ASCC position statement #16: referee test for flexural strength acceptance

Concrete Construction, Oct, 2004

Specifications from the Federal Aviation Administration (FAA), some highway departments, city and county engineers, and consulting engineers include requirements for flexural strength based on beam tests. Because results of beam tests are extremely variable, many highway departments and consulting engineers use only compressive strength requirements in their specifications, or allow in-place flexural strength to be estimated based on concrete cylinder compressive strength tests.

Specifications that require beam testing often contain no provisions for referee testing. This is unusual in the concrete industry because for many years ACI 318, "Building Code Requirements for Structural Concrete," and ACI 301, "Specifications for Structural Concrete," have included core testing as the referee method when cylinder strength test results are low.

When project specifications include requirements for flexural strength based on beam testing, the specifications should also provide guidance for investigation of low beam strength test results. The investigation should include a review of the testing procedures, a consideration of whether the low-strength beam represents a significant reduction in load-carrying capacity of the pavement or slab, and the effect on overall load-carrying capacity of slab thicknesses and k-values that exceed the design requirements. A referee method of testing, such as compressive strength tests of drilled cores, should also be stated. A commonly stated specification clause is that core test results must meet the requirements of ACI 318 or ACI 301 for structural adequacy.

ASCC agrees with the following recommendations from the American Concrete Pavement Association (ACPA) publication "Methods of Concrete Strength Evaluation of Pavement Acceptance," June 1997, and the National Ready Mixed Concrete Association (NRMCA) publication "Flexural Strength of Concrete," CIP 16, 2000:

* Compressive strength should be used for acceptance testing

* Mix-specific correlation between compressive strength and flexural strength should be used, and

* Contract specifications should clearly include referee testing provisions, including what and how the referee testing will be done.

When project specifications include pay factors based on beam strengths, it is especially important that the specifications also include procedures for investigating low beam-strength test results and for a referee method of testing when the test results are low. ASCC concrete contractors will work with specifiers in developing specifications that address these issues. If you have any questions, contact your ASCC concrete contractor or the ASCC Technical Hotline at 800-331-0668.

ASCC Position Statements

The American Society of Concrete Contractors has developed this series of position statements to address critical installation and finishing issues that contractors face every day. Written by industry experts, the statements clarify what can and cannot be expected of concrete. These documents will minimize disputes and facilitate communication among those responsible for specification, supervision, and execution of concrete construction. The first 12 position statements are now available as a set from ASCC. For a list of the topics or to order go to www.ascconline.org or call 866-788-2722.

Position statements we've published:

#1--Hard Trowel Finish on Air-Entrained Concrete (March 2003, p. 66)

#2--Location of Rolled Welded-Wire Fabric (June, p. 50)

#3--Coatings that Affect Bond to Reinforcement (September, p. 52)

#4--Trowel Marks on Concrete Floors (August, p. 44)

#5--Specifications for Crack Repair (October, p. 46)

#6--Division 3 versus Division 9 Floor Flatness Tolerances (November, p. 55)

#7--Birdbaths on Concrete Slabs (December, p. 48)

#8--Bugholes in Formed Concrete (January 2004, p. 60)

#9--Slab Thickness Tolerances (February, p. 104)

#10--Moisture-Sensitive Floor Coverings on Concrete Slabs (March, p. 58)

#11--Appearance Requirements for Concrete Slabs (April, p. 24)

#12--Responsibility for Buying Concrete (June, p. 42)

#13--Use of ACI Committee Reports and Guides in Projects and Specifications (July, p. 67)

#14--Anchor Bolt Tolerances (August, p. 24)

#15--Setting Time Expectations for Hard Trowel Finishing (September, p. 20)

#16--Referee Test for Flexural Strength Acceptance (October, this page)

Other position statements are still being finalized.