Compressive Strength of Grout

[lular515]

I Run a Lab and have been faced lately with grout compressive strength retrogression from its 28 day strength at 56 days.
Material was understandably experienced to gain strength from its 7 day to 28 day but strength has consistently reduced at its 56 day break
Example: 7 Day break 5890psi. 28 day break 6300psi and 56 days 5640psi.
All Laboratory ASTM procedures duly satisfied, Is there any explanation or experience of such
Just a bit more information: We had about 48 days of this pour and results seem to follow the same trend of 56day reduced strength except very few
7 samples made per mix, 2 for 7 days 3 for 28 days and 2 for 56 days.

 

[weldstan]

This could be poorly poured and cured specimen. Hopefully, you have a backup specimen for testing.

 

I don't know grout, but it seems to me that if the 56 day strength meets the applicable ASTM standard (not procedure) then it should be acceptable. However if it is significantly off what the manufacturer advertises then you probably are right to be concerned.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."