one of my colleague had casted M50 grade of concrete.
when he tested for 7 days compression strength it was around 45 N/mm2. Later for 28 days also the strength of the concrete is same. what will be the reason to get the same strength?
Dear Sai,
Can you mention other relevant parameters and properties? Such as, whether it was geopolymer concrete or simple PCC. Similarly, the cement used was OPC or PPC?. Moreover, how many samples were used for determination of average compressive strength at respective days of curing? Are the results were similar for other strength properties such as flexural and splitting tensile strength ?
Dear Sai,
Can you mention other relevant parameters and properties? Such as, whether it was geopolymer concrete or simple PCC. Similarly, the cement used was OPC or PPC?. Moreover, how many samples were used for determination of average compressive strength at respective days of curing? Are the results were similar for other strength properties such as flexural and splitting tensile strength ?
Number of reasons could cause this. I suggest look into the curing conditions. Usually for compressive strength test samples are kept in water for entire curing time to ensure maximum hydration of portlandcement. If, for example, samples were dry, hydration could have stopped, so there were no gain of compressive strength. Accurate answer requires more details (Curing conditions - water or air and temperature; type of binder, sample size, mixture composition, etc.) on experiment.
OPC 53 grade cement
Sample size: 150X150 mm
no.of sample for each testing :3
Curing: Wet
no admixture is used.
please discard readings obtained. please recast and test the cube with utmost care and strict supervision. there are n numbers of reasons for this wrong results. go for fresh samples. and you will get correct results.
Microstructural studies like XRD SEM analysis may confirm the exact mechanical behaviour.
Dear Sai, try to figure out the standard deviation between the results. If the standard deviation is similar for both the curing days then you have to search for other attributes. As per Indian Standards, the percentage variation from the mean shall be less than 15%. If this is not the case, then see the chemical composition of OPC by XRF(what was the alumina content). Else, recast the specimens (at least 5 cubes per testing age) or prepare flexural specimens and see the variation.
The cement could have been in contact with moisture. If the bag of cement were left open for a long time, it could have been damaged. See cement you used in experiment if it has lumps it can't be used any longer
That should not happen (7 days strength similar to 28 days strength).
One possible factor is the curing conditions.
Please refer to ASTM C192 (or C31) for guidance.
I agree with Bharat above. If all precautions have been taken throughout the whole process of curing and testing (of at least 3 samples) )have been followed, then you will need to ensure that the materials used are not contaminated. A chemical composition of the constituents will prove useful.
Is early age tensile strength equally high? Because in structures where crack control is required, obtaining 28 day's tensile strength of concrete after just 7 days is in fact not very beneficial.
Using that concrete in structures where the imposed deformations due to temperature caused by cement hydratation are high, produces vast amounts of reinforcement needed for crack control reasons and therefore very uneconomic design.
Best Regards,
Bartosz Grzeszykowski
There should a strength development with time at a reduced rate, and in general the strength at 7 days is about 65-75% of that at 28 days. However, the type and regime of curing, and ambient temperature influence the strength development. The rate of loading during testing is another factor that influence the strength.
7 days strength can never be the same as that of 28 days. Repeat the experiment
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