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Concrete Repair Question

 
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Old 05-15-2012, 04:36 PM   #1
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Concrete Repair Question


I got a question about concrete slab repair. I have poured slabs for about 14 years now and it appears i have my first cracking slab. My question is on how to fix it. It's a large patio slab with no load on top of it. The majority of the slab is 6 inch flatwork but in one area the yard dropped down and the concrete has a 15 inch above grade bean for about 8 feet. I have been using some additives and high strength integral fiber for the last few years on all my flatwork with zero surface cracking. Before we poured I was told by my engineer and the additive companies that that one run really didn't need rebar due to the upgrades on the concrete( about 8000 psi and around triple the flexural strength. It's also poured on solid bed rock. I ended up adjusting that beam to 18 inches wide the day of the pour because i was starting to question the info I was given. The concrete was exposed and visible for 4 months during construction and it never had even a hairline crack. Well, I trusted the info i was given and low and behold we had a bunch of rain after a long dry spell and this thing is cracking( has about an 1/8 inch crack running up the exterior beam and about 7 feet across the top of the slab. I contacted the ready mix supplier and informed them that I think that one of the trucks wasn't mixed correctly because the crack is in one spot that took almost all of one truck. Of course they said I could of over hydrated the concrete or took to long to get it out of the truck and other stuff that pointed at me and they steered clear of any possibility of them making a mistake at all. We poured 30 yards in under 2 hours.
My contract with my customer states everything i used and if I did business like almost everyone else in town I wouldn't have a problem leaving this up to the customer but I just don't do people like that. So my question now is what is the most economical way to remedy the situation and stop this thing from moving. Has anyone fallen to the marketing of the "newer better technology" and had to clean up the mess? I'm all ears at this point as i'm kicking myself in the butt over this.
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Old 05-15-2012, 04:59 PM   #2
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Re: Concrete Repair Question


are you a contractor? or a do it yourselfer? if a contractor...this question might be better posed in the construction side?

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Old 05-15-2012, 05:06 PM   #3
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Re: Concrete Repair Question


Concrete does 2 things........
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Old 05-15-2012, 05:35 PM   #4
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Re: Concrete Repair Question


I may be missing something here but why is a no load patio 6" thick and made with 8000psi mix?? Seems completely unnecessary if you ask me but then again, I'm not a cement guy.
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Old 05-15-2012, 06:16 PM   #5
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Re: Concrete Repair Question


Cracking could be structural and it could be shrinkage.

I don't know why you need High Strength concrete for a patio. When high strength concrete is made, to accomplish that they lowering the water cement content. I don't know where you from, but around here when they make High Strength concrete, very often silica fume is added to prevent forming of calcium hydroxide crystals, because they reduce the strength when cement is bonding.
Having your slab on solid bed rock, also not a good idea when working with high strength concrete, because the footing surface could have large uneven surface that could create voids during a slab pour, because 8,000psi is a high density concrete, and if the company using weaker aggregates, they may not be strong enough resist the loads which concrete can sustain, and that could cause the crack as well... Especially mentioned that after a few good rains it cracked.If anything, you should use high performance concrete instead of high strength concrete.
There also could be other factors....and as repair goes if that is a patio, it has to be taking out and replaced, because it will show no matter what you do, and the crack will only spread more.
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Old 05-15-2012, 07:24 PM   #6
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Re: Concrete Repair Question


do flatwork contractors ever sell an installation as being "crack free"? my understanding of concrete is that its unpredictable and even with perfect prep it can surprise us.
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Old 05-15-2012, 09:12 PM   #7
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Re: Concrete Repair Question


3 rules of concrete

Guaranteed to be heavy
Guaranteed to crack
Guaranteed no one will steal it
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Old 05-15-2012, 09:38 PM   #8
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Re: Concrete Repair Question


And it won't catch on fire...
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Old 05-15-2012, 10:09 PM   #9
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Re: Concrete Repair Question


Quote:
Originally Posted by Chris Johnson View Post
3 rules of concrete

Guaranteed to be heavy
Guaranteed to crack
Guaranteed no one will steal it
guaranteed to be cool
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Old 05-15-2012, 10:43 PM   #10
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Re: Concrete Repair Question


Quote:
Originally Posted by Chris Johnson View Post
3 rules of concrete

Guaranteed to be heavy
Guaranteed to crack
Guaranteed no one will steal it
Quote:
Originally Posted by TimelessQuality View Post
And it won't catch on fire...

