Non shrink grout strengths

It does not happen frequently but also not an uncommon phenomenon. It could be due to a host of factors: improper test procedure, fault line in the cube, cube disturbed during casting (read: some dummy moved the tray at the wrong time), etc, etc.

This is why you cast spares. Test another sample at 14 and see what results. At this point I wouldn't be too concerned about it.

Apparently it was more than a couple bad samples. 28 day samples were recently tested and they also came back at less strength than expected. Probably too much water was added into the mixture. Thanks for the feedback though.

Cubes are very susceptible to disturbance. 3"x6" cylinders offer better consistency in results.
One other issue you have is that non-shrink grouts achieve their lack of shrinkage (which is inherent to portland cement products) through admixtures that cause slight expansion. In place, non-shrink grouts are usually very confined. When tested, they are removed from the cube molds and can actually expand with time (very slight expansion, but sometimes enough to cause microcracking) as hydration and chemical expansion reactions occur. At 3 days they have had little time to expand. At 7 days in an unconfined state, they have likely expanded a bit more and it can continue through full curing.

To provide a better correlation with strength gain, leave the cubes in their molds until just before testing. Let us know your results. Also, if you want to carry this further, do a petrographic analysis of the cubes when unconfined to look for microcracking.

Also, don't over specify the required minimum grout strength. In my experience, sampling, curing, transporting, and/or testing are not often done correctly and lower strength is not uncommon. Don't specify more strength than you need. Just because 10 ksi grout may be available, don't specify it if you don't need it that strong. You may need only 5 ksi.

Thank you all for the feedback. Although we were only looking for 5000 psi the product specs called out for an 8400 psi @ 28 days so they considered that the grout failed in terms of not achieving its strength @ 28 days.

As long as you achieved the required 5000 psi, tell them the grout is just what they ordered and paid for.