214.4R-03
Guide for Obtaining Cores and Interpreting Compressive Strength Results
2003, 16pp.
$55.50

Core testing is the most direct method to determine the compressive strength of concrete in a structure. Generally, cores are obtained either to assess whether suspect concrete in a new structure complies with strength-based acceptance criteria or to evaluate the structural capacity of an existing structure based on the actual in-place concrete strength. In either case, the process of obtaining core specimens and interpreting the strength test results is often confounded by various factors that affect either the in-place strength of the concrete or the measured strength of the test specimen. The scatter in strength test data, which is unavoidable given the inherent randomness of in-place concrete strengths and the additional uncertainty attributable to the preparation and testing of the specimen, may further complicate compliance and evaluation decisions.

This guide summarizes current practices for obtaining cores and interpreting core compressive strength test results. Factors that affect the in-place concrete strength are reviewed so locations for sampling can be selected that are consistent with the objectives of the investigation. Strength correction factors are presented for converting the measured strength of non-standard core-test specimens to the strength of equivalent specimens with standard diameters, length-to-diameter ratios, and moisture conditioning. This guide also provides guidance for checking strength compliance of concrete in a structure under construction and methods for determining an equivalent specified strength to assess the capacity of an existing structure.

Keywords: compressive strength; core; hardened concrete; sampling; test.

Contents:
Chapter 1—Introduction, p. 214.4R-2

Chapter 2—Variation of in-place concrete strength in structures, p. 214.4R-2

2.1—Bleeding

2.2—Consolidation

2.3—Curing

2.4—Microcracking

2.5—Overall variability of in-place strengths

Chapter 3—Planning the testing program, p. 214.4R-4

3.1—Checking concrete in a new structure using strength-

based acceptance criteria

3.2—Evaluating the capacity of an existing structure using

in-place strengths

Chapter 4—Obtaining specimens for testing,

p. 214.4R-5

Chapter 5—Testing the cores, p. 214.4R-6

Chapter 6—Analyzing strength test data, p. 214.4R-6

6.1—ASTM C 42/C 42M precision statements

6.2—Review of core strength correction factors

6.3—Statistical analysis techniques

Chapter 7—Investigation of low-strength test results in new construction using ACI 318, p. 214.4R-9

Chapter 8—Determining an equivalent fcvalue for evaluating the structural capacity of an existing structure, p. 214.4R-9

8.1—Conversion of core strengths to equivalent in-place strengths

8.2—Uncertainty of estimated in-place strengths

8.3—Percentage of in-place strengths less than fc

8.4—Methods to estimate the equivalent specified strength

Chapter 9—Summary, p. 214.4R-12

Chapter 10—References, p. 214.4R-13

10.1—Referenced standards and reports

10.2—Cited references

10.3—Other references

Appendix—Example calculations, p. 214.4R-15

A1—Outlier identification in accordance with ASTM E 178 criteria

A2—Student’s t test for significance of difference between observed average values

A3—Equivalent specified strength by tolerance factor approach

A4—Equivalent specified strength by alternate approach