Chapter 7 Cast-In-Place Concrete
the area in question in accordance with ASTM C 42 is permitted. In such cases, three cores shall be taken for each strength test more than 500 psi (3.45 MPa) below the specified value of f’c.
NCSBC-1905.6.5.3 Condition of cores. If concrete in the structure will be dry under service conditions, cores shall be air dried at temperatures between 60°F (16°C) and 80°F (27°C) and relative humidity less than 60 percent for seven days before testing and shall be tested dry. If concrete in the structure will be more than superficially wet under service conditions, cores shall be immersed in water for at least 40 hours and be tested wet.
NCSBC-1905.6.5.4 Test results. Concrete in an area represented by core test shall be considered structurally adequate if the average of three cores is equal to at least 85 percent of f’c and if no single core is less than 75 percent of f’c. Additional testing of cores extracted from locations represented by erratic core strength results is permitted.
NCSBC-1905.6.5.5 Strength evaluation. If the criteria of Section 1905.6.5.4 are not met and if the structural adequacy remains in doubt, the building official is permitted to order a strength evaluation in accordance with ACI 318, Chapter 20, for the questionable portion of the structure, take other appropriate action.
SICC-704.1 Testing required. Material tests for concrete properties and strength, for determining the compressive strength of concrete, prior to removal of concrete formwork and shoring, reshoring, stressing post-tensioning tendons, loading of vertical building elements, erection of structural steel, and for verifying adequacy of concrete protection and curing methods during cold weather, shall comply with the following:
a. Frequency of testing. Samples for strength tests of each class (concrete mix design) shall be taken in accordance with NCSBC-1905.6.2. Concrete samples for test cylinders shall be taken in accordance with ASTM C 172. Additional test cylinders for strength tests shall be cast if required by the AR, SER, or County approved documents. Additional cylinders to be field-cured shall be required to evaluate strengths of concrete prior to removal of shores and concrete formwork, prior to stressing of post- tensioning tendons, loading of vertical building elements, erection of structural steel, and adequacy of concrete curing and protection methods during cold weather concreting conditions.
Laboratory-cured cylinders. Cylinders for strength tests shall comply with this section and NCSBC-
Cylinders for strength tests shall be cast, stored, transported and laboratory-cured in
accordance with ASTM C 31. Tests shall be in accordance with ASTM C 39.
c. Field-cured cylinders. Field-cured cylinders, to evaluate strengths of concrete prior to removal of concrete formwork and shoring, reshoring, prior to stressing post-tensioning tendons, and to determine adequacy of curing and protection of concrete during cold weather, shall comply with this section and NCSBC-1905.6.4. Field-cured cylinders shall be cured as closely as possible to the location of placement of the concrete pour they represent, and be exposed as nearly as possible to the same temperature and moisture environment, in accordance with ACI 318 and ASTM C 31. Tests shall be in accordance with ASTM C 39.
Cylinders may be fabricated on the ground or on the slab, and moved to the curing location no more than 30 minutes after fabrication. If fabricated on the ground, cylinders shall be placed in a temporary open storage location, protected by no more than insulated blankets, remain undisturbed for a minimum of 16 hours but no more than 24 hours after molding, and then be relocated into or on the structure as closely as is practicable to the concrete they represent. If molded on the slab, cylinders shall be placed into or on the structure as closely as is practicable to the concrete they represent immediately after molding.
Equivalency may be achieved by storing uncapped cylinders on or immediately adjacent to the structural concrete placement as soon as practical after casting (and until six hours or less prior to testing), and