10.050 Low Concrete Strengths on California School Projects

Q We have a school project in California where the specified concrete strength is 4000 psi at 28-days. On one specific pour, the following strengths were obtained:

7-day strength = 2780 psi
28-day strength = 3890 psi (average of 2 cylinders)
56-day strength = 4150 psi (1 cylinder)

Do you report the results as meeting the requirements of the DSA approved documents?

— Testing Lab Manager in Northern California

A 2/09 – Response prepared by David Chippero, the Special Inspection Division Manager at Testing Engineers, Inc.

California Building Code, Title 24, Part 2, Chapter 1905.A.6.3 Strength Test Specimens states:  Strength test acceptance criteria shall comply with the provisions of ACI 318, Section 5.6.3.  Section notes:  Concrete shall be considered satisfactory if both of the following requirements are met:

     A) Every arithmetic average of any three consecutive strength tests equals or exceeds f’c.
B) No strength test falls below f’c by more than 500 psi when f’c is 5000 psi or less.

Using this guideline, the results above would be acceptable if the 28-day cylinders, when averaged with three consecutive strength test results on the project are equal to or greater than 4000 psi. This assumes than no individual test was less than 3500 psi.

The Division of the State Architect holds a different position regarding low strength concrete test results. DSA believes that the LEA approved laboratory should report all failing test results immediately as a non-conformance. It is then up to the design professional and DSA to determine a corrective action plan. If an approved stamped change order is not received from DSA, the failing results should be reported on your laboratory verified report, DSA Form 291. In the 2007 California Administrative Code, Title 24, Park 1, Section 4-335b, Performance of Tests, states:  Where a sample has failed to pass the required tests the architect of engineer, subject to the approval of DSA, may permit retest of the sampled material. Section 4-335d, Test Reports, also notes:  Reports of test results of materials not found to be in compliance with the requirements of the plans and specifications shall be forwarded immediately to DSA, the architect, the structural engineer, ad the project inspector.

So, although the 56-day strength test met the 28-day f’c requirements, DSA does not consider the results to be valid. The test report must be distributed noting “the results do not meet the requirements of the DSA approved documents“. there are no provisions in the California Building/Administrative Code, Title 24 that allow the use of a 56-day test result in lieu of the required 28-day test result. However, a 56-day test result may be useful to the design professional and DSA in arriving at a corrective action plan.


Reference Documents

2007 California Administrative Code, California Code of Regulations, Title 24, Part I

2007 California Building Code, California Code of Regulations, Title 24, Part 2, Volume 2

Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary