Quick Answer

Radiant PEX loops are typically embedded 40–80 mm below the surface, but in poorly placed pours they can be as shallow as 20 mm. On a standard 100 mm slab, grinding removes 1–3 mm total, which is safe if the pipe was placed to spec. Before grinding, pressure-test the loop to 3–4 bar for 30 minutes, locate pipes with a cable/pipe detector, and map them on the floor. Use a lighter down-pressure (heated concrete is 5–15 °C warmer at the surface and grinds faster) and avoid edge grinders within 30 cm of walls where installers often run supply and return headers.

Why Heated Slabs Grind Differently

• Warmer paste — a slab kept at 20 °C by radiant heat is softer than a cold warehouse floor, so segments wear faster and cut deeper.
• Longer drying — radiant heat accelerates hydration; a 28-day heated slab is often at full strength while a cold slab is not.
• Risk of pipe puncture — a cut PEX loop means draining the system, jackhammering the floor, and patching concrete. Repair cost commonly exceeds $5,000.

Pre-Grinding Safety Steps

Step	Action	Why
1	Pressure-test loop to 3–4 bar	Confirms no hidden leaks; gives baseline gauge reading
2	Map PEX runs with pipe detector	Highlights shallow header zones near walls
3	Check design drawings	Typical depth = 40–60 mm above pipe, pipe spacing 150–300 mm
4	Lower slab temp to 15°C	Stabilizes grinding rate and reduces thermal shock
5	Mark shallow zones on tape	Crew knows where to lift the head

Tooling and Setup

Because the slab is warmer, it behaves like a medium-soft concrete. Switch from the bond you would use on cold warehouse concrete to one grade harder. A triple-round trapezoid disc in medium bond will cut cleanly without grabbing. Reduce added weight by 20 % — the goal is a consistent 1 mm cut, not maximum removal.

• Run the grinder on a linear pass pattern, never circular, so depth stays uniform.
• Watch slurry color: gray is normal; white milky slurry means you are hitting the top of a shallow pipe zone and should lift.
• Stop immediately if you hear a sharp metallic scrape or see a fine plastic curl.

Edge Work Near Walls

The highest-risk zone is 30–50 cm around the perimeter, where PEX loops turn up to manifolds or run along walls. Hand-grind this zone with light pressure and an S-segment cup wheel rather than the aggressive PCD. Many crews skip the perimeter edge grind entirely and leave a 5 cm hand-held finish — acceptable if the customer signs off.

After Grinding

Re-pressure-test the loop after the job. A 0.1 bar drop over 24 hours indicates a micro-puncture. Most heated slabs take densifier normally; lithium is preferred because it does not rely on film. Run the system at low heat through the first winter to avoid thermal shock to the newly exposed paste.

Frequently Asked Questions

Q: Can I polish over a newly poured heated slab?

A: Wait a minimum of 7–14 days if the slab was actively heated during cure; the paste reaches strength faster but you still need a full 28 days before densifier application.

Q: How deep can I safely grind?

On a spec slab (pipe 50 mm deep), 3 mm total removal is safe. On a slab with unknown pipe depth, stay under 1.5 mm until you have verified the layout.

Q: What if I hit a PEX line?

Shut off the manifold, drain the loop, cut out the affected concrete, splice the PEX with a compression coupling, patch with rapid-set mortar, and re-grind. Document for the GC — insurance usually covers contractor-caused damage.

Q: Does densifier affect the heating performance?

No. Densifier reacts only with the top 2–3 mm of paste; heat transfer through the slab is unchanged.

Cost of a Puncture (and How to Avoid It)

A PEX loop puncture typically costs $1,500–5,000 to repair: locate the leak, jackhammer a 1 m² patch, splice the pipe, pressure-test, patch concrete, and re-grind. On a 500 m² job, that eats the entire margin. Prevention costs 30 minutes of mapping with a pipe detector — the cheapest insurance on the site. Most GCs now require pipe mapping as a pre-grind step and will pay for the detector rental.

When to Refuse the Job

If design drawings are missing, the slab was poured recently, or the pipe detector shows loops shallower than 20 mm in working zones, walk away. You can still do a cosmetic honed finish at under 1 mm depth, but full grinding to expose aggregate is off the table. Put the refusal in writing — it protects you when the next contractor finds a puncture.

Conclusion

Grinding over radiant-heated slabs is safe if you pressure-test, map pipes, and tune setup for a warmer, softer slab. Use medium-bond trapezoid discs, reduce down-pressure, and baby the perimeter edges. With the right zig-zag trapezoid disc setup, a 500 m² retail floor can be ground, polished and handed back in 4 days without a single punctured loop.