Concrete cylinders do not gain strength as expected

[Robert168]

Recently, I did a concrete test and it did not turn out as I expected.

The test consisted of a group of 4 cylinders, two of which were tested on day 7 and the other two on day 28. The average compressive strength was 22 mpa for the day 7 break, but only 25 mpa for the day 28 break.

From tests done on the other clinders of a similar mix design, the cylinders would have an average compressive strength of 20 mpa for the 7 day's break and 31 mpa for the 28 day's. Therefore, the cylinders, having a compressive strength of 22 mpa on day 7, should produce a compressive strength of more than 31 mpa.

This is what had happened. My question is:
1. If a 7 day break has a compressive strength higher than average, it is possible that the 28 day break from the same test cylinders shows a compressive strength lower than average.
2. If possible, what may be the reason or reasons.

Thank you in advance.

 

[BigH]

Robert168 - can you provide some information on the mix design - did you use plasticizers? Type of cement? Did you cure the cylinders as per normal lab specimens or were they cured under site conditions. I will look up some records I have on tests on a project in Laos I have where we used normal portland cement.

 

[Robert168]

Thank you, BigH, for your response.

The 4 cylinders were cured in the lab tank as per CSA procedures as all other cylinders.

The tests on the other cylinders indicate that the strengths between the 7 day's break and the 28 day's break agree with each other. In other words, if the strength is higher than the average on 7 day break, it will also be higher than the average on 28 day break.

I just want to know whether there are any factors,either during the process of mixing or during the molding of cylinders, which could increase the rate of strength gain for the first 7 days and slow the rate later on.

I don't have the mix design. As far as I know, no plasticizers were used.

 

[iandig]

In answer to your original post, it is possible for the concrete to have a higher strength gain during the first 7 days, with a lower gain between 7 and 28 dyas. I am assuming that you are using the same cement and water cement ratio, and that the mix has not changed.
What can happen is that if the concrete is subjected to higher curing temperatures within the early stages, this can accelerate the curing of the concrete, thus resulting in a higher early strength, but the long-term strength will not increase at the typical rate. If the cement type chages, then you should also note that different cement blends can have very different strength gains, ie. a 'standard OPC' is likely to achieve 2/3s of 28 day strength at 7 dyas, whereas a PFA/GGBS is only likely to get 1/2 28 day strength at 7 days. The use of admixtures, or incorrect admixture additioan can also dramatically effect the strength gain.

 

[Robert168]

Thank you all for your responses. They have helped me a lot with a complete analysis of the test.

A very possible factor for the issue is that the concrete was not mixed thoroughly in the concrete truck, as shown by the difference in strength between the cores taken. It was different between the highest and the lowest by 3 mpa within a short distance, like 2m.

So the normal mixture for cylinders before the molding was not enough, and the cylinders richer in cement may show high early strength.

Generally speaking, the test data agree well with each other and the whole process is under control.

Thank you again for your help.

 

[JConc]

Your comment about the concrete not mixed in the truck would most likely only be true if water or AEA was added on site and then the load not mixed a minimum of 30 turns. Cores vary due to the destructive nature of drilling and possibly varying consolidation/no consolidation in structure. Another possible reason your strength gain from 7 to 28 days was lower than expected may be that the concrete was of a higher slump or just more water in mix. Compare the density.

 

[BigH]

One other point - about the fineness of your cement. Here in India they use Grade 33, 43, and 53 cement (each is the MPa strength of cement cube at 28 days). Obviously the higher grade, the finer the grind of the cement. We find that the older rules of thumb for the coarser ground don't apply and the strength gain is faster within 7 days and slower thereafter. At least this is my experience.