

can breathe and should be protected from standing water, bottom

units should be protected from splash back; units absorb enormous

quantities of paint; anchorage of units needs careful planning to

prevent pullout, especially for dynamic loads; the face is weak

and any tough surface may pull away due to the interface pulling

off the parent material. Finally, it should be mentioned that

caution should be exercised in transporting and handling the

units to avoid damage [30,40].

16
3 . 4 Design Considerations

3.4.1 The European Experience

A manual entitled, "Autoclaved Aerated Concrete-Design and

Technology" was produced by the Euro-International Committee for

Concrete (CEB) [1]. As indicated in the Preface, the manual is "a

source of information on the technology of this material and

methods of construction associated with it, but it is not

directly referable to the Code," (i.e., the CEB-International

Federation for Prestressing (FIP) Model Code for Concrete). The

manual addresses the manufacture and properties of aerated

concrete, design considerations, and economic considerations.

Many typical construction details and application photographs are

presented.

The manual indicates that codes and practices in some countries

give advice on calculation methods for allowable stresses in

autoclaved aerated concrete structural members [1]. It also

advises to check the design principles for autoclaved aerated

concrete structural units with full-scale tests. Results of

testing of autoclaved aerated concrete under dynamic repeated

loads indicate that the fields of application of this material

must be chosen carefully [41].

Briesemann [42] in his paper concerning criteria for the design

of structural elements of autoclaved aerated concrete indicated

that knowledge relating to their design is not yet complete. He

17
stated that structural elements of this material have been used

in buildings for over 50 years and they have proved to be

satisfactory in practice. With regard to reinforced structural

elements, there is a lack of knowledge about the effects of loads

applied to only part of the surface of a component and about the

design for simultaneous loading by shear and torsional forces.

Briesemann stated further that known design rules have not been

verified by scientific experiments like those for reinforced

concrete structures and may allow for excessively large safety

coefficients. This applies, for example, to the coefficient of

creep (ratio of the creep deformation to the elastic deformation)

of masonry constructions of autoclaved aerated concrete blocks [42].

3.4.2 The U.S. Experience

As previously noted, autoclaved aerated concrete precast units

are not presently produced in the United States and Canada. The

American Concrete Institute (ACI) publication entitled, "Guide

for Low Density Precast concrete Floor, Roof, and Wall Units,"

outlines practices of design and fabrication of low density, 50

lbm/ft 3 (800 kg/m3

) and under, precast reinforced concrete floor,

roof, and wall units which will result in members of adequate

load capacity, durability, appearance, and over-all serviceability

for the function intended [43]. Both cellular and low density

aggregate precast concrete are included in the guide. Units

covered by the guide should be protected from the free entry of

water. The guide gives allowable design stresses for concrete

18
and reinforcement and provisions for anchorage of tensile steel

reinforcement. Other provisions are given such as corrosion

protection for reinforcement, permissible span deflections, and

test of individual flexural units. The compressive strength of

the low density concrete used in precast units should have a

minimum compressive strength of 300 Ibf/in^ (2.1 MPa). The

design allowable stresses in the concrete should conform to the

requirements of ACI 318 [44], except for shear stress permitted

in an unreinforced web and the compressive stress allowed in low

density precast concrete load bearing walls. Dunn [26] discussed

some provisions of the ACI guide [43] with regard to precast

autoclaved aerated concrete units and he also discussed some

physical properties of these units.

The Council of American Building Officials (CABO) and the B.S.

Department of Housing and Urban Development (HUD) have issued a

report [45] and a Materials Release [46], respectively, for the

use of a proprietary autoclaved aerated concrete. The CABO

report defines allowable uses for roof, floor, curtain wall, and

partition panels and blocks. The HUD Materials Release Is for a

thin-joint block wall system.

3.4.3 Moisture Considerations

Autoclaved aerated concrete contains moisture from the manufacturing

process. The moisture content at the time of delivery to the

building site may be 20 - 35 percent by weight (10 - 18 percent

19
by volume) [1]. The moisture dries out gradually in normally one

to two years and reaches equilibrium in external constructions of

about 4-6 percent by weight (2-3 percent by volume) [1].

Moisture may also accumulate in the material during construction

and after the building is completed (e.g., from rain and

condensation) . It is necessary to assure that the design provides

for drying out of the initial moisture as well as maintaining low

moisture content in the building under service conditions. One

literature reference indicated that, if the inside humidity is 60

percent or less, the use of autoclaved aerated concrete is

practically unlimited [22]. Higher humidity requires measures to

limit vapor diffusion. In general, autoclaved aerated concrete

in equilibrium with a normal environment (65 percent R.H. and

68°F or 20°C) tends to have a moisture content of about 3 percent

by volume [ 47 ]

As indicated in the ACI Guide 515.1R-79, concrete masonry walls

above grade should normally be painted on the exterior for

dampproofing purposes [48]. Because of the higher porosity and

moisture absorption of autoclaved aerated concrete, as compared

to dense concrete, a surface treatment should be considered for

exterior surfaces of autoclaved aerated concrete masonry and wall

panels. The external treatments that are currently used are

mortar for masonry walls and paints for wall panels [1]. The

paint used should be nearly impermeable to water but should have

some permeability to water vapor so that, if moisture should

20
become trapped or condensed within the wall, it can slowly escape

to the exterior. Protections for masonry have included one to

three coats of mortar (not exceeding 0.8 in. or 20 mm) for

external application, and for internal use, a thin application of

resin or gypsum plaster, lime mortar, or paint [1,49].

For decorative purposes, ceramic tiles, colored sand or granite,

marble chips, broken glass, and other materials can be used on

wall panels [37]. External walls may also have a prefinished

surface [ 50 ]

.

21
4. PROPERTIES OF AUTOCLAVED AERATED CONCRETE

The physical and mechanical properties of autoclaved aerated

concrete may be made to vary over a considerable range, depending

on the quantities of the raw materials used, the mix design, and

curing conditions [17,18]. It is important to note that many

properties of cellular concrete can be given as functions of bulk

density and thickness [24,51], This section of the report

presents a brief summary of the most often reported properties of

autoclaved aerated concrete. A summary table of property values

has not been provided because of their wide variation between

products due to factors such as density, structure, thickness,

moisture, raw materials, and production conditions. The properties

summarized herein are based essentially on the list given in the

CEB Manual [1]

o Capillarity

o Coefficient of Thermal Expansion

o Cracking

o Creep

o Density

o Deterioration Resistance

o Fire Resistance

o Frost Resistance

o Moisture Expansion and Shrinkage

o Moisture Migration

o Permeability

o Shaping and Working

22
o Specific Heat

o Solubility in Water

o Sound Absorption

o Strength (compressive, tensile, shear and bond, modulus

of elasticity)

o Structure

o Temperature Resistance

o Thermal Conductivity

4 . 1 Capillarity

Capillarity is important for wetting and drying out the materials.

The capillary suction is slow in aerated concrete compared with some

other porous materials such as clay bricks [1]. Capillary

suction takes place almost exclusively through the micropores in

the cell walls and is negligible for the macropores [1].

4 . 2 Coefficient of Thermal Expansion

The coefficient of linear thermal expansion for autoclaved

aerated concrete is 4.5 x 10“ 6 per °F (8 x 10“ 6 per °C) compared

to 6.8 x 10

- 6 per °F (12 x 10”^ per °C) for mild steel reinforcement [6,21,28]. Dense concrete and steel have about the same

value of coefficient of linear thermal expansion.

4 . 3 Cracking

One of the major concerns with autoclaved aerated concrete is

that of cracking, both externally and internally [28]. Cracking

23
may be caused by shrinkage, creep, temperature, moisture effects,

or by deformation and settlement of foundations. It is not

always possible to separate the causes and effects of cracking.

The repair of cracks in occupied houses and apartments can be

difficult and expensive. In a study by Cividini [52], it was

reported that prestress of autoclaved aerated concrete slabs, due

to differential drying and cooling after autoclaving, may in some

cases increase crack resistance to such an extent that the slab

cross-section may remain without cracks during its service life.

4 . 4 Creep

For all autoclaved materials, creep is low in relation to the

elastic deformation [18]. Factors affecting creep of autoclaved

aerated concrete are stress level, moisture content, ambient

temperature and relative humidity. A creep coefficient (creep

strain divided by the momentary deformation) is indicated as 0.8

to 1.2 for autoclaved aerated concrete having a density of 31 lbm/ft^

(500 kg/m^) [1]. According to Kruml [53], factors influencing the

quantitative value of creep have not been sufficiently defined

and evaluated. For this reason, he noted that the creep coefficient

often differs in the literature. Also, creep is indicated to be

proportional to the applied stress up to a stress of approximately

50 percent of the strength of the autoclaved aerated concrete [1].

