You Can’t Do THAT

Posted by in Cracking, Masonry | June 04, 2019

We have been told by some masons that a new concrete block wall can’t be toothed into an existing concrete block wall.


Yes and No.

Apparently, it can be done, as shown in Figure 1.

New block wall toothed into existing block wall
Figure 1 New block wall toothed into existing block wall.

However, since the new blocks will shrink from moisture loss until they stabilize in their environment, shrinkage cracking may occur at the toothed joint. 

Usually shrinkage cracking is very narrow.  Since it is very narrow, exterior dampproofing will usually bridge across the cracking and continue to prevent the entry of ground water or surface runoff. 

However, if the block were left out in the rain and swelled from moisture absorption prior to installation, the shrinkage cracking may not be narrow and the dampproofing might not be able to bridge across the cracking. 

One way to avoid shrinkage cracking in this situation is to place a vertical expansion joint near the toothed joint so that length of new wall is relatively short and when it shrinks it is less likely to pull away from the existing wall and crack.  But then, the vertical expansion joint will need to be properly treated with a membrane, backer rod, sealant, etc. to prevent the entry of air and water.  Another way to reduce the risk of shrinkage cracking is to dry the block units before they are installed (like in the old days).  If they are installed dry (drier), they are more likely to expand from moisture absorption than to shrink from moisture loss, thus reducing the risk of shrinkage cracking.

Or just let it crack and then return to rout and re-point or seal the cracked joint.