So I finally got this slab poured yesterday, by the guy who is my contractor friend. Let me preface my question(s) with the disclaimer that he had put an addition on my barn which included pouring a slab for the floor that turned out just fine.
He used rebar for this job. The slab was ~six in., ~35X30 (I made it bigger than originally planned, I mean really, you can’t have too big a slab in front of the house). The slab is in four “squares.” As regarding the rebar, he laid it out in a grid which extended under the cross forms (not the perimeter). That is, each square did not have its own grid and the bar was laying on the ground. I brought to his attention that there wouldn’t be much reinforcement that way and he assured me that they would lift it up during the pour so it would end up in the bulk of the concrete. Seeing as how the grid was on the ground under the cross forms I didn’t really see how that was going to work so well. And in fact I don’t think it did.
While pouring the second square they had too much concrete and so shoveled the excess into what turned out to be square four. In the mean time it was walked on, a lot, and by the time square four was poured it was pretty hard. Is that concrete now part of the slab, or is the slab thinner in those areas by the thickness of the hardened material?
In two of the squares they didn’t do such a good job screeding and there are undulations. I mean, put a level across the top and there is an 1/2 – 3/4 in. space underneath. Is this something I should make an issue of? I really don’t consider this to be acceptable but if it sounds like it’s within some industry acceptable standard I can live with it. If it isn’t, how can it be made reasonably flat?
He used pressure treated 2X4 for the cross frame, which means there is concrete beneath the wood. Is that an issue, like is that now a two inch slab with a piece of wood on top of it? The wood was left in place as the expansion joints. Also, the wood sits proud of the concrete surface for some lengths depending on how well they screed. Should I ask that it be knocked down? It looks to me that it’s going to be a problem the first time I plow this winter.
I’d like to hear some comments, mainly constructive in nature if possible.
Thanks,
G
Replies
good luck
You just had about 20 cu. yards of concrete placed and finished yesterday. That amount of material weighs about 20 tons. I don't think the day after the pour is the best time to consider your options for a poorly prepared and finished concrete job of that scale. Just about anything you do to try to fix the problems you point out will be expensive and problematic. I doubt your contractor will want to deal with your complaints at this point either. He'd have to admit that he has a hell of a mess on his hands.
For example:
1.According to the NAHB Residential Performace Guidelines dwelling area slabs should not " have pits, depressions, or areas of uneveness 3/8" in 32 inches ". Sounds like you have an exterior patio that can have allowances for a drainage plane, but you still have "areas of eveness" that are horrendous and bound to be water and ice problems.
2. The 2x4's are definitely not "expansion joints". The rebar and concrete run under them. They're nothing more than screeds--screeds that are going to be chewed up by your plow pretty soon. Maybe remove them and mortar in slices of cobble stone or brick pavers that can be set flush at this point. Problem is: who's gonna pay for that?
3. Since the rebar wasn't stopped at the screeds, then it's anyone's guess as to where the slab will crack; either by relieving its plastic shrinkage stress or load stress. Alot depends on the sub grade preperation, actaul rebar placement, soil conditions, and site drainage conditions. The chances of a 6" x 35'x30' slab (with continuous rebar and 2x4 depressions at quarters) not cracking at all are slim to none. Good luck with that.
Slab for what??
It's in front of the house. I have a packed item 4 driveway. I wanted an area where I wouldn't need to be on my back in the dirt under my car. Have had enough of that. Also, when we get out of the car when it's raining we won't be tracking so much mud into the house. And I can wash the car without standing in mud.
I heat with an outdoor water fired boiler. The driveway is a loop and triaxle trucks will be passing ovet the slab, as will the propane truck and, in rare instances, the fuel oil truck.
I'm very sorry, but it's tough to come up with constructive comments on this one. I'll try: A least you won't have to lie in the mud to put on the tire chains.
As I think you know, the job of placing and finishing the concrete was botched. The rebar should have been supported by chairs, the 2x4 expansion joints are grossly oversized and unnecessary for the purpose, and the surface undulations and roughness is amateurish at best. The guy did a lousy job, pure and simple. How you deal with it and him is an open question.
The rebar situation can't be fixed at this point. The surface could be topped with some sort of high strength, super stick concrete stuff. I wouldn't trust your guy to apply it. The 2x4's are best removed and concrete placed into the gap.
"friend?"
Did you have a contract that specified the scope of work? If this was me, and the job was botched, I'd be demanding a tear-out and re-do. And he wouldn't be my friend at the end of this unless he made it right.
mine and his responses
I spoke with my guy yesterday and voiced my displeasure. His response to the 2X4's is that if there is going to be any cracking it will be there. More or less a stress relief type of thing. I decided against telling him about martensitic phase transformation toughened zirconia.
His response to the rebar was that doing it that way would prevent differential heaving of the squares, and stands by the lift as you pour. I could see how if each square had its own grid the lifting technique might work. However it was continuous (hence his heaving argument) and under the expansion joint 2X4’s and lifting didn’t accomplish much. The slab was poured on top of the existing hard pan, there's no layer of crushed stone underneath, which was my decision. I also mentioned it being inadvisable to leave ferrous metals in contact with the ground.
