Partial Slab Demolition 2

[Boiler106]

We are making a new opening in a mild (no PT) 2 way elevated slab in the US. We want to remove a portion of the slab, but keep the existing slab rebar, in order to move a beam. Full depth sawcutting is NOT an option.

Reading through ACI546, the committee seems to draw the conclusion that hydrodemolition is the only method which will not damage structural concrete that is to remain. They indicate that hammers heavier than 30lbs can cause microfracturing in the concrete.

This doesn't feel right to me, and I was wondering if anyone has experience specifying demolition methods, where full depth sawcutting is NOT an option.

thanks!

 

[Canuck67]

Boiler,

Jackhammers are often used for through slab demolition. For a small opening, the mobilization costs for the hydrodemolition equipment would be cost prohibitive. Yes, 30 lb jackhammers cause microfracturing. The perimeter of the demolition area is usually sawcut on the top surface to a depth of about 3/4" or less if concrete cover is minimal. (Don't forget to shore the slab if opening is large). Then starting from the centre of the area, the concrete is jackhammered out using a 30lb or heavier jackhammer. Stop approximately 3" from the edge of the opening and switch to a smaller 15lb jackhammer, the concrete bruising and microfracturing will be significantly reduced. Finish the opening with the smaller jackhammer. If you are reviewing the perimeter, sometimes you can see some fractured concrete and a hand hammer can be used to remove and ensure you have sound concrete around the perimeter. The perimeter of the soffit is also usually cleaned up using a grinder to provide the equivalent of a sawcut on the underside. The purpose of the perimeter sawcuts is to prevent feather edging of the patch.

 

[manstrom]

I just did something similar on a home reno. DID this, not designed this.

You can cut into the slab pretty deep with a gas powered concrete saw and a diamond blade. 4" to 5" cut depth. Once you get as deep as you can go, chip the rest out with a jackhammer. The saw does a pretty efficient job of cleanly destroying anything. Cut any rebar with an angle grinder. Pros shouldn't have much trouble with it.

Saws like this

http://www.contractorsdirect.com/Concrete-Tools/Concrete-Cut-Off-Saw

 

[Boiler106]

manstrom, the issue isnt so much 'can it be done' but rather can it be done without causing microfractures in the slab beyond the sawcut line

 

[jayrod12]

manstrom. I believe you missed the point the OP made about wanting to keep the slab rebar intact.

I agree with Canuck67's method. I have seen the guys use an angle grinder with a diamond cutting blade to do the same saw-cuts (3/4" deep) on the underside of the slab prior to beginning the jackhammering.

 

[Boiler106]

Canuck, might there be a proper way to specify this type of demolition?

 

[Canuck67]

Very Briefly,
1. Contractor to supply, install and maintain shoring.
2. Agree on area to be removed.
3. Sawcut perimeter of repair (removal) patches to provide a vertical surface a minimum of 3/4" deep. DO NOT CUT INTO OR DAMAGE REINFORCING. (we have all been on those projects where initial quality control was not in place an rebar is at or near the surface).
4. Remove all concrete using 15lb and 30lb jackhammers. 30lb jackhammers shall be used for initial removal. 15lb jackhammers shall be used for all final chipping and surface preparation to the satisfaction of the engineers until sound concrete is reached. (you can define if they are to do the last X inches of removal around the perimeter using 15lb jackhammers).
5. At this point the engineer would spot check visually and perhaps with a hammer to ensure the concrete on the edge was sound and no unsound concrete.
6. Sandblast the repair area.
7. Bonding agent if you so choose.
8. Pour, vibrate and finish concrete patch.

 

[CELinOttawa]

You mean bond breaker... Haven't seen a low bid contractor apply a bonding agent properly yet. Just specify SSD condition and save your client money, and yourself the probable headaches.