In creep studies by Nielson [54] using unreinforced beams, he

reported creep coefficients, after one year for dry material at

24
43% R.H. , that ranged from 0.2 to 0.3. For water-saturated

material, the creep coefficients ranged from 0.2 to 0.6. These

values are somewhat less than those found for normal concrete.

In a study by Houst, Alou, and Wittmann [55], larger creep

deformations for moist specimens were reported than for dry ones.

4 . 5 Density

Density of autoclaved aerated concrete generally refers to ovendry material. At the time that the material is delivered to the

building site, the practical density is higher because of the

moisture content and presence of any reinforcement. The density

of autoclaved aerated concrete is normally reported in the range

of 19 - 62 lbm/ft 3 (300 - 1000 kg/m3 ), with 31 lbm/ft 3

(500 kg/m3

) or more being the most common density for loadbearing products and reinforced flexural members [1,56]. Bave

[38] reported in 1980 that a density of 22 lbm/ft 3 (350 kg/m3

) is

now used for structural units of autoclaved aerated concrete such

as loadbearing walls, floors, and roofs. The dry density of the

solid material (without pores or air cells) is 150 lbm/ft 3 (2500

kg/m3

)

.

4 . 6 Deterioration Resistance

Autoclaved aerated concrete, because of its porosity and relatively

low alkalinity (pH = 9 - 10.5), does not provide the same corrosion

protection of reinforcement as that provided by dense concrete [1],

25
Thus, it is recommended that reinforcement should be protected

against corrosion by adequate surface treatment [1], In areas of

relative humidity less than 50 percent at equilibrium moisture

content, corrosion of reinforcement is reported likely to be

negligible [ 1 ]

There also may be a risk of corrosion of nails and steel anchors

because of the initial moisture content of the material, or

moisture from condensation or rain penetration [1], Acids in the

form of liquid and gases penetrate aerated concrete more easily

than dense concrete and their deteriorating effect is more rapid

than for dense concrete. Autoclaved aerated concrete should

therefore be protected from acidic liquids or fumes. The porous

structure of autoclaved aerated concrete allows air to penetrate

the material readily and, depending on the availability of carbon

dioxide and atmospheric humidity, a decrease in alkalinity of the

material occurs [25]. During autoclave treatment in high

pressure steam, a reaction takes place between the calcium

hydroxide and finely ground silicate aggregate components to form

calcium silicate hydrates. Because of this reaction, the lime

availability is greatly reduced and the calcium hydroxide content

of the concrete is lowered. Thus, it was found that 10 in.

(24 cm) thick autoclaved aerated concrete members had been carbonated

throughout after only two years of service [25]. The CEB Manual

[1] indicates that carbonation of autoclaved aerated concrete is

generally not of concern if the carbon dioxide content of the air

26
is as normally expected (0.03 percent). For carbon dioxide

concentrations above about 0.2 percent, carbonation of the

calcium silicate hydrate of fresh, moist autoclaved aerated

concrete can proceed rapidly and may lead to shrinkage cracking

[1]. Manufacturer's recommendations should be followed if

autoclaved aerated concrete is to be exposed to carbon dioxide

concentration above about 0.2 percent.

One literature reference has noted that autoclaved aerated

concrete is resistant to insects, termites, and fungi [21].

4 . 7 Fire Resistance

Autoclaved aerated concrete is non-combustible [1]. Because of

its low thermal conductivity, heat migration takes place at a

much lower rate than in dense concrete. Consequently, the fire

ratings for autoclaved aerated concrete units issued in some

countries in Europe indicate that this material provides for very

good fire resistance compared to other building materials [1,28].

One reference [21] indicated that, for the same thickness, the

use of cellular concrete in place of normal concrete has been

reported to increase the fire resistance by 30 to 40 percent. In

comparison, another reference [8] stated that a 4.0 in. (10 cm) thick

aerated concrete slab gives fire resistance of 2 hours against

one hour for an 4.4 in. (11 cm) thick brick wall or a 4.0 in.

(10 cm) thick normal concrete slab. As another example of the

fire rating of autoclaved aerated concrete, the Fire Offices

27
Committee in the U.K. recognizes that a 6.0 in. (15 cm) thick

loadbearing fire break of the material will give six hours fire

resistance [3]. The fire rating of loadbearing roof and floor

flexural units depends upon density and the thickness of the

cover of the reinforcement. Autoclaved aerated concrete may be

used as a cladding to protect other materials such as steel

members or to increase the fire rating of normal concrete walls

[ 1 ].

In the early 1960s, the U.S. National Bureau of Standards

conducted fire tests of lightweight, cellular precast concrete

(reportedly of the in situ gas-forming type) panels for floors

and roofs [57]. It was found that fire endurances were up to 2

hours for 6 in. (15 cm) thick slabs tested under load, and up to

4 hours for other slabs not loaded. It was also found that,

within the narrow density range of 35 - 50 lbm/ft^ (560 - 800

kg/m^), slabs made of higher density cellular concrete provided

longer fire endurance than those of lower density, both in terms

of heat transfer through the slab and in terms of continued

structural stability. The use of greater cover for the reinforcing

bars, in the range of 3/4 to 1-1/4 in. (19 to 32 mm), even

without increasing the total thickness of the cellular concrete,

will result in longer fire endurance.

28
4.8 Frost Resistance

The extensive use of aerated concrete in countries with cold

climates indicates that the material has resistance to frost.

Nevertheless, a risk of frost damage exists if the moisture

content in any part of the construction unit exceeds the critical

moisture content of the autoclaved aerated concrete. The

critical moisture content can be considered as the moisture

content of the material at which, upon freezing and thawing, a

loss in strength properties or damage occurs. The critical

moisture is, for example, about 40 percent by volume for aerated

concrete manufactured in Sweden with a density of 31 lbm/ft^

(500 kg/m^) [1]. However, the moisture content is usually

highest during construction, rarely exceeding 15 - 20 percent by

volume, before drying out takes place [1]. Under extreme

conditions, higher moisture contents may occur close to the

surface of the unit which increases the risk of frost damage.

Edlind [28] reported that heavier autoclaved aerated concretes

are satisfactory with regard to frost resistance but at densities

below about 31 lbm/ft^ (500 kg/rtv^ ) liability to frost damage

increases

.

From tests of 30 cycles of freezing for 5 hours and thawing for 3

hours, Purins [58] concluded that the critical moisture content

was between 50 - 70 percent by volume and is dependent on the bulk

density. Critical moisture content was 50 percent by volume for

density of 25 lbm/ft^ (400 kg/m^ ) and 65 - 70 percent by volume for

29
density of 40 lbm/ft^ (650 kg/m^). Tensile strength after the

tests decreased considerably, from 60 - 70 lbf/in^ (0.41 - 0.48 MPa)

to 4 - 20 lbf/in^ (0.03 - 0.14 MPa). This decrease was attributed

to internal damage occurring during the test, although reportably

not visible from the outside [58].

In a similar study, Roulet [59] reported on the resistance to

frost action of three different densities of autoclaved aerated

concrete. The densities were 19, 25, and 31 lbm/ft^ (300, 400,

and 500 kg/m^ )

. The frost expansion upon cooling to -4°F (-20°C)

and their length during thawing was measured. With dry samples,

there was linear thermal expansion. For samples with low water

content (degree of saturation of 20 - 40 percent by weight), they

exhibited contraction on freezing, and after thawing no significant

permanent expansion was measurable. Specimens having a critical

water content (60 - 70 percent by weight), which is low compared

to the total porosity or the zero pressure capillary water

content, exhibited a large expansion while freezing to about -4°F

(-20°C). After thawing, the sample with critical water content

showed a permanent expansion. If the samples were nearly

saturated with water, they became very brittle after one freezethaw cycle. Roulet [59] also reported that samples with a high

degree of saturation upon freezing became brittle and cracked.

However, scanning electron micrographs gave no evidence of a

structure change due to freezing.

30
In a related study, tests carried out on factors such as loss of

weight on freezing, tensile strength after freezing, and changes

in length and volume after repeated cycles indicated that the

critical degree of saturation was about 35-40 percent of pore

saturation [60]. Pore saturation denotes that the total pore

volume is filled with moisture. The dynamic modulus of elasticity

was determined for each cycle for samples in the thawed condition.

Damage occurred at a degree of pore saturation of 38 - 45 percent

[60]. However, a measure of the critical degree of moisture was

also obtained by studying the moisture-dependent deformation of

samples. It varied to some extent due to the quality of the

material and seemed to be in the range of 50 - 60 percent [60].

4 .

9

Moisture Expansion and Shrinkage

Autoclaved aerated concrete is similar to dense concrete in that

it expands on wetting and shrinks on drying [28]. Drying shrinkage

varies with density and method of manufacture. Values of drying

shrinkage range from 0.1 to 0.5 percent when measured from

saturated condition to a condition of equilibrium at 45 percent R.H.

