Concrete strength doubt 1

I have been asked to check on a possibility of interplot cube strength result for 28 days because the test date was missed. For the supposedly 28 days cube strength test, it was only done, 2 weeks after the due date. Now the client is questioning the validity of the concrete strength.

Does it make any different when the test is carried out ? if after the test, the result shown much higher strength than the required 40 Mpa.

 

[Ron]

shfran...
It makes a difference as to the test date for standardization and specification purposes. The practical difference is less significant. For example, strength gain curves developed for specific concrete mixes are based on testing at established intervals, usually 3,7,28,56, and 90 days. From these curves, we know that normal strength concrete will typically gain about 65 to 70 percent of its strength in the first 7 days (under standard curing conditions), and will achieve at least 100 percent of its design strength at 28 days, assuming all is well with the concrete.

In actuality, the 28-day strength is usually a bit higher than the design compressive strength for statistical confidence.

For your situation, you can probably estimate the strength at 28-days by looking at a strength gain curve for that particular mix, usually available from the supplier. This is only necessary for demonstration purposes as I would consider the issue moot, since the strength at 42 days exceeded the necessary strength (though there are situations where the design compressive strength at 28 days must be adhered to for contractual, and occasionally, structural reasons). Impress upon those involved that the level of accuracy in strength gain is less important than the strength itself.

 

[iyrana]

HI!
I have a problem with cube strengths after 28days. When we tested cubes for 7 days it gains 75% of its 28days strength but after 28days its strength gain becomes reverse means if its gain 3000psi after 7days it gains 2800psi after 28days. Pls tell me the problem with the cube strengths and also the tolerence for cube failure out of 6.

 

[Ron]

iryana....
Concrete strength does not reverse. It is likely that you have a sample integrity issue. Cubes strengths can be influenced by minor changes in the specimen, such as micro-cracking. If this occurs, strengths will appear to go down, though that is not really the case. I would suggest you do companion testing with 3"x6" cylinders along with the cubes, comparing the results.

In response to your second question, you may look at the small number of specimens and conclude that no more than 1 specimen should be below the strength.