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One of the concrete batches had low strength

  • 1.  One of the concrete batches had low strength

    Posted 11 November 2019 23:04

    Riverina Water is constructing two 11ML water supply reservoirs (post tensioned).


    Compressive test results indicate that an out of spec load was placed in the 260m3 floor pour.


    The results indicate that the 6m3 out of spec concrete will reach 20-25MPa rather than 40MPa. The same mix typically achieves 48-53 MPa.  

    The floor area with out of spec concrete has been identified using concrete test hammer. It is located in the ring beam and has a number of anchorages in it.


    After consultation with the design firm, the contractor is proposing to delay full stressing of the slab until the concrete out of spec concrete reaches its 28 days 40 MPa.


    We are concerned about the effect that low strength concrete will have on durability/ long term performance of the structure.

    Has anyone had a similar experience? What was the outcome?

    Can you please contact us to provide details.

    Anjanee Bichani
    Project Engineer - Networks
    Riverina Water County Council

    Mobile: 0437 217 509 Direct:  02 6922 0627


  • 2.  RE: One of the concrete batches had low strength

    Posted 19 November 2019 20:42
    Dear Anjanee,
    From what you have written, I understand that it has not been 28 days since the concrete was poured. However, if it has been about 7 days, concrete must have reached about 60% of its characteristic strength (28 days). In your case, if it has been more than a week or so, there is a good possibility that it will reach 40 MPa by the end of 28 days after the placement of the concrete.
    On the other hand, Schmidt Hammer test measures the surface hardness value of the concrete and the hardness values are correlated to the strength of the concrete. In other words, it only gives us an indirect idea about the concrete strength. By the 28th day, if there is still doubt on the concrete strength, the best way to overcome it is to take core samples from the concrete and test it.
    I hope this answer was useful for you.
    Kindest regards.
    Veysel Yazici

  • 3.  RE: One of the concrete batches had low strength

    Posted 19 November 2019 20:45
    I find it surprising that a batch of concrete designated for a project would be the incorrect MPA, however as suggested the concrete is not meeting compression standards, curing rates of concrete say that after 28 days the concrete is around 90 plus percentile of its cure, that's when it can be determined that the concrete has not met requirements and that could mean remediation by removal of the concrete.
    Were the batches slumped at each arrival? my theory is not the MPA defective strength but the batch in question was wetter and over vibed, this causes horizontal stratification and loss of durability

  • 4.  RE: One of the concrete batches had low strength

    Posted 19 November 2019 20:46

    If the indication is that the mix will meet 20-25MPa at 28 days then it is not likely to ever meet the design compressive strength of 40MPa.  The consultant's advice is technically OK, that is wait till the concrete reaches 40MPa, that is the design strength.  However, if the concrete is 25MPa at 28 days even under moist curing for 6 months (180 days) the maximum additional strength gained is likely to be less than 25% of the 28 day strength, that is 25MPa x 125% = 31.25MPa at 180days of moist curing.  Now most contractors will apply a barrier curing system which is typically only good for 3-7 days curing.  I doubt your contractor is moist curing and I doubt they intend to cure for 6 months.

    So where does that leave you.  I would suggest that if the contractor is insisting on this approach then be very cautious.  I am not saying that a contractor controlled testing authority would ever issue a false test certificate.  But maybe an unscrupulous contractor might attempt to amend such a certificate.

    One option I have used in the past when a contractor is attempting to pass off substandard works is to give the contractor a choice.
    1.  The contractor removes and replaces the defective material at their own cost; or
    2.  The contractor warrants the work for an extended defects liability period, where you retain a bond equivalent to the cost of replacing the defective works.

    Generally the contractor will remove and replace the defective material at their own cost.

    Ian Greenham
    Director Technical Services
    Orange City Council
    02 6393 8000


  • 5.  RE: One of the concrete batches had low strength

    Posted 19 November 2019 22:13
    Good morning Anjanee.

    Was it a 7 day sample that was tested? Sometimes a sample can fail in an unexpected way due to a defect in the sample. For critical pours I normally take 4-5 samples and break them at 7 days, 14 days, 21 days and 28 days (the fifth sample just in case the 28 day sample isn't up to strength - then you can wait another 2 weeks to break the last one) . That means you only get 7 sleepless nights if your first sample isn't as expected.

    Maybe 25 MPa is strong enough anyway? Depends where it's located and what portion of the overall pour volume. That's up to your consultant to advise on.



  • 6.  RE: One of the concrete batches had low strength

    Posted 01 December 2019 22:50
    Thanks all for valuable input and suggestions for us to take onboard. We are putting some thoughts together to work towards satisfactory solution to the council.
    Your time on time matter is much appreciated.