Also will not be the right color...

and it will get hard...
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Old 05-15-2012, 11:04 PM   #11
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Re: Concrete Repair Question


Quote:
Originally Posted by greg24k View Post
...you should use high performance concrete
what is high performance concrete and under what circumstances would it be used?
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Old 05-15-2012, 11:22 PM   #12
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Re: Concrete Repair Question


Quote:
Originally Posted by 72chevy4x4 View Post
what is high performance concrete and under what circumstances would it be used?
High performance concrete is a mix design matched to specs & job site criteria...

Sounds like a bunch of mumbo jumbo...

but just think about it...
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Old 05-16-2012, 04:14 AM   #13
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Re: Concrete Repair Question


The higher the strength of concrete, the more prone to shrinkage and cracking. Every commercial job I bid for site concrete, the engineers spec out 3,500 psi with wire. This is for sidewalks and dumpster pads.
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Old 05-16-2012, 04:27 AM   #14
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Re: Concrete Repair Question


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Originally Posted by rino1494 View Post
The higher the strength of concrete, the more prone to shrinkage and cracking. Every commercial job I bid for site concrete, the engineers spec out 3,500 psi with wire. This is for sidewalks and dumpster pads.

80% of what I do is masonry. all the concrete I pour is for footings or patios getting a stone overlay. I'm very versed on flatwork for flat pours. The additive I ad is called moxie 1800 and the tough fiber( poly steel integral fiber). The moxie triples the flexural strength and doubles the psi. When you start with 4000psi concrete you have extremely durable and flexion resistant concrete. I have used this for a long time to protect the stonework above the concrete. I have never had a slab crack. I don't ever even see surface cracking. The only thing different about this pour is that the outside perimeter beam was a little taller due to the grade change. Anyway thats why I pour the way I pour. Someone mentioned the contractors section I'm interested in posting it in that area wherever that is. Thanks
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Old 05-16-2012, 06:03 AM   #15
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Re: Concrete Repair Question


Quote:
Originally Posted by 72chevy4x4 View Post
what is high performance concrete and under what circumstances would it be used?
It has longer life span, easy placement, less maintenance, tougher values, etc... used mostly on highways, bridges,etc.

The reason I mentioned that is because, he used 8,000psi mix adding all kinds of additives to the mix, he would be better off ordering High Performance mix, and you could land 747 on it, that thing would never crack
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Last edited by greg24k; 05-16-2012 at 06:07 AM.
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Old 05-16-2012, 06:18 AM   #16
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Re: Concrete Repair Question


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80% of what I do is masonry. all the concrete I pour is for footings or patios getting a stone overlay. I'm very versed on flatwork for flat pours. The additive I ad is called moxie 1800 and the tough fiber( poly steel integral fiber). The moxie triples the flexural strength and doubles the psi. When you start with 4000psi concrete you have extremely durable and flexion resistant concrete. I have used this for a long time to protect the stonework above the concrete. I have never had a slab crack. I don't ever even see surface cracking. The only thing different about this pour is that the outside perimeter beam was a little taller due to the grade change. Anyway thats why I pour the way I pour. Someone mentioned the contractors section I'm interested in posting it in that area wherever that is. Thanks
Why you don't order Fibermesh concrete if the stone going over it, Fibermesh provides top to bottom and side to side uniform reinforcement and it also cost effective not to mention it is a superior replacement of wire mesh and has all other great characteristic.
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Old 05-16-2012, 07:13 AM   #17
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Re: Concrete Repair Question


14 yrs. & never had a slab crack ? How's it feel to have your cherry popped. It does not matter what psi the concrete is if you don't have the proper reinforcement it will fail. Fibers (steel or fiberglass) don't add structural strength. There are better ways to get 8,000 psi then fibers, because with 4,000 psi concrete you still need to watch your W/C ratio. 1st problem was you listened to the wrong engineer & a admix. salesman. You do need reinforcement where you are pouring on bedrock, because the base next to it will move differently the than the bedrock. Also you needed L bars in the beam to tie into the slab. Did you put any control joints in ? As for fixing you can expoxy inject the crack, but you will still see it unless you scarf the top surface of the whole slab & pour a top mix over it. That slab will also crack some more this summer when that slab heats up. Did the redi-mix supplier specify they were providing 8,000 psi concrete (which I doubt) ? If they did then have a testing co. come out with a swiss hammer & test it. It probably will be low then call the redi-mix co. & tell them their concrete psi was low then they will start to help you.
Here are your problems : sections of the base move differently, improper reinforcement design, lack of control joints, & CONCRETE SLABS CRACK.
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Old 05-16-2012, 10:25 AM   #18
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Re: Concrete Repair Question


