Crack filler for garage floor?

[Andehh]

We had some movement where a subbie used insulation under the screed in the garage floor. Found out after a year of use, but builder came out and dug out and redid the area. 

 

However, I now have this slight join between the two different slabs, and with my epoxy resin coating it really shows. 

 

Before the builder rectified it, when the original crack was there I tried some two part crack filler but it was gritty and impossible to get a smooth/flush finish.

 

Any recommendations for how to make it as seemless as possible, but also slightly flexible with the two different slabs? 

[Nickfromwales]

Grind it out and use a mastic that’s designed for this job.

 

You can hire a grinder on wheels like an electrician uses for chasing out. Then put a blade in that’s wide enough to give you a decent bit of space for the mastic to have some purchase.