4.1 This test method provides standardized procedures for obtaining cored specimens to determine the compressive strength and splitting tensile strength of shotcrete during pre-construction, during construction, and from older shotcrete structures.

4.2 Generally, test specimens are obtained in situ when doubt exists about the in-place shotcrete quality due either to low strength test results during construction or signs of distress in the structure. Other uses of this method are to provide specimens for acceptance testing, construction control and to assess the condition, quality and uniformity of the shotcrete in accordance with Practice C823/C823M (see Note 1).

4.3 Specimens obtained by this method are used to verify the thickness of shotcrete and aid in the visual assessment of the shotcrete quality, workmanship, defects, shotcrete-to-substrate bond and the condition of any reinforcement used in the shotcrete.4

4.4 Shotcrete strength is affected by the location of the shotcrete in a structure. Vertical, sub-horizontal and overhead elements of the shotcrete structure may show variability. Core strength is affected by core orientation relative to direction of shotcrete application. These factors shall be considered in planning the locations for obtaining shotcrete samples and in interpreting strength test results.

4.5 Shotcrete is applied in single or multiple layers, as plain shotcrete, reinforced shotcrete, or fiber-reinforced shotcrete. Core samples containing wire-mesh and reinforcing bars shall not be used for compressive strength testing.

4.6 The strength of shotcrete is affected by moisture content, the specified moisture conditioning procedure for cores is intended to provide test specimens with reproducible moisture contents that minimize within-laboratory and between-laboratory variations.

4.7 Sample acquisition may require a combination of core-drilling, sawing, and grinding that may have the potential to adversely affect the sample condition if care is not taken during sampling and testing.

4.8 A sampling plan shall be established that indicates the number of samples and their locations (see Note 1). The acceptance criteria for shotcrete core strengths shall be established by the specifier of tests (see Note 2).

Note 1: Practice C823/C823M provides guidance on the development of a sampling plan for concrete in constructions. A number of methods that supplement the examination of hardened shotcrete by means of representative core samples and testing are discussed in ACI 506.4R.

Note 2: The core strength acceptance criteria cited in ACI 318 are based on a comparison of molded cylinder specimens and cores. Shotcrete strengths are always based on core specimens, therefore the 85 % factor used in core to cylinder comparisons is not relevant to the evaluation of shotcrete core strengths. A typical shotcrete specification reflecting standard industry practice is contained in ACI 506.2.