SLU hairline crack issue
I poured Mapei Ultraplan 1 Plus over a plywood subfloor and found hairline cracks after a week. The more pronounced cracks seem to follow the path where the thick section of the pour meets a thinner section.
What can/should I do about this?
Was this caused by shrinkage?
Background
My floor had a 1-1/4" slope (from back wall to door) and is on the 2nd floor, so I built up the lowest sections with 3/4" or 1/2" OSB (depending on need) to avoid the added weight of the SLU. (see attached images)
Details
2 layers of 3/4" ply subfloor on entire bath (1 1/2" total)
lowest sections built up with an additional 3/4" and 1/2" of OSB to reduce SLU required (glued & screwed)
subfloor cleaned & primed with Mapei Primer T
I mixed SLU per directions on bag for water / bag ratio (5.75 QTS per bag)
I used a spiked roller to break surface tension and spread evenly.
I tried my best to keep the temperature of the room around 70-75 deg F for around 5 hours after the pour.
The pour looked near perfect with no cracks for a week.
The Mapei Ultraplan 1 Plus instructions say to turn off the forced air for 2-3 days while the SLU is curing, but I am living in the house and it was cold that week in Denver so I did not. could this be the issue?
Images
Hairline cracks as well as the boundary of the built up section. The deepest part of the pour is to the top of the visible boundary.
This image shows approximate depth of SLU poured in each area.
The Black line shows the boundary where the deeper pour meets the section built up with ply.
The most pronounced crack follows the black line.
Hairline cracks outlined for better visibility of location
The thicker side of the pour on the right with a slight lip of less than 1/32". not very noticable
See the floor built up the most near the door with 3/4" osb
Hairline cracks outlined for better visibility of location
Close up of cracks
Thanks for the feedback Ben.
I do agree either I did something wrong or got a bad batch of SLU.
I followed directions exactly, except for turning off the forced air in my house off for 2-3 days (it was cold).
The room temperature stayed between 68-72 during curing.
To answer your questions:
The Mapei SLU manufacture date was Jan 20, 2024, which I purchased at Floor & Decor.
I was not willy nilly with my mix. I've watched many videos of how to do this by professionals (tilecoach, homerenovision, etc) and am not too concerned about how I mixed it.
How I measured: I used a measuring bucket to get the near exact amount of 5.75 Quarts.
Mixing: I used a Harbor Freight mixing drill at high speed with a 30 inch concrete mixing paddle. (my dedicated concrete/mud/slu mixing setup) I've used it before to mix SLU without any issues.
As for dealing with the cracks, Im not concerned about the look of it.
If the cracks wont be an issue structurally, I will go ahead and add an uncoupling membrane, then tile over it.