I am testing my project samples for different design mixes, i.e., 3 samples of each mix. One sample from each mix gives very high strength compared to the other 2; all precautions were taken while mixing, compacting, curing the samples, and density was observed almost the same in all 3 samples.
Need Concrete Expert discussion on this issue ………..
The problem I see in preparing of samples! May be the worker increase layers or the compaction. Even may be make low w/c. Check the preparation process! If everything ok. They might be cure single sample from each patch with hoter water than other!
Generally, three samples per mix doesn't yield in reliable results!
Dear Mohammad Nadeem Akhtar
As you know, there should be three specimens for each test, and their average will be the desired strength. If the strength of one of them were too high or too low, you should exclusion it from averaging. It means the average of two specimens would be the answer.
Please note that there is something wrong with the testing procedure if this happened to approximately all your samples.
Thank you, Dr. Mehran, for a clear explanation, but what are the recommendations from ACI? As per my understanding, ACI recommended testing 30 samples to develop the standard deviation, and we need to check the multiple samples from different design mixes. In this case, I cannot cast 30 samples of each design mix. I agree with your suggestion of an average of 2 samples would be considered the desired strength.
Thank you, Dr. Abdullah, for a clear explanation. Yes, I agree with your explanation that 3 samples are not enough to develop the standard deviation. As per my understanding, ACI recommended testing 30 samples to develop the standard deviation. I need to add some waste materials in the mixes, which are not available in bulk, and we want to use the same material properties throughout the project. In this case, I cannot cast 30 samples of each design mix.
Below is an excellent suggestion ….Thank you!
They might be a single cure sample from each patch with hotter water than others!
Good afternoon, I have seen a coefficient of variation in my samples of 18% approximately (of mortars). As our colleagues commented, 3 samples are recommended for each specimen but that's why you could omit one of them.
About conformity control for concrete compressive strength, EN 206 standard states : "Where two or more specimens are made from one sample and the range of the test values is more than 15 % of the mean then the results shall be disregarded unless an investigation reveals an acceptable reason to justify disregarding an individual test value".
If there is no doubt on concrete fabrication/curing/preparation...You can also investigate the testing practice (calibration, load rate,...).
Regards
Maxime
if you are testing your project samples for different design mixes, i.e., 3 samples of each mix. if one sample from each mix gives very high strength compared to the other 2; so allowable difference not more than 15 % in this case this sample was rejected and calculate means strength only remains two specimen, if you found more than 15 % there are some deficiencies occurred during mix design, used aggregate (fine or coarse, mixing, compacting, curing the samples, and density or any thing else).
Thank you very much, Prof Maxime, for a clear explanation. Do you recommend me ACI or ASTM standard where I could find the standard between average value and individual values acceptable variation
Thank you very much, Fateh Ali but I am looking ACI or ASTM recommendation regarding this issue; if you know the exact code regarding these observations, kindly share
Dear Mohammad Nadeem Akhtar
The compressive strength requirements can vary from 2500 psi (17 MPa) for residential concrete to 4000 psi (28 MPa) and higher in commercial structures according CIDA.
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