Some independent contractors know these rules already from longtime experience, assuming the codes haven't changed. So not having a copy of the code doesn't necessarily mean they're quacks.Below is a scan of the Ontario Building Code, Section 9 (Residential) regarding Concrete Compressive Strengths.
You will note that 32 MPA is the minimum specified compressive strength for what the OP is proposing - not 20 MPA (Weekend Guy), not 25 MPA (Hammerstein).
35 MPA gets you a slightly better cement content.
End of story.
BTW, a good test for all you guys out there wanting to hire a Contractor to do some home Reno Work - "Do you own a current copy of the Ontario Building Code?"
No = HACK. I love how many Contractors run around spouting off about Code this and Code that, and they don't even OWN a Building Code.
Concrete expert needed.
- Thread starter tedhead
- Start date
Below is a scan of the Ontario Building Code, Section 9 (Residential) regarding Concrete Compressive Strengths.
You will note that 32 MPA is the minimum specified compressive strength for what the OP is proposing - not 20 MPA (Weekend Guy), not 25 MPA (Hammerstein).
35 MPA gets you a slightly better cement content.
End of story.
BTW, a good test for all you guys out there wanting to hire a Contractor to do some home Reno Work - "Do you own a current copy of the Ontario Building Code?"
No = HACK. I love how many Contractors run around spouting off about Code this and Code that, and they don't even OWN a Building Code.
A couple of things:
1) You're telling the OP to use 35 MPa concrete and you're referencing the code for the compressive strength of unreinforced concrete, yet you're suggesting the use of 15M rebar at 300mm c/c. This is overkill.
2) The reference you make to exterior flatwork requiring a compressive strength of 32 MPa for unreinforced concrete is in the same category as garage floors and carports. This category would be subjected to loading from vehicles, permanent storage of heavy items, and anything else someone might cram into their garage or exterior storage area. The OP is asking about a concrete pad for a backyard deck. I'm not sure what he's planning on using his deck for, but I doubt there would be an F-150 parked there.
3) Section 9.16 speaks to Floors-On-Ground:
9.16.4.5. Compressive Strength
(1) Where dampproofing is not provided the concrete used for floors-on-ground shall have a
compressive strength of not less than 25 MPa after 28 days.
(2) Where dampproofing is provided as described in Article 9.13.2.7., the concrete used for
floors-on-ground shall have a compressive strength of not less than 15 MPa after 28 days.
4) Section 9.16.2.2. Support of Floors speaks to the fill requirements for floors-on-ground that will be subjected to freezing temperatures. The OP's concrete pad could likely fall into the category of Floors-On-Ground, and therefore the compressive strengths outlined in 9.16.4.5. should not be dismissed.
5) The OPSS spec for sidewalks requires 30 MPa unreinforced concrete, which is designed for live loads resulting from pedestrian traffic (not an insignificant loading!). Again, you are telling the OP to use 35 MPa concrete with 15M rebar for concrete pad under a backyard deck!
Bottom line: it's not the end of story as suggested by JTK, and further investigation and opinions are warranted to save you some hard-earned $$$.
Uh, read what you posted.
An exposed slab is not damp proofed. And it’s not a concrete FLOOR. Therefore, you are incorrect again.
Yes, I told the OP to use 35 MPA concrete. The OBC calls for 32. Do you think that that is a huge difference? Really?
Uh, no.
35 MPA is readily available and typically spec’d by anyone in the industry who knows his ass from his elbow.
The OPSS allows for 30 MPA concrete for sidewalks you are correct (cause I posted that link as well). 30 MPA with 355 kgs of cement was the old MTO standard for everything. Now, they’ve gone to High Performance Cocnretes, etc – like 50 MPA and Silica Fume (neither of which I would ever recommend, even for a bridge) But they kept the 30 MPA for sidewalks.
BUT, the OBC calls for 32 MPA (not 20, and not 25) and since we’re talking a residential application, the OBC trumps MTO. However, it’s really splitting hairs because the MTO 30 MPA mix has 355 kgs of cement in it to resist freeze thaw damage, etc. and will come in around 40 MPA due to its high cement content.
Blah blah blah.
I’ve shown a scan of the OBC which clearly calls for 32 MPA concrete.
The 15 M rebar is based on my experience with mesh being for chumps and hacks and the cost difference would be minimal. Fibres help control shrinkage cracking (maybe), but they do NOTHING to prevent differential movement along a crack. Rebar (or arguably mesh) will definitely prevent differential movement and control cracking. You can add the synthetic fibres to the concrete for a few extra bucks and use it on top of the rebar.
