Extending a concrete base?

27 Mar 2009
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United Kingdom
I planning on extending a concrete base that my greenhouse is currently sat on. One edging face of it is beginning to crumble away so I thought that extending that edge by a further 12" should help preserve it?

I seem to recall hearing that PVA solution should be applied to the edging face of the existing concrete base so the new concrete will bond to it.

Is this true?
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No that's not true.

Unless you drill in some jointing bars, there will always be potential for the new piece to separate slightly from the existing piece
You are not giving much away here. If the concrete was badly mixed or too lean, there is nothing much you can do other then to replace it. If the concrete is OK but the crumbling has got something to do with freezing and defrosting cycles then to water proof it could preserve the top surface.
Extending the existing (crumbling) slab with not preserve the existing. In fact it may encourage it to deteriorate quicker. PVA is a waste of time.
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Heres a photo of the concrete base and greenhouse. Ive no idea how old the base is. As you can see the base is crumbling in places, and may eventually need breaking up and relaying. Im about to lay a path upto the greenhouse and thought I ought to do what I can for the base whilst I'm at it. I could drill out and insert some jointing bars I guess...


Looking at the close up it looks as though the concrete base was laid with very little cement and a lot of rubbish. The top 1" looks OK, I wonder if some one overlaid a decent screed over the original. This could be way in, that is to encapsulate the rubbish within a covering of a couple of inches of very rich fine concrete.
Select a bit say 1m long. Dig down a narrow trench 6" down and 2" wide as close to the green house as possible. Excavate under the screed an inch or so. Render up with a 1:2:1 cement/sharp sand/pea shingle and put slope on outer side, so water runs off. Repeat. . . Best to do it in sections else the green house may crack the edge of screed its sitting on.
That top 1" is actually a narrow bed of mortar running along the perimeter of the base that I put there to sit my greenhouse on in the hope of getting a nice seal with the base.

I know the base isn't particularly good, and will all need replacing in its entirety one day. But I'm hoping to delay that as long as possible.

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