We see buildings after buildings without a crack in AAC masonry and then we see a project that is full of cracks in the masonry walls. Why is it so?
The number one culprit is drying shrinkage.
AAC blocks during the process of manufacturing, as they come out of autoclave, contain high amount of residual moisture to the tune of 25%. This residual moisture leaves the blocks predominantly in the first 48 hours. As the moisture leaves the blocks they shrink. Imagine you have installed these blocks in a masonry wall within the first 48 hours and then the shrinkage happens inside the wall. The shrinkage is 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 72 hours before they go in a masonry wall, then you will find that the cracks in your building would reduced by 80%.
Now what about the remaining 20% of the cracks? There are two reasons for these remaining 20% cracks. The first one is workmanship. You need to pack the wall with the upper beam before you put any mortar in that gap. This can be done by using 10 mm down aggregates. Our RCC beam is never 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 packed with the aggregates then the point load from beam gets converted into a udl and stress concentration in the wall is reduced eliminating the cracking in the wall.
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%.
