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Posted: Wed Sep 02, 2015 4:19 pm
by sawtooth
I have a concrete base in the garden which used to have a prefab concrete garage on it.
I want to extend the base by about 5ft across the whole width of the garage (18ft wide).
I understand that the new concrete pad will need to be tied to the existing base.
What is the best way to do this to ensure that the new part wont ever separate from the old?
I will ask a builder to do the work for me but for my own peace of mind want to undertstand what is involved. I read on here about drilling holes in the end of the existing pad and inserting metal bars. Would this be the way to go for a joint of this type?
Thanks in advance for any advice you can give.
Posted: Thu Sep 03, 2015 12:12 am
by lutonlagerlout
assuming the existing is of sound construction you could drill and resin fix rebar every 400 or so and tie that to your new mesh
however a proper job would be to rip out and replace
LLL
Posted: Thu Sep 03, 2015 9:19 am
by sawtooth
Thanks LLL
The existing base seems fine but there is a about 8ft unused space around the back of the garage that I cant use unless I extend the garage into it. There used to be a tree in that space but that has since blown down hence why previous owners couldn't go back any further.
If I ask the builder to use rebar and probably more substantial base and thickness of concrete than existing (100mm I think). Would that be adequate? I don't really want the cost and hassle of ripping and replacing the whole slab but accept that would be the best job.
3/4 of the garage (approx. 16ft) will remain on the existing base, the final 1/4 (approx. 5ft) will be supported by the new concrete pad.
Posted: Thu Sep 03, 2015 10:52 am
by seanandruby
Why would you want to rip out a sound concrete slab on a garage base? Dowels and mesh will be more than adequate on a good sub base.
Posted: Thu Sep 03, 2015 5:30 pm
by sawtooth
Thanks.
I was hoping that rebar and mesh reinforced concrete would be adequate.
Its also been suggested that if do that I would also need to then cover the whole slab with 2 inches of new concrete. Is this really necessary? I can see it adding strength but need to avoid doing this unless its doomed to fail if I don't.
Here is a piccie of the plot, its hard to see but the wedge shaped spare land is 8ft deep on the left and 12ft deep on the right.
Posted: Thu Sep 03, 2015 7:25 pm
by lutonlagerlout
seanandruby wrote:Why would you want to rip out a sound concrete slab on a garage base? Dowels and mesh will be more than adequate on a good sub base.
to do a proper job Sean
we all know about settlement,the existing slab has probably settles as much as it wants to
if the new bit settles 2-3mm ,then you have a crack in your garage
the OP has just mentioned a tree,which as we all know had a root system which will certainly affect the heave of the ground now its gone
LLL
Posted: Thu Sep 03, 2015 8:17 pm
by sawtooth
The tree has been gone for about 2 years now and it left a big hole which was only back filled with loose soil at the time. The builder knows about this, will a good well compacted base and over 100mm concrete with rebar and mesh reinforcement be enough to stop any problems?
Posted: Thu Sep 03, 2015 10:00 pm
by lutonlagerlout
what are you planning to use this base for sawtooth?
one of the commonest reasons I see drives fail is where trees have been dug out and eventually the ground drops
LLL
Posted: Thu Sep 03, 2015 10:44 pm
by sawtooth
A double prefab sectional concrete garage was on the original concrete pad for the past 10 yrs or so. I was planning to put up the same (new) garage but 5ft-6ft longer to make better use of the land. I hate to think of that land going to waste, as you can see on the picture once the garage is up the wedge of land is unaccessible and practically wasted.
I was going to ask, would the sectional concrete be more forgiving to any small settlement if it happened? I'm no builder but I can see that the concrete panels bolted together would have a certain amount of give in them....or not. I'm only thinking worse case scenario if the new pad started to sag a little.
What if I went overboard and asked the builder to dig proper footings like for a house. Perhaps not as deep as that but a similar trench like footing. Would that help guard against the concrete pad sinking?
Thanks for your all your help. I have a garage that was delivered that is currently on the floor waiting for the base to be extended so I am up against it. :>(
Posted: Thu Sep 03, 2015 10:51 pm
by sawtooth
A rough sketch of the site.
Posted: Fri Sep 04, 2015 12:01 am
by lutonlagerlout
a sectional garage would probably be ok as IIRC the bottom panel sits on a mastic joint,plus as you say the bolts would allow some movement
trench fill footings would be overkill for this
LLL 
Posted: Fri Sep 04, 2015 1:20 am
by sawtooth
Would you suggest placing anything between the 2 concrete slabs where they meet? Or is the idea to stick the 2 slabs together?
Posted: Fri Sep 04, 2015 7:10 am
by seanandruby
Shouldn' need anything because it will be a contraction joint. You can place a repate in the top, say 10/20 ml so you can use a sealant
In the gap. Look at the main index for joints in concrete.
Posted: Sat Sep 05, 2015 11:36 am
by sawtooth
So if the builder follows the instructions below for this type of joint then we are good to go?
Pardon my ignorance but the idea is for the new pad to stick/adhere to the old slab. The dowels in the old slab are stuck tight, the dowels in the new slab have a sleeve that allows the rebar to move.
So am I right in thinking that its anticipated the old and new slab may separate at the point at which they are joined? The rebate about an inch deep that will be created is to encourage the crack to happen at this point? And if we dont do that the crack could happend elsewhere and be more disastrous and random?