hollow sound under self level compound

[jhaug]

I'm installing a bathroom in my basement, about 1 month ago I put down henry self level compund to take some of the dips out of the concrete floor. I could be wrong, but I don't think it required a primer. Anyway, just today I noticed some hairline cracks, which lead to me tapping and noticing some hollow spots. I whacked it few times with a hammer and it did not readily come up. I will putting down redguard and then tiling for finished floor. The affected areas are no larger than say 18 sq in. What is proper protocol in this situation? Do I have to remove all hollow areas, prime and repour. Will its present condition pose serious problems if I just continue with install? any help is greatly appreciated.

[hj]

If you tap the tile after it is installed it will also probably have the hollow sound. But I go into bathrooms all the time where I can tell the tile is loose, but the homeowners haven't a clue it has happened. If the compound did not bond at that point, there is a good chance it did not bond elsewhere even if it does not ring hollow.

[jadnashua]

The leveling compound will likely debond. Most configurations require the primer - it is NOT optional. Without knowing the specific one you used (Henry makes several), it is hard to say, but the instructions will tell you for sure. Cracks often are a sign of not measuring the water properly - too much causes it to have excessive shrinkage. It could also be inadequate mixing. Most are very specific about the type of mixing paddle, time, and speed of the required mixing. The leveling compound contains a lot more than just cement and sand. There are often all sorts of proprietary things like laytex, that must be wetted properly to work and get coated by the cement into a matrix. Fail to do that, and it won't work well.

Redgard is primarily a waterproofing material. Although it does have some minor decoupling capabilities, if you are worried about that, I think you'd be better off with something like Ditra, which can both waterproof and decouple. To get the waterproofing, you need to use Kerdi Band on the seams. If you are hoping to keep water vapor from migrating through the slab, Redgard will just debond from the surface eventually. Ditra, with its channels, will allow a path to the edges of the room for that water vapor to escape.

[jhaug]

I used henrys 547 universal. I reread the instructions and don't see it calling for a primer. Anyway, the area poured is about 18 sq ft out of apx 80 sq ft, and will be primarily covered by a washer and dryer, if I use ditra, do you think I can get away with not removing the floor and repouring?

[jadnashua]

The vibrations from the washer and dryer might just make it worse. I'd ask this over at www.johnbridge.com where they specialize in tiling. They have the MDS, but not the detailed installation instructions on their website. The best thing for you to do is to call their Technical Services Phone:
1-866-443-4455 or
724-203-5000
and get a good answer; anything else is a guess. It would be a pain to install tile and then have it fail.

[jhaug]

thanks for the info!