Special Mix Design Loads Of Cracks A Year Later. - Concrete & Paving - Contractor Talk

Special Mix Design Loads Of Cracks A Year Later.

 
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Old 07-12-2018, 10:37 AM   #1
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Special Mix Design Loads Of Cracks A Year Later.


I did a job renovating a 100 year old school last year. I don't remember what the mix design was but I know it had tons of extra gravel in it. A year later there are cracks where there shouldn't be any. Randomly coming off of straight sections of expansion and tons of other weird places. Could that mix design have something to do with it?

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Old 07-12-2018, 11:21 AM   #2
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Re: Special Mix Design Loads Of Cracks A Year Later.


Not really enough info to go on.

Was the mix pretty lean?

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Old 07-12-2018, 11:21 AM   #3
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Re: Special Mix Design Loads Of Cracks A Year Later.


Maybe...that extra gravel you are describing is curious.

Was it tested by a lab?

Was an inspector at the batch plant when being batched?

Inspector on sight upon delivery?

Slump? Time in transit?

Weather conditions when placed?

Water added when placed?

Kept wet during initial cure?

Placement of steel in relation to top of of concrete?

Who designed the mix? Anything questioned at that time?

Reason for special mix design? Sounds like a lot of rock.

Additives?
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Old 07-12-2018, 11:52 AM   #4
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Re: Special Mix Design Loads Of Cracks A Year Later.


Quote:
Originally Posted by PancakeBrock View Post
I did a job renovating a 100 year old school last year. I don't remember what the mix design was but I know it had tons of extra gravel in it. A year later there are cracks where there shouldn't be any. Randomly coming off of straight sections of expansion and tons of other weird places. Could that mix design have something to do with it?

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Uhhh...




....yes?



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Old 07-12-2018, 01:45 PM   #5
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Re: Special Mix Design Loads Of Cracks A Year Later.


This was a job for a public school. The GC was supposed to have 30 tests done and they only did 3. The three cylinders they took from one pour tested good on the break. I called the PM and asked for a copy of the mix design just now and he is supposed to be sending it. The design had so much rock the courtyard I had to pour with a big crane pump continually clogged the line in the pump. It was a hell of a few days pouring with the pump. Time in transit maybe 10 minutes. Everything was poured between September and November so cool temperatures but not freezing. Everything except for the stuff that got pumped was 3-4" slump. This job I didn't provide the concrete the GC did and I have no clue who did the design. Here's some pictures of the random cracks everywhere.

So many cracks ran right next to my saw joints but didn't start appearing until maybe the last month.

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Old 07-12-2018, 02:10 PM   #6
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Re: Special Mix Design Loads Of Cracks A Year Later.


Is the school district complaining about it?

Also how thick is the concrete and how deep are the control joints? How long did you wait until cutting?

One trick is if you have two joints coming off an inside corner like in your picture, make one deeper than the other so it is more likely to crack first. Otherwise the two lines “fight for the crack” and neither may get it we have even put a small champher strip underneath the slab, lined up with the control joint on top. Good planning eliminates weird cracking.
Also narrow wedges, I put that little 12” line going askance anyway, because it wants to go there.
The biggest square you can have is 10x10, do you have some bigger pics?



Sounds like this might fall on the GC though




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Old 07-12-2018, 02:16 PM   #7
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Re: Special Mix Design Loads Of Cracks A Year Later.


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Is the school district complaining about it?

Also how thick is the concrete and how deep are the control joints? How long did you wait until cutting?

One trick is if you have two joints coming off an inside corner like in your picture, make one deeper than the other so it is more likely to crack first. Otherwise the two lines “fight for the crack” and neither may get it we have even put a small champher strip underneath the slab, lined up with the control joint on top. Good planning eliminates weird cracking.
Also narrow wedges, I put that little 12” line going askance anyway, because it wants to go there.
The biggest square you can have is 10x10, do you have some bigger pics?



Sounds like this might fall on the GC though




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Those were the only pictures I took. I got asked to come to a meeting about the school districts complaints with the GC. It's a 3 hour drive.

I had a section that was 20' x 60'. Joints were cut every 10' and there is a crack running all the way down the 60' stretch maybe 4' off the cut. Concrete is all 4" and joints are all 1". I've poured tons of concrete I've been doing this for 10 years and have never seen anything like it.

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Old 07-12-2018, 02:28 PM   #8
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Re: Special Mix Design Loads Of Cracks A Year Later.


Here are some of the pictures of the stuff we did. One of those sets of steps even cracked pretty much down the middle of the step then shot off to the side up top.

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Old 07-20-2018, 06:19 PM   #9
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Re: Special Mix Design Loads Of Cracks A Year Later.


Why don't you know the mix design? It should have been on the plans you were provided if an engineer drew them up.
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Old 07-20-2018, 06:21 PM   #10
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Re: Special Mix Design Loads Of Cracks A Year Later.


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Why don't you know the mix design? It should have been on the plans you were provided if an engineer drew them up.
The plans and spec book said a mix design had to be submitted and approved. The GC provided concrete and did the mix design. I was told to pour it.

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Old 07-20-2018, 07:11 PM   #11
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Re: Special Mix Design Loads Of Cracks A Year Later.


who placed the base?

was it inspected & tested?

