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Thread: Concrete strength

  1. #1
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    Cool Concrete strength

    Now have to deal with this suspended slab situation:

    RESULTS: 3500psi required at 28 days :-


    Test Core 1: 2856psi at 59 days

    Test Core 2: 2985psi at 59 days

    Test Core 3: 1851psi at 59 days

    The cube tests failed at 28 days, Schmidt hammer test confirmed it.
    Enjoy a good rum on the rocks at sunset.

  2. #2
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    Default Re: Concrete strength

    You got bad concrete. Who gets to rip it out and re-pour???

  3. #3
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    Default Re: Concrete strength

    Ugh I feel your pain. What does your engineer say? Being a suspended slab, doesn't sound like an easy fix.

    Chad
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  4. #4
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    Default Re: Concrete strength

    Dangit! Sorry. I'm starting to dislike concrete contractors.

  5. #5
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    Default Re: Concrete strength

    Quote Originally Posted by bluedog225 View Post
    Dangit! Sorry. I'm starting to dislike concrete contractors.
    Good concrete guys are hard to find, but in this case sounds like this is not as much the fault of the concrete sub but rather the mix plant. I wonder if the submitted the mix design for approval?

    Chad
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  6. #6
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    Default Re: Concrete strength

    You ask your engineer if the actual strength is sufficient.
    Life is complex.

  7. #7
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    Default Re: Concrete strength

    I wonder if a couple of inches of really strong well-bonded concrete on top, to take the compression loads, could be the answer.
    Any engineers like to comment?

  8. #8
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    Default Re: Concrete strength

    Look at the bright side. Think of all the money you saved not paying for a batch plant inspector...

  9. #9
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    Default Re: Concrete strength

    Amazing!

    The Romans got concrete right 2000 years ago. Some of their aqueducts and buildings are still in use.

    Anyone who can't get concrete right today is a fraud and a thief in my book.

    Hope the voyage is a long one.
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  10. #10
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    Default Re: Concrete strength

    I told the contractor, absolutely no water added. Turned my back for a few minutes and they had added 20 gallons.

  11. #11
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    Default Re: Concrete strength

    I once helped with a ferro cement boat. The plasterers and the cement mixers (small batches continuously) were all-stars of skill. The majority of helpers were, like myself, were there to carry heavy stuff from hither to yon. That was the day when the goal was to never lose the wet edge so the team was in action, shifts, for about 48 hours.

  12. #12
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    Default Re: Concrete strength

    Quote Originally Posted by bluedog225 View Post
    I told the contractor, absolutely no water added. Turned my back for a few minutes and they had added 20 gallons.

    And that is the point you remind him that when you have to tear out and re-pour that it is on his dime.

    I really hate when subs don't listen to what you ask for. They don't always know why you want something a particular way.

    Kinda like how the sub thought it would be better to turn the step footing up rather than down as shown. Cost him the two stiffener walls that had to be built to brace what was supposed to act as a small retaining wall.

    Chad
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  13. #13
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    Default Re: Concrete strength

    Around here the phrase used is "Well, that's the way we always done it..."

    I can't begin to tell you how much that grates on me.
    Hope for the best, but plan for the worst.

  14. #14
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    Default Re: Concrete strength

    Quote Originally Posted by Draketail View Post
    You got bad concrete. Who gets to rip it out and re-pour???
    Concrete redirects-mix supplier.

    Quote Originally Posted by cs View Post
    Ugh I feel your pain. What does your engineer say? Being a suspended slab, doesn't sound like an easy fix.
    Chad
    See below
    Quote Originally Posted by Too Little Time View Post
    You ask your engineer if the actual strength is sufficient.
    One suggestion is to take strength, slab thickness and 'back engineer' it to determine the minimum concrete strength requires (including the minimum safety contingency) and see if these strengths are OK. Chad[/QUOTE]

    [QUOTE=cs;5745340]Good concrete guys are hard to find, but in this case sounds like this is not as much the fault of the concrete sub but rather the mix plant. I wonder if the submitted the mix design for approval? Redi-mix plant took 3 cubes - 1 failed at 28 days. Contractor took cubes - Independently tested, all failed at 28 days. Redi-mix plant did Schmidt hammer test, wa well apart at about 50+ days - all failed. Public Works the did 3 core tests at 59 days - all failed.


