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Bruce[_8_] Bruce[_8_] is offline
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Default butt joining floor slabs

On Tue, 20 Apr 2010 10:41:01 +0100, Tim Watts wrote:

If you do - this trick is fairly easy:

Drill into the side of the old slab at foot intervals and resin-mortar in
some heavy studs (M10-M12), say 4" into the old slab and 4" protruding.
That should provide a good mechanical lock.



That's not anywhere near long enough. Thermal movements, shrinkage
and creep will be enough to break the bond, and the dowel bars (that's
what they are called) will be worthless. I would recommend 200mm into
the old slab and 200mm protruding. Otherwise, don't bother. Also, I
would avoid studs and use plain mild steel bar - not the ribbed kind
either.


I've seen this done on a much larger scale when they widened the M25 bridge
decks near Jnc 7-8 - bloody great big bars where set into the sides of the
original deck and then the new slabs were poured incorporating these bars.
Couldn't see the details of how they tied the rebars to the mortared in
bars as the shuttering was up at that point - I assume they were well tied.



There is no need to tie the dowel bars to the rebar. They are
performing two quite separate functions, each of which involves
interaction between steel and concrete. Steel to steel interaction is
neither necessary nor particularly desirable, as it can induce
cracking of the concrete.

If a waterproof joint between the old and new slabs is important (the
OP has not made this clear) fix a small softwood fillet about 10 mm
wide by 15 mm high against the top edge of the existing slab. After
the new slab has cured (at least a week) carefully remove this fillet
and clean out the resulting slot.

Then almost fill the slot with a good quality polyurethane mastic.
Leave around 3mm clear at the top to allow upward expansion. This
will give a watertight joint while allowing for the inevitable small
amount of movement.

However, if a waterproof joint is not needed, this can be omitted.