[qaz112]

Hi concrete base question

I have a new brick out building about 5 by 3 metres, built on an old concrete base. In the 3M dimension there is about 7 inches height discrepancy between ends. At the higher end I want to level the surface for about 2 ½ ft outwards from the wall [to put freestanding steel shelves up].

My question is: if I put down a polythene damp proof membrane will the new concrete base that I intend to pour, tend to slide downhill on the polythene and the slope of the existing base

If this is the case then could I pour the new base slightly lower and without the polythene, then eventually pour another base on top of it using polythene between the two

Thanks

[smearcase]

If you are concerned about the new concrete moving/sliding, why not drill some holes (larger diameter than dowels) in the existing concrete and grout rebar dowels into the holes, and place the leveling course of new concrete?

[qaz112]

Thanks, that sounds sensible.. I wonder though if the weight would hold the new base in place anyway

Cheers

[smearcase]

You could also use an epoxy material between the old and new as well as roughing up the surface of the existing concrete, as well as create a toe wall at the low end. You could also use expansion anchor bolts instead of or in combination with the dowels. The expansion anchors with bolts could be installed before any concrete is installed with the bolt entirely embedded in the new concrete, or the anchored drilled in and the bolt head countersunk after the concrete has set. Also watch the minimum depth of new concrete, best to be at least 3 inches to avoid cracking and breaking off. This can be done by adding height to the slab or by removing a portion of the old concrete at the most shallow end. I vote no on any poly whatsoever- serves no purpose that I can imagine.
Epoxy info: Bonding Agents | Sika Corporation U.S.

[smoothy]

I agree with smearcase on the poly sheet... if you was pouring a new slab on a compacted gravel base it would be far different.

[qaz112]

Interesting, this brings up the question of is the damp coarse necessary [ polythene is a plastic sheet, like plastic bags]. If I don't use it what stops the damp coming up through the old base? [it definitely can do so?], I believe it is normal in uk to use such a damp proof coarse when pouring a base, but there is not one in place here

I have established that if I didn't use the dpc I would just rely on weight to keep slab in place, both surfaces will be rough enough to key into each other anyway , and I will rough up a bit more as suggested... what do people think?

[smoothy]

The poly sheet would prevent the bonding... you'd have to pour it thicker AND put a wire mesh to prevent cracking because it would act like two separate slabs.

Not sure how much anchors and pins would change that... but I defer to smearcase on his opinion there.. as I'm sure he's got a lot more experience than I have.

[smearcase]

I went back and re-read your original post a little more carefully as I should have to start with. I had mis-read thinking that you wanted to level up the whole slab.
To just create level steel shelves, my first inclination would be install some anchors in the existing concrete, possibly screw all-thread bolts into the anchors, then into some angles bolted or welded to the steel shelf supports (uprights which usually have pre-drilled holes). With nuts and washers above and below where the bolt passes through the angle, you can level it as you please and cutoff or not cutoff the excess bolt length. Nuts and washers at the surface of the existing concrete, installed on well placed anchors should certainly support the shelves.
Or, as an alternate, even bolt on some treated lumber to provide a level base.
If you already have a moisture problem in the existing slab, you may need to install underdrain on the perimeter of the slab or if severe, consider a sump pump to relieve the pressure.
If you definitely require concrete and are concerned that bolts or rebars (grouted into the existing concrete) and/or expansion anchors and epoxy are insufficient-- remove the existing concrete where the leveling pad is to be placed and pour a full-depth (down to subgrade) pad. Install poly under that new pad and epoxy all edges to repel moisture.

[qaz112]

I think we are losing the thread, please refer to my post 6 above.

I know there are dozens of ways to level a steel shelf and I don't have a lake under the building, just moist soil. And bolts, pins, epoxy resin and pegs etc shouldn't be needed, its just a shed [outside of building regs], plus the fact that builders managed OK before those products were available

But thanks anyway

[smearcase]

You have about 1 1/2 inches fall in that 2 1/2 feet.
Put a 2x4 under the shelf legs/bottom. It's plumb.

[qaz112]

Hey smearcase, are you just not reading the post or are you trying to wind me up, I said: "i know there are dozens of ways to level a steel shelf"

Anyway I want the level area for other reasons, wish id never mentioned the shelf now :)

[qaz112]

Smear case, why are u telling me this again? i say again: "i know there are dozens of ways to level a steel shelf"

Starting to wish id never mentioned them now, :) I need the level surface for other reasons too

[qaz112]

Sorry I repeated the post , it didn't show first time

[smearcase]

Let us know how it comes out. Feedback is always valuable here as it helps us help others in the future.

[qaz112]

Is there any point, the advice/posts is getting a bit off the mark. It will be OK, the weight and rough surface will keep it in place