View Full Version : Prepping Concrete pad for garage floor!

05-02-2013, 05:37 PM
So I am prepping my own pad to be poured for the garage I am building this year. Will be getting someone out to do the finishing.
Just wondering what kind of gravel I need to order to put underneather it? Will be a 4" pad with 8" around the sides.

Also i called 'cal before you dig', and i found out that my electrical and my gas line runs right underneath where I am building my garage. I saw on the city of Calgary site that I can not build over a gas line.
My friend at work bought his house brand new and the pad was already poured over the gas line. Can I get an easement to build over it? and than if anything does ever go wrong, I would be responsible for paying to fit it if they have to tear through my garage to get to the gas line?

05-02-2013, 08:13 PM
Should be 150mm of 20mm minus crush gravel, compacted to 100% SPMDD

Most important thing is the compaction more so than the material if you don't want your foundation to crack during settlement or be affected by frost heave.

You will probably need your gas and electrical to be relocated prior to building... best bet is to check with 311 and have the city sign off on your plans before you start work.

05-02-2013, 08:54 PM
Originally posted by Sugarphreak
Should be 150mm of 20mm minus crush gravel, compacted to 100% SPMDD

what is 100% spmdd?
I am renting a plate packer and packing every 3 inches.
Going to call the city, they signed off on the permit and everything but they didnt even know about the lines. I only found out about them after i got the permit...

05-02-2013, 08:58 PM
It is a technical term for compaction

SPMDD = Standard Proctor Maximum Dry Density

Because you are building a structure with no piling or footings, it is fairly critical that proper compaction levels are achieved if you want it to last. Technically speaking the loads are going to be on the edges of the slab (the portion where you are thickening it to 8") so ensure that is the most thoroughly compacted area. Also make sure you use a full 150mm (6") or 20mm minus gravel on the edges, it isn't as critical in the center.

People really underestimate the importance that compaction plays for structures, if you get poor settlement it will fuck your whole garage up.

You should be able to get good compaction with plate packer, just spend some extra time going around the edges. Also make sure you are not compacting when it is too wet or frozen, it should be dry material and not leave moisture on your hand if you pick it up.

05-02-2013, 09:08 PM
Be careful how you compact the gravel when it's overtop of a gas line. The rental plate compactors should be fine, but it's possible to damage that line during the compaction process.

05-02-2013, 10:11 PM
You might have to have the gas line and whatnot else moved from underneath - not a big deal but a good idea in any case.

Something most don't do is put in enough reinforcing steel and shrinkage of the concrete when it cures (yep, it's like your blue jeans ....) results in cracks (concrete strength in tension is minimal at best and actually considered to be zero in engineering design).

Put in a grid of 10M rebar at 12" (300 mm) centers and add a couple more continuous length-wise for your slab edge thickening (footing). The actual amount for shrinkage crack control is 0.2% by volume (for a 1000x150 cross-section you would need steel area of 0.2/100 x 1000 x 150 = 300 mm^2 of steel x-sec area. 10M bars have 100 mm^2 of x-sec area per bar so that means you should install a minimum of 3 pcs of 10M bar for every 1000 mm width of slab ... or 3 pcs 10M per meter.....a tiny bit more is 10M @ spacing of 300 mm or 1 foot. That will give you a slight excess of steel to handle any other small bending stresses due to improper compaction.

Speaking of compaction - do not pack down much more 150 mm (6") of material at a time ... do it in lifts to get proper compaction and go over each spot at least 3x with a plate tamper and you should be good. A tiny bit of water will aid compaction (optimum water content ... won't get into that). Like Sugarphreak said, it should not be sopping wet ... if too much water, let it dry......you will see it pumping and then you know it is too much.

Last of all, seal the slab with a sealer ... and keep it slightly damp on the surface for a day or so (very light spray ... or use wetted burlap). Reason is that concrete needs some water to cure .... and it will be stronger then. It should be allowed to set first though ... Also some kind of water-proof membrance or sealer should be used to prevent salt water from getting into the concrete and rotting out the rebar. That causes pop-outs and spalling and can wreck a slab in a season or so, which is not so good.

So use 10M bars (1/2" dia roughly) at 12" on-centre each way.

Something else that gets missed in residential is that there should a clear space/gap of 75 mm (3") - between the surface the concrete is set against and the rebar. If you only have 4" of concrete that would result in very little cover on top so often the bars are placed sort of in the middle of the slab. You might want to get some plastic chairs to set the rebar grid upon to keep clearance ... also 2" (50 mm) clearance at the sides.

Is that enough? :) :) :)


05-03-2013, 06:49 PM
That is perfect thank you for the helpful response.

One thing though, isnt it kind of a big deal to move the gas line though? wont that cost quite a bit of $$$