Reducing small[ish] hump in concrete

FreeBear

EssexExile wrote: »

I would use that diamond blade in preference to the others mentioned. It will cut quicker & last longer.

One diamond blade would indeed outlast a pack of five discs. Cutting a series of grooves in to the slab at 25-50mm spacing and then chiseling out the lumps would be the way to go. Get a scrutch chisel and a good heavy hammer, and please wear some eye protection.

If you do any cooking, imagine you have made a large tray of fudge and need to cut it to bite sized squares - Now go do the same on the hearth.

Silver-Surfer_2

red_imps_2003 wrote: »

Thank you. I intend to purchase both, as my understanding is that they work in different ways. Am I right in thinking that the diamond blade is designed to 'slice' through concrete (and similar materials) whereas the grinding blades are designed to 'sand' an area down? I suspect I may need to do both to get the area flat enough to lay soundboards over the top. Am off to buy dust mask, ear defenders and goggle snow too. And a massive dust sheet!

There are two types of discs grinding and cutting. Well three actual as you can get them that do both.

Doing it the way people have described on here you need a cutting disc either diamond or an abrasive one.

phil24_7

I would have personally used a kango (like a big drill but it doesn't spin) with a chisel blade in it. They are very easy to use and leave a reasonable finish once you get the hang of them. Another option would be a hammer drill set to hammer only (same principle as the kango but less powerful). Both of these will create much less dust than an angle grinder.

red_imps_2003

phil24_7 wrote: »

I would have personally used a kango (like a big drill but it doesn't spin) with a chisel blade in it. They are very easy to use and leave a reasonable finish once you get the hang of them. Another option would be a hammer drill set to hammer only (same principle as the kango but less powerful). Both of these will create much less dust than an angle grinder.

Thank you. I am hoping to have a go at it tomorrow so it is probably a bit late to get hold of a kango. I do have a hammer drill, though, so will see if it has a hammer-only function. I only have drill bits for it (max 10mm); would you suggest obtaining something more akin to a chisel for it instead? I may give that a go before resorting to the hammer and chisel and/or the angle grinder. I think my main concern with large impact tools is the possibility of creating a substantial crack across the entirety of the concrete. I have no idea how deep it is or what it is underneath it. I'm also worried about introducing cracks to the newly-plastered adjacent wall.

Looking at the 'lump' again a couple of nights ago I don't think it is as large as I first thought. It's barely perceptible to the naked eye except when I look at the skirting, where there is a 20mm gap around the whole room except for a section a few inches long where this mound in the concrete meets it. I reckon I probably need to lose about 7-8mm from the highest part. I need to be able to slide the 1.5mm-thick soundboards under the skirting (and still leave a 2-3mm gap).

There are also lots of pea-sized (or smaller, even) lumps in it, which I would like to flatten down so they don't puncture the flooring. I'll attack these with a chisel or drill in the first instance and try to grind everything flat once I've got the worst off.

Silver-Surfer_2

Unless you have an SDS drill you're unlikely to have a hammer only function on the drill.

red_imps_2003

Silver-Surfer wrote: »

Unless you have an SDS drill you're unlikely to have a hammer only function on the drill.

It's a cheapy drill, so you are probably right.

phil24_7

red_imps_2003 wrote: »

Thank you. I am hoping to have a go at it tomorrow so it is probably a bit late to get hold of a kango. I do have a hammer drill, though, so will see if it has a hammer-only function. I only have drill bits for it (max 10mm); would you suggest obtaining something more akin to a chisel for it instead? I may give that a go before resorting to the hammer and chisel and/or the angle grinder. I think my main concern with large impact tools is the possibility of creating a substantial crack across the entirety of the concrete. I have no idea how deep it is or what it is underneath it. I'm also worried about introducing cracks to the newly-plastered adjacent wall.

The concrete will probably be 3 or 4 inches thick and supported by wood underneath. It may crack if you are too brutal but it wont cause any problems, especially as it will be covered.

As S-S mentions, you wont likely have the right type of drill and you would need a chisel bit for it. Someone you know may have a suitable drill they may lend you, especially if you tell them they can keep the chisel bit.

You will unlikely cause damage to the surrounding area as the floor joists will absorb and soften any vibrations. I battered the crap out of my upstairs hearth when renovating a room and no damage to anything else.

Looking at the 'lump' again a couple of nights ago I don't think it is as large as I first thought. It's barely perceptible to the naked eye except when I look at the skirting, where there is a 20mm gap around the whole room except for a section a few inches long where this mound in the concrete meets it. I reckon I probably need to lose about 7-8mm from the highest part. I need to be able to slide the 1.5mm-thick soundboards under the skirting (and still leave a 2-3mm gap).

Get a long spirit level or straight edge to highlight the high spots (and low spots). Mark these with permanent marker and take a photo or 3 to highlight where needs knocking back.

There are also lots of pea-sized (or smaller, even) lumps in it, which I would like to flatten down so they don't puncture the flooring. I'll attack these with a chisel or drill in the first instance and try to grind everything flat once I've got the worst off.

Please see the notes above.

Good luck

red_imps_2003

phil24_7 wrote: »

Please see the notes above.

Good luck

Thank you. All good observations and suggestions. And all appreciated.

cyclonebri1

red_imps_2003 wrote: »

Jack hammering down the offending section seems sensible (and quicker/cheaper) and I could easily raise and level using sand. My concern with the jack hammer is that I have never used one before and I am anxious about potential collateral damage. Is there a risk the adjoining load-bearing wall could crack, for instance? The bump is very close to it. How much control would I have over chipping away at the area concerned?

I've never heard anything so stupid in my life, talk about a sledge hammer to crack a walnut.
Do that and you'll have a crater to fix FFSAKE, repeat do this at your peril.

red_imps_2003

Don't worry, I haven't pursued the jackhammer route. Am looking more at a mixture of cutting, Chiselling and grinding.