Several new technologies have entered the Indian construction industry in recent times. One of the prominent ones among them has that been of autoclaved aerated concrete blocks (AAC). It is here that Methra Industries with their Cell O Con AAC Blocks have emerged as a leading name in recent times. In this article, P.Venkatesan, Managing Director of Methra Industries, gives detailed insights on the manufacturing process of AAC blocks, the various cost benefits of using AAC blocks, the environment benefits, before finally making an appeal to the different stakeholders for utilization of AAC blocks for helping economic growth of the country along with saving the environment.
Considered one of the foremost authorities in the field, Venkatesan, is a post graduate in structural engineering from Anna University. He has over three decades of experience in the construction industry. Backed by his wide ranging experience he has been successful in making Cell O Con AAC blocks a preferred choice among the different stake-holders in the construction industry. Prior to his starting the venture, he also had a successful stint with PWD.
It gives me great pleasure in announcing that Cell O Con AAC blocks has completed two successful years. Within the short two year period, we have spread our presence to the whole of South India viz Tamil Nadu, Karnataka, Kerala and South of Andhra Pradesh. This has been made possible because of the company’s strong R &D focus and stringent adherence to quality standards.
Another remarkable achievement has been the fact that within the short period it has also been able to export Cell O Con AAC Blocks to various parts of the globe. It is a proof that even though overseas buyers have a number of options before them, they have gone ahead and chosen Cell O Con AAC Blocks , which goes on to prove the high quality standards of the product.
We also proudly announce the launch of “Cell O Con Dry Mix”, compatible for AAC Blocks Methra Industries has also come out with its engineered architectural product- “Cell O Con AAC bull nose elevation band”, which reduces the cost of concrete and formwork. The product has already become the favourite with architects, tech-nocrats and builders, helping in enhancing aesthetics of buildings.
AAC Blocks Manufacturing Process
AAC – is a precast product manufactured by combining pulverised fly ash, cement, lime, water and an aeration agent – aluminium (paste or powder), poured into a mould. Once added to the concrete, aluminium (powder or paste) reacts with lime and fly ash, resulting in the formation of millions of microscopic hydrogen bubbles. The hydrogen bubbles cause the concrete to expand to roughly double the times its original volume. The hydrogen subsequently evaporates leaving a high closed – cell aerated concrete. The now aerated concrete is cut into blocks which are then steam and pressure – cured in an autoclave.
AAC, unlike traditional concrete masonry units, is a solid block / material system with integrated insulative and structural components and is available in the form of a variety of products, which can be used in both load and non-load bearing applications. Complete load bearing applications, however, are limited to low-rise constructions with some specified specifications as advised by the manufacturer / structural engineers, though the infill wall blocks to take advantage of AAC’s insulative, fire proofing and other benefits on mid – and high – rise projects.
The manufacturing of AAC also results in a product that is lightweight, strong, inorganic, non- toxic, and does not shrink, rot, warp, corrode off-gas or otherwise deteriorates. Due to its light weight and dimensional accuracy, AAC can be assembled with minimal waste and a reduced need for additional equipment. AAC is also termite and mold resistant, and nearly fireproof. This extreme durability makes the product virtually maintenance free, eliminating the need for repair materials, pesticides, and chemical treatments, while also lowering operating costs.
AAC has High Thermal Efficiency
Autoclaved aerated concrete is further considered a sustainable building product because of its excellent insulating qualities resulting in increased energy efficiency. AAC’s thermal efficiency stems from the three factors. First, AAC structures results in solid wall construction with integrated insulation. Entire wall coverage prevents the thermal bridging associated with conventional stud – framed walls, which essentially leave cold gaps around every stud and header.
Second, the solid wall construction of AAC structures creates an airtight building envelope, minimizing uncontrolled air changes while helping maintain desired indoor temperatures and maximizing the efficiency of HVAC equipment.
Third, AAC structure benefit from the added value of thermal mass and low thermal conductivity. Though an eight inch wall has a steady state R – value and the benefits of thermal mass, however, vary by location and greatest in regions where the outdoor temperature fluctuates above and below the desired indoor temperature over a 24 – hour period.
AAC Reduces Noise Pollution and Improves Indoor Air Quality
As additional quality contributing the sustainability of a building products is its ability to reduce and absorb noise or to improve or maintain indoor air quality. Due to its millions of independent air cells, which dampen sound transmission. AAC has excellent sound insulation and absorption qualities.
