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Posted: Mon Sep 05, 2005 8:25 pm
by hudd
Replacing my old 16 x 8ft garage with 20 x 10ft concrete sectional garage. Got 3" deep concrete base in place already but only 16 x 8ft. Contractor has suggested leaving the base and concreting over top has said would be a sound base, may just get hairline crack where old concrete base joins hardcore. Have choice can go over top or I can dig out. Any advice appreciated?
Posted: Thu Sep 15, 2005 2:46 pm
by Tony McC
This "concreting over the top" business - how much concrete would be placed on top of existing? Owt less than 100mm will crack, regardless of what's underneath.
In theory, you could 'upgrade' the add-on area by preparing a damned good sub-base up to the level of the existing concrete, and then casting a new, fibre-reinforced 100mm slab over the lot, but there is a risk of differential settlement at the boundary between the sub-base and old garage base, so I would be tempted to use a keeled construction, where the depth of concrete in the 'new' areas is increased to 150mm (minimum) with a sheet of steel mesh thrown in for additional tensional strength.
However, I'd need to know the positional relationship between the old and the new - will the new slab sit centrally over the old, with an extra 2 feet at either end and a foot to each edge, or will it be plonked to one side?