Dr. Mohammad Arif Kamal
Dept. of Architecture, Aligarh Muslim University,
Autoclaved aerated concrete (AAC) is a derivative of fly ash that is combined with cement, lime, gypsum and aluminum powder as foaming agent. AAC is produced as blocks and panels. The blocks can be used for load bearing and non-load bearing walls. Autoclaved aerated concrete, is concrete that has been manufactured to contain lots of closed air pockets. Lightweight and fairly energy efficient, it is produced by adding a foaming agent to concrete in a mould, then wire-cutting blocks or panels from the resulting ‘cake’ and ‘cooking’ them with steam (autoclaving). Aerated concrete is a lightweight material and has high thermal insulation value. Fig. 1 shows an AAC block. Due to the light weight and high strength-to-weight ratio of aerated concrete products, their use results in appreciable economy in the structural members, and thereby save cement and steel reinforcement.
AAC was perfected in 1924 by the Swedish architect and inventor Dr. Johan Axel Eriksson, working with Professor Henrik Kreüger at the Royal Institute of Technology . It is an approved eco-friendly building material that comes from industrial waste and is made from non-toxic ingredients. With AAC, the construction process can be about 20 per cent faster. It weighs only about 50 per cent of a standard concrete block and possesses high thermal insulation and is acoustics-friendly. It also has better fire resistance than fly ash and is non-combustible. It’s non-allergic and hence maintains the quality of air within a building without changing its properties over time. AAC is used in a wide range of commercial, industrial, and residential applications and has been in use in Europe for over 70 years, the Middle East for the past 40 years, and South America and Australia for approximately 20 years. According to one report, AAC now
accounts for over 40% of all construction in the United Kingdom and more than 60% of construction in Germany .
Specifications of AAC Blocks
Size: The blocks are made of 625 mm length, 250 mm height and of various thicknesses: 100, 125, 150, 200, 225, 250, 300 mm with a tolerance of ±1.5 mm.
Density: The block density is 600 to 650 kg/cum, whereas the bricks have a density of 1750 to 2000 kg/cum. The wet density is approximate 800 kg/cum in comparison to that of bricks (2400 kg/cum).
Compressive Strength: 35 to 50 kg/cm2 (as per IS: 2185).
Fire Resistance: 2 to 6 hours depending upon thickness.
Sound Reduction Index: 45 for 200 mm thick block walls (against 50 for 230 mm thick wall).
Thermal Conductivity: 0.16 kw/m C° against 0.70 of bricks, thus recurring energy cost is reduced in air conditioning.
Water Absorption: In AAC curing takes place at high temperatures and pressure in the presence of saturated steam. During curing, part of the siliceous material (flyash) reacts chemically with the calcareous ingredients such as lime liberated by the hydration of cement to form a Micro-Crystalline structure of Tobermorite with much lower specific surface and is characterized by pores formed by the release of H2 gas during Casting-Rising stage of production.
Raw Materials used in Manufacturing of AAC Blocks
AAC Comprise of the following raw materials as mentioned in Table 1.
Laying of Blocks
In laying AAC blocks, the procedure is same as that for conventional half brick work except that the block should be slightly wet with sprinkler before use and not soaked as in the case of bricks.
i. Before laying the first course the alignment of the wall is marked over the DPC.
ii. The blocks for 1st course should first be laid dry without mortar along a stretched thread between properly located corners of the wall in order to determine the correct position of the blocks, including those of cross walls joining it and also to adjust their spacing within the wall length.
iii. When the blocks are set in proper position, the two corner blocks are removed. Specified mortar bed is spread for the required bed thickness (10mm).
iv. The blocks are laid back in place with true level and plumb.
v. The thread is then stretched tightly along the faces of the two corner blocks and the faces of the intermediate blocks are adjusted to coincide with the thread line.
vi. Each intermediate block is removed and re-laid with mortar.
vii. After every three or four blocks layers are laid, their correct alignment level and verticality is checked.
viii. In the vertical joints the mortar is applied only on the face of the blocks.
Electrical and Plumbing Installations
Electrical and plumbing installations in AAC masonry are placed in routed chases. Care should be taken when laying out chases to ensure that the structural integrity of the AAC elements is maintained. Do not cut reinforcing steel or reduce the structural thickness of the AAC elements except where permitted by the designer. In vertically spanning AAC elements, horizontal routing should be permitted only in areas with low flexural and compressive stresses. In horizontally spanning AAC elements, vertical routing should be minimized.
