Today whilst plastering with Adobe onto a cement block stem wall we encountered a problem. We were aware that using fibre in an adobe mix was necessary to spread the cracks more evenly across a surface but when we started on our second mix and realised we had forgotten to add grass we stood in reflection for a few moments, and decided to continue anyway. That second mix would only cover around 1.5 square metres anyway so if all went wrong it was not a lot of work to re-do.
A hot afternoon passed and I sprayed the wall with water to try to stop it from drying too quickly, but despite this, large cracks appeared in the section which had been plastered without a fibre additive. I have seen cracks of this size on various walls we have worked on and they have never been an issue but perhaps because we were plastering on to cement blocks which do not easily accept a clay render, they were slightly more vulnerable to the effects of the cracking that was happening
The cracks were so deep that they were able to pull the various sections of the wall away from each other, and the render itself separated from the wall, creating an air gap between the wall and render.
As you can see from the photo above, the cracks are all focused in large fissures, which is the problem we all know when a mix has too much clay, dried too quickly or does not have enough fibre. But this sort of crack is generally repairable, unless the render layer itself actually moves so much that the wall and render become separate.
When this happens the only thing to do is to remove the sections which have air gaps (you can test by tapping on the wall with your finger and listening for a hollow sound).
Generally cracks are not too much of an issue and clay will always show some sort of cracking as it drys and contracts, but if you do not mix your adobe correctly, using the right ratios of clay, sand and fibre when necessary, cracks can actually cause wall and finish failure, which means…doing it again
As you can see in the image above, the wet section across the middle. On the left are barely any cracks, and the render has taken strongly to the cement block wall, but the right shows deep cracks that eventually formed air gaps behind the render, despite not drying fully.
Check out more posts like this in my ‘Materials Testing’ Section