    Quote Originally Posted by birlinn View Post
    I wonder if a couple of inches of really strong well-bonded concrete on top, to take the compression loads, could be the answer.
    Any engineers like to comment?
    Interesting. Construction has progressed. Raises questions on thickness bonding, door lintel height adjustment etc.

    Quote Originally Posted by bluedog225 View Post
    I told the contractor, absolutely no water added. Turned my back for a few minutes and they had added 20 gallons.
    The Contractor says that no water was added. I have to believe him, I wasn't there. LOL

    BTW Observation
    . . . at 59 days the core test results are :

    Core 1: 2856psi, 81.600% and 644psi below specified 3500psi strength
    Core 2: 2985psi, 85.286% and 515psi below specified 3500psi strength
    Core 3: 1851psi, 52.887% and 1649psi below specified 3500psi strength

    The ACI Building Code, ACI 318, and the Standard Specifications for Structural Concrete, ACI 301, recognize that when mixtures are proportioned to meet the requirements of the standards, low strength results will occur about once or twice in 100 tests due to normal variability.
    Under these provisions, for specified strength less than 5000 psi (35 MPa), concrete is acceptable and complies with the specification if:

    1. No single test is lower than the specified strength by more
      than 500 psi (3.5 MPa), and
    2. The average of three consecutive tests equals or exceeds the specified strength.
    3. Source: https://www.nrmca.org/aboutconcrete/cips/09p.pdf

    Note that :
    i) all tests are below 500psi of the specified strength
    ii) the average strength of samples does not equal or exceed the specified strength.

    According to the above the concrete is not acceptable.

    . . . . concern that the safety of the Client and future residents is paramount and that time is of the essence as the construction continues.

    Rafters are now in place and ring beam is ready for casting.

    Ooer.

    Enjoy a good rum on the rocks at sunset.

  15. #15
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    Default Re: Concrete strength

    What does the lawyer say?

  16. #16
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    Default Re: Concrete strength

    Quote Originally Posted by StevenBauer View Post
    What does the lawyer say?
    Not quite there yet.
    Enjoy a good rum on the rocks at sunset.

  17. #17
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    Default Re: Concrete strength

    Times a wasting. You don't want to be the one liable for the deaths caused by the collapse, do you?

  18. #18
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    Default Re: Concrete strength

    Quote Originally Posted by StevenBauer View Post
    Times a wasting. You don't want to be the one liable for the deaths caused by the collapse, do you?
    Building is under construction.
    Enjoy a good rum on the rocks at sunset.

  19. #19
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    Default Re: Concrete strength

    With all these test failing I'm more and more inclined to believe that it was in the mix and not the actual concrete placement. I think this should go back on the batch plant. I still wonder about the mix design. Were admixtures approved and/or used? Flyash? (although you can get faster stronger mixes with flyash).

    What was the weather like during curing? Was the concrete wet cured or with a curing agent? Usually that is more of spalling issue rather than strength? Was a slump test done prior to pouring and if so did it pass?

    Not knowing more about the building design I can't imagine a topping would work for some of the reasons you mention above. If it is not possible to tear out and re-pour I wonder about supporting it from below with a steel frame? Once again not knowing the design I can only speculate.

    Pictures?

    Chad
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  20. #20
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    Default Re: Concrete strength

    I'll try an get some pics for your entertainment.
    Enjoy a good rum on the rocks at sunset.

  21. #21
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    Default Re: Concrete strength

    One person stated on this thread that they added water. Bad! More water means less strength, but easier to pour. You may know that. There is a slump test. Investigate that.

  22. #22
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    Default Re: Concrete strength

    Quote Originally Posted by Rum_Pirate View Post
    Building is under construction.
    So it's okay if the construction workers get killed? How will you explain that to their families?

  23. #23
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    Default Re: Concrete strength

    AKA these parts "She'll be right. mate"..
    Quote Originally Posted by mmd View Post
    Around here the phrase used is "Well, that's the way we always done it..."

    I can't begin to tell you how much that grates on me.
    Xanthorrea

  24. #24
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    Default Re: Concrete strength

    Quote Originally Posted by ahp View Post
    One person stated on this thread that they added water. Bad! More water means less strength, but easier to pour. You may know that. There is a slump test. Investigate that.
    To my knowledge no water was added.
    Enjoy a good rum on the rocks at sunset.