I gave the guy a good long term design that will perform. If you think it’s wise to save maybe a grand on materials for s substandard design that will not perform nearly as well as mine – well, knock yourself out.
But don’t go for a permit – cause they’ll tell you you need 32 MPA.
No offense buddy, but I’ve never been one to do half assed anything. It only comes back to bite you in the ass.
No, it pretty much makes them hacks in my eyes. It is IMPOSSIBLE to know the OBC by heart.
You can buy "Code and Construction Guide for Housing" from the gov't book store for $90.00. It's basically Part 9 (Residential) of the OBC in both an easy to understand version with helpful hints and a copy of Part 9.
For 90 bucks, there's no excuse not to own this if you're a Contractor.
Here's the link:
https://www.publications.serviceontario.ca/ecomlinks/510082.pdf
This is very important, and is rarely mentioned in many of the do-it-yourself books.
Read the whole sentence:
If the slab is not sawcut within say 12 to 18 hours, the slab will curl and crack no matter how much or little rebar is placed within the conrete.
Contractor yes, but tradesperson or independent operator doing your patio hardly. Some moonlighters who work for larger companies will know about the proper specs for exterior concrete, but even the ready-mix companies will advise on the right mix because they usually ask what's it going to be used for.
The OBC isn't the be all and end all for concrete slabs (it's pretty scant when it deals with concrete flatwork - there's more info in the CSA standards). An experienced tradeperson doing just one aspect of a job like a slab doesn't have to be an engineer or memorize the entire OBC to necessarily know what's required for the work of his trade.
All said, I can't argue against your standard of care.
I did read that part, and I'm saying that I don't necessarily agree when you say that if the slab is uncut, it will crack regardless of the extent of reinforcing.
Concrete will not always crack. It can be laid in a manner that prevents cracking, and for appropriate applications cracking can be eliminated or confined to the surface layers.
It is difficult to do this, and as such, many concrete installations will crack. The saw cuts control the cracking.
like he said, but leave out the rebar and the sealer...saw cut depth to 1/3 the slab thickness
no fiber or rebar is needed for a slab on grade, (they don't use it in sidewalks) just a good base and saw cutting
Some sidewalks are reinforced I can assure you. (Most are not.) But sidewalks crack and heave quite often. I doubt the OP would want this (I wouldn't) so rebar (or if you're cheap - mesh) will prevent heaving along cracks.
Sidewalks are also not saw cut, they tool joints into the fresh concrete - the idea being the same - to induce cracking at a known location - though it seldom works. Long pours will always shrink crack. It's a given.
The sealer is a joint sealer and is there to prevent water getting into the sawcuts and freezing and causeing damage.
Uh-oh!
Your municipality (and your neighbours) might have an opinion worth checking out, as well as TERBians. And they might have offered different advice if you'd put the question that way.
But mebbe I'm just imagining?
OK. Are you by any stretch, thinking you're getting advice about pouring a parking pad in your backyard, accessed thru your garage?
Your municipality (and your neighbours) might have an opinion worth checking out, as well as TERBians. And they might have offered different advice if you'd put the question that way.
But mebbe I'm just imagining?
Thanks James!!! Great information. Our foundation walls are triple brick, parged on the outside, and we have an old concrete walk running alongside the house in the driveway That walk has tilted & cracked over the years so now it traps water against the parging instead of channeling it away. Our basement hasn't leaked in the 15 years we've lived there but the only thing keeping the water out is probably that layer of parging. We started off planning to just rip & replace the driveway and the deck but now we're digging up half the house!! Hard to know where to stop.
Yeah! First I'll fuck everything up....then I'll blame my contractor so Mike can come in and rebuild half the house. If only.....
This is great! Thanks again!
Here is a diagram on how to properly build a weeping tile system around your foundation.
It's true, if the slab area is large enough the concrete will crack. If you cast a slab 1m by 1m odds are it will not develop cracks. Now cast a slab 8 m by 12m without any sawcuts and I guarantee it will crack regardless if the slab is heavily reinforced or not. I have seen it happen about 100 times in the last 4 to 6 years alone, no lies. As I noted before, concrete contains water, one expects it to crack. The skill of the design engineer is to force the slab to crack within the deep sawcuts. By keeping the sawcuts relatively close and by weakening the slabs at the location of the sawcuts the chances are very high that the cracks will develop within the sawn joint. Another thing to keep in mind is that square slabs and square sawcut pattern that runs perpendicular to each other should be maintained as much as possible.
Your answer to my reply is a bit off topic but I would never assume that concrete won't crack unless appropriate steps are taken. Yes, I know about control joints.
