Tiles popped up at new concrete fill

Hi everyone,

I've looked through the different forum categories and I think this is the right place for me to post this question. I have a tiled bathroom that was done a couple of years ago. The tiles have popped up at the shower and the bathtub. I initially thought it was a waterproofing issue but when I removed the tiles and shower to look at the slab, it looked to me that it was the concrete fill that was the issue. The new concrete somehow swelled and caused the tiles to pop up and get damaged.

My question is: what do you think caused the concrete fill to do that? Before I fix it I want to know the exact issue to prevent it from happening again. My guess is the builder used a dodgy concrete mix and maybe put the tiles on top before the concrete complete cured.

I've attached here 3 photos. The first photo shows the floor with the holes before the builder did the work. The second photo shows the swelled concrete at the bath drain. The third photo shows the swelled concrete at the shower drain.

Thank you!

 

Fist off, concrete does not swell, when cured it does not absorb water like a piece of wood.

Question, for the areas that were filled, how were they prepared?

Were they filled with sand, compacted, and wet concrete filled and smoothed?

 

Thank you. Yeah I thought it would be strange for concrete to swell, but I couldn't find any other explanation.

I don't know exactly what they did, so I just broke out the concrete to look. It looks like they poured concrete directly on the round gravel. Looking at the top of the concrete fill, it probably wasn't smoothed either. The photo shows the transition from the original slab to the concrete fill. As you can see, there is a damp proof membrane under the original concrete, but there's nothing under the concrete that filled the hole. Maybe moisture under the slab got into the concrete fill and somehow pushed it up? The round gravel under the slab is damp but not wet. The concrete itself is a little damp too and crumbly, definitely softer than the original concrete around it.

 

Not seeing any standard practices for the construction of the shower bed, pan liner, pre-slope and with the large format tile, can't imagine how they sloped it toward a drain that is not designed for use in a tiled shower. The lack of a pan liner meant that water was seeping into the mortar and not the drain causing swelling of whatever was underneath. To me, the whole install was not done correctly.