ACI 214.4-21 Obtaining Cores and Interpreting Core Compressive Strength Results—Guide.
3.3—Curing Proper curing procedures, which control the tempera- ture and moisture environment, are essential for quality concrete. Low initial curing temperatures reduce initial strength development rate but can result in higher long- term strength. Conversely, high initial-curing temperatures increase initial strength development but reduce long-term strength (ACI 308R). High initial temperatures generated by cementitious mate- rial hydration can signifcantly reduce strength of the inte- rior regions of massive elements (ACI 305R). For example, results in Fig. 3.3 indicate the strength of cores obtained from the middle of mock 30 x 30 in. (760 x 760 mm) columns is consistently less than the strength of cores obtained from exterior faces (Cook 1989). Mock columns were cast using high-strength concrete with an average 28-day standard cylinder strength exceeding 11,200 psi (77 MPa). Simi- larly, data analysis from large specimens reported by Yuan et al. (1991), Mak et al. (1990, 1993), Burg and Ost (1992), and Miao et al. (1993) indicate a strength loss of approxi- mately 3 percent of the average strength in the specimen for every 10°F (5°C) increase of average maximum tempera- ture sustained during early cement hydration (Bartlett and MacGregor 1996a). Maximum temperatures recorded in these specimens varied between 110 and 200°F (45 and 95°C).
4.1—Investigating concrete in a new structure using strength-based acceptance criteria To investigate low-strength test results in accordance with ACI 301, three cores are required for each strength test from that part of the structure cast from concrete represented by the low-strength test result. The investigator should only sample areas where suspect concrete was placed. In some situations, such as thin or heavily reinforced sections, it is difcult or impracticable to obtain cores meeting all the length and diameter requirements of ASTM C42/C42M. Nevertheless, cores can allow a relative compar- ison of two or more portions of a structure representing diferent concrete batches. For example, consider two sets of columns placed with the same concrete specifed mixture proportion: one acceptable based on standard strength tests and one questionable because of low-strength test results. Nondestructive testing methods (ACI 228.1R) may indicate that the concrete quality in suspect columns exceeds that in acceptable columns. Alternatively, 2 in. (50 mm) diam- eter cores can be taken from columns where 1 in. (25 mm) maximum size aggregate was used. After trimming the cores, however, ℓ/d may be less than 1.0 because the cover is only 2 in. (50 mm) and reinforcement bars cannot be cut. Although strength tests of short cores may not accurately represent the column strength, a comparison of the short core strengths can indicate that the questionable concrete strength is comparable to that in the other placement or that further investigation is warranted.ACI 214.4 pdf download.