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Aerated Concrete Blocks: A Sustainable And Versatile Material

B. L. Rajput1 and A. L. Agarwal2

1Assistant Professor, NICMAR, 25/1 Balewadi, Pune 411045

2Professor, NICMAR, 25/1 Balewadi, Pune 411045

Abstract: Large varieties of materials are being manufactured and consumed by the Indian construction industry. Although growth in the construction sector in India has boosted the demand for various materials, yet concrete remains one of the most extensively consumed construction materials. Replacing bricks with concrete blocks is widely accepted as it saves the fertile topsoil of cultivable land. Concrete blocks are used in partition walls of various thicknesses as well as in load-bearing walls. In order to deal with problem of the weight of concrete on structure and to make the design economical, efforts are being made to reduce the concrete block load by making it perforated or cellular. This paper attempts to highlight the latest developments in aerated concrete blocks as well as their properties, testing requirements as per the IS code, applications and merits over conventional bricks.

Introduction

The Indian construction industry is one of the largest in terms of expenditure, the volume of raw materials/natural resources consumed, the volume of materials and products manufactured, employment generation, environmental impacts, etc. Large varieties of construction related materials are manufactured and consumed in the Indian construction industry. Growth in the construction sector in India has boosted the demand for various construction materials as shown in Table 1[1].

Table 1: Projected demand for building materials

Material	Year 2000	Year 2020
Bricks(nos)	150 x 109	246 x 109
Structural steel (tons)	11 x 106	30 x 106
Cement (tons)	96 x 106	255 x 106

PLZ See the Pfd Page No 307 for Table

Most of the building materials are energy intensive materials, and their extensive use can drain the energy resources and adversely affect the environment. Hence there is a need for optimum utilization of available energy resources and raw materials to produce simple, energy efficient, environment friendly and sustainable building alternatives and techniques to satisfy the increasing demand for building construction. The following are some of the parameters which need to be taken into consideration for developing sustainable alternative building technologies.

• Use of eco-friendly materials and techniques

• Minimum use of high energy materials

• Use of renewable energy sources

• Minimum transportation and use of locally available materials

• Utilization and/or recycling of wastes of building materials

Autoclaved Aerated Concrete

Autoclaved aerated concrete (AAC) is a lightweight and eco-friendly construction material. Axel Eriksson, a Swedish architect, invented this innovative material in mid 1920 [3]. AAC is usually used as blocks but comes in a variety of forms ranging from blocks, lintels, walls, floors to roof panels. AAC blocks are used for load-bearing structure walls, partition walls, inner leaf of cavity walls or as backing for brick masonry, external walls for RCC framed structures when protected from weather by rendering or by some other efficient treatment. The requirements of AAC blocks having a density up to 1000 kg/m3 are covered in BIS code IS 2185 (Part 3).

Production Process

A very brief description of the production process of AAC is given in IS 2185 (Part 3), which is presented here. AAC blocks involve the use of aerated concrete, which is made by introducing air or other gas into a slurry composed of cement or lime and siliceous filler. Cement complying with Indian Standards may be used at the discretion of the manufacturer. Use of fly ash conforming to IS 3812 – 1981 may be permitted to a limit of 20 %. The lime used should satisfy the requirements for class C lime specified in IS 712- 1973. The aggregates used for the manufacture of AAC blocks should confirm to IS 383- 1970, except for the grading, which may be made to suit the product, and the silica content, which should not be less than 80 %. Admixtures may be added either as additive to cement during manufacturing or as admixtures to the concrete mix. There are several ways in which air cells or other voids may be formed in

the slurry so as to result in a cellular structure after autoclaving:

• By formation of gas by chemical reaction within the mass during the liquid or plastic stage

• By introducing air, either by adding to the slurry in the mixer a preformed stable foam such as is used in fire fighting or with the use of air entraining agents

Generally autoclaving (High Pressure Steam curing) is done for about 14 to 18 hours. In autoclaving, concrete is subjected to a maximum temperature of about 1750c, which corresponds to a steam pressure of about 8.5 Kg/cm2. The highly foamed closed cell concrete is then cut to size after the evaporation of hydrogen. AAC blocks can be given a variety of surface textures ranging from a very fine close texture to a coarse open texture by the proper selection, grading and proportioning of the aggregates at the time of manufacturing or by treating the face of units while still green by wire brushing or by slightly eroding the surface by playing a fine spray of water upon it.

Physical Properties

Table 2 provides a comparison of various properties of different walling materials like clay bricks, fly-ash bricks, hollow and solid concrete blocks. It shows that AAC blocks are excellent walling materials in comparison with other materials.

Sampling

For testing AAC blocks on site, samples need to be taken from a lot. The detailed sampling method has been described in IS 2185 (Part3) for sampling of AAC blocks.

When it is not practicable to do the sampling by the described method, the samples need to be taken by using a random sampling method. In this method, the sample should be taken from the top of the stack at the accessible site and from the interior of the stacks by opening trenches from the top. The samples of blocks should be marked for future identification of the consignment they represent. The blocks need to be kept under cover and protected from extreme conditions of temperature, relative humidity and wind until they are required for the test.

