Aerated Autoclaved Concrete

 WHAT IS AUTOCLAVED AERATED CONCRETE

Autoclaved aerated concrete (AAC) is a building material which consists of various sized elements that form a complete building system. In most cases, every portion of the structural and insulation requirements of a building are satisfied with the one material. The elimination of a number of other materials, and the fact that AAC is an inherently “green” building system, result in a “healthier” building. Low energy required in production, low raw material consumption, ease of use in construction, high energy efficiency, better indoor air quality, and recyclability add up to make AAC a very environmentally friendly building material and system.

Autoclaved Aerated Concrete (AAC) is a fully integrated building system of blocks and panels used in the construction of walls, floors and roofs.  It may be used in either a load-bearing capacity or as a non-load bearing component such as cladding or infill.  The manufacturing process for AAC was developed in Sweden by an architect who was looking for a building material with the properties of wood without the disadvantages of wood: combustibility, decay and insect damage.  He succeeded in producing a highly cellular, lightweight masonry material, tobermorite which is now known as aerated autoclaved concrete.

SURFACE OF GREEN AAC BLOCK

FINAL PRODUCT – AUTOCLAVED

 MANUFACTURE

RAW MATERIALS

AAC consists of basic materials that are widely available. These include sand, cement, lime, gypsum, water, and an expansion agent. Silica sand, the raw material used in the greatest volume in AAC, is one of the world’s most abundant natural resources. The finished product is up to five times the volume of the raw materials used, with an air content of between 70% to 80% (depending on the required strength and density.) Due to this large increase in volume, AAC is very resource efficient. The following chart demonstrates the volumes obtainable from one cubic meter of raw materials for AAC and various other common building materials.

 THE PROCESS

Aerated concrete is made by introducing air or other gas into a slurry composed of Portland cement or lime and a siliceous filler , so that when the mixture sets hard, a uniform cellular structure is formed. In the gasification method , a finely powdered metal, preferably aluminum , is added to the slurry and this reacts with the lime which has been used as the cementing agent.

To start with ,first all the ingredients which constitute AAC ,that is sand , cement , lime , gypsum ,water and expansion agent (aluminum) are fed to the mixer to form a slurry. The slurry is then transferred to the moulds which are bought to the mixer by the help of rails.

The mould are kept about 1/3rd empty because the reaction between expansion agent and other component of AAC causes the slurry to expand .From here the mould is transferred to the riser section where rising of the mix takes place to fill the mould to overflow. The extent to which the moulds are filled with the slurry depends upon the density of the product which is to be made , and so on the amount of expansion to be expected.

Cake after cutting ready to be autoclaved

After a period of 3 – 6 hrs the casting will have set enough to withstand cutting. after this the mould is removed and the ‘green’ AAC cake is transferred to the cutting machine. First trimming is done and then parallel cuts are made usually in two directions at right angles , by tensioned wires so as to produce the required blocks .Now the green AAC is ready for autoclaving. Autoclaving is curing with the help of steam under high pressure. The Autoclaves are frequently about 8 ft in diameter and about 80 ft long. So it can hold 12 mould at a time. The aerated concrete within the moulds remains in the autoclaves for around 14 – 18 hrs during which the pressure achieved is 11 bars corresponding to a temp of 180 degrees. After the cooling period the product is taken out ready to be taken to the site or to stock pile.

CAKE CUTTING SLEDGE

AUTOCLAVES

                                LINTELS                BLOCKS                  U- BLOCKS

   High pressure steam curing is practically unavoidable in making aerated  concrete of first quality when cement is used as a binder ,and absolutely essential when lime is used. With Portland cement , the initial development of strength in the product depends primarily on the normal setting of the cement , and autoclaving is used to improve the characteristics.

REINFORCED PANELS

For making reinforced slabs the method is same. The reinforcement rods are cut ,welded into suitable mats and dipped into the coating mixture to protect them from corrosion. The reinforcement is kept in the moulds before the slurry is put, so when finally we have slabs they contain reinforcement.