Unprotected exterior AAC deteriorates when exposed to cycles of freezing and thawing while saturated. To prevent such freeze-thaw deterioration, and to enhance the aesthetics and abrasion resistance to AAC, exterior finishes should be used. They should be compatible with the underlying AAC in terms of thermal expansion and modulus of elasticity, and should be vapor permeable. Many different types of exterior finishes are available. Polymer-modified stuccos, paints or finish systems are the most common exterior finish for AAC. They increase the AAC’s water-penetration resistance, while allowing the passage of water vapor. Heavy acrylic-based paints containing aggregates are also used to increase abrasion resistance. There is generally no need to level the surface, and horizontal and vertical joints may be chamfered as an architectural feature, or may be filled.
Interior finishes are used to enhance the aesthetics and durability of AAC. They should be compatible with the underlying AAC in terms of thermal expansion and modulus of elasticity, and should be vapor permeable.
i. Many different types of interior finishes are available. Interior AAC wall panels may have a thin coat of a mineral-based plaster to achieve a smooth finished surface. Lightweight interior gypsum-based plaster may provide a thicker coating to level and straighten walls, and to provide a base for decorative interior paints or wall finishes. Interior plasters have bonding agents to enhance their adhesion and flexibility, and are commonly installed by either spraying or troweling.
ii. For commercial applications requiring high durability and low maintenance, acrylic-based coatings are often used. Some contain aggregates to enhance abrasion resistance.
iii. When ceramic wall tile is to be applied over AAC, surface preparation is normally necessary only when the AAC surface requires levelling. In such cases, a Portland cement or gypsum-based parge coat is applied to the AAC surface before setting the ceramic tile. The ceramic tile should then be adhered to the parged wall using either a cement-based thin-set mortar or an organic adhesive. In moist areas such as showers, only a Portland cement-based parge coat should be used, and the ceramic tile should be set with cement-based, thin-set mortar only.
For concealed or piping, block wall can be chased using hand or electric router. Depths of vertical chases should be limited to one-third of the wall thickness and horizontal chases to one-sixth of the wall thickness. Holes in block wall can be made with a standard hand or electric drill (Fig. 2). The chases shall be refilled with leaner mortar and chicken mesh shall be applied on that area and cured.
Precast or cast-in-situ concrete lintels can be used in block masonry, over all openings. Lintels shall always rest on full block with minimum bearing as under. Below the openings RCC band should be provided with reinforcement to avoid diagonal tension cracks. The bond beam to be extended up to 300 mm from window corners both the sides. Table 2 shows the minimum bearing on each side for the different opening sizes.
Followings are the points that should be considered while plastering the AAC walls:
i. Do not soak the wall before plastering. The wall shall be moistened evenly before applying the plaster. A fog spray is recommended for this purpose.
ii. In external situations, plastering has to be carried out in two coats, applying SBR coating with sand on block surface will enhance the bonding and minimize thickness of plastering.
iii. It is recommended to use cement mortar 1:6 for internal and external plastering works and preferably use PPC cement for masonry and plastering works to minimize shrinkage cracks.
iv. Plastering thickness can be minimized to 10 mm and 15 mm for internal and external walls.
Precautions while Laying the Aac Masonry
The cracks occurring in block masonry and in plastering are of any structural problem involving stability and safety to the structure. However, it is advised to minimize the same to have good appearance and maintenance free.
i. Do not store the blocks on unleveled surface.
ii. Do not use wet blocks for masonry construction.
iii. Do not make the holes on block masonry for scaffolding supports.
iv. Do not soak the blocks before use.
v. Do not hammer the block masonry for service lines, chases etc.
vi. Do not completely wet the block masonry before plastering works.
vii. Do not chase the blocks back to back for lesser thickness blocks.
Autoclaved Aerated Concrete (AAC) blocks is one of the most sustainable building materials in contemporary building construction. The basic cost of AAC Blocks is about `3,200/- to `3,600/- per cum at factory excluding transportation. The saving is also achieved due to reduction in thickness of plaster due to even size of blocks. Economy is achieved in multistoried residential buildings by using AAC blocks instead of bricks due to substantial reduction in dead weight, thereby reduction in cost of framed structure. At high altitude where transportation cost is high AAC blocks can be economical. The unique product’s flexibility and characteristics allow for high-speed and energy-efficient construction methods. Due to its relatively low consumption of readily available raw materials, excellent durability, energy efficiency, relative cost effectiveness, and ability to be recycled, AAC is well deserving of its ‘green’ designation.