Sl No	Properties	Common burnt clay building bricks IS 1077	Fly-ash bricks IS12894 - 2002	Hollow & solid concrete blocks IS2185 (Part - I)	AAC block IS2185 (Part III)
1	Size As per IS mm Normal	190 x 90 x 90 190 x 90 x 40 230 x 110 x 70	190 x 90 x 90 190 x 90 x 40 230 x 110 x 70 230 x 110 x 30	400, 500, 600 length 200, 100 mm height 50, 75, 100, 150, 200, 250 or 300 mm width	400, 500, 600 length 200, 250, 300 height 100, 150, 200, or 250 mm width
2	Density kg/m3	1700-1800	1700	Hollow A- Min 1500 B- 1000 to 1500 Solid Not less than 1800	451 to 1000
3	Compressive strength N/mm2	3.5 to 35	3.5 to 35	A-3.5,4.5,5.5,7.0 B-2.0,3.0,5.0	1.5 to 7
4	Drying shrinkage	Not available	Shall not exceed 0.15%	Shall not exceed 0.1%	Shall not be more than 0.05% for grade 1 & 0.10% for grade 2 blocks
5	Thermal conductivity W/m oK	0.82	20 to 30% less than those of concrete blocks	0.70 - 1.28	0.9-0.22
6	Sound insulation	50 db for 230 mm thick wall	Normal	Good	45 db for 200mm thick wall
7	Water absorption	Shall not be more than 20% by weight up to 12.5 class & 15% of weight for more class	Shall not be more than 20% by mass up to 12.5 class & 15% by mass for higher class	Shall not be more than 10% by Mass	8 % *
8	Efflorescence	Shall not be more than moderate up to class 12.5 & slight for higher classes	Shall not be more than moderate up to class 12.5 & slight for higher classes	Not applicable	Not applicable
9	Cost	Rs. 4-5/ brick	Rs 6/brick	Rs. 20 to 30 per block	Rs. 80 - 90/block

Table 2: Comparison of physical properties of different walling materials

PLZ See the Pfd Page No 309 for Table

(Source: IS codes and manufacturing brochures of Ecolite - JVS Comatsco India Pvt. Ltd, Aerocon, Hyderabad, and

Siporex India Pvt. Ltd* Brochure of Magicrete AAC Blocks)

After sampling, testing should be undertaken as soon as possible. All the 24 blocks need to be checked for dimensions and for visual defects. Out of the 24 blocks, 12 blocks are subjected to the compressive strength test, 3 blocks to the test for density, 3 bocks to the test for thermal conductivity and 3 blocks to the test for drying shrinkage. The remaining 3 blocks should be reserved for re-test for drying shrinkage if the need arises.

Criteria For Conformity

The concept and production of AAC is a decade old, and it is manufactured across the globe. Some of the codes practiced are listed below [3].

• Indian Standard : IS2185 (Part 3)
• Australia Standard : AS3700:1997
• Germany Standard : DIN4165:1990
• American Standard : ASTMC1693:1999
• China Standard : GB11968:2006
• England Standard : BS-EN771-4:2003
• Japan Standard : JISA5416:1995
• Euro Standard : EN771-4:2003
• Vietnam Standard : TCVN7959:2011

Along with above standards, a quality management system under ISO 9001:2008 needs to be followed in the manufacturing of AAC blocks.

Benefits Of Autoclaved Aerated Concrete Blocks Environment Friendly

An environmental friendly and energy-conserving AAC block meets all the requirements of our modern age. There are absolutely no pollutants or hazardous wastes generated in the process, and there is no waste of precious raw materials [4]. AAC blocks can be used as an alternative to traditional clay bricks.

Traditional clay bricks are produced from clay soil. Clay soil is a vital reserve that takes thousands of years to form. With the continuous use of AAC blocks, the consumption of naturally occurring clay soil can be reduced. Once clay bricks are moulded and dried, they go in for the elaborate firing process. The firing process is inefficient and polluting. It then leads to a high loss of energy, and incomplete and unequal firing further results in poor quality bricks and high percentage of SPM (solid particulate matter), which is released into the atmosphere and attributes to environmental issues like ozone depletion and global warming [5]. AAC blocks provide excellent thermal insulation and reduce energy consumption quite drastically. The easy workability of AAC blocks helps to eliminate unnecessary wastage on jobsite too.

Durability

The durability of any material is an important factor which needs to be taken into consideration in the selection of building construction materials. AAC blocks have excellent durability. There are numerous structures worldwide, many over 60 years old, in excellent condition [6]. AAC blocks can provide good resistance to harsh climatic conditions and will not degrade over a period of time. This in turn reduces the maintenance cost of the construction, reducing its life cycle cost. Because of being less porous and having lesser joints, AAC blocks provide excellent resistance to termite attacks which prevents the need for chemical protection. Due to their being lightweight, walls made with this material have the property of good earthquake resistance.