 PROPERTIES

1) Compressive strength     40 – 45 kg./ sq. cm for slabs

                                               35 – 40 kg. / sq. cm for blocks

2) Dry density                       640 kg. / cu. mts for slabs

                                               550 kg. / cu. mts for blocks

3) Modulus of elasticity        0.21 x 10^5 kg. / cu. mts

4) Modular ratio                    1:100

5) Permissible stresses in:

               a.)  Bending compression     15 kg. / sq. mts

               b.)  Shear                                1.0 kg. / sq. mts

               c.)  Bond                                  1.0 kg. / sq. mts

6) Limiting deflection  span/ 300

 PERMISSIBLE LOADS :

1.  ROOF SLABS          Live load of 150 kg. / sq. mts + terracing and water  proofing of 7.5 cm
2.  FLOOR SLABS        Live load of 250 kg / sq mts + floor finish of 4 cm
3.  LINTELS                    Spans upto 7.5 m , loads upto 1500 kg / m

ADVANTAGES OF AAC

 APPEARANCE

Autoclaved Aerated Concrete is very light colored. It contains many small voids (similar to those in aerated chocolate bars) that can be clearly seen when looked at closely. The closed air pockets contribute to the material’s insulating properties and also its aerated nature. Although there is no direct path for water to pass through the material, an appropriate coating is required to prevent water penetrating into the AAC material. AAC can be sculpted with wood working tools, but its softness means that it is rarely used as an exposed finish owing to its need for surface protection.

 STRUCTURAL CAPABILITY

The compressive strength of AAC is very good and load-bearing structures up to 3 storey high can be safely erected. Entire building structures can be made in AAC from walls to floors and roofing with reinforced lintels, blocks and floor, wall and roofing panels available from the manufacturers.

THERMAL MASS

The thermal performance of AAC, as for other high-mass materials, is dependent on the climate in which it is used. With its mixture of lightweight concrete and air pockets, AAC has a moderate overall level of thermal mass performance. The temperature moderating thermal mass is most useful in climates with high cooling needs.

 INSULATION

AAC has reasonably good insulation qualities. In most Australian climates the need for supplementary insulation can be avoided. A 200mm thick AAC wall gives an R-Value rating of 1.43 for AAC with 5% moisture content by weight. AAC wall surface temperatures were measured over a 24hour period on a west wall, which was painted black to increase surface temperature. The outside wall temperature fluctuated by as much as 126˚F. The inside temperature remained at a pleasant 68˚F without air conditioning with a mere 3.6˚F variation. Additionally, the peak temperature was shifted to a later time of the day when energy is no longer required to mechanically adjust the indoor temperature.

SOUND INSULATION

With its closed air pockets, AAC can provide very good sound insulation. As with all masonry construction, care must be taken to avoid gaps and unfilled joints that can allow unwanted sound transmission. Combining the AAC wall with an insulated asymmetric cavity system will provide a wall with excellent sound insulation properties.

 SUSTAINABILITY (environmental impact)

Weight for weight, AAC has manufacturing, embodied energy and GH emission impacts similar to those of concrete, but can be up to one quarter to one fifth that of concrete based on volume. AAC products or building solutions may have lower embodied energy per m2 than a concrete alternative. Its much higher insulation value reduces heating and cooling energy consumption. AAC has some significant environmental advantages over conventional construction materials addressing longevity, insulation and structural demands in one material. As an energy and material investment it can often be justified for buildings intended to have a long life.

 FIRE AND VERMIN RESISTANT

AAC is inorganic and incombustible and is thus especially suited for fire-rated applications. Depending on the application and the thickness of the blocks or panels, fire ratings up to 4 hours can be achieved. AAC does not harbor or encourage vermin. AAC is non-combustible. A 4-inch thick non-load-bearing or a 6-inch thick load-bearing E-Crete wall, provides 4-hour fire rating. This far exceeds the requirements of the Standard Building Code, and provides a significant level of protection against loss of life and property. Toxic fumes generated from traditional materials burning pose a danger. AAC is an inorganic material that does not burn. The melting point of AAC is over 2900 ºF, more than twice the typical temperature in a building fire of 1200 ºF. The use of AAC eliminates the need for applying costly fireproofing.

TOXICITY AND BREATHABILITY

The aerated nature of the material facilitates breathability. There are no toxic substances and no odour in the final product. However, AAC is a concrete product, and similar precautions should be taken as when handling and cutting concrete products. Personal protective equipment (such as gloves, eye wear, respiratory masks) is required during cutting due to the fine dust that is produced by concrete products. If low-toxic, vapors permeable coatings are used on the walls and care is taken not to trap moisture where it can condense Autoclaved Aerated Concrete is about one-fifth the density of normal concrete blocks.

DURABILITY AND MOISTURE RESISTANT

The purposely lightweight nature of AAC makes it prone to impact damage. With the surface protected to resist moisture penetration it is not affected by harsh climatic conditions and will not degrade under normal atmospheric conditions. The level of maintenance required by the material varies with type of finish applied.

The porous nature of the material can allow moisture to penetrate the material to a depth but appropriate design (damp proof coarse layers and appropriate coating systems) prevents this happening. AAC will not easily degrade structurally when exposed to moisture, but its thermal performance may suffer.

