In general there is no chance that loss of strength at 56 and 91 days in compression to 28 days if all conditions are ideal because as time increases hydration process take place and unhydrated CH further consumed and strength increases.

I think it may be instrumental error which you have already mentioned or may be the chance of improper curing such as acid attack or other phenomenon takes place which decreased the strength in that period or may be the chance of improper proportion of superplasticizers used may be very high doses or may be expired, overall improper mixing has been done.

It is believed that strength of concrete increases with time due to hydration of cement process and refinement of pores during these periods. Even there is a debate to consider 90 days strength rather than 28 days strength as factually concrete during construction is rarely subjected to full load within 90 days.

It may be due to error in testing or curing of cubes that you are not getting correct results.

Modern cement also gives 70-80 % strength within 7 days and possibility is that the strength gain may not be significant after 28 days but reduction in strength is not expected unless until something is wrong with the materials.

Yes, Dr. Om Prakash Chhangani and Dr. Shashikant Kumar I agree with your explanation; because of the same design mix, there is no materials issue, all precautionary measurements were taken while casting, compaction, and curing. So, my point of view this is an issue of an instrumental error at the time of testing

No

Thank you Dr. Bismark Mensah I agree

Dr. Mohammed Nadeem Akhtar, ideally the answer to the question is no but there have been instances like that. This was attributed to the casting process like the intensity of the tamping.

Thank you Dr. Bismark Mensah for this point

Mohammad,

I have seen this type of behaviour only once. In that case i was heat cured specimens at one day compared to 28 day water cured specimens. The 28 day samples were improperly handled after casting, and were not put into the curing bath for 2 days after demoulding.

In general there is no chance that loss of compressive strength in function of time, if all conditions are ideal because as time increases hydration process take place and unhydrated CH further consumed and strength increases.

Thank you Dr. Rachedi Mokhtar for sharing the technical discussion

The probability of human error in mishandling the specimens may be considered, if the speciemens were to suffer impact in its early days, that could lead to losses in strength in the future.

Thank you Dr. Gustavo Toigo for a nice technical explanation

As discussed by others, chances of errors during proportioning, mixing, compaction, curing, testing etc. may be there. one of the solution can be redo the testing with all precautionary measure as per standards you follows for testing.

Thank you Dr. Bhavin G. Buddhdev for the explanation

In addition to the above-described probability of errors during dosing, mixing, compaction, curing, testing, etc., such an effect can be observed with a number of features of the composition of cement and fillers that can lead to acid, salt or alkaline corrosion of concrete. The most probable for the described case may be an increased content of magnesia and sulfate phases in cement and / or fine filler.

Strength retrogression has been observed in very high strength concrete (see works of De Larrard and others) but usually at ages beyond 90 d (perhaps due to self-desiccation as CH formed earlier in hydration reacts with silica fume for example). If all specimens were prepared in the same manner and at the same time (from the same batch) and cured under equivalent conditions, result is rather unexpected. Are the differences in the strengths at the three ages statistically significant (given the variability observed in the replicate specimens tested at each age)?

Thank Dr. D. P. Bentz for discussion

Theoretically, no. However, it can happen in experimental work due to the heterogeneous nature of concrete constituents and practices during casting and curing of concrete samples.

Yes, if the cement is not sulphate resistant then in a sulphate environment, such as the ocean, concrete can deterioate. Similar if the cement has an high alkali content, concrete made with alkali reactive aggregates can lose strength and fall apart. Cements with high ground blast furnace slag 35-65% will perform better for long term durability. OPC cement grades of Low Heat (used in dams) and sulphate resistant ( both with lower C3S) will have better durability than high strengths OPC with C3S over 60%. The standard sulphate resistant test with mortars demonstrates the reduction of strengths of out of specific SR cements.

Thank you Dr. Peter Benkendorff for a nice valuable discussion

One other possibility: was the cement used a high alumina cement. These have been known to have a later age strength regression due to temperature conversion.

Thank you Prof Gordon Chirgwin for the discussion

In my opinion, It occurs due to the casting and compacting of the samples.

Logically it's not possible for optimum concrete design. In your case, it might have occurred due to the error in mix design, curing temperature or any false test outcomes.

Probably due to sampling

It is known that; many of specification stipulates that samples compressive strength of concrete must be at least "fck+delta (MPa)" at 28 days.May 20, 2015

https://www.researchgate.net › post

How can I interpret compressive strength of concrete at 28 days?

https://www.researchgate.net>post

What is the reason for attaining same strength of concrete for 7 days and,kindly check this link.

https://www.hindswi.com>amse

Prediction of compressive strenghth of concrete using Artifcial Neural,this link may be useful.

It is an interesting question. The reduction in strength for 56 days and 91 days as compared to 28days quite possible due to various reasons. First of all needs to confirm concrete specimens prepared for 28 days, 56 days and 91 days were same. If it is same then it may be due to poor preparation of concrete specimen that includes compaction methods adopted in all these cases... Another important point is environmental conditions and adding proper water during these days of curing periods. ok all the best..

In my opinion, there are some errors such as : sampling compaction, mix design and test machine reading.

Dear;

Logically it's not possible but different samples may probably cause such results.