Moreover, because AAC construction has very low air infiltration and is mold resistant, indoor air quality of AAC structures improved relative to competitors, AAC performance as both a structural and insulation material also eliminates the need for other materials (like fibreglass insulation) that may contribute to poor indoor air quality.
AAC structure Are Well – Suited to Withstand Fires, Earthquakes and Other Natural Disasters
AAC is inorganic, non-combustible, and virtually fireproof. It has received a 4 hour UL fire rating and has a melting point of over 2900 degrees Fahrenheit. As a result, the use of AAC eliminates the need for additional fireproofing materials. Moreover, because AAC is completely inert, it does not emit toxic gases even when exposed to fire.
AAC buildings can be engineered for earthquake and hurricane prone areas, and such buildings have performed well, due to its light weight, the losses and damages will be very less. They also were immune from fires started during the earthquake and even acted as firebreaks. The ability of AAC structure to withstand fire and natural disasters minimize waste, contamination to the surrounding environment.
AAC Structures Are Price Competitive
Due to AAC’s low density the structural load (dead load) will be very less and in turn the structural members can be designed accordingly. In turn the requirement of concrete and steel will be less for foundation and all structural members of the building. Because of the bigger size of AAC blocks the number of joints will be less and in turn the requirement of cement mortar will be less. The labour required to lay the AAC blocks will also be considerably lesser and time will be saved too. Due to the thermal efficiency of AAC blocks the cost of energy will be lowered due to lower operating and maintenance costs, the structure’s longer life span and lower initial outlays for heating and cooling systems. When looked at from the point of view of an individual or an entire country, this amounts to tremendous savings. That is why this technology holds so much promise for a country like India, which is facing acute scarcity of electricity.
A Truly Green Material
In the manufacture of clay bricks, the top soil is depleted and natural resources are eroded, resulting in ecological imbalances. But by utilising fly ash which is the waste material generated by thermal power stations, we will be saving the natural resources on one hand and putting the industrial waste into productive use. This makes Cell O Con one of the best examples of a truly “green product”, in every sense of the term. Sand is another alternative to fly ash for manufacturing AAC blocks. However, sand cannot be recycled like fly ash and therefore manufacturing AAC blocks using sand cannot be considered a green process and the resultant product a green product. Using sand will only erode the natural river bed, thereby changing its course and result in ecological imbalances.
As every structural engineer knows, that adequate care needs to be given to masonry wall members, since it gives full shape and structure to the building. I have realized that everyone neglecting the wall member in the structure will mean that importance of the masonry wall will have been neglected by all parties’ concerned – structural consultants, builders, promoters, contractors, etc. This will mean not following the specifications laid down in BIS 6041 of 1985 and BIS 1905 of 1987, which will lead to problems like, shear crack, horizontal crack and settlement crack etc., in the wall at a later date. I therefore appeal to all friends, contractors, constructors, builders, etc, to follow certain important guidelines given below before and during construction of AAC masonry wall for better achievement.
Let us take a closer look at some key points about Cell O Con AAC Blocks.
– Compressive Strength
– Water Absorption
– Laying Procedure
The Compressive Strength Test:
The test specimen should be a cube, of size 200mm x 200mm x 200mm or 150 mm x 150mm x 150 mm (cut from block a block) and tested with a machine of least count 1kN. Load has to be applied perpendicular to the aeration (please check with the manufacturer for ascertaining the correct side). The cube has to be oven dried at 50 C before testing. Testing must be carried out at room temperature. This test conforms to BIS 6441. (Part 5)-1972. Blocks of thickness less than 150mm are not suitable for testing compressive strength. If this block is tested, it will yield lesser values of compressive strength.
Water Absorption Test:
There is no specific water absorption test prescribed in BIS 2185-3 or BS 6073-1 for water absorption of AAC blocks. These blocks, being lighter, float on water and hence any test involving submersion is not possible. The test for water absorption by capillarity measures the rate at which moisture is taken up by the blocks in exposed fair–faced walling. BS EN 771-4 requires the water absorption by capillarity to be measured over a period of 10 minutes in accordance with BS EN 772-11 if rendering is not done. However we recommend rendering for fair faced walls.
