Bricks to blocks — Changing paradigm of construction

Jagvir Goyal.

Advent of many new materials has brought significant changes in the concept of residential buildings in India. Path breaking designs are being suggested by the architects. The most basic building material, the brick, too, has witnessed a change.

Now, as the awareness for building earthquake resistant houses increases, RCC framed structures are being preferred by people building houses on large size plots. For such structural frameworks, AAC blocks are being preferred over bricks to raise the walls.

AAC is short form of Autoclaved Aerated Concrete. Autoclaved Aerated Concrete blocks though in production for the last three decades in India, have not seen much use in the residential sector at individual level. But now, even individuals are using these whenever they opt for RCC framed structure for their house.

Size of AAC blocks

AAC blocks are much larger in size than the normal bricks. The normal length of these blocks is 600 mm that is about 2 feet though these are manufactured with lengths of 400 mm and 300 mm also. The width is 200 mm i.e. 8 inches. AAC blocks of 4, 6 and 10 inch thickness are also produced. The height of AAC blocks is kept as 75 mm to 300 mm i.e. 3 inch to 1 foot. Blocks of all sizes are thus produced and one may choose blocks of dimensions in accordance with the site requirement. Commonly used sizes of AAC blocks are 16 inch x 8 inch x 8 inch, 16 inch x 8 inch x 6 inch and 16 inch x 8 inch x 4 inch.

Solid & Hollow blocks

Solid as well as hollow AAC blocks can be produced and are in use. Hollow blocks have hollow slots left in their body thus making them still lighter and more resistant to heat and sound due to air cavity created in them. These, however, need more careful handling at site and one has to be careful while cutting chase in them to conceal any light conduits in them. Solid AAC blocks are used more as users consider them safer than hollow blocks.

Advantages over bricks

The biggest advantage of using AAC blocks instead of bricks in walls is their thermal insulation property. Aerated concrete transmits less heat than normal concrete due to low thermal conductivity. R value of AAC blocks is checked before selecting them. R value is a measure of thermal resistance of materials. Higher is the R value, more is thermal resistance of blocks. This results in cooler homes and lesser air conditioning load. Another advantage of AAC blocks is their light weight which causes lesser load on foundations resulting in economical design of foundations due to reduced dead load. Larger size of blocks also results in lesser number of joints and lesser consumption of mortar in AAC block masonry. Their surface finish being much better than that of bricks, there is saving in plaster work also. Earthquake-resistant design demands that a building should be light in weight. That purpose is also served by AAC blocks.

Sound Insulation

AAC blocks provide good sound insulation. These are rated on Sound Transmission Class (STC) basis. One may look for STC value of AAC blocks if sound insulation is a particular requirement. STC rating is worked out by taking average of sounds of 16 different frequencies, measured in decibels, stopped by the blocks. AAC blocks can provide STC of 40 and above. 

Efflorescence

Another major advantage of using AAC blocks in walls is the elimination of problem of efflorescence in walls. Efflorescence, commonly known as ‘Shora’ problem is so much prevalent in brick masonry walls that people often ask for an alternative to bricks as efflorescence problem keeps recurring. 

Below DPC

Use of AAC blocks in foundations and below DPC level should be avoided. In RCC framed structures, RCC foundations are laid and RCC columns are raised on them. Plinth beams are laid at plinth level and AAC block masonry is raised over them. Column foundations themselves are designed to bear the load of building and AAC block masonry in between the columns below the DPC level plinth beam is not required.   

Precautions during use

While using AAC blocks in wall panels of RCC framed structures, the AAC block masonry work should be delayed as much as possible after the completion of column beam framework. This step will allow the concrete structure to undergo changes, if any, due to structural movements and initial settlement of earth below column foundations and will help in avoiding any cracks in AAC block walls. In windows, a bond beam having nominal reinforcement should be provided at sill level. Similarly vertical studs of RCC should be provided on either side of window frames. Lintel will always be there at top of window. The reinforcement in bond beam can be simple 8 mm bars. Vertical studs will also help in provision of proper anchorage window frame holdfasts.   

Mild steel bars

Wherever AAC block masonry is provided in partition walls, it is reinforced by providing suitable mild steel or tor steel bars at regular horizontal intervals. Sometimes, bars at vertical intervals are also provided. 6 mm diameter mild steel bars are commonly used and provided at every third course of AAC block masonry.

Cost factor

If only the cost of bricks and AAC blocks is compared, AAC blocks prove costlier. However, when the cost of masonry is compared, AAC block masonry proves cheaper than brick masonry. One cubic meter of bricks has 450 bricks in it which cost about Rs 1800. Depending upon the size of AAC blocks used, the number of blocks per cubic meter can be worked out. On the average, 1 cubic meter of blocks costs Rs 3000. In masonry, cost of mortar used in joints is saved due to less number of joints in AAC block masonry. Also, some cost is saved in labour also as blocks are larger in size than bricks but easily handled due to their being light weight. More saving occurs in plaster work also as surface of concrete blocks is much smoother than that of brick masonry and lesser thickness of plaster is required. 

(The writer is HOD and Engineer-in-Chief, Civil Engineering Department in a Punjab PSU)

Autoclaved Aerated Concret

By autoclaved concrete we mean concrete that has been steam cured in an autoclave. By aerated concrete, we mean concrete that has been made light weight by aeration method. Under the aeration method, gas is chemically generated in the concrete through chemical reaction or air is introduced in it when the cement sand mixture is still in slurry form. Millions of tiny air or gas cells get created in the concrete. After autoclaving, which is done for a period of 15 to 18 hours under particular pressure and high temperature conditions, the aerated concrete blocks produced can have their weight as low as 500 kg per cubic meter while the weight of normal concrete is in the range of 2,000 kg per cubic meter. Aerated concrete is also known as cellular concrete.

Load-bearing capacity

AAC blocks can be used both in load bearing walls as well as non-load bearing or partition walls. Their maximum use is being made in RCC-framed structures where these blocks fill up the wall panel spaces between column and beam network. Whenever these blocks are used in load bearing walls, the wall thickness shouldn’t be kept  less than 200 mm though for internal load bearing walls, sometimes, 150 mm thick walls and blocks are also used. For external walls, however, the wall and block thickness should be kept as 200 mm or more.