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zzr-matt
What type of ply was used?
12mm WBP
What type of ply was used?
looks like the grout joint is to wide to hold the thin grout bed you washed it down to thin leaving the tile edges prone to damage and probaly mixed the grout to dry need to rake out do again what sponge did you wash it off with a cheap sponge will pull out the grout .try washing off with an ardex sponge not wet just damp and do one sweep at a time cleaning spong after each sweep
Sorry if its bad advice I thought you have to wait for grout to start setting before you buff off mate
bit i was on about ... it does matter how long you wait 🙂doesnt matter how long after grouting you sponge off.
me personally, I dont think its deflection, i would use bal gt1 admix with a bal grout and this will enhance the flexibility more than your pre bagged bal flexible grouts
If its not a deflection issue why would he have to use a flexible additive?
It could be a number of things as already stated...