We have tested the surface water absorption by fixing the glass piece of 75 mm depth in the periphery of the specimen 600mm x 200mm x200mm size block with water tight (without water leak) and water to be filled fully for 75mm depth and to be kept in room temperature to avoid evaporation of about 24 hrs or 48 Hrs. After the specified time the specimen to be cut and the water penetration due to capillary action to be verified. This proves that water penetration due to capillary action is about less than 5% by surface area.
I have shown through the model picture for laying the CELLO CON AAC Blocks in which the provision of sill bond beam, lintel bond beam, stud reinforcement for converting the wall load into the uniformly distributed load to avoid any type of shear crack and horizontal cracks. Many suggestions have been given below for how to lay and use CELLO CON AAC Blocks & Cement Mortar in a detailed manner.
The Mortar and Bond
To understand the behaviour and visualise the performance of the masonry, one has to understand the basic characteristics of the unit, properties of mortar and their interrelations which determine the efficiency of the bond between them.
When mortar is applied on the surface of a masonry unit, it absorbs water from the mortar, with it the finer particles of the cementing material travel to fill up the voids at the interface, forming the bond layer.
For the mortar to have a moderate rate of absorption by the masonry unit and good workability in application and spreading of the mortar, it is essential that the mortar should have high water retaining capacity. Such a mortar, being plastic in nature, does not segregate even with low absorption masonry units. The mortars with low water retaining capacity are poor in workability. If the initial rate of absorption is high (IRA), the blocks absorb water too fast. This causes formation of channels in the mortar joints which create voids on drying, thus making the joint permeable. On the contrary, if the IRA is low, the water from joining the mortar accumulates at the interface as a result of segregation. This on drying creates a clear cut crack at the interface and forms a source leakage.
Drying Shrinkage / Moisture Movement
The drying shrinkage (moisture movement) of the mortar is to be avoided in the AAC masonry construction by using thin lean mortar with allowable water absorptions. The mortar to be used for masonry should always be weaker than the masonry unit. This facilitates an even distribution of drying and ensures shrinkage cracks occur at a few places passing through masonry units. They occur at weaker sections of the wall. Because, they are less in number, they are wider and hence affect the impermeability to a greater extent.
Significance of pre-wetting of the masonry units with high IRA and not pre-wetting those with lower IRA must be borne in mind. Use of good workable, water retentive mortar results in good quality bond, better compressive strength, shear resistance and impermeability.
Mortar for AAC Blocks Masonry
– Conventional Cement Mortar
– Dry Mix Mortar
Mortars based on mineral binders like lime, cement, or gypsum has been used for more than 8000years in the construction of buildings. These mortars have mainly been used for laying stones, brick, and concrete block, AAC Blocks (masonry mortars) and for coating walls (rendering mortars). The mortar thickness has to be 8mm to 10mm with required water to avoid the moisture movement. Until the 1950s cement made mineral mortars were exclusively produced and applied by so called job – site mixing technology. Thus the cement, the most common mineral binder, is mixed with fillers (sand) before water is added to create the wet mortar for application. For this conventional cement mortar the requirement of the sand (filler) will be much more for masonry wall, because of more thick mortar required. In this case of using conventional mortar, the AAC blocks require a lean mortar thickness of 8mm to 10mm and curing is necessary for joints.
Similar to the way in which job – site mixed cement mortar was substituted by the economically and ecologically more favourable ready – mix dry mortar, job – site mixing technology for masonry rendering mortars was replaced by factory – mixed dry mortars also called dry – mix mortars. Dry mix mortars or dry mortars are produced in specially designed dry – mix mortars plants in which mineral binders and aggregates (sand) are mixed together in the appropriate way. With this dry -mix mortar technology, the laying of AAC Blocks requires only 3mm to 5mm and curing is not required. The process ensures savings of energy, water and labour.
The AAC blocks being a soft material because of the aerations need to be handled very carefully. The light weight nature of the blocks helps in reduction of dead load on the superstructure. There are chances of some damages occurring, if the soft AAC blocks are not handled carefully.
The following guidelines are to be followed while laying Cell O Con AAC blocks.
– Laying of Cell O Con AAC blocks masonry shall be in accordance with the recommendations of IS 6041 of 1985 and IS 1905 of 1987.
– Laying of blocks may be started either at the corners first or it may be started from one end and proceed to the other direction.
