Why there are more complaints of Cracks in Walls made with AAC Blocks :
During the manufacturing, when AAC blocks come out after autoclaving, they have high amount of residual moisture from 24 to 28%.
(This is the reason most of the manufacturers do not despatch the blocks to the market as the strength of the blocks is less and they tend to chip or crack during loading and unloading of the blocks.)
This residual moisture leaves the blocks predominantly in the first 48 hours. As the moisture leaves the blocks they shrink. Imagine we have got installed these blocks during a masonry wall within the primary 48 hours then the shrinkage happens inside the wall. The shrinkage may be a massive force which tears the wall vertically resulting into a crack at the centre of the length of the wall.
On the other hand if the blocks are allowed to “dry down” for a period of 48 hours to 96 hours before they go in a masonry wall, then you will find that the cracks in your building would be reduced by 80%.
Now what about the remaining 20% of the cracks?
There may be two reasons for the remaining 20% of cracks in the walls made with AAC Blocks. The first one is workmanship.
We need to pack the wall with the upper beam before we set any mortar therein gap. We can use 10 mm size aggregates for the same. Our RCC beam is not in ‘line and level’. This leaves variable gaps between the top of the wall and the bottom of the beam. This gap creates a point load on the wall due to deflection in the upper beam. Once the wall is full of the aggregates then the purpose load from beam gets converted into a udl and stress concentration within the wall is reduced eliminating the cracking in the wall.
Compressive strength or Tensile strength
The other reason for cracking is the strength of the blocks which needs to be minimum 4Mpa which brings down the drying shrinkage value to less than 0.05%.
Another cause is using strong mortar. Use of higher grade of cement to make mortar. Use of rich cement mortar i.e. the cement content is higher than specified in the design mix.
Jointing Mortar Compressive Strength is higher than AAC block.
Now if block compressive strength is less than its joint mortar strength with less construction joint, the surface tension of mortar will be very high. And it will be crack because of very less tolerance limit in AAC block.
After knowing all these things, a person named Mr. Ramakrishna has come out from the company and started his own dry mix unit. He is selling more than 600 tons of dry mix products in Hyderabad whereas the company he was working earlier, is not able to sell the same throughout India.
The main cause is that the lack of expansion joints, we should never run blockwork longer than 6m without joints.
Even for every 1200 mm height, we should have bed reinforcement.
If the plant rejections are more, it will also reflect in loading, unloading and in the site (after construction). For example, Ascolite, ecolite, shreeshakthi plants have fewer rejections (below 3%) whereas in corporate companies rejection levels are more than 5% and the cracks after construction are more. Big corporate company which uses dry fly ash in their batch, have more complaints of cracks in the market. In the enquiry it was found that their rejection levels are more than 10%.