Non-monolithic footers/slab pour?

[pago cruiser]

For any good experienced concrete guys...

Have a problem just cropped up today.
Obtained Pre-Slab inspection today, and the Contractor is going to be pouring Wednesday.
Garage is freestanding, 36x26, 2'x2' footers, #4's at 12"OC on 2.5" dobies (should have heard the Contractor whine about using these...), 6" 4000 psi slab. Double Halos in the footers.
I live on a hilltop at just over 4000' elevation. IRC windspeed design is 105 MPH. I have personally measured 70 MPH in gusts 2X in the last 10 years.

The engineer has (2) 48"W Bracing Panels at all corners, as well as between the overhead doors, and slightly off center in the back wall. Sill bolts are 1/2" x 14", min 48" OC, with a true 12" embedment. Several other internal steel members at TJI's to top plate and studs to bottom plate.

Each one of the (3) 48" Panels in the front (corner, between doors, corner) has about 8100# of force applied by a true 100MPH wind; each of these Panels has (3) Sill Bolts. The bolts are not overloaded. The monolithic pour footer/slab has sufficient mass to resist this; I need 162 cubic feet and I have 180 cubic feet in the footer length; same numbers on the back wall, and similar numbers to the sidewalls.

Now the Contractor wants to pour in 2 steps. Footers first, slab last. This is about a 42-yard pour, and these is some problem with the trucks, I guess. he did not elaborate before he hurried off.

Problem:
1. How are you going to insert 12" sill bolts when the 6" slab is poured on top of the previous days 24" footers?
2. I will still need that 162 cubic feet of mass; the 6" slab will provide about 20 cubic feet, assuming 12' of length for bearing of each Bracing Panel. I asked him this - just before he left the site in a hurry after disclosing the non-mono pour, and he said they would "place vertical rebar into footers, bend them over, and place about mid thickness of the slab". Seriously? (2) #4 rebar at each panel does not appear sufficient to join the slab to the footer with an uplift of 8000 lbs. I do not see how this will suffice.

Wish I knew more.
I have invested too much into this to have it compromised by sloppy workmanship at this most critical point.
Speaking of workmanship, we get down to the 20's each night, and he has no plans to use, and does not own, any concrete blankets. "We never use them, and have never had a problem". So I pre-purchased the roofing for the garage, and have sitting in a stack near the slab. Once he leaves, I will cover the slab with the (32) 5/8" plywood sheaths, and cover that with plastic. I am pretty confident that will avoid any surface freezing.

Sigh.

Am I missing something with the rebar?

TIA

 

[ConCretin]

Rebar is very strong in tension but it needs enough embedment to develop it's full capacity. Without more detail, I couldn't tell you if the rebar is adequate but those anchor bolts obviously need more than 6" embedment. How did you even pass a pre-slab inspection with this set up. It's clearly not per the engineered drawings. Your contractor seems to be making it up as he goes.

To start, I'd insist the anchor bolts be installed in the footing placement and pass through the slab. Edit; I removed my suggestion of longer anchor bolts. I think the original bolts would be fine as long as they have full embedment.

With regard to the cold temps. I call BS on his claim that he's never had a problem leaving a freshly placed slab exposed in 20 agree weather. I'm assuming he's using calcium, which accelerates the set and the heat of hydration but that's still an invitation for a frozen concrete surface. At a bare minimum, you should maintain the temperature of the concrete at 50 degrees for 48 hours before exposing it to freezing temps. Keep in mind that your slab is going to be pretty soft when you cover it and will be susceptible to damage.

There's more than a few warning signs here. I know it's painful but you might want to tap the brakes with this guy before you have a disaster on your hands. Trust your gut.

 

[PCustoms]

There's more than a few warning signs here. I know it's painful but you might want to tap the brakes with this guy before you have a disaster on your hands. Trust your gut.

I can't provide more accurate technical details then LL, but when I read the OP my first thought was to call off the pour until you get things worked out to your satisfaction.

 

[mike93lx]

You have control over pushing out the pour. It's your money.

Dont let the guy do work that you don't want and lean on the engineer regarding any deviation from the plan, not a contractor

 

[pago cruiser]

Thanks for the answers guys. Still waiting to talk to the Contractor.
Does anyone know how many or what size or embedment of rebar would be necessary to tie the footer and slab together without loss of strength? Currently, the footer rebar IS NOT connected to the slab rebar. I have seen that 30 diameters is a standard; but I am not going to get 30 diameters in a 6' slab... And I could find nothing on the "on-center" spacing along the garage perimeter. 6"? 12"?

