Any one ever experienced that 7 days compressive strength is higher to 28 days strfength??? What were the probable reason for such discrepancy ?

Yes we have experienced that higher strength result on7th day because while taking sample the proportion of aggregate and cement has been overadded due to lack of supervision. 

Also higher result may get coz of a close eye preservation.

special cares on the sample may cause that kind of results ( ex: adding some aggregate,good sample compaction ,keep in23 deg celsius )

thanks for sharing your experience Mr. Steven Gill

Description:

It is observed that, on3rd August2015 and6th August2015 the28 day’s concrete compressive strength test results are found less than28MPa. The detail of Test Result is given below.

Sl. No

Casting date

3 Days Test Result

7 Days Test Result

28 Days Test Result

1

6th July2015

17.0MPa

25.7MPa

26.4MPa

2

9th July2015

18.8MPa

25.2MPA

23.7MPa

Justification:

We have completed89% of concrete works and we have conducted325no’s Compressive Strength Test for this project. This is the only two strength test results we observed less than28MPa.

As per the ACI318M-Section-5.6.3.3 the cylinder Strength Test of concrete shall be considered satisfactory if both of the following requirements are met.

1. The average of any three consecutive strength tests is equal to, or greater than, the specified strength, f'c and

2. When the specified strength is5000 psi (35MPa) or less, no individual strength test (average of two cylinders) is more than500 psi (3.5MPa) below the specified strength, f'c.

In the first case, the28 Days compressive strength test results are meeting the above both criteria’s and to be considered as satisfactory.

Also the ACI Standard is saying “Strength test failing to meet these criteria will occur occasionally (probably about once in100 tests).” So far we have conducted325no’s Compressive Strength Test for this project.

Moreover, the28 days compressive Strength we observed on6th August2015 is23.7MPa, where as we got the7days test result25.2MPa for the same batch, which is meeting our average7 Days Compressive Strength Test Result (25.7MPa). So, we assume that there is an error happened in the cylinder identification numbering.

BEST REGARDS

STEVEN GILL