grout testing

[Zulak]

I'm getting testing results from grout used in pressure-grouted auger piles, and the 7-day results reflect approximately 50% of the desired 28-day requirements, but the 28-day breaks are all fine. Is this typical of grout? Concrete usually yields approximately 70% of design strength at 7 days. Is it the difference in testing methods (cubes for grout vs. cylinders for concrete), or maybe the aggregate in the concrete affect early compressive strength, while grout can't. Just curious.

 

[jheidt2543]

Technically, the only strength test you are REALLY interested in is the 28 day, that is the basis of the design. The "standard" 3, 7, and 14 day tests were really developed as a guide for formwork removal. I believe you will find that each mix design, whether concrete or grout, will have a different rate of strength gain. In your case there is no form work removal with an auger cast pile, however the intermediate tests could be used as a basis to install the next structural element.