[28]. The low drying shrinkage of an autoclaved product as

compared to that for air-cured cellular concrete, has been

attributed to chemical reactions that take place during autoclaving

[ 21 ].

The test results for shrinkage may vary considerably with

different materials and methods of tests. In practice, the

31
shrinkage between delivery moisture content and equilibrium

moisture content is of interest [1]. Excessive drying below

equilibrium at 43 percent relative humidity results in an

increase in shrinkage of the material. An example of shrinkage

was given for materials where the moisture content was reduced

from about 75 percent to 3 percent by mass. The materials were

tested from water-saturated condition to equilibrium at 43

percent R.H. and 68°F (20°C). The shrinkage for this case was

0.03 percent [1]. The density of the materials was not reported.

4.10 Moisture Migration

Autoclaved aerated concrete, due to its higher porosity, has higher

moisture absorption than dense concrete. Moisture may migrate

through capillarity, or through diffusion or both. For normal

moisture content conditions, the moisture migration is mainly by

diffusion and with increasing moisture content, it is due more to

capillarity [1]. Moisture migration is due almost entirely to

capillarity when the moisture content is above 40 percent by mass

(20 percent by volume). The moisture migration is also influenced

by the pore structure, the dimensions of the unit, the thermal

conductivity of the material, temperature, vapor pressure, and

movements of the air to which the surfaces of the construction

units are exposed.

32
4.11 Permeability

The permeability of autoclaved aerated concrete to air varies

with the moisture content of the material. It decreases with an

increase in moisture content in the pores [6]. Nevertheless, the

CEB Manual [1] has indicated that the permeability of dry

material at normal pressure differences, such as caused by wind,

is very low and may be considered negligible. It is noted that

significant penetration can take place through improperly formed

joints and other connections in the building.

The water vapor permeability by diffusion also decreases with an

increase in the moisture content of the material. The diffusion

factor of aerated concrete (ratio between the diffusion of water

vapor through a layer of air to that through a layer of material

of the same thickness) has been determined to be 5 - 7 under air

dry conditions for material having a density of 31 - 38 lbm/ft^

(500 - 600 kg/m3

) [1]

.

4.12 Shaping and Working

During construction, autoclaved aerated concrete can be easily

cut, nailed, drilled, milled, or shaped to accomodate building

needs such as utilities, or other construction requirements

[1,28,61]. In most cases, ordinary woodworking tools can be

used. Reinforced flexural members should not be cut on the

construction site without authorization of the manufacture since

anchorage of reinforcement may be affected [1].

33
Attachment of other buildings units, materials, or accessories to

autoclaved aerated concrete is more critical than those in denser

materials such as brick or normal concrete, since ultimate

failure loads (pullout) are lower [62].

4.13 Specific Heat

Specific heat of a material is the heat required to raise the

temperature of unit mass of the material one degree. It is a

measure of the capacity of a material to store heat. For

autoclaved aerated concrete the value of specific heat is about

0.24 - 0.26 Btu/lbm°F (1.0 - 1.1 kJ/kg°C) for material of normal

moisture content (2-3 percent by volume or 4 - 6 percent by weight)

[ 1 ].

4.14 Solubility in Water

The strength-related components of autoclaved aerated concrete

have only very low solubilities in water. However, depending on

the raw material and mix proportions the product may contain

small quantities of soluble salts which, under certain climatic

conditions such as those resulting in slow evaporation, may

crystallize on the surface to form efflorescence [1].

4.15 Sound Absorption

Autoclaved aerated concrete is considered to provide good sound

insulation, having sound absorption properties somewhat better

than that for smooth dense concrete [1,21,28]. The sound

34
absorption coefficient of untreated autoclaved aerated concrete

was reported to be 0.00, 0.15, 0.25, 0.20, 0.20, and 0.20 for

frequencies (H z ) of 125, 250, 500, 1000, 2000, and 4000 respectively

[1]. If the aerated concrete is painted or coated, its sound

insulation effectiveness is reduced. In general, for solid

construction, sound transmission depends on the mass of the

structure. Cavity wall construction gives greater sound reduction

than solid construction for the same total mass [20].

4 . 16 Strength

Considerable data have been generated regarding the strength of

autoclaved aerated concrete including compression, tension, shear

and bond strength, and modulus of elasticity. Strength tends to

increase with an increase in density, while thermal resistance

tends to increase with a decrease in density [28]. According to

Shrivastava [21], there is a straight line relationship between

strength and density. Grimer and Brewer [63] tested four aerated

concretes of varying density, 33 - 43 lbm/ft^ (530 - 690 kg/m^ )

,

and found that there was a slight systematic increase in strength

with depth. The strength increases were accompanied by increase

in specific gravity. The moisture content of autoclaved aerated

concrete also has an influence on its strength. Svanholm [64]

reported that strength decreases with an increase in moisture

content. This was found for compressive and tensile strength and

modulus of elasticity.

35
4.16.1 Compressive Strength

The compressive strength of autoclaved aerated concrete increases

with an increase in density [1,8]. The average value of compressive

strength measured using cubes for a dry density of 25 lbm/ft 3

(400 kg/m^ ) was reported to be about 290 lbf/in^ (2 MPa), and for

a dry density of 44 lbm/ft^ (700 kg/m^ )

,

it was reported to be

about 870 lbf/in^ (6 MPa) [1]. Another report showed that for a

density range of 31 - 44 lbm/ft^ (500 - 700 kg/m^ ) , the compressive

strength was 290 - 580 lbf/in^ (2-4 MPa) [8]. The relationship

between the cube compressive strength and the density of autoclaved

aerated concrete is shown in Figure 3 [1].

Examples of values of compressive strength and modulus of

elasticity for three densities of dry autoclaved aerated concrete

(Table 2) were given by Edlind [28]. He noted that the compressive

strength depends on test conditions such as test specimen size,

its planeness or type of capping material, and its moisture

content. The compressive strength decreases with an increase in

moisture content of the material. The relationship between the

compressive strength and the moisture content of autoclaved

aerated concrete is shown in Figure 4 [1]. With moisture

contents of 5 and 10 percent, the compressive strength is reduced

about 20 and 25 percent, respectively. In another study by

Dworjadkin and Malinowski [65], it was reported that the compressive

strength of an autoclaved aerated concrete with maximum water

36
saturation (40 - 42 percent by volume) was 40 - 50 percent less

than that of the air-dried material.

Compressive strength has, in at least one case, been found to

decrease with time of exposure. For example, block samples

exposed outdoors up to one year were tested by Grimer and Brewer

[66] who found changes in strength (as indicated by compressive

strength, as well as splitting strength, and modulus of rupture)

with time. The decrease ranged from 10 - 15 percent for these

properties

.

4.16.2 Tensile Strength

The tensile strength of autoclaved aerated concrete is normally

1/4 to 1/6 of the compressive strength [1]. Edlind [28] reported

the tensile strength to be about 20 percent of the compressive

strength. A moisture gradient within the test specimen has a

large effect on the tensile strength test result. The measurement

of this property is even more sensitive to test conditions than the

measurement of the compressive strength.

The tensile strength in bending is normally somewhat higher than

when measured in direct tension. A moisture gradient in the

specimen also affects this property.

Grimer and Brewer [66] reported that the modulus of rupture of

specimens uncovered and stored outdoors changed considerably (up

37
to 50 percent decrease) from one month to one year. For specimens

covered and also exposed outdoors, no significant change in

strength occurred as compared to specimens stored indoors.

The modulus of rupture for a density range of 31 - 47 lbm/ft^ (500

- 750 kg/m^) was about 100 - 190 lbf/in^ (0.7 - 1.3 MPa) [8].

Cracking in reinforced flexural members was observed to occur

between 1.5 and 2.0 times the working load, under which condition

the tensile stress can approach 50 percent of the compressive

strength of the material.

4.16.3 Shear and Bond Strength

The shear rupture strength, based on pure punching shear, can be

assumed to be 25 - 30 percent of the compressive strength [1]. The

value for pure shear can be taken as that for direct tension, if

information about shear strength is not available. The shear

strength was also reported to be about 1/8 of the compressive

strength and to range from 36 - 72 lbf/in^ (0.25 - 0.5 MPa) [8].

In tests reported by Regan [67], the nominal shear stresses in

reinforced autoclaved aerated concrete beams at the cracking

stage were in the range 51 - 58 lbf/in^ (0.35 - 0.4 MPa). This

corresponds to the upper limit of values obtained from unreinforced

(shear) slabs. The test results suggest that once shear cracking

occurs in an autoclaved aerated concrete member, the truss action

of the stirrups has to support the full shear with practically no

38
separate contribution from the concrete. The increases in

strength produced by vertical and transverse stirrups were

modest, about 17 percent of the strength of comparable unreinforced

beams. However, the use of inclined stirrups was much more

effective and increased the ultimate load by almost 50 percent [67].