Quote:
Originally Posted by chew View Post
14 yrs. & never had a slab crack ? How's it feel to have your cherry popped. It does not matter what psi the concrete is if you don't have the proper reinforcement it will fail. Fibers (steel or fiberglass) don't add structural strength. There are better ways to get 8,000 psi then fibers, because with 4,000 psi concrete you still need to watch your W/C ratio. 1st problem was you listened to the wrong engineer & a admix. salesman. You do need reinforcement where you are pouring on bedrock, because the base next to it will move differently the than the bedrock. Also you needed L bars in the beam to tie into the slab. Did you put any control joints in ? As for fixing you can expoxy inject the crack, but you will still see it unless you scarf the top surface of the whole slab & pour a top mix over it. That slab will also crack some more this summer when that slab heats up. Did the redi-mix supplier specify they were providing 8,000 psi concrete (which I doubt) ? If they did then have a testing co. come out with a swiss hammer & test it. It probably will be low then call the redi-mix co. & tell them their concrete psi was low then they will start to help you.
Here are your problems : sections of the base move differently, improper reinforcement design, lack of control joints, & CONCRETE SLABS CRACK.
Doesn't feel awesome. You have to remember that this is a patio and 90% of the entire slab is a flat 5 inch thick patio slab. The perimeter beam on that one side is the only area with an elevated beam or really a beam that drops down to meet the grade change. The w/c ratio was fine but i'm hesitant to believe i got 4000 psi concrete to begin with. The area that cracked is the area where the first truck was emptied. I think that batch was the problem. Do you have info on which epoxy would be best to use. My biggest concern is if this thing will eventually settle or if this problem will keep on going. Even though i stated everything in the contract and the owner knows it, I want to treat the guy right. Like I said, this is my first cracking issue. I guess stuff can and will happen even if you try to provide the best install possible. The only thing I'm kicking myself in the butt over is not putting that rebar in the beam. I know better but took bad advice against my gut feeling. Thanks for the feedback
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Old 05-16-2012, 10:33 AM   #19
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Re: Concrete Repair Question


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Doesn't feel awesome. You have to remember that this is a patio and 90% of the entire slab is a flat 5 inch thick patio slab. The perimeter beam on that one side is the only area with an elevated beam or really a beam that drops down to meet the grade change. The w/c ratio was fine but i'm hesitant to believe i got 4000 psi concrete to begin with. The area that cracked is the area where the first truck was emptied. I think that batch was the problem. Do you have info on which epoxy would be best to use. My biggest concern is if this thing will eventually settle or if this problem will keep on going. Even though i stated everything in the contract and the owner knows it, I want to treat the guy right. Like I said, this is my first cracking issue. I guess stuff can and will happen even if you try to provide the best install possible. The only thing I'm kicking myself in the butt over is not putting that rebar in the beam. I know better but took bad advice against my gut feeling. Thanks for the feedback
I forgot to mention that i called the ready mix co and they basically said that unless someone doesn't get core tests done and take them to court, they don't offer any assistance. The manager actually chuckled and said " we've very rarely been at fault". They are a big company and act like they are doing you a favor when they bring you concrete. I've had a problem with this since day one with this company. If I treated my clients like they do i wouldn't have any. The epoxy being visible won't be a problem due to the fact that stone is laid over the entire top of the slab and the face is stucco.
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Old 05-16-2012, 10:39 AM   #20
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Re: Concrete Repair Question


Quote:
Originally Posted by greg24k View Post
Cracking could be structural and it could be shrinkage.

I don't know why you need High Strength concrete for a patio. When high strength concrete is made, to accomplish that they lowering the water cement content. I don't know where you from, but around here when they make High Strength concrete, very often silica fume is added to prevent forming of calcium hydroxide crystals, because they reduce the strength when cement is bonding.
Having your slab on solid bed rock, also not a good idea when working with high strength concrete, because the footing surface could have large uneven surface that could create voids during a slab pour, because 8,000psi is a high density concrete, and if the company using weaker aggregates, they may not be strong enough resist the loads which concrete can sustain, and that could cause the crack as well... Especially mentioned that after a few good rains it cracked.If anything, you should use high performance concrete instead of high strength concrete.
There also could be other factors....and as repair goes if that is a patio, it has to be taking out and replaced, because it will show no matter what you do, and the crack will only spread more.
I miss quoted myself. It is high performance concrete. High psi is just one of the specs of the enhancement. It also has much higher flexural strength, it's vapor proof and much more dense. I added 2 things moxie 1800 admix and the fiber plus I ordered 4000 psi concrete from the plant and altered the aggregate.

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