Diamond Angle Grinder Wheel

[Jon McElwain]

I am going to attempt to save my back concrete patio by grinding out a 4-6" wide "dado" across a crack that runs across the middle and filling it with flagstone. The patio cracked due to settling, and I had it mud jacked about 5 years ago. It appears to have stopped moving, but I know better than to try to fill the proposed dado with something like tile that will allow the crack to transfer through. The flagstone can be set in a flexible concrete crack sealer, or maybe sand like you would for a flagstone path or pad. I envision a somewhat artistic meandering line that will cover/hide the crack. I've attached a sketch of what I'm trying to do.

I've posted in Power Tools to see if anyone has an idea on diamond angle grinder wheels that would fit my 4", 4.5", or 7" electric angle grinders. I know I can rent a gas powered demo saw or other tools, but I am exploring the possibility of using my own tools first. I thought of making several passes and chipping out the waist with my SDS rotary hammer.

Thoughts?

335700

[Tom M King]

What's the total lineal feet of cuts you will need to make, and how deep? I'm thinking the cuts can be 3/8+ to 1/2" apart, but it would depend on the aggregate size in the mix. I've done something similar more than a few times, but not exactly like that.

If it's a job smaller than requires a rental, I'll buy a 30 buck 4-1/2" grinder from Walmart expecting to throw it away fairly quickly, but don't want to put one of my good grinders in concrete dust. I have a helper hold a slow running water hose on the wheel to knock down the dust. Water flow is varied until there is no dust. The tool is double insulated, but I still have a second helper holding the cord plug ready to unplug it quickly if needed. I've done this more than a few times in anything from concrete, to brick, to stone.

Once I bought the cheapest circular saw for the same such use, and used it with the same waterhose method. I used the circular saw method when we had to cut a 12' long rabbet in the edge of a concrete carport floor that we were converting to a greenhouse.

What you pay for in the range of diamond wheel pricing is longevity pretty much directly proportional to cost. I sort of remember buying a pack of three in Lowes for the rabbet, and we only used up two. You know when one is done. They don't start cutting slower. They just reach an abrupt point where it won't cut at all.

However you do it, wear whatever breathing protection you need to avoid breathing concrete dust.

[Jon McElwain]

The crack is probably 15 linear feet. So, if we're going 6" wide, and cutting every half inch, that's 13 cuts to get 6" at 1/2" between center of cuts. 13 cuts, times 15' long equals 195' l.f. of cutting. I've run concrete demo saws for days on end - I could probably cut through 6" of concrete at a rate of maybe 50-75' per hour? It's been a while though - I could be off. I'm thinking this will be a little slower with a smaller tool - albeit not cutting as deep. Perhaps 4 hours of continuous cutting?

[Tom M King]

That's about the same lineal feet we did on the rabbet, but ours was only 1-1/4" deep. I don't remember it taking us over an hour to do it. I could go in from the end for about half of the width for the bottom of the rabbet though. Sounds like a rental is the best bet. I've rented concrete saws from Home Depot.

[Rich Riddle]

I am betting it will take a lot longer than an hour to do even a sloppy job. An hour would be more than three feet a minute. Bent over that's quite a speed to achieve in concrete. If you get it done in a day, I'd say you did a decent day's work. You'll need something to make your back feel better when it's done. It's not the cutting the concrete that will be the only issue. You have maintaining a continuous depth as the blades wear, chiseling out the gaps. making it all "level" or "straight." It's a large undertaking.

[Jason Mikits]

I would suggest a diamond blade for a circular saw. It's easy to maintain the depth, but takes awhile to cut. Cutting grooves slightly deeper than needed and hammering out the waste sounds like a good plan. As others have mentioned, I would strongly suggest proper PPE and using a shop vac, or since it's outside, possibly a strong fan at your back to help with the dust. I cut a few hundred feet of score lines in my concrete ~1.5" deep in a few hours this way and the cheap diamond blade still cuts my score lines as my concrete fetish grows. :D

[Bob Grier]

I have used a circular saw with diamond blade to cut straight and curved edges in concrete and used angle grinder to clean up concrete surfaces and grind swales in concrete patios for drainage. I vote for the circular saw with a little water for dust control and a fan at your back and particle mask. Circular saw blades are less expensive and will be easier to control for a neater edge.

You might consider options for minimizing the width or depth of cutting to save work chipping concrete or you might consider just cutting the line each side of the cut to a deeper depth and then use a hammer to break the concrete out. Then fill with porous bedding material to set your accent pieces and use a flexible concrete caulk to finish the surface joints.

I think it might be easier to make cut each side of the flagstone in 2 passes with circular saw with second pass following the first pass but deeper to make it easier to break the concrete out with a hammer without damaging the patio. Could be that chipping shallow swale would be less work but I think it might be more difficult and take longer. If you try a short section and get discouraged, then you could opt for cutting edges deeper and then breaking the concrete. Breaking concrete with hammer is probably contingent on the existing crack going all the way through the slab.

[Brian Lamb]

I have done this to put electrical in the slab to get to an island, or the center of the shop where the table saw is located. I bought a cheap 7 1/4" circular saw and swapped out to a diamond blade. It's a messy project, lots of dust, but make a couple passes and demo hammer out the center. If you need more depth than you can get with the circular saw, you'll want to rent a gas powered block saw... They rent/make electric versions too, but they are gutless in comparison to the gas versions... and you are outside already, so gas fumes shouldn't be an issue.

[Eric Schmid]

You might find it difficult to cut curves with concrete on both sides of the cut. The smaller diameter the blade the easier it is going to be. Removing some of the waste before making your final border cuts will give you room to maneuver the curves.

I have concrete cutting tools from 4" grinders, circular saws, 14" gas powered cut off saw, etc. I would probably use the circular saw and grinder for the borders and the cut-off saw for the waste cuts. If I only had one to do your job with, the circular saw would be my choice. It will be the easiest to control without additional accessories.

Pearl Abrasives makes a roller wheel for making curves cuts and a water attachment for circular saws, but they are fairly expensive for one time use.

A diamond cup for the grinder would allow you to smooth out the edges; cutting inside curves can be challenging.

Finally, I'd recommend looking into polymeric sand as an alternative to mortar. I doubt the mortar will hold up. Even if your base is no longer settling, just the freeze thaw cycles will probably pop the joints.

[Tom M King]

I forgot the leaf blower. A handheld leaf blower sends dust away if you can't use water. It's what I use for cutting chimney flashing slots (see the Chimney Flashing page on my website, but I can't post a link here).