Based on a statistical evaluation of previously published test

results, Briesemann [68] developed semi-empirical methods of

calculating the ultimate shearing strength of autoclaved aerated

concrete slabs without web reinforcement, as well as the loads

resulting in diagonal cracks and shear failure in beams with

shear reinforcement. This design method was reported as being

useful for components of autoclaved aerated concrete because the

test specimens and the components used in practice were identical.

The best agreement between the calculated methods and test

results was achieved with a linear equation for slabs without

shear reinforcement and with a quadratic equation for beams

without shear reinforcement.

The bond of reinforcement in autoclaved aerated concrete is

normally a fairly plastic phenomenon with the peak stress

attained at a slip of about 0.0004 in. (0.01 mm) and maintained

for movements up to about 0.008 in. (0.2 mm) [67]. Because of

the relatively low bond strength, the reinforcement in autoclaved

aerated concrete members often relies on welded cross-bars or

stirrups which transfer load by bearing [8,26,43,67].

39
Regan [67] suggests from pullout tests a limiting value of bond

stress for coated plain (smooth round) bars of 180 lbf/in2 (1.25

MPa) based on the perimeter of the bar not including the coating.

The bond strength of plain bars embedded in autoclaved aerated

concrete may differ depending on the type of protective coating

used. Other pullout tests [26] on small diameter plain bars

coated with a cement-latex coating embedded in autoclaved aerated

concrete having a compressive strength of 500 lbf/in (3.4 MPa)

yielded bond stresses of 150 - 200 lbf/in2 (1.0 - 1.4 MPa) at a

slip of 0.001 in. (0.03 mm) at the free end. Dunn [26] stated

that these values are generally greater than those calculated at

design load, and actual load tests on precast units confirm that

while the bond is adequate for design loads, slip can occur at

about 1.25 times design load. Because of this, Dunn [26] further

states that welded anchor bars or cross-bars as required in the

AC I Guide for Low Density Precast Floor, Roof, and Wall Units

[43] are needed to provide an adequate factor of safety.

4.16.4 Modulus of Elasticity

The modulus of elasticity is about one tenth that of dense

concrete [24]. It has been given as a function of the density

and compressive strength of the material. As an example, the

modulus of elasticity for autoclaved aerated concrete having a

density range of 31 - 44 lbm/ft 3 (500 - 700 kg/m3

) was reported

to be 200 - 400 x 10 3 lbf/in2 (1.4 - 2.8 x 10 3 MPa) [8]. The

moisture content also affects the modulus value. The modulus

40
decreases with an increase in moisture content, similar to the

way it affects the compressive strength. Moisture effects can be

taken into account by using the compressive strength value for

the wet material according to the formula below. The formula has

been reported to correspond well with test values of some autoclaved

aerated concrete produced in Sweden having a density range of 25

- 44 lbm/ft 3 (400-700 kg/m3

) and a range in moisture contents of

3-10 percent by volume [1].

E0 = kY^ry

where

E0

is the tangent modulus, MPa

Ydry is the dry density, kg/m3

f c is the compressive strength, MPa, for wet material

k is an empirical constant, the value reported to vary

between 1 . 5 and 2.0 [ 1

]

It was noted [1] that this formula can be used by the designer to

estimate elastic deformations of unreinforced material even

though values obtained from this formula may differ from test

results for other autoclaved aerated concrete products.

4.17 Structure

The structure of autoclaved aerated concrete is characterized by

pores or cells which are formed by the hydrogen gas, air, and

water during the molding and expanding phases of production [1].

41
The micro-cellular structure of autoclaved aerated concrete is

shown in Figure 5

Scanning electron microscopy (SEM) micrographs of the cell

structure of the hardened material, which is generally considered

to be uniform, have been reported [1,58,59,69,70], The pore

structure is important with regard to physical properties of the

material such as strength, thermal conductivity, capillarity, and

frost resistance. Depending on the pore size and their physical

characteristics, they can be divided into micropores and macropores.

An example of the distribution of pore sizes in autoclaved

aerated concrete is given in Figure 6 [1], Larsson and Purins

[60] reported pore size for various kinds of autoclaved aerated

concrete investigated in freezing tests. The macropores were

determined to have radii of between 64,000 and 550,000 A and

micropores to have radii of between 40 and 64,000 A. They also

reported that the pore size distribution was different for the

various autoclaved aerated concretes investigated. Micropores

may be considered to be capillary active, and the distribution of

cell sizes within the micropore range is important with regard to

properties such as freezing and moisture migration [1]. The

macropores are usually almost spherical in shape; however, for

cases where they are more oval in shape, it may be assumed that

the physical characteristics of the autoclaved aerated concrete

may differ somewhat in the directions of the major and minor axes

42
[1], If the cells are irregularly shaped rather than being

spherical, excessive reduction in strength occurs [21].

In a study by Schwiete and Ludwig [71], the porosities of some

autoclaved aerated concrete specimens were found to be about 70 -

80 percent. They considered the concept of effective porosity to

assess the micropore content of the specimens. They reported

that the effective porosities ranged from 4 - 13 percent. Moisture

contents of the specimens were about 2 percent by volume and the

bulk densities were 44 lbm/in^ (700 kg/m^).

Purins [58] conducted an examination of pore structure and he

reported no entirely closed pores in autoclaved aerated concrete.

His evidence was electron micrographs of the material with

magnifications of up to 26,000. He observed that the structure

was capillary, both at the bottom and in the walls of the pores,

and concluded that no closed pores existed in the material.

4.18 Temperature Resistance

Shrinkage cracking will result if autoclaved aerated concrete is

exposed to dry air at a high temperature for a long time, and

excessive drying eventually takes place. For this reason, structural

aerated concrete should not be used in constructions that are

continuously exposed to dry air at a temperature above 122 -

158°F (50 - 70°C) without protection against excessive drying [1].

For non-structural applications, aerated concrete can be exposed

43
to much higher temperatures, up to 1300°F (700°C), if appropriate

precautions are taken against excessive drying. The CEB Manual

[1] indicates that, for such high temperature use, manufacturers'

design considerations must be strictly followed. The melting

point of aerated concrete is 1800 - 2200°F (1000 - 1200°C),

depending on the materials used [1],

4.19 Thermal Conductivity

The relatively low thermal conductivity of any cellular concrete

is due to air-filled pores [26]. The thermal conductivity, k

value, of aerated concrete depends primarily on its density.

Other factors such as moisture content, temperature level, pore

structure, and raw materials also effect the thermal conductivity

[1]. The relationship between the thermal conductivity and the

density of dry specimens of autoclaved aerated concrete is given

in Figure 7 [1]. Based on dry density, the average value of

thermal conductivity for a density of 25 lbm/ft^ (400 kg/m^ ) was

reported to be about 0.55 Btu* in. /f

2 *h* °F (0.08 W/mK)

,

and for a

density of 50 lbm/ft^ (800 kg/m^ )

,

it was reported to be about 1.5

Btu* in. /f t^ «h* °F (0.21 W/mK) [1,10]. It was reported by Leitch

[3] that k-values associated with autoclaved aerated concrete, as

with unautoclaved materials, go as low 0.55 Btu* in. /f t^ -h* °F

(0.08 W/mK)

As expected, thermal conductivity increases with an increase in

moisture content [1,10,64,72,73]. Data in the CEB Manual [1] indicate

44
that this relationship is linear for moisture contents up to 20

percent by weight. As an illustration of the relationship, the

average value of thermal conductivity for a material with a dry

density of 31 lbm/ft^ (500 kg/m^) was shown to be about 0.76 Btu*in.

/ft^-h-°F (0.11 W/mK) at zero moisture content, whereas it was

shown to be 1.1 Btu* in. /f t^ *h* °F (0.15 W/mK) at 10 percent

moisture content by weight [1]. It was interesting to note, that

moisture factors (ratio of thermal conductivity of moist material

to that of dry material), determined experimentally by Loudon

[72], correlated better with moisture content by weight than

moisture content by volume.

For lightweight concretes in general, Valore [74] suggested a

single moisture factor that consists of a 6 percent increase in

thermal conductivity for each one percent of moisture, by weight,

relative to oven dry density. A constant moisture content of 4

percent, by volume, was proposed for lightweight concrete walls having

a density of 25 lbm/ft^ (400 kg/m^ ) , protected from rain penetration.

45
5 . ENERGY CONSIDERATIONS

As stated in the introduction to this report, a major advantage

influencing the use of autoclaved aerated concrete products has

been the relatively low thermal conductivity combined with loadcarrying capacity. This factor has been noted by numerous

authors who have reported on the properties and performance of

autoclaved aerated concretes [e.g., 1,5-7,10,19,21,24,28,38].

