26.12.4 Investigation of Low Strength-Test Results
(a) If any strength test of standard-cured cylinders falls below fc' by more than the limit allowed for acceptance, or if tests of field-cured cylinders indicate deficiencies in protection and curing, steps shall be taken to ensure that structural adequacy of the structure is not jeopardized.
(b) If the likelihood of low-strength concrete is confirmed and calculations indicate that structural adequacy is significantly reduced, tests of cores drilled from the area in question in accordance with ASTM C42 shall be permitted. In such cases, three cores shall be taken for each strength test that falls below fc' by more than the limit allowed for acceptance.
(c) Cores shall be obtained, moisture-conditioned by storage in watertight bags or containers, transported to the testing agency, and tested in accordance with ASTM C42. Cores shall be tested between 48 hours and 7 days after coring unless otherwise approved by the licensed design professional. The specifier of tests referenced in ASTM C42 shall be the licensed design professional or the building official.
(d) Concrete in an area represented by core tests shall be considered structurally adequate if (1) and (2) are satisfied:
(1) The average of three cores is equal to at least 85 percent of fc'.
(2) No single core is less than 75 percent of fc'.
(e) Additional testing of cores extracted from locations represented by erratic core strength results shall be permitted.
(f) If criteria for evaluating structural adequacy based on core strength results are not met, and if the structural adequacy remains in doubt, the responsible authority shall be permitted to order a strength evaluation in accordance with Chapter 27 for the questionable portion of the structure or take other appropriate action.