 

[bluedog225]

From experience. Stop the job. Stop paying him. Get it worked out. And be thoroughly comfortable with the plan before you proceed. And have each and every piece of material on site before authorizing the pour.

I had void boxes and rain to deal with. Turned out mediocre. I had two big pours. Once the gray mud is in place, they took off. Once the concrete trucks show up, you are out of options.

 

[readhead]

Stop. If that is what the concrete guy wants to do then have him contact the engineer and pay for the redesign. Otherwise he needs to follow the plan if for no other reason than liability.

 

[WNYflyer]

Thanks for the answers guys. Still waiting to talk to the Contractor.
Does anyone know how many or what size or embedment of rebar would be necessary to tie the footer and slab together without loss of strength? Currently, the footer rebar IS NOT connected to the slab rebar. I have seen that 30 diameters is a standard; but I am not going to get 30 diameters in a 6' slab... And I could find nothing on the "on-center" spacing along the garage perimeter. 6"? 12"?

Since you had an engineer involved in your design tell your contractor to submit to your engineer for approval any changes the contractor proposes to the original design. Any of the engineers time should paid for by the contractor. I would proceed with nothing until your engineer has had a look see and weighs in.

Of course being residential construction your contractor is most likely not going to like this.

 

[ConCretin]

Thanks for the answers guys. Still waiting to talk to the Contractor.
Does anyone know how many or what size or embedment of rebar would be necessary to tie the footer and slab together without loss of strength? Currently, the footer rebar IS NOT connected to the slab rebar. I have seen that 30 diameters is a standard; but I am not going to get 30 diameters in a 6' slab... And I could find nothing on the "on-center" spacing along the garage perimeter. 6"? 12"?

If the anchor bolts are imbedded in the footing, there really isn't much need for rebar connecting the slab to the footing. You could add a #3 or #4 bar every couple feet for good measure. Put an L shaped bar in the footing and bend it over into the slab. 18" of embedment in both elements would be fine.

If you are considering putting the anchor bolts in the slab only and tying the slab to the footing as your contractor seems to be suggesting, that is just a bad idea.

 

[billconner]

18" anchor bolts in footing?

 

[GlenW]

Can't comment much on the loading, but do have experience with the temps. I had the same thing told to me. Even at around 27F (-3C) with calcium added, heated aggregate and water, I still ended up with some of the top flaking. Looks like just the top 1/8" froze overnight and will need some finishing work to get a nice surface back. About 10 spots with 6"-8" circles. Too bad as he power troweled till it was almost shiny. Looked awesome. It was hard enough to walk on with no marks at all, yet still froze as it hadn't cured enough.

 

[pago cruiser]

Thanks again guys.
After some interesting discussions on how he was going to mitigate cutting the footer in half... and inserting 12" sill bolts into a 6" slab pour, we are back to a mono pour.
Apparently, the problem was that the concrete vendor had 2 trucks down; and we have 5 sequential trucks needed.
Concrete trucks fixed as of this am; hopefully will go smooth tomorrow....
In spite of the fact that Pop always taught us kids that hope is not a plan...

This reminds me of that old Jimmy Buffet song from Don't Stop the Carnival; "...running this place borders on hand to hand combat...".

I have waited 10 years for this project; right now I really wish it was over...

GlenW - did you have any cover on your slab? Or Plastic? Do you have a feel for how long that 27F temp was maintained? While we will drop to the mid-20's at night, by 1000 we are in the 50's. Supposed to reach 65 by 1500. Wednesday night (day of the pour) we have a low of 29F, and a high of 65 by mid afternoon.

 

[GlenW]

Best guess was it was only below freezing for 4-5hrs. If I remember right, it was similar to what you're expecting. It was up at least in the 50's by the next afternoon, but was below 40 during the pour, and just dropped below overnight. It was poured early as well, so I had hoped it would have cured enough by nightfall.

No, I didn't have any cover at all. I asked about that and was told it would be fine...guess not. I was going to tent it with simple vapor barrier plastic and put a small space heater under it. I had covid at the time, so I was in no shape to argue or make any efforts above what was asked. LOL! Good Luck with your project!

