10.001 Establishing Concrete Strengths with Core Tests @ 85%

I’m investigating an older concrete building and would like to use ACI 318 Section 5.6.5 to confirm the existing concrete strengths. Can you outline a procedure for specifying the work and explain the rule of 85%?

— Unknown

3/24/05 – Response prepared by Terry Egland, Principle, Testing Engineers, Inc.

A nondestructive test method, such as probe penetration, impact hammer or ultrasonic pulse velocity may be useful in surveying structural members for areas of lower strength concrete. From this preliminary view point use ASTM C823-00 “Standard Practice for Examination and Sampling of Hardened Concrete in Construction” to formulate specific areas of investigation. The selected areas then can be specified for investigation for concrete strength according to ASTM C42-04 “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete”. Section 3.2 states “Generally, test specimens are obtained when doubt exists about the in-place concrete quality” and “use of this method is to provide strength information on older structures.”

According to International Building Code (IBC) Section 1905.6.5.2, three cores will be taken for each strength test. And Section 1905.6.5.4 states, “the average of three cores is equal to at least 85% of f’c“.

The rule of 85% can be best explained by ASTM C42-04 Section 3.5: “There is no universal relationship between the compressive strength of a core and the corresponding compressive strength of standard-cured molded cylinders. The relationship is affected by many factors such as the strength level of the concrete, the in-place temperature and moisture history, and the strength gain characteristics of the concrete. Historically, it has been assumed that core strengths are generally 85% of the corresponding standard-cured cylinder strengths, but this is not applicable to all situations.”

The commentary of ACI 318 Section R5.6.5 also states “Core tests having an average of 85% of the specified strengths are realistic. To expect core tests to be equal to f’c is not realistic, since differences in the size of specimens, conditions of obtaining samples, and procedures for curing, do not permit equal values to be obtained.”

NOTE: According to ACI 214.4R-03 “Guide for Obtaining Cores & Interpreting Compressive Strength Results” the preceding method is NOT an option when evaluating for structural capacity.

For further information ASTM references Neville, A., “Core Tests: Easy to Perform, Not Easy to Interpret”, Concrete International, Vol.23 No. 11 November 2001, pp. 59-68.