– The first course of the AAC blocks masonry shall be laid with greater care making sure that it is properly aligned levelled and plumbed as this may assist the mason in laying succeeding courses to obtain a straight and truly vertical wall.
– The AAC wall laying pattern is only by stretcher course and not suitable for header course.
– The vertical joints shall not be in straight line, overlapping of blocks in the structure course shall be minimum of 250mm over the bottom course.
– For providing the closer in the structure course in AAC wall by cutting the block to the required size not less than 250mm by ordinary angular cutting machine( by dry cutting machine)
– Before laying AAC Blocks apply 8mm to 10mm thick lean cement mortar uniformly over the block and it requires the curing for the joints after laying.
– If the dry mix mortar using for laying AAC Blocks, apply only 3mm to 5mm thick uniformly over the block and it does not require any curing after laying.
– Wet the blocks on all sides of mortar contact surface by brush or cotton waste and place them one by one and press it firmly to have proper bond without any level difference and fill the joints between the blocks by same mortar.
– The RCC sill bond beam to be provided at sill level (below window) for 75mm thick with minimum reinforcement for 100 mm, 150mm, 200mm, and 230 mm thick walls as per the picture shown.
– RCC bond beam to be provided at lintel Level (above the window level other than the regular lintels and openings) for 50mm thick to be provided with minimum reinforcement for 100 mm, 150mmm, 200mm and 230 mm thick walls as per the picture shown.
– For 100mm thick wall the stud reinforcement is advisable as per picture shown.
– The design of wall should be such that cutting of blocks is minimised when leaving space for doors and windows.
– The blocks should not be laid continuously for more than 4 meter span length wall. If necessary to lay more than 4 meter span length wall the gap should be given 75mm width for every 4 meter and the gap should be filled with lean plain cement concrete mix by using graded chips of size 12mm and less.
– The mortar shall not be spread so much ahead of the actual laying of the units that tend to stiffen and lose its plasticity thereby resulting in poor bond.
– Mortar consistency as per requirement of site must be maintained at the point of laying over bed.
– Mortar joint shall be struck off flush with wall surface and when the mortar starts stiffening, it shall be compressed tightly to have a key for plastering.
– Design of Cell O Con AAC block masonry is based on the same para-meters as that of brick masonry, as Indian Standard BIS: 1905 (Masonry Code) applicable to brick masonry is also applicable with the same provisions to AAC block masonry.
– The slenderness ratio of AAC masonry walls shall be as per guidelines of Indian Standard for masonry walls.
– Guidelines for chases/cutting Cell O Con AAC walls as per the Indian Standard BIS: 1905 need to be followed.
– The cutting/chases, recesses etc should be done without damage to the surrounding masonry to use such tools for cutting which depend upon rotary motion not on heavy impact action ( i.e. dry cutting machine)
– The cutting portions should be filled by the same lean mortar, which is used for CELL O CON AAC masonry (sieved sand to be used as mortar) but not up to the surface of the block and make key to the plastering of the walls and layer of galvanised chicken mesh is to be embedded in the plaster over the width of the chase with proper overlap.
– For 100mm AAC wall chases/ cutting is advisable after plastering.
Moreover it is advisable to use the stud reinforcement as shown in the picture to avoid any type of crack.
The CELLO CON AAC Block is the concrete block using cement and lime as the raw material for manufacturing which is having bonding properties and it is not require any scoring marks in the blocks to have bonding with cement mortar or dry mix mortar during laying and rendering (plastering).
An appeal to all stakeholders in the construction industry:
The author wishes to make an appeal to all stake holders in the construction sector to consider the various aspects mentioned in this article during the design stage of the structure itself, in-order to drastically reduce the cost of construction by way of less dead load and in turn reduction in the use of steel and concrete. Further reduction in cost is ensured by way of lesser cement mortar (fewer joints, less thickness, lean mortar, less labour). It would also not be out of place to highlight how structures built with Cell O Con blocks can lower the dependence on air conditioners due to their less thermal conductivity and superior insulation properties. Please re-member ‘Energy Saved is Energy Produced’.
Builders need to come forward to use AAC blocks in industrial, commercial and residential projects as this can not only bring down the overall cost of construction, but also help in creating a cleaner, greener environment.For further details:
Methra Industries India Pvt. Ltd.
62 Panchali Amman Koil Street,
Arumbakkam, Chennai – 600106