The rebar was rebar, not mesh or similar product. It was 3000# concrete according to the invoice, that's the only information I have. No fiberglass. I'll post some pictures.
My main concern here is to what extent has the structural integrity been compromised. We didn't get too deep into the cosmetic issues, which to the extent that they can, will be fairly easy to remediate. No I didn't have a contract or written scope of work, although I did give that some thought earlier and decided I wouldn't need it..... BTW, I have a M.S. in ceramic engineering and intuitively know the answers to my questions but I need to hear it from those who can provide informed opinion. That way I have reinforcement for my argument and be better educated, both as a dumb homeowner and a dedicated DIY'er
I left it that I would get back to him next week after having had a chance to consult with my go-to concrete guys, namely, you guys. He has offered to tear it up and start from scratch but I don't know if he'll still stand by that next week. I'd like to avoid that, if possible, and that's one of the reasons for my posts.
I'm still a film guy but my sister in law is here with a digital camera and I'll get some photos.
G
you're answering your own question
You've outlined three main issues, two of which suggest the inherent structural integrity of this slab is compromised, and one of which (3/4 deflections of the finished surface) will exacerbate those (water pooling in areas of likely cracks) as well as make the slab useless for keeping the area dry. Seems his offer to tearout and repour is correct. The trick will be for him to do it properly, rather than poorly while using justifications for the time-saving measures that don't hold up to best practices nor scientific scrutiny...If I were you, I'd insist on proper placement of the rebar, proper construction of control joints, and a decent attempt to actually screed/finish the slab in a suitable way.
My guess is that the way he did it normally works pretty well, but he was rushed/shorthanded/whatever for your project and it got out of hand. Lifting the rebar CAN work, sort of, but only if it actually gets done. Achieving a smooth, level surface is also a matter of time and available hands at critical points in the pour. Using a 2x4 in the middle of a large slab is often done, to facilitate screeding, but the board is pulled and the depression filled in before the concrete sets too far. In this case, he fell behind and the board got left there.
No, it doesnn't
DanH wrote:
My guess is that the way he did it normally works pretty well...
No, it doesn't. That's why the job turned out unsatisfactory.
G,
I think this slab is fine for its intended purpose. There is no real load on it and no rebar is required for a driveway. It might be ugly but will suport cars and trucks.
KK
What's a "real" load
I appreciate you sharing your opinion. IMO, your opinion is why many concrete driveways uncontrollably crack... and are ugly.
The facts speak otherwise. The concrete material tself constitutes a 20 ton dead load. Add to that the live load weight of a periodic "triaxle truck" and you have signicant 'real' loads. The only reason rebar is not required by code for a driveway is because there is no immediate danger to life or limb if it cracks. That is the same reason a permit (plan review) is not generally required for a ground level concrete patios-- at least around here.
However, in both cases proper slab reinforcement, which may include reinforcing wire or rebar, should be designed (not ignored) per best concrete work practices.
deadnuts,
I've only been doing construction since '77 so I'm still learning. Most slabs crack because of too much water added and an uncompacted base. Wire does nothing to improve the tensile strength of a slab but fiber will add a little. Your interpertation of code is off also.
With 6 inches of concrete and rebar that may or may not be in the correct plane I still say his slab is fine for a driveway.
KK
doing isn't always knowing
coonass wrote:
Wire does nothing to improve the tensile strength of a slab....
YOur right; you do have alot to still learn. I'm wishing you all the best in making sure that happens.
Most of what you wrote seems pretty ordinary to me. The only think I would take issue with is the undulations.
If you run a hoes on the slab, does all the water run off? If it doesn't, then I'd say you have a danged good reason to complain.
photos and follow up
I posted photos here:
https://www.flickr.com/photos/toogbrand/sets/72157646652446120/
My friend further states that expansion joints accommodate expansion/contraction through the thickness, not horizontally. Is that true?
His solution is to remove the wood and replace with appropriate expansion joint material, cut the slabs loose from one another and grind out the surface discontinuities with one of those grinders that looks like a floor polisher. And provide a five year warranty that if it fails he'll then remove the slabs and redo the job correctly.
We've had plenty of rain lately, no standing water.
That's one amateurish looking piece of work. Given that this is his level of expertise, I wouldn't hold out much hope for his grinding solution. It's not the process that's questionable, just the operator.
As for his explanation of why expansion joints are used: Your friend is full of beans.
If it were me, at this point I think I'd just live with what I've got. But, I'd demand a substantial refund or deduction in the bill. No way did he perform to professional standards.
If you wanted it perfect that should be spelled out in advance. For it's intended purpose it sounds pretty normal/average - not the best practice, but most slabs aren't. Tearing out a slab like that is rediculous.
IdahoDon,
With all due respect, you can't be serious....
As I said, I already knew the answers to my questions, but wanted some informed opinion to show my friend. This is singularly the worst concrete work I have ever seen. And I used to work for a company that built condo developments so I have seen my share of concrete poured.
And rediculous.....is spelled ridiculous.
But thanks for checking in.
Thanks for taking the place of my spell checker. I have no doubt you've seen better - we all have - but crappy work isn't uncommon.
I wouldn't pay for crappy concrete work like that, but I would have seen more of the guys previous work and not hired him.