Concrete expert needed.

[bebe]

Hi, Everyone! Just wondering if anyone here is in the concrete business. I am looking to form a concrete pad for my backyard deck 25X40ft. Looking for a good price of advice since I am on a budget Thanks.

If you use 150 mm slab, I would make the deck muliples of 12, say 24 by 36 feet, this way you can cut the slab every 12 feet. For a 150 slab you are best not to place your saw cuts more than 12 feet apart. You want the grid to be a perfect checker board, any sharp corners will increase the chance for cracks.

 

[bebe]

More to the point, is whether the concrete truck can get to the location behind the garage.

Good point, pump trucks cost a ton.

 

[james t kirk]

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.

 

[james t kirk]

It is best not to place re-bar under the location of the proposed saw cut if you want to prevent/minimize cracks. The goal is to create a line of weakness where you want the slab to crack. The current theory is to toss the rebar in the garbage and use fibremesh. Never use green rebar.

I would up the slump to 125mm.

Negative.

Never go more than 100 mm slump.

125 mm requires too much water and too much water = many shrinkage cracks. See the Scan of the OBC above. See the Water to Cement Ratio of 0.45. You can not get a W/C ratio of 0.45 with a 125 mm slump. 125 mm slump is a lot easier to place I admit, but it will crack a hell of a lot more.

And there is nothing wrong with Epoxy Coated Rebar provided the Epoxy is not knicked. The knock against the Green stuff is that it will pit if the coating is knicked because all the cathodic reaction in the concrete is concentrated at the knick. No knick, no problem. Also, the rebar in this design is strictly to control cracking and differential movement associated with a cracked slab. It is not rebar that is required to resist bending. Hence even if it were to fail, it would not matter.

Besides, I originally stated to put the rebar right in the middle of the slab. That gives you 75 mm cover from the soil and 75 mm cover from the Environment. Black bar will be fine with this cover.

And no, I do not like mesh. I consider it a joke for the most part. Maybe, maybe for a basement floor. But that would be about it.

 

[bebe]

Negative.

Never go more than 100 mm slump.

125 mm requires too much water and too much water = many shrinkage cracks. See the Scan of the OBC above. See the Water to Cement Ratio of 0.45. You can not get a W/C ratio of 0.45 with a 125 mm slump. 125 mm slump is a lot easier to place I admit, but it will crack a hell of a lot more.

And there is nothing wrong with Epoxy Coated Rebar provided the Epoxy is not knicked. The knock against the Green stuff is that it will pit if the coating is knicked because all the cathodic reaction in the concrete is concentrated at the knick. No knick, no problem. Also, the rebar in this design is strictly to control cracking and differential movement associated with a cracked slab. It is not rebar that is required to resist bending. Hence even if it were to fail, it would not matter.

Green bar is dead - PERIOD, knicks or no knicks, it was a passing fad. Stainless Steel rebar is the latest and greatest.

Rebar will never stop cracks in concrete, it changes how the slab cracks. By eliminating or reducing the rebar at a sawcut the stresses should be released there, however 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.

You could always add a SRA (Shrinkage Reducing Agent) but they are costly, been on the market for about 10 years now.

 

[bebe]

And no, I do not like mesh. I consider it a joke for the most part. Maybe, maybe for a basement floor. But that would be about it.

Never use welded wire mesh, I agree 100%. Fibre is what the Material Engineers are pushing at the moment.

 

[james t kirk]

Green bar is dead - PERIOD, knicks or no knicks, it was a passing fad. Stainless Steel rebar is the latest and greatest.

Rebar will never stop cracks in concrete, it changes how the slab cracks. By eliminating or reducing the rebar at a sawcut the stresses should be released there, however 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.

You could always add a SRA (Shrinkage Reducing Agent) but they are costly, been on the market for about 10 years now.

MTO still specifies Epoxy Coated rebar. It's been around since the mid 70's (it used to be Brown). Until MTO outlaws epoxy coated, it's still out there. Stainless is six times the cost of black bar btw....

I did not say that rebar will stop cracking. It won't. All it will do is prevent differenial settlements along a crack (one side higher than the other) which is why I want it there. A 0.5 mm shrinkage crack does not concern me in the least (provided there is rebar across the crack.) Saw cutting is fine for slabs on grade and what not, but you never sawcut a structural slab (like a bridge deck, or a garage floor). I have seen bridge decks with hundreds of shrinkage cracks and I have seen bridge decks with none. The CPCA seems to believe that improper curing is the major culprit. I'm more convinced it's concrete that is too hot. You ask for 35 MPA and you get 65.

Here is a link to the OPSS spec for sidewalk concrete. You will note that slump for even a crappy sidewalk is specified at 70 mm +/- 20 mm.

http://www.raqsb.mto.gov.on.ca/tech...4965852570c9006ae62d/$FILE/OPSS 351 Nov05.pdf

 

[bebe]

MTO still specifies Epoxy Coated rebar. It's been around since the mid 70's (it used to be Brown). Until MTO outlaws epoxy coated, it's still out there. Stainless is six times the cost of black bar btw....

