10.005 Shotcrete Core Diameter

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UBC Section 1924.10 states that shotcrete with maximum nominal aggregate larger than 3/8-inch shall be tested using 3-inch diameter cores or 3-inch cubes. Shotcrete with maximum nominal aggregate of 3/8-inch or smaller shall be tested using 2-inch diameter cores or 2-inch cubes.

ASTM C1140 requires shotcrete be tested as drilled cores or sawed cubes, and references C42 and C513, respectively, for obtaining the specimens. Cores shall be at least 3.70-inches in diameter for load bearing structural shotcrete. Cores for non-load bearing shotcrete, or when it is impossible to obtain cores with length-diameter ratios greater than or equal to 1, are not prohibited. Cubes shall be 2-inches to 4-inches in size, with no size requirement based on aggregate size.

COMMENTS
ASTM Subcommittee C09.46, who has jurisdiction over C1140 will be replacing the requirements of C42 with a new ASTM designated as C1604-05 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete. This new standard has the following requirements:

8. CORES FOR COMPRESSIVE STRENGTH

8.1 Diameter – The diameter of core specifications for the determination of compressive strength in load bearing structural members shall be at least 3.0 in. [75 mm] (see Note 4).

Core diameters less than 3.0 in. [75 mm] shall be permitted as directed by the specifier of the tests.

Note 4 – The compressive strengths of 2-in. [50-mm] diameter cores are known to be somewhat lower and more variable than those of 3-in. [75-mm] diameter cores. In addition, smaller diameter cores appear to be more sensitive to the effect of the length-diameter ratio.

This new standard will bring us better into alignment with IBC, UBC and CBC