Bave [6,10,38] has summarized the development and growth in use

of autoclaved aerated concrete, particularly noting the factors

affecting energy conservation. The use of this material originated

in northern European countries where thermal insulation has been

a long time concern due to cold climates and a dependency on

imported fuel. Subsequently, its use expanded to warm climates

where its low thermal conductivity offered advantages for energy

savings in cooling buildings. Bave [6] has indicated that

autoclaved aerated concrete, in many geographic locations,

satisfies both thermal and loadbearing requirements while

offering an overall energy saving. He did not indicate the

details of these requirements.

In addition to energy considerations associated with use of a low

thermal conductivity material, Bave [6,10] also pointed out

qualitatively other factors regarding energy savings in the use

of autoclaved aerated concrete. These factors included raw

materials, the production process, and transportation and

46
erection of the manufactured units. As an example, he stated

that the relatively low weight of aerated concrete building units

allows cuts in the energy consumption due to handling and

erection on site.

Kohler [7] has presented the results of a study on the energy

costs for the production and use of autoclaved aerated concrete

taking into account direct and indirect energy costs. He

compared the energy costs for production of different types of

walls having equivalent thermal and loadbearing properties.

Among his findings, he reported that the separation of the thermal

and loadbearing functions (i.e., composite wall with thermal

insulation and structural element) leads generally to lower

overall energy costs. For example, he found that the energy

costs for a composite autoclaved aerated concrete wall with

expanded polystyrene insulation were about 8 percent less than

those for the equivalent wall comprised only of autoclaved aerated

concrete

.

Kohler [7] also modeled the energy performance of complete

buildings whereby the use of autoclaved aerated concrete floors,

walls, and ceilings was compared to the same elements of conventional

materials. A significant finding was that, in the case of

energy-efficient houses, "it was not possible to reach the

insulation requirements by aerated concrete only even by using

very large sections". He indicated that, in these cases,

47
autoclaved aerated concrete has to be used together with a

material which is specifically a thermal insulation.

In this regard, Kohler’s findings may be related to current

practice (as recommended by ASHRAE) in the U.S. for opaque walls

and roofs of new buildings except low-rise residential. For

relatively severe climatic locations having 6500 heating degree

days or more, the minimum total thermal resistance (R-value) of

walls is about 20; whereas for roofs, it is about 30 [75]. An

autoclaved aerated concrete wall, 8 in. (20 cm) thick and having

a density of 31 lbm/ft^ (500 kg/m^ ) may have an R-value about 11

[28]. A 6 in. (15 cm) roof slab of similar density material

would have an R-value of about 8. Obviously, for both the wall

and the roof, additional thermal insulation would be needed to

attain an R-value of 20 to 30. In contrast, in areas of the U.S.

having mild climates, autoclaved aerated concrete alone may

satisfy insulation needs.

Recently Frey and Briesemann [5] studied the factors affecting

the primary energy costs to produce autoclaved aerated concrete.

They found that, for the plants studied, raw materials production,

raw materials transportation, and actual factory production

accounted for 60 percent, 5 percent, and 35 percent of the energy

costs, respectively. They indicated that the process is currently

very energy efficient, and consequently, substantial energy

savings cannot be expected in the future. Where reductions are

48
accomplished, they will be associated with improvements in

cementitious binder technology.

49
6 . STANDARDS AND RELATED DOCUMENTS

Since autoclaved aerated concrete has been used extensively

worldwide, standards and related documents have been developed in

many countries. With the exception of the previously mentioned

CABO [45] and HUD [46] documents, no standards and codes have been

developed in the U.S. Recently, RILEM^ Technical Committee 7

8 -MCA

on Model Code for Aerated Concrete Based on RILEM Testing Methods

[76] has compiled, as a working document, a list of "Standards

and Regulations Concerning Building and Buildings Constructed

with Autoclaved Aerated Concrete." The titles of these documents,

as listed by the RILEM committee, are given in Appendix A. A

summary of the contents of these documents is beyond the scope of

this review. The contents vary among standards, and some treat a

broader range of requirements and use conditions than others. In

general, the standards provide: general information on the

autoclaved aerated concrete product, minimum requirements for

material properties, requirements for the dimensions of specific

products (e.g., blocks), test methods, requirements for quality

control of the production process, and marking.

^RILEM is a French acronym for International Union of

Testing and Research Laboratories for Materials and Structures.

50
7 . SUMMARY

This report is a review of the properties and performance of

autoclaved aerated concrete. This is a construction material

that has gained widespread use in many areas of the world.

Little use has occurred in Canada and the United States. It is a

lightweight porous concrete whose cellular structure is generally

obtained through an in situ gas-producing reaction. Subsequent

autoclaving in the factory provides strength, dimensional

stability, and other properties of the finished product. These

materials combine a relatively low thermal conductivity with a

bearing capacity suitable for use in structural applications.

This combination of properties might make them attractive for

energy-conserving applications. For this reason, the review was

conducted at the request of the U.S. Department of Energy.

The review addressed an overview of the manufacturing process,

uses of autoclaved aerated concrete in building constructions,

properties, energy considerations, the availability of code-related

documents, and standards. The literature on these materials is

extensive. Uses include block and panel construction in both

loadbearing and non- loadbearing applications for walls, floors,

and roofs. With regard to energy considerations, the review

pointed out that, for energy efficient applications under severe

climate conditions, the autoclaved aerated concrete unit alone

may be insufficient to provide the targeted minimum value of

thermal resistance. In these cases, autoclaved aerated concrete

51
has to be used together with additional thermal insulation. In

contrast, in areas of the U.S. having mild climates, autoclaved

aerated concrete alone may satisfy insulation needs.

52
8 . ACKNOWLEDGMENTS

This study was sponsored by the U.S. Department of Energy, Office

of Conservation and Renewable Energy. The authors appreciate the

support and assistance of William Gerken (DOE) and David McElroy

( ORNL ) who both provided liaison between DOE and NBS . The

authors also thank their NBS colleagues, James Pielert, James

Clifton, Geoffrey Frohnsdorff, and Daniel Gross who gave important

review comments on this report. The authors acknowledge the

excellent support of Denise Herbert for typing the manuscript.

Finally, special thanks are expressed to Samuel Aroni, University

of California at Los Angeles, Poul Nerenst, H + H Industri,

Denmark, and Gunnar Bave, Siporex, Sweden, who visited NBS to

discuss their experiences regarding the properties and performance

of autoclaved aerated concretes. Their valuable review comments

on this report are appreciated.

53
9. REFERENCES

1. "Autoclaved Aerated Concrete -- CEB Manual of Design and

Technology," Comite Euro-International du Beton (CEB), The

Construction Press, New York (1978), 90 pages.

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3. Leitch, F.N., "The Properties of Aerated Concrete in

Service" Proceedings of the Second International Congress

on Lightweight Concrete, the Concrete Society, London (14-15

April 1980) pp. 97-113.

4. Bessey, G.E., "The History and Present Day Development of

the Autoclaved Calcium Silicate Building Products Industries,"

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Building Products, University of London, Society of the

Chemical Industry (18-21 May 1965), pp. 3-6.

5. Frey, E. and Briesemann, D. , "More Recent Calculations of

the Primary Energy Costs of Autoclaved Aerated Concrete,"

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6. Bave, G. , "Climatic Conditions and Energy," in "Autoclaved

Aerated Concrete, Moisture and Properties," F.H. Wittmann,

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7. Kohler, N. , "Global Energetic Budget of Aerated Concrete," in

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Wittmann, Ed., Elsevier, Amsterdam (1983), pp. 13-26.

8. "Autoclaved Aerated Concrete," Building Digest 86, Central

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in Load-Bearing Structures for Residential and Public

Buildings in the U.S.S.R.," Proceedings, 1st International

Congress on Lightweight Concrete, Vol. 2, Cement and

Concrete Association, London (May 1968), p. 179.

10. Bave, G. , "Aerated Concrete - How it Conserves Energy,"

Housing Science, Vol. 5, No. 1 (1981), pp. 73-81.

11. Bessey, G.E. , "Author's Introduction," Proceedings, 1st

International Congress on Lightweight Concrete, Vol. 2,

Cement and Concrete Association, London (May 1968), p. 161.

12. Bessey, G.E., "The World Development and Economic

Significance of the Aerated Concrete Industry," Proceedings,

1st International Congress on Lightweight Concrete, Vol. 1,

54
Cement and Concrete Association, London (May 1968),

pp. 203-212.

13. Valore, Rudolph C., Jr., "Cellular Concretes, Part 1,

Composition and Methods of Preparation," ACI Journal Vol. 25

(May 1954) , pp. 773-796.

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15. Houst, Y. and Wittmann, F.H., "Bibliography on Autoclaved

Aerated Concrete," in "Autoclaved Aerated Concrete, Moisture

and Properties," F.H. Wittmann, Ed., Elsevier, Amsterdam

(1983), pp. 325-369.

16. "Autoclaved Aerated Concrete, Moisture and Properties,"

Wittmann, F.H., Developments in Civil Engineering, 6,

Elsevier, Amsterdam (1983), 380 pages.

