10.004 7-Day Concrete Strength is a Percentage of What?

Q Our testing laboratory routinely supplies our customers with 7-day concrete break results. At what percentage of the specified strength should we state there might be a problem with the final result?

— Unknown

A 07/28/05 – Response prepared by Jeffry Cannon, Laboratory Manager, Kleinfelder, Inc., Sacramento, CA


Different concrete mixtures will gain strength at different rates, so there is no universally applicable rule about what a 7-day strength versus 28-day strength ratio will be. This is also true because of the numerous types and brands of cement, and the hundreds of different admixtures routinely used in concrete today. But as a general rule of thumb for normal-weight concrete that does not contain unusual ingredients, 7-day strengths that are less than 65% of the required 28-day strength should warrant close attention.
NOTE:  If the concrete mix contains fly ash, especially large amounts of fly ash, strength gain will be slower than mixes that do not contain fly ash.