Chapter 2 Literature Review
3.1 Study on walls and insulation capabilities of different materials and methods of
3.1.4 Autoclaved Aerated Concrete (AAC)
The suggested solution for newly planned constructions is to use AAC as wall filling.
AAC is a versatile lightweight construction material and usually used as blocks. Compared with normal “dense” concrete, aircrete has a low density and excellent insulation properties.
The low density is achieved by the formation of air voids to produce a cellular structure (see Figure 3.3). These voids are typically 1mm - 5mm across and give the material its characteristic appearance. Blocks typically have strengths ranging from 3-9 Nmm-2 (when tested in accordance with BS EN 771-1:2000). Densities range from about 300 to 750 kg m-3; for comparison, medium density concrete blocks have a typical density range of 1350-1500 kg m-3 and dense concrete blocks a range of 2300-2500 kg m-3 (“Autoclaved aerated concrete (AAC, Aircrete)” Understanding Cement, Web. 2013).
Figure 3.3. Cellular structure of AAC with air voids. (Web. 2013)
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AAC is not a “new” innovation. Autoclaved Aerated Concrete has been around for over 80 years. Invented in 1923, AAC has been used extensively in Europe and Asia. It comprises over 40% of all construction in the United Kingdom and 60% in Germany. More AAC is produced worldwide than any other building material with the exception of regular concrete (“Autoclaved Aerated Concrete also known as AAC.” Aac-autoclavedaeratedconcrete.com, Web 2013).
The main advantages and preferences of AAC are:
Thermal protection.
When used to build external walls, AAC along meet all strict requirements made by various countries without use of any other auxiliary thermal protecting materials. It can be illustrated that AAC with thickness of 4-5cm performs the same function of thermal protection as one layer of common bricks. Therefore AAC is not only the structure material but also the thermal materials. The R-value for 20 cm thick AAC block is 8.12 (h·ft²·°F/Btu) compared to 20 cm thick concrete block 1.11 (h·ft²·°F/Btu) (Institute for Sustainable Futures, “Your Home – Technical manual.” 2010).
Lightweight.
The dry specific gravity of AAC is between 300kg/m3 to 650kg/m3. This is about 1/3 of common bricks and 1/4 of common concrete. Therefore it reduces the total weight of the construction and is very easy to process on-site. AAC is easily cut to any required shape.
AAC's light weight saves labor.
Sound Insulation.
The test result shows that AAC has excellent acoustic performance. Walls could reduce noise range from 30-52dB depending on thickness of walls and different surface disposed (“Advantages of Ytong Block”. Xella International Gmbh., Web. 2013).
Fire Resistance.
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The raw material adopted and AAC itself are absolutely non-combustible. Experiment results show that 10-cm thick walls made of AAC could stand against fire for at least 4 hours.
Hence AAC is the perfect building material for fireproof walls (“Autoclaved Aerated Concrete also known as AAC.” Aac-autoclavedaeratedconcrete.com, Web. 2013).
Convenience.
AAC can be sawn, drilled, nailed and machined using normal wood working tools.
Also, simple construction details allow the designer, detailer and contractor to quickly and confidently complete project without any anxiety over difficulty or voluminous details.
Standardized sizes makes AAC excellent tool for architects to adapt exact dimensions and shapes to the prefabricate blocks.
Green Design.
The manufacturing of AAC materials is a pollution free process that makes the best use of a minimum amount of energy and natural resources, resulting in a premier green building material. All the materials are completely natural. The block consists of sand, lime and cement - natural abundantly available raw materials that are obtained from responsibly managed extraction sites.
High energy efficiency is one of the determining characteristics of autoclaved aerated concrete. AAC's cellular structure gives it a thermal efficiency 10 times higher than that of aggregate concrete and two to three times better than clay brick. Consequently, buildings made of AAC are warm in winter and cool in summer. With buildings accounting for 40% of the EU's energy requirements, greater use of AAC, both in construction and renovation, offers an immediate solution to cutting the energy consumption of residential and non-residential buildings.
AAC's excellent inherent thermal insulation properties reduce the need for space heating and cooling, also cutting carbon dioxide emissions and combating climate change and also make the use of additional insulation materials unnecessary. Insulation requirements can be met by using AAC alone. By contrast, aggregate concrete, clay brick and calcium silicate
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masonry units need to be used in combination with insulation products, thereby adding to their cost and environmental impact.
AAC is energy-efficient over its whole life cycle. Its production requires less energy than other construction materials and its light weight saves energy in transportation (European Autoclaved Aerated Concrete Association, Energy Efficiency, 2011).
