Hollow Sounds And Space Under Edge Of Tiles?

[mobad]

We are having our kitchen redone due to problems with our suspended flooring.
As a result we have put a concrete floor down.
We have just had our tiler start on the job - laying 40m2 of porcelain tiles (60*60)

We have just inspected some of the tiles that were laid and have noticed that a couple of them sound hollow in places and when we look around the edges of the tiles, you can see gaps between the base of the tile and concrete flooring.

Please see pictures attached.

Please see video of the tiler tiling - Kitchen_20170901_122447.mp4 - https://drive.google.com/open?id=0Bz6kL5nCR1XKbkg4MWFpREFValE

*We also have underfloor heating which had been left on by the builders before the tiler started the job and was only switched off between 30mins-1hr before hand. Not sure if any relevance to the above?


Is this normal or do we have a problem?!

 

[L95]

Well it's not been dot & dabbed so that's a good start. The concrete doesn't look overly level and as your tiler is using leveling clips to give you a good finish, it may be a side effect of that. I would personally talk to your tiler regarding the hollow sounds but as far as I can personally see he seems to be doing everything as expected.

 

[tilerman]

Personally, I back butter all large format tiles. He was not doing that. Also, what type screed is it, traditional sand/cement or a poured anhydrite?

 

[foxhole]

Those unsupported corners could snap when weight applied or accidentally dropped.

 

[chappers]

I agree essentially it appears he is doing all the right things there but there at a few issues.
He's not really getting enough adhesive to the edges of the tiles hence the gaps you can see, tiles of that format should be fully bedded once pressed down.
He's using lash clips which when tightened can lift the tiles together rather than pulling them down to each other and to exacerbate the problem it doesn't look like he is using a deep enough notched trowel, I would use a minimum 12 mm trowel for those sized tiles and back skim to ensure full bed coverage. It does appear that there are some issues with the evenness of the screed and in those circumstances I would probably up the trowel size to 15mm or more particularly as he's using lash clips. I would be a bit worried that he lifted the tile and then only banged a blob of muck under the corner to level it back up and then relaid it. Almost certainly avoid under that one.
Then on top of that the UFH should have been off for at least 48hrs ideally but slowly ramping up the temperature when turning it back on is more important and preferably leave it off for a week too.
The only sure way to find out if the tiles are fully bedded is to lift some, my guess however is that they're not, normal procedure would be to lay a couple and lift them again whilst still wet if you were unsure of getting full adhesion, and gaps under tiles would be an indication that you weren't
The gaps under the edges may fill with grout if he is thorough enough and you may never have any problems but my personal opinion is that this does have potential for problems, whether as Foxhole suggests cracked tiles or tiles coming loose.