17. Valore, Rudolph C., Jr., "Cellular Concretes, Part 2,

Physical Properties," ACI Journal, Vol. 25 (June 1954),

pp. 817-836.

18. Urmston, W.B. and Bessey, G.E., "Structural Lightweight

Concrete Using Lightweight Aggregates with Aeration,"

Chemistry and Industry (7 April 1973), pp. 300- 307.

19. Merlet, Jean-Daniel, "Cellular Autoclaved Concrete," CSTB

Magazine, No. 23 ( January-February 1984), p. 2.

20. Ironman, Ralph, "Cellular Concrete Gains Worldwide Strength,"

Concrete Products (July 1977).

21. Shrivastava, O.P., "Lightweight Aerated or Cellular Concrete

- A Review," Indian Concrete Journal (January 1977), pp. 18-

23.

22. Kritov, V.A., Skatynsky, V.I., Chikota, E.I., and Udachkin,

I.B., "Building Enclosing Structures Made of Energy Saving

Autoclave Concrete," Proceedings, CIB 83 (1983), pp. 145-

152.

23. Bennett, R.P., Furgeaud, R. and Paljak, I., "Le Beton

Cellulaire Autoclave: Proprietes et Utilisations," Annales

de L' Institute Technique du Batiment et des Travaux Public,

No. 376 (October 1979), pp. 74-102.

24. "Autoclaved Aerated Concrete," Building Research Establishment

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Watford (June 1975), 4 pages.

55
25 . Schulze, W. and Gunzler, J., "Corrosion Protection of the

Reinforcement in Lightweight Concrete," Proceedings, 1st

International Congress on Lightweight Concrete, Vol. 1,

Cement and Concrete Association, London (May 1968), pp. 111-

122 .

26. Dunn, R. Hunter, "Precast Low Density Concrete Units," Paper

SP 29.9, Lightweight Concrete, Publication SP 29, American

Concrete Institute, Detroit, MI (1971), pp. 147-159.

27. Crosier, C.M., "Effects of Variations in Curing Factors on

the Strength of Autoclaved Aerated Concrete," Paper SP 3 2-

10, in "Menzel Symposium on High Pressure Steam Curing,"

Publication SP-32, American Concrete Institute, Detroit, MI

(1972), pp. 193-219.

28. Edlind, O. , "Some Properties and Practical Aspects of

Hardened Aerated Concrete: Swedish Experience," Proceedings,

1st International Congress on Lightweight Concrete, Vol. 1,

Cement and Concrete Association, London (May 1968), pp. 77-87.

29. Svanholm, G. , "The Use of Storey High Units of Aerated

Concrete for External Walls and for Cellars," Proceedings,

1st International Congress on Lightweight Concrete, Vol. 2,

Cement and Concrete Association, London (May 1968), pp. 76-

82.

30. Wennstrom, I., "The Use of Aerated Concrete Units for LowRise Housing: Planning and Design, Architectural Properties

and Some Experience with Completed Houses," Proceedings, 1st

International Congress on Lightweight Concrete, Vol. 1,

Cement and Concrete Association, London (May 1968), pp. 65-

'75.

31. Nettleton, C.N., "Advantages and Limitations of Large

Aerated Concrete Units," Proceedings, 1st International

Congress on Lightweight Concrete, Vol. 2, Cement and

Concrete Association, London (May 1968), pp. 164-165.

32. Makarichev, A. A.

,

"Author's Introduction," Proceedings, 1st

International Congress on Lightweight Concrete, Vol. 2, Cement

and Concrete Association, London (May 1968), pp. 161-163.

33. Gemmel, C., "Developments in the Use of Aerated Concrete in

Sweden," Proceedings, 1st International Congress on Lightweight

Concrete, Vol. 2, Cement and Concrete Association, London

(May 1968), pp, 90-91.

34. Purvis, J.F., "Finishes on Aerated Concrete," Proceedings of

the Symposium, Autoclaved Calcium Silicate Building Products,

University of London, Society of the Chemical Industry (18—

21 May 1965), pp. 252-258.

56
35. Stromberg, J . , "Some Technical Data Concerning a New Type of

Tongue-and-Groove Joints for Roof Slabs of Cellular Concrete,"

Proceedings of the Symposium, Autoclaved Calcium Silicate

Building Products, University of London, Society of the

Chemical Industry (18-21 May 1965), pp. 234-238.

36. Bave, G, , "New Applications of Siporex Large Size Aerated

Concrete Units," Proceedings of the Symposium, Autoclaved

Calcium Silicate Building Products, University of London,

Society of the Chemical Industry (18-21 May 1965), pp. 230-

233 .

37. Ulanichev, P.V., "The Various Products of an Aerated

Concrete Factory in the U.S.S.R," Proceedings, 1st

International Congress on Lightweight Concrete, Vol. 2,

Cement and Concrete Association, London (May 1968),

pp. 165-166.

38. Bave, G. , "Aerated Lightweight Concrete - Current Technology,,"

Proceedings of the Second International Congress on Lighweight

Concrete, The Concrete Society, London (14-15, April 1980),

pp . 28-35.

39. Tietz, S.B., "Some Problems with Aerated Concrete, "Proceedings,

1st International Congress on Lightweight Concrete, Vol. 2,

Cement and Concrete Association, London (May 1968), pp. 188-

189.

40. Eden, N.B., Manthrope, A.R. , Miell, S.A., Szymanek, P.H.,

and Watson, K.L. , "Autoclaved Aerated Concrete From Slate

Waste - Part 1: Some Property/Density Relationships,"

International Journal of Lightweight Concrete, Vol. 2, No. 2

(June 1980), pp. 95-100.

41. Polyakov, S.V. and Y.I. Kotov, "The Strength and Deformation

of Aerated and Lightweight-aggregate Concrete Under Repeated

Dynamic Loads," Proceedings, 1st International Congress on

Lightweight Concrete, Vol. 2, Cement and Concrete Association,

London (May 1968), pp. 113-118.

42. Briesemann, Dietmar, "Structural Elements and Masonry of

Autoclaved Aerated Concrete," in "Autoclaved Aerated

Concrete, Moisture and Properties," F.H. Wittmann, Ed., Elsevier,

Amsterdam (1983), pp. 257-266.

43. "Guide for Low Density Precast Concrete Floor, Roof, and

Wall Units," ACI 523.2R68, Report by ACI Committee 523,

American Concrete Institute, Detroit, MI (Revised 1982), 6

pages

.

57
44 . "Building Code Requirements for Reinforced Concrete (ACI

318-83)," ACI 318-83, American Concrete Institute, Detroit,

MI (September 1984), 111 pages.

45. Council of American Building Officials, Report No. NRB-192,

(April 1984), 4 pages.

46. U.S. Department of Housing and Urban Development, Materials

Release No. 1062a (March 19, 1984), 5 pages.

47. Kinniburgh, W. , "Moisture Content and Moisture Migration in

Aerated Concrete," Proceedings of the Symposium, Autoclaved

Calcium Silicate Building Products, University of London,

Society of the Chemical Industry (18-21 May 1965), pp. 174-

181.

48. "A Guide to the Use of Waterproofing, Dampproofing, Protective

and Decorative Barrier Systems for Concrete," ACI 515.1R-79,

Report by ACI Committee 515, American Concrete Institute,

Detroit, MI (September 1984), 41 pages.

49. Ahlstedt, E.T., "The Quality of Aerated Concrete Products in

Sweden," Proceedings, 1st International Congress on Lightweight

Concrete, Vol. 2, Cement and Concrete Association, London

(May 1968), pp. 103-104.

50. Aldrin, L. "Dimensional Accuracy of Aerated Concrete Wall

Units," Proceedings, 1st International Congress on Lightweight

Concrete, Vol. 2, Cement and Concrete Association, London

(May 1968), pp. 82-84.

51. Nakano, S. and Tada, S., "Optimum Performance Design for

Cellular Concrete Outer Wall," Proceedings of the 3rd

ASTM/CIB/RILEM Symposium, Performance Concept in Building

(March 29,30,31 - April 1,2, 1982) pp. 373-385.

52. Cividini, B., "Investigation of Long Time Deflection on

Reinforced Aerated Concrete Slabs," in "Autoclaved Aerated

Concrete, Moisture and Properties," F.H. Wittmann, Ed.,

Elsevier, Amsterdam (1983), pp. 267-282.

53. Kruml, R. , "Influence of Saturation Degree on Autoclaved

Aerated Concretes on their Creep," in "Autoclaved Aerated

Concrete, Moisture and Properties," F.H. Wittmann, Ed.,

Elsevier, Amsterdam (1983), pp. 249-256.

54. Nielsen, A., "Some Experiments on the Creep of Aerated

Concrete," Proceedings, 1st International Congress on

Lightweight Concrete, Vol. 2, Cement and Concrete Association,

London (May 1968), pp. 105-106.