IMO, there is a flaw in the mix design.

problems can occur if concrete is too wet when placed.

i am also of the opinion that concrete can be spec'd that is too dry to reasonably work with. that 3-4" slump is very difficult to pump and have any decent time to work it. i've had the batch plant refuse to pump a 4" slump as it can/will plug a pump. discussions with the architect & engineer have allowed the slump to go to 5" with very specific instructions to the testing lab that 5" slump is the max. allowed. the 4-1/2"-5" slump is easier to work with.

the mix design w/the extra rock, low slump, especially the way you describe the pump operation seems like a recipe for disaster.

if there are no issues raised with the base it would be time for the gc to contact the architect & engineer.

is the school dist. complaining?
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Old 07-20-2018, 08:59 PM   #12
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Re: Special Mix Design Loads Of Cracks A Year Later.


Quote:
Originally Posted by griz View Post
who placed the base?

was it inspected & tested?

IMO, there is a flaw in the mix design.

problems can occur if concrete is too wet when placed.

i am also of the opinion that concrete can be spec'd that is too dry to reasonably work with. that 3-4" slump is very difficult to pump and have any decent time to work it. i've had the batch plant refuse to pump a 4" slump as it can/will plug a pump. discussions with the architect & engineer have allowed the slump to go to 5" with very specific instructions to the testing lab that 5" slump is the max. allowed. the 4-1/2"-5" slump is easier to work with.

the mix design w/the extra rock, low slump, especially the way you describe the pump operation seems like a recipe for disaster.

if there are no issues raised with the base it would be time for the gc to contact the architect & engineer.

is the school dist. complaining?
I did some stuff on an air Force Base that the slump had to be less then a 1. We used the truss screed to screed that one out but yes concrete can definitely be spec'd too low of a slump. I placed the base but the GC provided the material and it was the cheap stuff dirt the concrete plant scraped out of the field next to their plant. It did pass compaction tests though. The base seems fine and nothing appears to be sinking it just looks like it's falling apart. I told the super he needed to take some core samples and he came back and said he knows it's probably not going to pass and they want me to tear a few sections out and repour them just so the district will finally sign off and let them be done with the job. They are going to pay for me to re do those sections which is probably close to 3 yards of concrete.

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Old 07-20-2018, 09:05 PM   #13
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Re: Special Mix Design Loads Of Cracks A Year Later.


will you be cutting rebar & have to dowel in?

inspected?

what mix design?

something you may want to bring up is the color of the replaced concrete will be different.

been there done that...school people are a different lot...

good luck
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Old 07-21-2018, 05:19 AM   #14
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Re: Special Mix Design Loads Of Cracks A Year Later.


Quote:
Originally Posted by griz View Post
who placed the base?


i am also of the opinion that concrete can be spec'd that is too dry to reasonably work with. that 3-4" slump is very difficult to pump and have any decent time to work it.
I agree completely with that. You need to find that sweet spot between workable, but not compromising the 28 day strength.

Working a true 3-4" is a hassle right out of the truck, I can't imagine how stiff it gets after running through 100 feet of pump hose.

I pour almost everything at a "truck driver's 5", which is probably not a true 5 most of the time, but very workable without being runny.
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Old 07-21-2018, 05:23 AM   #15
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Re: Special Mix Design Loads Of Cracks A Year Later.


Quote:
Originally Posted by griz View Post
will you be cutting rebar & have to dowel in?


been there done that...school people are a different lot...
I know a guy that did a set of huge bluestone treads for a school with something like a 7" rise. One of them measured at something like 7 - 1/8" so they stiffed him a portion of the final payment.

He probably should have checked the tread thickness before installing them all, but it wasn't a dangerous amount of variation.
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Old 07-21-2018, 08:58 AM   #16
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Re: Special Mix Design Loads Of Cracks A Year Later.


Quote:
Originally Posted by griz View Post
will you be cutting rebar & have to dowel in?

inspected?

what mix design?

something you may want to bring up is the color of the replaced concrete will be different.

been there done that...school people are a different lot...

good luck
If I can I'd rather just do a regular 3000 mix with fibermesh. There is wire mesh in it and only rebar down the thickend edges. I'll definitely be Dowling it in. I doubt it's going to be inspected. This is pretty bad and I don't blame them but blaming the concrete guy when I did everything correct and it seems like something was really wrong with the actual mix and the GC isn't going to say that it is because they could loose big replacing it.

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Old 07-21-2018, 03:34 PM   #17
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Re: Special Mix Design Loads Of Cracks A Year Later.


Who is covering replacement cost for those sections? Just curious.


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Old 07-21-2018, 03:40 PM   #18
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Re: Special Mix Design Loads Of Cracks A Year Later.


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Who is covering replacement cost for those sections? Just curious.


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Old 07-21-2018, 04:00 PM   #19
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Re: Special Mix Design Loads Of Cracks A Year Later.


have the gc order the mud so if something goes haywire it's not on you.

i'm surprised the architect & school district didn't take some action before now.

if they core it, check the breaks and work placement back through the mix design, and if someone really wants to point fingers it will either be the concrete company or the engineer/architect or a combo of them
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Old 07-21-2018, 04:04 PM   #20
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Re: Special Mix Design Loads Of Cracks A Year Later.


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Originally Posted by griz View Post
have the gc order the mud so if something goes haywire it's not on you.

i'm surprised the architect & school district didn't take some action before now.

if they core it, check the breaks and work placement back through the mix design, and if someone really wants to point fingers it will either be the concrete company or the engineer/architect or a combo of them
Exactly why the GC doesn't want to core it.

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