Standards for 7-day and 28-day strength test results

Here it is, direct from ACI, a discussion of 7 and 28 day strength. See our strength test discussion a couple of posts down.

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"Technical Questions - ACI Concrete Knowledge Center


Q. We test concrete cylinders for compressive strength. Most specifications that we see require a compressive strength of 3000 psi (20.7 MPa) and testing at 7 and 28 days. What ACI standard stipulates the percent of the specified compressive strength that the cylinder must meet in order to pass the compressive strength test at both the 7- and 28-day ages?

A. ACI 318-02, "Building Code Requirements for Structural Concrete," and ACI 301-99, "Specifications for Structural Concrete," contain standards information related to this question.

Section 5.6.2.4 of ACI 318-02 requires a strength test to be the average of the strengths of two cylinders made from the same sample of concrete and tested at 28 days or at the test age designated for determination of the specified compressive strength.

ACI 318-02 doesn't state a percentage of the specified compressive strength that must be reached at 28 days. In accordance with Section 5.6.3.3 of ACI 318-02, the strength level of an individual class of concrete is considered to be satisfactory if every arithmetic average of any three consecutive strength tests equals or exceeds the specified compressive strength and no individual strength test (average of two cylinders) falls below the specified compressive strength by more than 500 psi (3.45 MPa) when the specified compressive strength is 5000 psi (34.5 MPa) or less.

Results from the 7-day tests mentioned are usually not used for acceptance purposes, and thus there isn't a percent of the specified compressive strength that the cylinder must meet in order to pass the compressive strength test. Section 1.6.4.2.e of ACI 301-99, for instance, requires molding and curing three cylinders from each concrete sample and testing one specimen at 7 days for information and two specimens at 28 days for acceptance, unless otherwise specified.

The 7-day test result is used to monitor early strength gain and is often estimated to be about 75% of the 28-day strength (Kosmatka, Kerkhoff, and Panarese, “Design and Control of Concrete Mixtures,” PCA, 2002). Neville (Properties of Concrete, 4th Ed.,” Prentice Hall, 1995), however, suggests that if the 28-day strength is to be estimated at 7 days, a relationship between the 28-day and 7-day strengths has to be established experimentally for the given concrete. For this reason, he states that various expressions for the ratio of the two strengths (expressions that were discussed in the previous edition of his book) are no longer thought to be reliable.

Regardless of the reliability of the estimate for 28-day strength, 7-day strength test results are useful to the contractor and concrete producer as an early warning signal. With today's fast-track concrete-placement schedules, it's essential for the contractor and concrete producer to know when 7-day test results are low. Then suitable steps can be taken promptly to adjust batch quantities, improve quality control procedures at the job site, and ensure that sampling, molding, and testing of the cylinders are being done in accordance with ASTM applicable standards.

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To purchase PCA documents referenced above, visit the bookstore at:
http://www.cement.org/bookstore/

To purchase the book by Neville, visit websites such as http://www.amazon.com


Posted 2/13/06.

Disclaimer: Questions in this column were asked by users of ACI documents and have been answered by ACI staff or by a member or members of ACI technical committees. The answers do not represent the official position of an ACI committee. Only a published committee document represents the formal consensus of the committee and the Institute. " >>> end of ACI post