58
55. Houst, Y. , Alou, F. , and Wittmann, F.H., "Influence of

Moisture Content on Mechanical Properties of Autoclaved

Aerated Concrete," in "Autoclaved Aerated Concrete, Moisture

and Properties," F.H. Wittmann, Ed., Elsevier, Amsterdam

(1983), pp. 219-234.

56. Nakano,S. and Tada, S., "Durability Criteria for Cellular

Concrete Outer Wall," Proceedings, 2nd International

Conference on the Durability of Building Materials and

Components, National Bureau of Standards (U.S.) (14-16

September 1981), pp. 232-241.

57. Ryan, J.V. and Bender, E.W. , "Fire Tests of Precast Cellular

Concrete Floors and Roofs," NBS Monograph 45 (April 12,

1962 ) , 12 pages

.

58. Purins, E., "The Frost Resistance of Aerated Concrete,"

Proceedings, 1st International Congress on Lightweight

Concrete, Vol. 2, Cement and Concrete Association, London

(May 1968) , pp. 94-100.

59. Roulet, C.A., "Expansion of Aerated Concrete Due to FrostDetermination of Critical Saturation," in "Autoclaved

Aerated Concrete, Moisture and Properties," F.H. Wittmann, Ed.,

Elsevier, Amsterdam (1983), pp. 157-170.

60. Larsson, L. and Purins, E., "Determination of the Critical

Degree of Saturation for Autoclaved Lightweight Aerated

Concrete by Studying Deformations in Single-Cycle Freezing

Tests," International Journal of Lightweight Concrete,

Vol. 2, No. 1 (1980), pp. 33-41.

61. Jennings, B.M., "Versatility and Serviceabilty of Aerated

Concrete Units," Proceedings, 1st International Congress on

Lightweight Concrete, Vol. 2, Cement and Concrete Association,

London (May 1968), pp. 82-86.

62. Custance, R.M. , "Fixings to Autoclaved Aerated Concrete

Building Elements," Proceedings of the Symposium, Autoclaved

Calcium Silicate Building Products, University of London,

Society of the Chemical Industry (18-21 May 1965), pp. 216-

219) .

63. Grimer, F.J. and Brewer, R.S., "The Within Cake Variation of

Autoclaved Aerated Concrete," Proceedings of the Symposium,

Autoclaved Calcium Silicate Building Products, University of

London, Society of the Chemical Industry (18-21 May 1965),

pp. 163-170.

64. Svanholm, G. , "Influence of Water Content on Properties," in

"Autoclaved Aerated Concrete, Moisture and Properties," F.H.

Wittmann, Ed., Elsevier, Amsterdam (1983), pp. 119-130.

59
65. Dworjadkin, A. and Maalinowski, R. , "Porosity, Compressive

Strength and Volume Changes of Autoclaved Aerated Concrete,"

Proceedings of the Symposium, Autoclaved Calcium Silicate

Building Products, University of London, Society of the

Chemical Industry (18-21 May 1965), pp. 174-181.

66. Grimer, F.J. and Brewer, R.S., "The Effect of the Environment

on the Strength of Autoclaved Aerated Concrete," Proceedings

of the Symposium, Autoclaved Calcium Silicate Building

Products, University of London, Society of the Chemical

Industry (18-21 May 1965), pp. 182-187.

67. Regan, P.E., "Shear in Reinforced Aerated Concrete,"

International Journal of Lightweight Concrete, Vol. 1, No. 2

(1979), pp. 47-61.

68. Briesemann, D. , "A Method of Calculating the Shear Resistance

of Reinforced Slabs and Beams of Autoclaved Aerated Concrete,"

International Journal of Lightweight Concrete, Vol. 2, No.

1, (1980), pp. 21-31.

69. Tada, S. and Nakano, S. "Microstructural Approach to

Properties of Moist Cellular Concrete," in "Autoclaved

Aerated Concrete, Moisture and Properties," F.H. Wittmann, Ed.,

Elsevier, Amsterdam (1983), pp. 71-90.

70. Prim, P. and Wittmann, F.H., "Structure and Water Absorption

of Aerated Concrete," in "Autoclaved Aerated Concrete,

Moisture and Properties," F.H. Wittmann, Ed., Elsevier,

Amsterdam (1983), pp. 55-69.

71. Schwiete, H.E. and Ludwig, U., "The Porosity and Gas

Permeability of Aerated Concrete," Proceedings of the

Symposium, Autoclaved Calcium Silicate Building Products,

University of London, Society of the Chemical Industry (18-

21 May 1965), pp. 188-194.

72. Loudon, A.G. , "The Effect of Moisture Content on Thermal

Conductivity," in "Autoclaved Aerated Concrete, Moisture and

Properties," F.H. Wittmann, Ed., Elsevier, Amsterdam (1983),

pp. 131-142.

73. Hums, Dieter, "Relation Between Humidity and Heat Conductivity

in Aerated Concrete," in "Autoclaved Aerated Concrete,

Moisture and Properties," F.H. Wittmann, Ed., Elsevier, Amsterdam

(1983), pp. 143-152.

74. Valore, R.C., Jr., "Calculation of U-Values of Hollow

Concrete Masonry," Concrete International - Design &

Construction, Vol. 2, No. 2 (February 1980), pp. 40-63.

60
75 . ASHRAE Standard ANSI/ASHRAE/IES 90.1 P, "Proposed American

National Standard for Energy Efficient Design of New

Buildings Except Low-Rise Residential Buildings," Chapter 8

(August 22, 1986), p. 8-58.

76. Wittmann, F.H. and Houst, Y. , "Standards and Regulations

Concerning Building and Buildings Constructed with Autoclaved

Aerated Concrete," 2nd Revised Version, Working Document,

RILEM Technical Committee 78-MCA.

61
COUNTRIES USING

AUTOCLAVED AERATED CONCRETE

Figure 1. Countries where Autoclaved Aerated Concrete (AAC) is

Used, As Described by Bave in 1983 [6].

62
STEPS IN MANUFACTURING PROCESS

Figure 2. Production of Autoclaved Aerated Concrete Products.

63
10

00 CL

2

x"

I—

o

LU

cr

l~

c

o

LU

>

CO

CO

LU

tr

CL

o

o

9 1305

6 19 31 44 56

(100) (300) (500) (700) (900)

DENSITY, lbm/ft 3 (kg/m 3)

Figure 3. Relationship Between the Cube Compressive Strength and

the Density of Autoclaved Aerated Concrete (From CEB

Manual [1], Courtesy of CEB and the Construction Press).

64
Ill

MOISTURE CONTENT, % by weight

Figure 4. Relationship Between the Compressive Strength and the

Moisture Content of Autoclaved Aerated Concrete (From

CEB Manual [1], Courtesy of CEB and the Construction

Press )

.

Figure 5. The Micro-Cellular Structure of Autoclaved Aerated

Concrete (Magnified to About 3 x Acutal Size).

65
THERMAL

C0NDUCTSV8TY,

W/mK

Figure 6

Figure 7

Density = 31 lbm/ft 3

rr-~\ --r- - —— i (2 mm)

.2 g Macropores

c o O Q.

Example of Distribution of Pore Sizes in Autoclaved

Aerated Concrete (from CEB Manual [1], Courtesy of CEB

and the Construction Press).

25 31 37 44 50

(400) (500) (600) (700) (800)

DENSITY, lbm/ft 3 (kg/m 3

)

1.73

1.39

1.04

0.69

0.35

>

H

>

h-G

D

Q

2

O

o

<

cc

LLJ I

o

r.

CM

c

5

The Relationship Between the Thermal Conductivity and

the Density of Dry specimens of Autoclaved Aerated

Concrete (from CEB Manual [1], Courtesy of CEB and the

Construction Press).

66
Table 1. Estimated 1987 Production of Autoclaved Aerated

Concrete m Some

Country

Belgium

Czechoslovakia

Denmark

Finland

France

Federal Republic of Germany

Japan

Netherlands

Poland

Sweden

Soviet Union

United Kingdom

Yugoslavia

Countries

ft 3 m3

8.8 X 10 6 250 X 10 6

105.9 X 10 6 3000 X 10 3

10.6 X 10 6 300 X 10 3

7.1 X 10 6 200 X 10 3

17.6 X 10 6 500 X 10 3

97.1 X 10 6 2750 X 10 3

105.9 X 10 6 3000 X 10 3

10.6 X 106 300 X 10 3

141.3 X 10 6 4000 X 10 3

8.8 X 10 6 250 X 10 3

187.2 X 10 6 5300 X 10 3

100.6 X 10 6 2850 X 10 3

35.3 X 10 6 1000 X 10 3

1 Gunnar Bave , Siporex, personal communication

Table 2. Examples of Values of

of Elasticity [28]

Compressive Strength and Modulus

Density Compressive Strength Modulus of Elasticity

kg/m3 lbm/f

t

3 MPa lbf /in3 MPa Ibf /in 2

400 25 1.5 215 1000 142,230

500 31 3.0 430 1400 199,130

650 41 6.5 924 2600 369,810

67
' ' ' - . .