The use of autoclaved aerated concrete has a range of environmental benefits:
Insulation: most important, the insulation properties of AAC will reduce the cooling and heating costs of buildings constructed with autoclaved aerated concrete, with constant savings over the lifetime of the building.
Materials: lime is one of the principal mix components and requires less energy to produce than Portland cement, which is fired at higher temperatures. Sand requires only milling before use, not heating. Lime may require less energy to manufacture compared with Portland cement but more CO2 is produced per ton (cement approx. 800-900 kg CO2/ton
compared to lime at 1000 kg CO2 per ton).
Carbonation: less obviously, the cellular structure of aircrete gives it a very high surface area. Over time, much of the material is likely to carbonate, largely offsetting the carbon dioxide produced in the manufacture of the lime and cement due to the calcining of limestone (“Autoclaved aerated concrete (AAC, Aircrete)” Understanding Cement, Web.
2013).
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3.1.5 “Baoshan” Case Study
In 2009 I was part of a construction project of an 8 apartment villa in Baoshan area, Hsinchu. The plan was to use traditional clay bricks as a walls filling. After my suggestion and cost calculations the owner accepted that we will use AAC blocks as the wall filling. Because there wasn’t any company that makes AAC in Taiwan I had to import all the AAC blocks from overseas. The cost of the material along with transportation and all necessary fees was at the time the exact price as the traditional clay bricks. Therefore the designer and the owner decided to use the AAC blocks as the building material for all walls of the villa. Because of relatively new construction material in Taiwan I had to personally train the contractor and help the contractor with building process. Figure 3.4 shows the construction process on site.
Figure 3.4. Construction process. Building with AAC blocks. (Krivak J., 2009)
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One of the main attributes of the AAC is the construction speed. Because of the size and lightweight the construction process is more than three times faster than clay bricks. This was not well received with the contractor due to their contract with owner that the contractor is paid by the time of work and not the amount of work that contractor had done. For example, if traditional clay bricks are used the contractor would need about three weeks to complete all the walls but with AAC blocks the work could be done only in one week. That means that contractor will lose earnings for 2 weeks.
This situation can be very easily solved with proper contracts between owner and the contractor. It is not standard that contractor is paid by the time spend for work but for the work that been done by quantity. If paid by time needed for work there is a speculation that the contractor would intensively slow their work speed due to higher earnings.
The benefit was the construction speed and much higher quality. The most difficult challenge was changing people's mind to accept this solution.
Price calculation: The cost of AAC was calculated as all expenses needed to buy and transport AAC on working site.
For this project was bought total of 87 m3 of AAC blocks of size (600*250*125mm) for inner walls and (600*250*250mm) for exterior walls.
Price for AAC blocks at Shanghai Ytong factory was at the time 64 US$ per 1m3 .
After all expenses, included fright, customs clearance, taxes and on site transport and import provision, the price for AAC blocks was established as 4140NT$ for 1m3 include adhesive needed for building with AAC.
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Very important factor in comparison between AAC and bricks is price for material, labor and time effectiveness. Table 3.3 shows the calculations for both AAC and bricks.
Because of the difference in thickness of the walls (25cm AAC, 20cm brick) the price will be calculated per m2 of the construction.
Table 3.3. Cost effectiveness for AAC and brick walls. (Krivak J., 2013)
Description Units AAC blocks
(600*250*250mm)
Clay brick (200*100*50mm) Cost of the material m
21040,-NT$ 1350,-NT$
Time factor 1 person 14m
2/day 3.5m
2/day
Labor price 1 day 3600,- NT$
Labor price per m
21 person 260,-NT$ 1030,-NT$
From the Table 3.3 it is clear that using AAC blocks as a wall building material is much more cost effective than using traditional clay bricks. As shown the cost of AAC material is slightly lower. The building effectiveness is 4 times higher than in clay bricks because of the size and lightness of the AAC blocks. As a result the labor price per m2 for walling is 4 times lower than bricks.
The measurements of the temperatures on surface were done by professional infrared thermometer Optex Thermo-Hunter PT-3S. Data were collected on August 7th,2013. Data were measured on the ouside and inside surface as well as inside and outside temperature.
Results are shown in Table 3.4. Weather conditions: Sunny with outside temperature of 36, 2
°C.
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Table 3.4. AAC wall temperature measurement data. (Krivak J., 2013)
Wall built with AAC blocks 25 cm thick.
Temperature decrease
Outside temperature 36.2°C -
Outside temperature on surface
31.6°C -4.6°C
Inside temperature on surface
27.4°C -8.8°C
Inside room temperature 27.1°C -9.1°C
The results showed that using AAC as a building material helped to increase thermal properties of the building and then lowered the initial cost of the construction as well as maintenance cost.
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