 

[kbs2244]

this is how it is done

 

[ConCretin]

Wednesday night (day of the pour) we have a low of 29F, and a high of 65 by mid afternoon.

What is the psi of the concrete? Are you adding an accelerator? If you have a decent amount of cement or an accelerator, you are probably fine with minimal protection. A layer of poly would probably trap enough air to work. A few hours of temps that are slightly below freezing aren't likely to overcome the heat the concrete produces. This of course assumes the forecast is correct so remain vigilant. Good luck. I hope it goes well tomorrow.

 

[billconner]

this is how it is done

I found ASCE 32 gives you a few more options for FPSF.

 

[Rusted Nut]

Concrete usually does get freeze damage the first night, there is sufficient heat from the concrete. It’s the 2nd and 3rd nights you need to be concerned about. Ask about air entrained mix. Trying renting some curing blankets. Not sure if they are readily available in Tucson though. If not, insulation covered with visqueen and weighted down will work.

 

[billconner]

Visqueen and straw

 

[pago cruiser]

Thanks again guys.

GlenW - Sorry to hear of your travails; sometimes we can only do so much, and hope the people we hire know wth they are doing; sometimes we find out the contrary.

KBS2244 - Lots of good info in that doc. Here is southern AZ, even at 4000 ft elevation, our frost line is about 2" below grade...

Rusted Nut/billconner - Once the Contractor left this afternoon, I headed over to ACE to get enough plastic Tarps to cover the ply to cover the slab. My plan is late tomorrow to ...carefully ...slide 4x8 roofing sheathes to cover the slab, starting at one end, and working my way to the other side, sliding the new ply over the just laid ply. Then start another row and repeat. Then lay out the Tarps over the ply, using spare 2x lumber or rocks to weight down the Tarps. I am pretty confident it will work; just not sure if my 68 YO back will cash the check that my brain is writing...

May have to offer $100 to a couple of the concrete finishers to return late in the day to lay out plywood. It is just... hard to get folks to even show up on site. Or maybe call one of the neighbors to see if his teenager is available; but this is the same kid I offered $100 for 6 hours work to help move stuff from a 20' container into a 24' container. He never bothered to show up, nor even call...

Tomorrow should prove interesting.

 

[Toolfool]

Maybe put the tarps down first, or else your plywood will soak up all the moisture from the fresh concrete and you'll end up with curled up plywood.

 

[pago cruiser]

Toolfool - yep, that's what we did.

Good news.
Mono pour went off today without a hitch. 36' x 26' x 6", #4's at 12" OC, 4000 psi, 24x24 footers, 45 yards.
Started at 0700 and the 10 guys left the site at about 1600.
Got them to help me cover the slab; used 6 mil plastic in (3) 12' wide sections, then I bought some 10 mil Tarp product at 30' x 20' to cover the 6 mil stuff, then we used 5/8" ply (roof pre-purchase) to cover the voids and weight down the edges.

Weather for the next few days is 29-31 lows at night, warming to 67-68 during the day. Then ... Sunday/Monday we are looking at 70% showers; guess I will need to get some more tarps and cover the ply to avoid it warping before I can get it on the roof. After that is gets a little more dicey, as following the rain we will get low 20's and highs only in the low 50s. But that will be 5-6 days after the pour, at which point I think... the slab should be ok.

Interesting Note 1: After we had covered the slab in the 2nd (black) plastic, I sat down at the perimeter in a couple locations and held my hand on the soil, then on the plastic, in both shady and sunny areas. We got up to 65F today. In the sunny soil/plastic, the plastic was easily 20F higher temp. In the shady area, it was a lower differential, but still easily 12-15F. Fascinating stuff.

Interesting Note 2: The slab is in a location that is maybe 50% wind blocked by the main house. Easily a 10F difference when the wind (maybe 10-15mph) was blowing on you versus in the lee of the main house. But the concrete that was exposed to the wind was surface drying faster than the part not exposed; so much so that the Contractor wanted to get the plastic on the wind-swept area while they were still "polishing" (as they say) the area in the lee. Based on this, once the Contractors left I moved our cars to act as a windbreak for the area exposed to the wind. I have never seen this phenomenon discussed, but it seemed a valid cause and effect... and mitigation with the placement of the cars.

I should really start a thread on the project...

Thanks again all.