It seems like low-strength breaks always happen at the worst time, when your schedule is already delayed and you are trying to move your project along. Under ACI and CSA Standards, a break is considered to be low when an individual compressive strength test (average of two cylinders) is more than 500 psi below the specified strength or if the average of three consecutive sets of tests does not equal or exceed the specified strength. These requirements account for the typical variability of concrete strength tests.
Understanding what test cylinders represent is important: Cylinders that are standard cured—according to ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field—are not intended to represent the in-place, early-age strength of the concrete even though they are often used that way. Field-cured cylinders under C 31 have to be kept at the jobsite under conditions as near as possible to the concrete they represent until they are tested. That’s difficult, so we are often stuck with strength results from lab-cured cylinders. Understanding what can contribute to failing strength tests either for standard- or field-cured cylinders is important.
Source:
Concrete Construction: Six Most Common Causes of Low Cylinder Breaks
https://www.concreteconstruction.net/how-to/materials/six-most-common-causes-of-low-cylinder-breaks
