How many joules does a rotary hammer need to bore through a concrete slab?

Question

I'm in need to buy a rotary hammer to make some holes in a slab about 12 cm thick of reinforced concrete. The holes will be between 5 and 10 cm in diameter, so I will have to hammer a round opening instead of drilling through (or drill lots of holes in a circle).

The rotary hammers in the market goes from 2,5 to 30J. I have no idea of how much energy I need to this task and I wasn't able to find it anywhere. The price goes up really high (and so is the weight of the tool).

The smaller models (about 2,7J) like Bosch GBH 2-24 costs about 7 days of rental of a bigger model (about 10J), so I'm inclined to buy one if it will work.

Answer 1

It's mostly going to be about speed - if you are patient and like handwork, a star drill and 4 lb / 2 kg hand sledge will drill holes in concrete or granite. If you plan to drill many small holes and then hammer out the plug, the smallest rotary hammer drills will generally do that just fine. The bigger ones may do it much faster, and will do it with bigger bits.

The "nice way" to drill finished holes of the size you want is with a core drill (which just drills around the edge, like a hole saw, leaving a plug or core) but those cost serious money even to rent/hire, usually.

Answer 2

Power drills have a maximum bit size specification shown as: ø. I would take this into account on hammer drills more than it's joules when deciding on a purchase. That's just how well it's going to work and tendency to get bogged-down. When renting, as long as the bit size you need chucks, it's not your concern how bad you dog it out. If it is, don't push so hard and give the bit some time to cool as well as the drill. If you are going to do anything ever again that could remotely be improved by using a hammer drill, buy one.

If this is a one-off, rent the cheapest one they have, butcher both the tool, the job, and be done with it. I used to smash through, too, after drilling a bazillion holes before I had a core bit set. Hilti TE72 is one of my best friends, shes discontinued though. Probably because it still works after 10+ years of being utterly dogged-out and only one 'proprietary' trigger replacement from....Bosch. I see these (so pretty) used for ~$800 and 4" SDS core bits for ~$150.

Answer 3

Do you require a well-made round hole, or just hack an opening for something then pour concrete over it?

If the hole needs to be well made, you will need core drill - those are pretty expensive, so you may want to hire a craftsman with proper equipment instead of buying it all yourself for single job.

If you just need a hole, regardless of how ugly it is, a regular rotary hammer will suffice.
When buying, look for two things:

1. It MUST have option of chiseling without rotation.
2. Read the label and make sure the impact is pneumatic and not mechanical. Around 3 Joules is the standard.

You will find in stores cheap devices that claim to be rotary hammers, but use mechanical impact and cannot get into regular wall. What you are looking for is "big can with a handle" and heavy, not something that looks like regular drill. You don't need to go overboard on power - 3 joules is standard for the job, 24 joules is a jackhammer territory. If your local hardware store offers warranty, buy cheaper model that doesnt look like its going to explode in your hands and just ride it to death. Make sure to buy couple chisel bits for it - those can unexpectedly snap, especially if they get stuck.
Important: Wear eye protection. Bits of concrete in your eyes are no fun.
When you have the tool, its just a matter of jacking a hole with it.

Additional safety note:
Rotary hammers of this size and power often do not include safety clutch, and they can and will twist your arm until breaking without stopping if you are drilling and get stuck in a rebar.
My advice is to use your left hand on the button-handle - this way if the device gets stuck but keeps rotating, it will slip out of your left hand and stop since you let the button go. If you hold with right hand, there is a good chance of device jamming into your hand, button still pressed, and that is quite unpleasant and dangerous.