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  • I have performed compressive testing of cubes using fly ash based cement . Three of them were demolded after 24 hours and 3 of them were demolded on the 5th day.
    It was observed that the strength of latter was found to be more than the former by about 27%.

    I'm unable to conclude my observation and kindly request you to attribute the reason to this observation.
    Also if I would have used portland cement, what could have been the expected results?

    Regards,
    Rishabh
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Replies

  • Ramani Aswath

    Member • Oct 30, 2013

    Concrete blocks go on curing over many days.
    It is not clear if you tested the first blocks as soon as demolded and the rest five days later.
    The strength can increase much more than 27% Concrete can go on increasing in strength over years.
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    You can see the graphs here:
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  • RISHABH LALA

    Member • Oct 31, 2013

    The cubes were tested for the seven day strength i.e. they were tested on the seventh day after mixing water.
    Although the average strength of the cubes which were demolded after 5 days was fouund to be more by 27%.
    Also note that the Superplasticizer named Sika plastiment 1000, was also used in the cubes 1% by weight of cement.
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  • Ramani Aswath

    Member • Oct 31, 2013

    The cubes were tested for the seven day strength i.e. they were tested on the seventh day after mixing water.
    In that case how were the demolded cubes stored. One possibility is that the demolded ones lost water too early while the ones in the mold for five days retained the water and cured better.
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  • RISHABH LALA

    Member • Oct 31, 2013

    The cubes were immediately stored in water(submerged) after demolding.
    Although the plasticizer, is known for forming a film over the cuncrete cubes, that prevents water loss from the concrete, which also means that it retained water and hence cured better.
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  • Ramani Aswath

    Member • Oct 31, 2013

    Interesting. The next possibility is temperature. Concrete curing is exothermic. Possibly the cubes in the mold were warmer than the demolded ones. A higher temperature can alter curing.
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  • RISHABH LALA

    Member • Nov 1, 2013

    Possibly! But we did not test this criterion. Can you brief us about how does the temperature possibly effect the curing?
    Although the cubes were kept at the room temperature.
    We are of the opinion that this phenomenal effect has been due to the Superplasticizer added.
    Although we still can't justify which effect has worked.
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  • Ramani Aswath

    Member • Nov 1, 2013

    When the Hiracud dam was being built crushed ice had to be used instead of water to delay the setting time till the concrete reached the pouring spot.
    Setting is a chemical process. The rate is doubled for every ten degree C rise in temperature. It is possible that in the mold the temperature was higher than the water immersed cubes by a few degrees, enough to show the observed difference.
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  • RISHABH LALA

    Member • Nov 4, 2013

    If this the correct reason,
    then a conclusion could be "The cubes should be demolded after 4-5 days, when kept at the room temperature; as this increases the temperature as we delay demolding; in order to achieve higher strength;provided the Superplasticizer is added"(roughly)!?
    Also this would go against the IS Code which says "Test cubes should be demoulded between 16 and 24 hours after they have been made. "
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  • Ramani Aswath

    Member • Nov 5, 2013

    Quite possible that you are right. However, if the ASTM test prescribes demolding at some specified time, the reason is for consistency of results. Whatever we discussed was just to explain the anomaly. Not to change the test.
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  • Stephenie Harada

    Member • Nov 10, 2013

    Thanks for info.

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