  25. #25
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    Default Re: Concrete strength

    Concrete supplier/producer is twisting every which way.
    33% of his cubes taken at departure from plant failed.
    +/- 50% of independent cube tests taken from truck on site failed.
    Schmit hammer test failed.
    Second Schmit hammer test failed 4 days apart, tried to say the slab was wet from train.
    All core tests failed.
    The concrete 'failed' see post # 14 according to AIC specs.

    Now wants an independent engineer to review concrete on site and the test results and report on if it is safe.
    Great chance [sarcasm] that he will say it is OK.
    Site visit is tomorrow, Friday.
    The Engineer will also be given the plans, including reinforcement.


    At present, so far seems that there are three solutions, but not for me to propose them :
    1. Demolish and rebuild $$$$$
    2. Cast and bond a slab to existing $$$$ has issues with stair tread heights and door heights.
    3. Support with steel frame and 'I' beams below $$$

    Waiting to receive the learned engineers report.

    Looks like heading to :
    StevenBauer

    What does the lawyer say?
    Enjoy a good rum on the rocks at sunset.

  26. #26
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    Default Re: Concrete strength

    Ask the "independent engineer" how well he is insured in the event that his findings turn out to be wrong and the building collapses and kills people.

  27. #27
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    Default Re: Concrete strength

    Quote Originally Posted by oldsub86 View Post
    Ask the "independent engineer" how well he is insured in the event that his findings turn out to be wrong and the building collapses and kills people.

    Will do.

    Will also ask if the construction should cease in the interim.
    Enjoy a good rum on the rocks at sunset.

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    Default Re: Concrete strength

    Quote Originally Posted by mmd View Post
    Around here the phrase used is "Well, that's the way we always done it..."

    I can't begin to tell you how much that grates on me.
    When working on software for clients I tell people, "That's how we've always done it" is not an answer to "Why do you do it that way?". If I don't do that, people will reply with that phrase at least 50% of the time. Even after telling them that, people will still say it.
    "If it ain't broke, you're not trying." - Red Green

  29. #29
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    Default Re: Concrete strength

    Do you know if the tests were taken before they added water or after? Could be important. Oh never mind, that wasn't the OP, my bad.

  30. #30
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    Default Re: Concrete strength

    Quote Originally Posted by oldsub86 View Post
    Ask the "independent engineer" how well he is insured in the event that his findings turn out to be wrong and the building collapses and kills people.
    Quote Originally Posted by Rum_Pirate View Post
    Will do.

    Will also ask if the construction should cease in the interim.
    I am not sure if those questions are appropriate. The engineer is most likely going to be asked is the structure currently meets the building code or not. That is much different than asking if it is safe.

    It is easy to mistreat a structure and make it unsafe.
    Life is complex.

  31. #31
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    Default Re: Concrete strength

    Meeting the building code is not quite the same thing as meeting the specs for the building. Code is minimum and some folks like to build better than that.

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    Default Re: Concrete strength

    Quote Originally Posted by Rum_Pirate View Post
    To my knowledge no water was added.
    Adding water is only a problem with lower MPa mixes: 35 and below.

    A high slump/high MPa mix is used for precision pours where vibration alone won't remove air bubbles, or where thin sections are required (cast kitchen benches, tables and so on).

    But it does sound like a bodgy mix. What are the spans and what is the thickness?
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  33. #33
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    Default Re: Concrete strength

    6” thickness
    One slab has a 23’0” span both ways but is divided into three sections by two parallel beams 12” wide and 16” deep below u/side of slab.
    has reber both ways in bottom of slab plus mesh in top and +/- 6’ bars over beam extending into slab. IIRC bars are 5/8”
    Last edited by Rum_Pirate; 12-08-2018 at 09:33 AM.
    Enjoy a good rum on the rocks at sunset.

  34. #34
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    Default Re: Concrete strength

    Quote Originally Posted by oldsub86 View Post
    Meeting the building code is not quite the same thing as meeting the specs for the building. Code is minimum and some folks like to build better than that.
    Core test result is below that of the AIC minimum. Conc plant co is squirming and delaying which will cost them more in long run.
    Enjoy a good rum on the rocks at sunset.

  35. #35
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    Default Re: Concrete strength

    Quote Originally Posted by oldsub86 View Post
    Meeting the building code is not quite the same thing as meeting the specs for the building. Code is minimum and some folks like to build better than that.
    That is true, but we already know that the building does not meet the specs.
    Life is complex.

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