I have an existing floor slab (poured last year) and will be pouring a new one to butt up against it. Do I need to join the new slab to the old in any way? Yes I could have put rebar sticking out of the original slab, but it was not really convenient at the time. Cheers, Simon.
sm_jamieson wibbled on Tuesday 20 April 2010 09:25
I won't answer whether you *need* to (depends on the base).
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.
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.
What was interesting is how long the shuttering and scaffold frame stayed up - many months IIRC.
Probably not necessary to be too fussy with a floor slab, but if the mesh in the new section rests on these studs it should be a pretty strong bond and will assure there's no way either slab will slip vertically relative to the other.
Anyway, the method I mentioned up top is exactly what I did when I had to backfill a new bit of concrete to repair excessive hackery to my concrete strip house foundation and I wanted to be sure the new concrete would stay attached to the existing strip.
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.
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.
Bruce wibbled on Tuesday 20 April 2010 11:54
Good to know. Do you reckon 300mm spacing is OK?
I have never seen closer spacing. In very large slabs, it is often wider, but 300mm seems pretty sound for this application.
This is a 5 inch thick kitchen floor slab - a 2 x 3m slab being poured up against a 4x5m slab, on top of 75mm celotex. No mesh in the slab. Will be screeded over both slabs, 75mm screed. Cheers, Simon.
In which case, there is probably no need to seal the joint. But even with dowel bars, you will inevitably get a fine crack at the joint. This will eventually be reflected through the screed and any tiles that span the joint, although the good, thick 75mm screed will help to minimise and/or delay this.
I would be tempted to ensure that there is a joint in the screed and also in the tiles exactly above the line of the joint between the slabs. That would help to confine any opening up to that one joint in the tiles, which could be re-grouted when needed, and avoid any risk of the tiles splitting.
On the other hand, you could just ignore it and screed right across. But there will be short term shrinkage of the new slab, and long term creep of both slabs, and while the dowel bars will help reduce it, they will not eliminate it. Laws of physics (and chemistry) and all that.
I guess most builders would just screed over and be done with it. Floor slab joints must be very common with extensions etc. Best thing is to make the dowel bars of a reasonable size I suppose. Simon.
Sometimes a degree of ignorance can be a good thing. Or, to put it another way, knowing too much can get in the way of an uncomplicated life.
The dowel bars and thick screed will probably do the trick.
Another way of minimising the problem is to include some chicken wire or brick reinforcement mesh in the screed where it goes over the joint between the slabs. One brick's width should be enough. Instead of one concentrated crack above the joint, it will create many tiny micro-cracks that are too small to see with the naked eye and won't affect the tiles above.
te:
Good idea, thank for the suggestion. I've got half a roll of brick mesh I used in the aircrete walls. Simon.
Job done. ;-)
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.