There are a number of proprietary finishes available (acrylic polymer based) which when applied over a sand and cement render provide a very durable and water resistant coating system to AAC block work. They need to be treated in a similar fashion with acrylic polymer based coatings prior to tiling in areas such as showers. The manufacturer can advise on the appropriate coating system, surface preparation and installation instructions to give good water repellent properties prior to tiling in wet areas.

Plasticized, thin coat finishes are common, but here a non-plasticized thick coat (10mm approximately) render was used for environmental reasons. Some variation in the amount of ‘show-through’ of the block work pattern can be seen in this example that also illustrates the use of glass blocks, as well as more conventional windows. The external plumbing was a choice made to reduce loss of internal space, avoid potential problems with wall cavities, and express the decision to avoid the use of PVC plastic in the construction.

WORKABILITY

Blocks are one-fifth of the weight of concrete and are produced in a variety of sizes, but although AAC is relatively easy to work, is light and easily carved, cut and sculpted, it generally requires careful and accurate placement so that skilled trades and good supervision are essential. Competent bricklayers or carpenters can work successfully with AAC. Very large block sizes may require two-handed lifting and be awkward to handle but can result in fewer joints and more rapid construction.

The construction process with AAC products results in a low waste component, as the off cuts can be re-used in the construction of the wall.

 LIGHTWEIGHT SO EASY TO WORK WITH

EASY TO CUT ,DRILL

 SEISMIC DESIGN

AAC has performed well for many years in seismically active and hurricane-prone regions around the world. AAC buildings have shown good resistance to earthquake forces. The non-combustible and fire resistant characteristics provide further advantage against fires commonly associated with earthquakes.

TOOLS USED

There are special tools which are to be used for fixing AAC’S and for their future use. Some of them are :

TAB WASHER

Pre-Spot Tabs

Can be Screwed, Nailed, or Shot

Works Well with Multiple Substrates

Covers Many Rigid Insulation Applications

WOOD SCREWS

 Climacoat ProtectionFor use with dimensional lumber and plywood.

STEEL SCREWS

 Climacoat ProtectionFor use with metal from 20- to 12-gauge studs

THE QUICKCORE BIT

 Boring Tool

Designed Expressly for AAC Applications

DURARASP

Tungsten-Carbide Rasp

Non-Gouging, Rounded Corners

Custom-Crafted and Serial-Numbered

AERATED WOOD RASP

 MKD Metal Claw Anchors

• Recessed Ceilings
• Sprinkler Systems
• Threaded Rods
• Gas lines
• Conduit
• Pipes

 VLF Frame Anchors

Door and Window Frames

Bathroom Fixtures

General Purpose

Curtain Rods

Hand Rails

A highly effective anchoring system for a wide variety of applications. This anchor is composed of a polyethylene expansion body formed around a hardened steel shaft and capped by a metal tip.

SOME OF THE BUILDINGS MADE OF AAC

 Reinforced AAC blocks used for the two storey building.

 

Reinforced AAC vertical wall slabs in a single storey dwelling

 Dwelling house built with AAC block masonry walls

SITE PHOTOGRAPHS

Sawing aerated concrete on the site. Combined saw and file for cutting the reinforcing bars

 Reinforced AAC factory roof in course of erection

Generator for in situ concrete

 Erecting an AAC dry jointed block wall

 AAC block wall during construction

APPLICATION

Roof slabs

Thermal insulation

On flat roofs the thermal insulating capacity has usually been increased by means of a screed made of insulating light weight concrete or of similar material between the load bearing structure and the water proofing membrane of asphalt or bituminous felt which is placed on it.where the roof is intended to be walked on or is used for other purposes which may involve wear and tear, an additional wearing surface is provided, usually in the form of precast concrete slab.

Roof covering

A common roof covering to exclude rain water consists of a single or double layer of bituminous felt. Before laying the surface must be coated with an asphalt solution or emulsion.Concrete or clay tiles may also be used.

Internal treatment

The soffit might be painted with a coat of lime wash or cement or silicate paint, but where the surface treatment is impermeable to vapors, the slabs should not be painted before the roof system is dried out. In addition to the ventilated roof space, the underside of the roof may also be protected with an additional water repellent coating.

Construction details

Solid and ventilated roofs of various types are made of aerated concrete. AAC slabs must be adequately anchored to the supporting structure. Steel straps are used for this purpose. In long buildings, suitable expansion joints must be provided.

Floor slabs 

Reinforced AAC floors have been used for industrial purposes under a loading of 80 ib/sq. ft and more ,over spans of up to 20 ft. The structural design of AAC floor slabs is based on the same principles as that of the roof systems. In some cases screed is added to serve as a base for the floor covering, but resulting also in greater stiffness and strength.