Quick Answer

Yes, expansion joints can be ground flat for polished concrete — but only after they are properly cleaned, backer rod is set, and a compatible joint filler is installed. Never grind over old, loose joint filler or exposed backer rod: it pulls out, tears segments, and leaves the joint deeper than before. Fill and cure, then grind flush with a coarse metal disc before the polish sequence.

Why Joints Are a Problem Area on Polished Floors

• Joint filler is often softer than concrete and erodes faster, leaving a low channel.
• Old filler hides dirt and debris that destroys diamond segments.
• Backer rod exposed at the surface catches the disc edge and rips segments.
• Rough joint edges chip during grinding and show as dark lines after polish.

Step-by-Step: Grind an Expansion Joint Flush

1. Route or clean the joint to a consistent width and depth.
2. Install backer rod at the right depth, leaving space for the filler.
3. Fill with a semi-rigid or rigid joint filler designed for saw-cut joints.
4. Allow full cure — never grind a soft filler.
5. Grind the whole floor, joint included, with a Double Arrow Segment Diamond Trapezoid Grinding Disc (JOY-TP-A2) or similar coarse metal.
6. Continue the grit sequence; the joint will sit flush with the slab.

When NOT to Grind the Joint Flat

• Working joints (structural movement joints) should stay open or use a flexible cap, not be ground flat.
• Joints wider than 12-15 mm usually need a different filling approach, not a flush grind.
• Unsure joints — saw a test joint open and check the condition before deciding.

Tools That Handle Joint Grinding Best

Coarse metal trapezoid discs handle the joint edge without chipping. Keep the machine moving across the joint — holding the disc on the joint line wears the center segments faster. A disc with a balanced segment pattern, like the Double Zigzag Segment Diamond Trapezoid Grinding Disc (JOY-TP-ZG2), spreads wear more evenly across the pass.

Frequently Asked Questions

Q: Can I just grind the joint edges without filling?

A: Edges chip and collect debris; the polish will not stay clean. Fill first, then grind the whole width.

Q: What filler is grindable after cure?

A: Semi-rigid polyurea and cementitious fillers grind cleanly after full cure. Flexible sealants generally do not.

Q: Will grinding the joint damage the diamond segments?

A: Only if you grind old filler, sand, or exposed backer rod. Clean joints cut fine with coarse metal.

Q: Should joints be ground in the same pass as the floor?

A: Yes, same pass with the same disc keeps the joint flush with the slab instead of leaving a step.

Conclusion

Grinding expansion joints flat is standard practice for polished concrete when the joint is cleaned, filled, and cured first. Use coarse metal discs for the flush pass, then carry the full grit sequence across the joint. Tell JOY TECH your joint width and filler type for a disc recommendation that keeps the pass clean.