17 . * . < • • • ' • , ; . € -; . *

..
APPENDIX A. LISTING OF STANDARDS AND RELATED DOCUMENTS

This appendix lists standards and related documents for autoclaved

aerated concrete products (Table Al). The listing was assembled

from the working document prepared by RILEM Technical Committee

7

8 -MCA on Model Code for Aerated Concrete Based on RILEM Testing

Methods [76] as part of its activities to recommend an application

practice for autoclaved aerated concrete. As is evident from the

listing, some of the titles of the documents are given in their

language of origin. In these cases, only the original language

was given in the RILEM working document. The ASTM documents

included in the listing are not specific for autoclaved aerated

concrete

.

A-l
Table Al. Standards and Related Documents Associated With

Autoclaved Aerated Concrete

Country Standard

Belgium NBN 538 (1962)

Blocs en beton pour maconnerie ordinaire

Denmark DS 420.1 (1967)

Vandrette baerende elementer af armeret autoklaveret

letbeton

Ds 420.1 (1967) (Supplement)

Vandrette baerende elementer af armeret autoklaveret

letbeton

DS 420.2 (1977)

Vertical load bearing elements of lightweight concrete

DS 420.3 (1980)

Letbeton vandretbaerende elementer

DS 438 (1981)

Blokmurvaerk

Federal DIN 4223 (1958)

Republic Bewehrte Dach- und Deckenplatten aus dampfgehartetem

of Germany Gas- und Schaumbeton

Entfurf DIN 4223 (1978)

Bewehrte Bauteile

DIN 4164 (1951)

Gas- und Schaumbeton - Herstullung, Verwendung und Prufung

DIN 4165 (1982)

Gasbeton-Blocksteine

DIN 4166 (1973)

Gasbeton-Bauplatten

Zulassungsbescheid Nr. Z 17.1-1387 (1981)

Mauerwerk aus Hebel-Plansteinen

Zulassungsbescheid Nr. Z 17.1-43 (1975)

Geschosshohe tragende Hebel-Wandtafeln aus dampfgehartetem

Gasbeton der Festigkeitsklassen G 35 und G 50

Zulassungsbescheid Nr. Z 2. 1-5.1 (1982)

Bewehrte Hebel-Deckenplatten aus dampfgehartetem Gasbeton

der Festigkeitsklassen GB 3.3 (GSB 35) und GB 4.4 (GSB 50)

A-2
Table A1 . Standards and Related Documents Asociated With

Autoclaved Aerated Concrete (continued)

Country

Finland

France

German

Democratic

Republic

Standard

Zulassungsbescheid Nr. Z 2. 1-5. 2 (1982)

Bewehrte Hebel-Dachplatten aus dampf gehartetem Gasbeton

der Festigkeitsklassen GB 3.3 (GSB 35) und GB 4.4 (GSB 50)-

Zulassungsbescheid Nr. Z 2.1-23 (1982)

Bewehrte Hebel-Sturze aus dampfgehartetem Gasbeton

GB 4.4 (GSB 50) ohne schragbewehrung

RIL 69 (1969)

Karkaistun kevytbetonin normit 1969

NF P 14-306 (1978) , Blocs en beton cellulaire autoclave pour murs

Avis no 16/75-20 (1976)

Avis sur le mur en blocs de beton cellulaire Ytong (Burcht)

Avis no 3/77-58 (1978)

Avis sur le plancher Siporex (Bernon)

Avis no 16/77-29 (1978)

Avis sur le procede de maconnerie Sipobloc ( Bernon )-

Durobloc (Bernon)

TGL 21098 (1972)

Gasbeton - Technische Forderungen

TGL 21098 (1972)

Gasbeton - Prufung

TGL 23374 (1971)

Wandplatten fur den Skelettbau - Fensterplatten aus

Stahlbeton

TGL 23374 (1974)

Wandplatten fur den Skelettbau - Aussenwandvollplatten aus

Gasbeton

TGL 23374/02 (1975)

Wandplatten fur den Skelettbau - Einschichtige

Aussenwandelemente aus Leicht- und Schwerbeton

TGL 25553 (1972)

Wandbausteine aus Gasbeton
Table A1 . Standards and Related Documents Associated With

Autoclaved Aerated Concrete (continued)

Country Standard

Great

Britain

Israel

Japan

Romania

Sweden

Switzerland

The

Netherlands

Turkey

BS 6073 (1981)

Precast concrete masonry units

Part Is Specification for precast concrete masonry units

Part 2 : Method for specifying precast concrete masonry

units

CP 110 (1972)

Code of practice for the structural use of concrete

Part Is Design, materials and workmanship

CP 121 (1973)

Code of practice for walling

Part 1: Brick and block masonry

Regulations D13(b) and D14 (1976)

The building regulations

SI 268 (1979)

Ytong blocks

Standard for structural design of autoclaved lightweight

concrete construction - noticed by the Ministry of

Construction - 1967

STAS 8036-81 (1981)

Beton cellular autoclavizat - Gazbeton - Conditii tehnice

generale

PFS 1980:3

Autoclaved aerated concrete products - products of

autoclaved aerated concrete without coarse aggregate

SIA 177 (1980)

Maconnerie

SIA 177/1 (1983)

Calcul des murs de maconnerie sollicites a la compression

Bouwblokken en -stenen van beton (concrete building blocks

and stones)

TS 453 (1977)

Gazbeton ypi malzeme ve elemantari (gas concrete material

and elements for building)

ATable Al . Standards and Related Documents Associated With

Autoclaved Aerated Concrete (continued)

Country Standard

USA ANSI/ASTM C 796-80

Foaming agents for use in producing cellular concrete using

preformed foam

ANSI/ASTM C 869-80

Foaming agents used in making preformed foam for cellular

concrete

USSR GOST 190 10-82 (1982)

Blocs en beton et en beton arme pour murs de batiments

GOST 25485-82 (1982)

Cellular concretes, technical specifications

Yugoslavia Civil engineering and architectural documentation IR-9

Vol. 337: Regulations on technical standards for the

design and erection of structures made of precast nonreinforced and reinforced aerated concrete units

JUS U.N1.300 (1981)

Celijasti beton - proizvodnja, primena i ispitvanje

pref abrikovanih elemenata od gasbetona i pjenobetona

osnovne odredbe

JUS U.N1.302 (1981)

Celijasti beton - armirane krovne i mdeukatne ploce od

plinebetona i pjenobetona

JUS U.N1.304 (1981)

Celijasti beton - armirane zidne ploce od plinobetona i

pjenobetona

JUS U.N1.306 (1981)

Celijasti beton - zidni blokovi od plinobetona i pjenobetona

JUS U . Nl .310 (1981)

Celijasti beton - zastita armature od korozije

A-5
NBS-114A irev. 2 -60 ) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See in struction s)

1. PUBLICATION OR

REPORT NO.

NBSIR 87-3670

2. Performing Organ. Report No. 3. Publication Date

MARCH 1988

4. TITLE AND SUBTITLE

A Review of Autoclaved Aerated Concrete Products

5. AUTHOR(S)

Robert G. Mathey and Walter J. Rossiter, Jr.

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City . State. ZIP)

U.S. Department of Energy

Office of Conservation and Renewable Energy

1000 Independence Ave., SW

Washington, DC 20585

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

This report is a review of the properties and performance of autoclaved aerated concrete. The material

has a relatively low thermal conductivity combined with loadbearing capacity for use in structural and

non- structural applications. This feature may make its use attractive for energy-conserving applications.

For this reason, the review was conducted at the request of the U.S. Department of Energy.

The review addressed an overview of the manufacturing process, uses of autoclaved aerated concrete in

building constructions, properties, energy considerations, the availability of code-related documents, and

standards. Uses include block and panel construction in both loadbearing and non- loadbearing applications

for walls, floors, and roofs. Many properties are reviewed including density, fire resistance, moisture

expansion and shrinkage, strength, structure, and thermal conductivity. With regard to energy considerations

the review points out that, for energy-efficient applications under severe climatic conditions, the

autoclaved aerated concrete unit alone may be insufficient to provide the targeted minimum value of

thermal resistance. In these cases, autoclaved aerated concrete has to be used together with additional

thermal insulation. In contrast, autoclaved aerated concrete alone may satisfy insulation needs in areas having mild climates.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

aerated; autoclaved; cellular; concrete; insulating concrete; lightweight;

manufacture; properties; standards; uses.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[X] Unlimited

Q31 For Official Distribution. Do Not Release to NTIS 79

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. 15. Price

Q£] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $13.95

USCOMM-DC 6043-P80
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