# Concrete repair question



## german (Jun 23, 2009)

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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## dayexco (Mar 4, 2006)

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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## blu (Jun 5, 2010)

Concrete does 2 things........


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## peteo (Jan 8, 2011)

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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## greg24k (May 19, 2007)

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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## duburban (Apr 10, 2008)

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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## Chris Johnson (Apr 19, 2007)

3 rules of concrete

Guaranteed to be heavy
Guaranteed to crack
Guaranteed no one will steal it


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## TimelessQuality (Sep 23, 2007)

And it won't catch on fire...


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## FramingPro (Jan 31, 2010)

Chris Johnson said:


> 3 rules of concrete
> 
> Guaranteed to be heavy
> Guaranteed to crack
> Guaranteed no one will steal it


guaranteed to be cool :cool2:


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## griz (Nov 26, 2009)

Chris Johnson said:


> 3 rules of concrete
> 
> Guaranteed to be heavy
> Guaranteed to crack
> Guaranteed no one will steal it





TimelessQuality said:


> And it won't catch on fire...



Also will not be the right color...:whistling:thumbup:

and it will get hard...:thumbup::laughing:


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## AustinDB (Sep 11, 2006)

greg24k said:


> ...you should use high performance concrete


what is high performance concrete and under what circumstances would it be used?


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## griz (Nov 26, 2009)

72chevy4x4 said:


> 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...:thumbsup:


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## rino1494 (Jan 31, 2006)

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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## german (Jun 23, 2009)

rino1494 said:


> 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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## greg24k (May 19, 2007)

72chevy4x4 said:


> 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 :laughing:


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## greg24k (May 19, 2007)

german said:


> 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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## chew (Apr 5, 2012)

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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## german (Jun 23, 2009)

chew said:


> 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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## german (Jun 23, 2009)

german said:


> 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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## german (Jun 23, 2009)

greg24k said:


> 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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## chew (Apr 5, 2012)

I don't think the problem is with the concrete, but try to get one their QC guys out to look at it along with your engineer & admix salesman. Don't let the engineer charge you for the visit you poured it the way he told you. I use my Swiss hammer 1st that way I know whether to core or not, so far no cores, but I also take clyinders on everything I pour. Where are you located ? I looked up Moxie 1800 ( I have never used it ) It looks to be just a waterproofer, but it looks like it would also knock out the air entrainment ( I have add air to all my mixes). If you added it and not the redi mix co. you mixed it wrong according to their directions. You can get epoxy from Hilti or use a Sika hi-mod epoxy. You could also get a concrete restoration co. to come out & epoxy inject it. I would not be to worried about the crack if you are putting stone on top just use a decoupling membrane & set the stone on top of it. I would also find another redi-mix supplier no customer service means no business from me.


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## german (Jun 23, 2009)

chew said:


> I don't think the problem is with the concrete, but try to get one their QC guys out to look at it along with your engineer & admix salesman. Don't let the engineer charge you for the visit you poured it the way he told you. I use my Swiss hammer 1st that way I know whether to core or not, so far no cores, but I also take clyinders on everything I pour. Where are you located ? I looked up Moxie 1800 ( I have never used it ) It looks to be just a waterproofer, but it looks like it would also knock out the air entrainment ( I have add air to all my mixes). If you added it and not the redi mix co. you mixed it wrong according to their directions. You can get epoxy from Hilti or use a Sika hi-mod epoxy. You could also get a concrete restoration co. to come out & epoxy inject it. I would not be to worried about the crack if you are putting stone on top just use a decoupling membrane & set the stone on top of it. I would also find another redi-mix supplier no customer service means no business from me.


Thanks so much for the info. I read some other comments about high performance concrete. The problem here is the inconsistencies from the ready mix plants. Nobody really has a lot of confidence in them. Sounds crazy but it's true.99.9% of the stuff poured here is standard 3000 psi mix and I think it is because the companies products get screwy if you vary from that. I know they have guys that know how to do stuff right but i don't think they make it down to where the trucks are actually being mixed very often. Every time i have gone in an office and spoken with the managers about how I need my concrete mixed, it's very obvious they don't know what the heck i'm talking about. I have been at the plant when they were mixing trucks and was very concerned when they were adding cement and aggregate to my mix with such variation between the 2 trucks I ordered, and they were both supposed to be the same mix. Most guys who actually care about their name and clients have tried to seek further education on enhancing the concrete themselves, beyond the plant such as additives and fiber. The companies here are often getting called out for sub par concrete, they just have so many outs that it's hard to prove they did something wrong. The moxie 1800 is much more than a water proofer. I think that the reason I only recently " just had my cherry popped" as you put it, is because of that stuff. I also use it in my mortar and stucco. It's a really, really good product. I know there is still much i could learn as far as concrete goes but i don't really do foundations so the enhancement for my flatwork and footings has been a strong step in a positive direction for me. I will follow the leads you gave and I really do appreciate the feedback.


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## german (Jun 23, 2009)

greg24k said:


> 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.


I have to say I have mixed emotions about using only fiber or mesh. From now on I will use both. The reason i do this is because when the concrete footing cracks, so does the stone on top. Our weather and soil here makes for a really bad combo. Super dry, and then it down pours and then dries up again. The ground is also full of clay and soft but then you will have huge chunks of limestone here and there. When I say huge i mean several city blocks big. So The ground in a 10 square mile area of town can shift a whole lot and then be totally fine in another.


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