My 28 day compressive breaks are lower than the 7 day breaks 2

[aaronc]

With a 2" x 2" cube and design strength of 2000 psi @ 28 day. The results for the 7 day comressive strength is higher. I have reviewed the test methodes and innitial curing procedures. I cannot find the reason for these results can some one assist, explane the reason or direct me to the answer.

 

[NickE]

We need a bit more info.

Such as:
What is the cube made out of? what is the temperature? 4in^2 area on a side?

You're loading the cube btw two platens, with a force of 8000lbf?

nick

 

[aaronc]

The cube is cast with 2000 psi grout, area is 3.97 sqI, and it is loaded between the 2" riser and head @ a rate of 300 lb per minute. the 7 day results are 27560 load, 6940 psi. The 28 day results are, 22965 load, 5780 and 27695 load, 6980

 

[rconner]

Don'tknow exactly what you arerunning into, but your data set certainly doesn't have a lot of "population" so as to determine variation.

 

[dgallup]

Your second 28 day load is slightly higher than your 7 day load. Looks like part to part variation is more significant than cure time. Did the low failure part have a small void or other defect?

 

[oldestguy]

An important thing about compression testing is the sort of connection between the loading surfaces and the surface of the specimen. That's why capping compounds or other methods are needed for eliminating test result variations from that source.

 

[arunmrao]

Are you testing on blended cement or on Portland cement. I am no cement technologist,but I have noticed such variances during my visit to cement plants. It was all attributed to the slag used .

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[Compositepro]

Are your specimens representative of your material? It would not make sense to test a two inch specimen if you have 1" aggregate in the cement. I don't think that your data says that 28 day strength is lower than 7 day. It says you have a lot of scatter.

 

[aaronc]

I am testing Portland cement. The specimen is representative of the placement, fine sand grout with no aggregate in the mix design.

 

[rconner]

Is it possible this is not "ordinary" (Type 1) but some other type with maybe some different time-dependent strength properties?

 

[aaronc]

I want to thank all of you for your input on this problem. I have used your imput and forwarded it on to the QA Manager for his report. Thank you again for your assistance.