Here is a link to the OPSS spec for sidewalk concrete. You will note that slump for even a crappy sidewalk is specified at 70 mm +/- 20 mm.

Its just a matter of time until the MTO will phase out green bar. Don't forget it is OUR tax dollars at work!!

If you want a 'workable' mix he should bite the bullet and dd Super P. or a midrange water reducer. Still 100mm slump concrete is flowable. Then of course we could discuss aggregate size... small flows better cracks more, large aggregate is the opposite.

Bottom Line - Concrete has and always will continue to crack no matter what we add, it contains water.

For my money I would build a patio with stone pavers. Bestway Stone has some nice stuff.

 

[james t kirk]

MTO did ban the use of Epoxy Coated Steel (very briefly) maybe 10 years ago. But the Rebar suppliers went NUTS screaming about how much money they had invested in the technology, blah blah blah.

At the end of the day, they changed the type of epoxy they used to a stronger epoxy and all was well again in epoxy coated rebare land.

There is a product out there which I think would be just fine called "Z-bar" Basically, it's galvanized, then epoxy coated (yellow) for a double layer of corrosion protection. I would think this stuff would be excellent.

http://www.specifyzbar.com/

 

[GPIDEAL]

It's concrete, not cement. Cement is the powder you use to make concrete.

Fibre can be metailic or nonmetalic. The nonmetalic is better for applications where you are worried about aesthetics. It may or may not help control cracking. Bear in mind, concrete wants to shrink and shrinkage cracks are the most common type of cracking one finds.

A slab on grade is fully supported and should not flex and thus should not crack due to bending. The fact that it is not being restrained by dowels also helps to prevent cracking. If one uses rebar, it will eliminate cracks opening up.

I know that Jim but those tradespersons are still called cement finishers or cement masons (maybe cuz they work with the cement paste at the surface). The Labourers' Union refers to them as cement finishers in the collective agreement. The WSIB refers to them as Concrete Finishers. The terms are used interchangeably, even when describing concrete structures, which as you pointed out, is technically incorrect.

Synthetic fibres are more for shrinkage cracks whereas steel fibres can eliminate the use of rebar for certain S.O.G.s (slabs-on-grade to the uninitiated, not 'shots-on-goal').

 

[GPIDEAL]

Never use welded wire mesh, I agree 100%. Fibre is what the Material Engineers are pushing at the moment.

WWM is still fine for the OP's application. Just make sure you lift it to the centre of the slab depth. Certain developers in the past didn't even use WWM because their subgrades were perfect (you could drive a ready-mix truck on them without forming wheel ruts) which is what James Kirk said about the importance of having a proper subgrade.

 

[slowpoke]

I'll be tearing down a deck in the near future to get the back of the house dug up and waterproofed. The deck is old and needs to be replaced anyway. I was thinking about a poured patio slab instead of a new deck but all this talk about cracking concrete is giving me pause. There are lots of attractive stone pavers and interlocking brick options available but I'm concerned that these will heave and shift. Can I use an extra deep base of sand or gravel with drainage tiles or some other method to eliminate shifting? I'm just north of the Danforth near Coxwell and the soil here is very sandy. We'll be digging right down to the footings and installing weeping tiles around the foundation so I could dig a bit more and put in drainage pipes in the patio area if it would keep the soil from heaving. Any free advice?

 

[james t kirk]

I'll be tearing down a deck in the near future to get the back of the house dug up and waterproofed. The deck is old and needs to be replaced anyway. I was thinking about a poured patio slab instead of a new deck but all this talk about cracking concrete is giving me pause. There are lots of attractive stone pavers and interlocking brick options available but I'm concerned that these will heave and shift. Can I use an extra deep base of sand or gravel with drainage tiles or some other method to eliminate shifting? I'm just north of the Danforth near Coxwell and the soil here is very sandy. We'll be digging right down to the footings and installing weeping tiles around the foundation so I could dig a bit more and put in drainage pipes in the patio area if it would keep the soil from heaving. Any free advice?

You owe me dinner on the Danforth.....

1. WRT to your weepers, etc.

Use the white 4" perforated PVC pipe from Home Depot. (Do not use the thin walled black shit wrapped in filter sock - it's fucking junk.) You will see the white PVC perforated pipe in the plumbing section along side the black ABS pipe. This is the good stuff. You want to install the weepers so that the bottom of the weepers is at the same elevation as the underside of your footings (or foundation wall). Always install the holes down!!! (So many idiots install the holes up thinking that the pipe catches the water that way, it does not. The pipe simply forms a conduit for the water to flow through. Now the good part, you want to encase the weepers in 6" of either 3/4" clear stone all round, or crushed stone (but I'm not so sure about using crush with weepers to be honest). The entire thing has to be wrapped with a Geotextile cloth (or filter cloth) to prevent fines from contaminating the weeper over time. You then connect the weepers to a sump, or a storm water drain. (Do not connect to a sanitary drain!!).