High Thermal Insulation

In a typical manufacturing process, large numbers of tiny cells are formed in AAC blocks. These tiny cells contribute to the thermal insulating property of the material, which helps in reduction in the heating and cooling costs of a building. It also has better thermal and sound insulation due to fewer joints. Experiments conducted on AAC block walls have shown that the average energy consumption of a room made with AAC blocks is less by approximately 25 % compared to that of a room made with clay bricks.

Fire Resistant

AAC blocks are totally inorganic and incombustible. As AAC is chemically an inorganic material, which  means that it does not burn, there is absolutely no chance of toxic fumes, which is often the case with traditional materials. The product is especially suited for fire rated applications. Most residential fires do not exceed temperature of 12000c, and AAC blocks do not break down until well over 30000c. This allows it to maintain its structural integrity even after a fire. Walls made of AAC blocks not only resist fire but actually help to stop it from spreading to other areas by serving as a firebreak in the case of homes being situated too close to form a barrier. A 100 mm AAC block wall has a 4 hour fire rating [5]

Sound Insulation

Because of the tiny cell structure of AAC blocks and solid wall construction, an AAC block wall provides excellent soundproofing, greatly reducing outside noise and providing a quieter, more comfortable interior for the occupants. The sound transmission class (STC) rating of a typical AAC wall which is about 200 mm thick ranges from 40–55 [6]. Hence, AAC blocks possess an excellent sound reduction capacity and reduce the echo effect in an empty room. AAC’s inherent sound insulating properties makes it ideal for controlling noise transmission between adjoining rooms and, hence, these blocks are preferred in the construction of flats, hotels, IT offices, recording studios, multiplexes, etc.

Easy Workability

AAC blocks can be easily cut or shaped into the desired dimensions. With AAC, almost any creative design can be achieved. The corners of the blocks can be cut in order to design arches or rounded edges. It can be sawn, drilled, nailed and machined using off-the-shelf tools. It can also be plastered and be finished with tile or paint, making it one of the most versatile building construction materials. Hence, AAC blocks can be used for all applications, including walls, roofs, floors and balconies, in both loadbearing and non–load-bearing applications.

Cost-effective construction

Tables 3 and 4 justify AAC blocks as cost-effective construction material.

PLZ See the Pfd Page No 312 for Table

Conclusion

The huge construction growth in India has boosted the demand for various construction materials. Due to rising costs, it is very difficult for the construction agencies to control the cost of materials. Also, most of these materials are energy extensive and transported over long distance. The extensive use of these materials can drain energy resources and adversely affect the environment. Hence, there is an urgent need to use simple, energy efficient, environment friendly and sustainable building materials and techniques for construction. AAC blocks fulfil all above requirements and can be used an alternative walling material. AAC is durable material in the most diverse climatic conditions. Construction with AAC proved to be economical due to speed in construction, less finishing required over it and very less maintenance thereafter. AAC may not be economical where small quantities are required for the construction and transportation cost itself makes them costlier compared to other alternative construction materials. The properties of AAC, such as thermal conductivity, unit weight, workability, sound insulation, permeability and fire-resistance, make it a versatile and futuristic construction material.

Note: The above calculations have been done considering only the saving due to materials cost, whereas other cost saving due to the light weight of the AAC, handling cost, construction labour cost and lesser structural weight on the foundation leads to a saving in the cost of the structural framework. The overall cost saving could be approximately up to 15%.

PLZ See the Pfd Page No 313 for Table

REFERENCES

1. B.V.Venkatarama Reddy, "Sustainable Building Technologies", Current science, Vol. 87, No.7 October 2004, pp 899-907.

2. APITCO Ltd., A.P., India, < www.apitco.org/Profiles/Aerated%20concrete.pdf > (accessed on 12/6/2013)

3. Autoclaved Aerated Concrete Blocks, Vietnam, < www.aeratedconcreteblock.com/ > (accessed on 15/6/2013)

4. Aercon Autoclaved Aerated Concrete, Florida, U.S., < www.aerconaac.com/ > (accessed on 22/6/2013)

5. ECOLITE Autoclaved aerated Concrete Blocks, JVS Cosmatsco Industries Pvt Ltd., Pune. < www.ecolite.in/pdfs/Ecolite_Brochure.pdf > (accessed on 20/6/2013)

6. M.K. Prabhakar, "AAC, saves time, money and ensures environment benefits", The Master Builder, Feb 2010, pp 72-83.

7. Tarmac Building Products Ltd., U.K.< http://www.tarmacbuildingproducts.co.uk> (accessed on 15/6/2013)

8. Shirke Group of Companies- Siporex India Pvt. Ltd., Pune.< http://www.shirkegroup.com/siporex.htm > (accessed on 12/6/2013)

9. IS 1077, Common Burnt Clay Building Bricks - Specification, Bureau of Indian Standards, New Delhi.

10. IS 12894 Pulverized Fuel Ash Lime Bricks Specification, Bureau of Indian Standards, New Delhi.

11. IS 2185 (Part I): Specification for Concrete Masonry Units (Hollows and Solid Concrete Blocks), Bureau of Indian Standards, New Delhi.

12. IS 2185 (Part III): (Autoclaved Cellular Aerated Concrete Blocks), Bureau of Indian Standards, New Delhi.