slab thickness

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In a previous post P.Fritz says...

That's when the trouble (and overtime for the engineer) really starts!
I have had to require that the contractor tear out the bad stuff and replace with new. Finger pointing gets started in earnest at that point.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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But it is always the contractor's fault :-)

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Bob Morrison wrote:

And I have had to tear out concrete that didn't break at the spec'd pressure after 28 days. I didn't pay for it though, the cement plant did. They paid for the new concrete, the labor, the disposal of the old concrete, everything.
When you do this to them a couple of times, they start paying close attention to the mix and their quality controls at the plant. Whenever I order concrete now, I always tell them that the testing lab will be there to take samples and do a slump test (whether they are or not). They are minding their Ps and Qs, then.
We have had the odd batch not break at 3500 or 3000, but it is usually in an area where it doesn't matter, so it doesn't have to be torn out. They sometimes break at 2900 or 3400, but for a slab on grade, that is fine. When you get into retaining walls and other critical systems, the heat is on, and if you have all of your paperwork in order, it is on the supplier.
Whenever I have to call the plant and tell them that their concrete is subpar, the first question I always get is How much water did you add? For that reason, I always have the drivers indicate on the delivery ticket if water was added and who requested it. Sometimes the drivers will add water on the way to the job to keep the mix turning. If it is a spec'd job with tests in place, that can come back to haunt them.
Moral: Always keep your delivery tickets, indicate water added, and test, test, test.
--
Robert Allison
Rimshot, Inc.
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Robert Allison wrote:

Can you actually catch them if they don't "'fess up", though?
When I drove one summer while in high school for a local mix service here (classmate's Dad owned the place) there would have been no way to prove it as they, at least, weren't monitoring onboard water consumption...I suppose w/ "real" construction, that's done now.
(Note--Otto only let us kids deliver to the oil well sites where you couldn't mess anything up other than bury it to the axles in mud... :) )
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Duane Bozarth wrote:

Well, if I have a delivery ticket that says no water was added, and the test breaks at 2800 for a 3500 PSI concrete, who is responsible? The plant! That is what I am talking about. If you order 3500 PSI mix and you don't get it, it is the cement plant that has to fix it.
--
Robert Allison
Rimshot, Inc.
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Robert Allison wrote: ....

That surely, is the "proof"...I was just curious during the finger-pointing phase if the driver tried the "not me!" trick whether with newer mix trucks water usage might be monitored...
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Duane Bozarth wrote:

One way to do that is to tell the plant that you want the water tanks full when they arrive at the jobsite. That leaves the driver the option of adding water and then stopping somewhere and refilling the tanks, but I don't think that they are that devious. Most drivers I have found to be pretty honest. I can't imagine a situation where one would want to ruin your mix just to be mean. Especially when they know that their jobs could be on the line for it.
You can also check the level of the water tank on the truck when it arrives and when it leaves, keeping in mind that if the truck is not on level ground, the readings could be different if it moves.
--
Robert Allison
Rimshot, Inc.
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And if it needs to be 'pumped', I guess the slump is less, right ..so it can flow thru the pump easier ?

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Rudy wrote:

Slump would then be greater, not less...
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Duane Bozarth wrote:

Exactly. And if you are pumping the mix, you have to let the plant know that in advance. It requires a certain size of aggregate and a 5-6" slump, which means that the plant has to adjust the mix so that when it cures, it is at the right PSI.
You can't just order standard 3000PSI mix, then add water to get it through the pump.
--
Robert Allison
Rimshot, Inc.
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Bob Morrison wrote:

Yes, but rebar will help prevent problems is there is a soft spot in the ground under the slab that settles a little. It can also prevent cracks from opening up or shifting vertically.
Matt
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In a previous post Matt Whiting says...

Matt:
Quite true. However, I tell the contractors to spend their time and effort on subgrade preparation. It's easier to do than correctly placing rebar, and with the current price of steel may even be cheaper.
A good unreinforced slab on properly prepared subgrade with adequate crack control should not have any problems.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

NY Code for concrete cover of reinforcing
TABLE 1907.7.1 MINIMUM CONCRETE COVER
MINIMUM
COVER
CONCRETE EXPOSURE inches
1. Concrete cast against and permanently exposed to earth 3
2. Concrete exposed to earth or weather
No. 6 through No. 18 bar 2
No. 5 bar, W31 or D31 wire, and smaller 1-1/2
If I'm reading that right, and you can figure out a way to cast the driveway above grade and then lower it into place (!), you can go with as little as a 3" slab. Otherwise you need 6" if it's cast in place and reinforced.
Weird thing. If you _reinforce_ it, code requires a thicker slab!
Bob's got it right, the slab doesn't move unless something underneath it does. Preparation - drainage, compaction, etc. - is the key.
R
R
R
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In a previous post RicodJour says...

Rico:
Your math is almost correct:
1-1/2" top cover 1/2" (actually 5/8" OD) for #4 Longitudinal bar 1/2" (actually 5/8" OD) for #4 transverse bar 3" bottom cover
Total = 5-3/4"
Using #3 bards drops the total to 5-1/2"
You would need at least that much concrete to get the steel to bond properly anyway.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Bob Morrison wrote:

Again with the math! You're such an engineer sometimes. ;)
They allow some leeway with the coverage - I believe it's +/- 3/8".
Oh, and coming from a woodworking background, I half-lap my rebar so it's only one layer thick. ;)
R
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RicodJour wrote:
> Again with the math! You're such an engineer sometimes. ;)

And do you sand those nasty ridges off the rebar to make them smooth as you would with wood? :-)
Matt
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Matt Whiting wrote:

Hell, no! That would take foever and give inferior results. I turn them down on the lathe.
Where have you been Matt? I'm guessing the spouse-enforced two week vacation.
R
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RicodJour wrote:

Good guess. Yes, just returned from two weeks driving around the southwest in NV, AZ, CO and UT. Nice country for a visit. A little warm (I have a shot of the car thermometer at 108 or something ridiculous like that), but with the low humidity I found it very comfortable compared to PA with our 95 and 95 conditions.
Matt
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Matt Whiting wrote:

I'm guessing Four Corners, Grand Canyon, Mesa Verde and maybe Bryce. How much did you lose in Vegas? Oh, right, what happens in Vegas stays in Vegas. ;)
I rode my bicycle through that area one summer. Never got above 124 F in the shade. I'll take the humidity.
R
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RicodJour wrote:

Well, this is pretty fat OT already, but we hit the above and in addition several things around Colorado Springs (Pikes Peak, Garden of the Gods, Cave of the Winds, etc.), as well as Zion in addition to Bryce and a few other things. It was a fun two weeks.
The temps stayed right around 100 in Vegas which was the warmest area. Colorado Springs was nice with temps in the upper 80s to lower 90s. Did have one whopper of a thunderstorm the afternoon that we ate lunch at Solo's, which is a restaurant inside an old airplane (KC-97 tanker). Man was it loud when the 1/2" hail began to hit the aluminum!
I'm not a gambler so I didn't even enter a casino in Vegas. The only money I lost there was on souvenirs! :-)
Matt
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