2. WRT to waterproofing.

Waterproofing is what keeps the water out. Many are under the misguided belief that the wall (especially a concrete wall) will keep the water out. It will not. A house is not a boat.

Do not skimp on water proofing. You've gone to all the trouble of digging out, installing weepers, etc. it's fucking stupid to use cheap waterproofing. (And by cheap, I mean that black shit in a can that you see idiots putting on with a roller. Utter crap.)

I have seen this stuff both in use and at trade shows and I'm most impressed with it. I would recommend you hire them to do your foundation walls.

http://www.advancedcoatings.on.ca/rub-r-wall-airtight.html

3. WRT to Styrofoam insulation

It’s the best way to insulate your basement as you keep the wall warm and there is a thermal break on the outside of the wall. But........

Be careful insulating with Rigid SM foam on the outside of foundation walls, especially in your part of the town. Termites are well known to dig through foam creating tunnels to get into your house. If the foam laps any wood rim joist, the termites will dig through the SM, right to the rim joist and then they are in and you have a huge problem on your hands.

4. Be careful insulating old masonry walls (rubble stone) or old concrete block walls. If you insulate on the inside, the wall will now be cold. The wall will wick moisture in the fall, then in the winter, since the wall is cold, the moisture freezes and blows apart the mortar or the block. The result is structural damage over time to your masonry foundation wall. Better to keep the wall warm in the winter through a bit of heat loss than to freeze an old masonry wall.

 

[james t kirk]

This is a GREAT site for excellent construction techniques and details for Residential.

It's American, but there's no difference between T.O. and Boston in terms of climate.

http://www.buildingscience.com/index_html

And here:

http://www.buildingscience.com/documents/profiles/etw-cold-climate-boston-profile/

 

[GPIDEAL]

James, so the fines will arise to contaminate the weepers over time even if you use clear stone (I can see how with Granular A)?

Styrafoam under a basement slab can usually be found in custom homes. It really does help keep the basement floor warm.

 

[Rockslinger]

More to the point, is whether the concrete truck can get to the location behind the garage.

Might have to use wheelbarrows. Hee, hee, hee.

 

[james t kirk]

James, so the fines will arise to contaminate the weepers over time even if you use clear stone (I can see how with Granular A)?

Styrafoam under a basement slab can usually be found in custom homes. It really does help keep the basement floor warm.

The purpose of the filter cloth is to keep the fines out.

That's why I like the clear stone around the pipe.

The filter cloth is on the outside of the clear stone, not around the weeper.

 

[TOraps23]

You want to use 35 MPA concrete with a 7% air entrainment with a slump of 75 mm.

Slab should be 150 mm thick and poured on top of a free draining material - like Granular A. If clay is present under the Granular, install weeping drains (holes down) below the granular layer to provide drainage.

Use 15 M rebar at 300 mm c/c located in the centre of the slab (hold it off the ground with concrete bricks) in both directions to control cracking.

Sawcut the slab down the centre, then in quarter panels in the other direction once the concrete has set up (usually within 12 hours) to control cracking (maybe).

Water cure the slab for 7 days (i.e. keep it CONTINUOUSLY soaking wet) to aid in hydration and to control shrinkage cracks (maybe)

Seal the sawcuts with a duoflex sealer once the curing is complete.

Over kill, wire mesh would be fine or concrete with fibers is even better but wont prevent cracks and you will not need to use steel . Saw cut next day in 2 meter section or smaller divide the slab by the amount of cuts you needs to get 2 meters or under. No need to water for 7 days it not a bridge but if you plan to pour on a hot humid day make sure you use a curing compound ( white or clear its up to you) sprayed on top of the concrete once it is finished this prevents the concrete from drying to fast and prevents suface cracks from occuring. Sealing cuts are not important because your pad should have fall so the water well not seat on your pad or in your saw cuts it will run off.

Leave it to the pros

 

[TOraps23]

I know that Jim but those tradespersons are still called cement finishers or cement masons (maybe cuz they work with the cement paste at the surface). The Labourers' Union refers to them as cement finishers in the collective agreement. The WSIB refers to them as Concrete Finishers. The terms are used interchangeably, even when describing concrete structures, which as you pointed out, is technically incorrect.

Synthetic fibres are more for shrinkage cracks whereas steel fibres can eliminate the use of rebar for certain S.O.G.s (slabs-on-grade to the uninitiated, not 'shots-on-goal').

thank you

 

[GPIDEAL]

... the slab will curl and crack no matter how much or little rebar is placed within the conrete.

I don't know about that part of your statement. Perhaps for regular floors with modest reinforcement but I've seen thick pads or structural suspended slabs with a lot of rebar and they don't crack like typical saw-free floors.