ACI PRC-214.4-21: Obtaining Cores and Interpreting Core Compressive Strength Results

Description

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

This guide summarizes practices for obtaining cores and interpreting core compressive strength test results. Factors that affect in-place concrete strength are reviewed so sampling locations that are consistent with objectives of the investigation can be selected. Strength correction factors are presented for converting measured strength of nonstandard core-test specimens to strength of equivalent specimens with standard diameters, length-to-diameter ratios, and moisture conditioning that have been derived using data for normalweight concrete with strengths between 2000 and 13,400 psi (14 and 92 MPa). This guide also provides direction for checking strength compliance of concrete in a structure under construction and methods for determining equivalent specified strength to assess existing structure capacity.

Keywords:

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

Document Details

Author: ACI Committee 214

Pages: 20

ISBN: 9781641951432

Categories: Cracking, Curing

Formats: Printed Document or Protected PDF/Web View

Table of Contents

CHAPTER 1—INTRODUCTION, p. 2

1.1—Introduction, p. 2

1.2—Scope, p. 2

CHAPTER 2—NOTATION AND DEFINITIONS, p. 3

2.1—Notation, p. 3

2.2—Definitions, p. 3

CHAPTER 3—VARIATION OF IN-PLACE CONCRETE STRENGTH IN STRUCTURES, p. 4

3.1—Bleeding, p. 4

3.2—Consolidation, p. 4

3.3—Curing, p. 4

3.4—Microcracking, p. 5

3.5—Overall variability of in-place concrete compressive strengths, p. 5

CHAPTER 4—PLANNING TESTING PROGRAM, p. 5

4.1—Investigating concrete in a new structure using strength-based acceptance criteria, p. 5

4.2—Evaluating existing structure capacity using in-place strengths, p. 5

CHAPTER 5—OBTAINING TEST SPECIMENS, p. 6

CHAPTER 6—CORE TESTING, p. 7

CHAPTER 7—ANALYZING STRENGTH TEST DATA, p. 7

7.1—ASTM C42/C42M precision statements, p. 7

7.2—Review of core strength correction factors, p. 8

7.3—Statistical analysis techniques, p. 9

CHAPTER 8—INVESTIGATION OF LOW-STRENGTH TEST RESULTS IN NEW CONSTRUCTION USING ACI 301, p. 10

8.1—Procedure for samples of more than three cores, p. 10

CHAPTER 9—DETERMINING AN EQUIVALENT fcʹ VALUE FOR EVALUATING STRUCTURAL CAPACITY OF AN EXISTING STRUCTURE, p. 12

9.1—Conversion of core strengths to equivalent in-place strengths, p. 12

9.2—Uncertainty of estimated in-place strengths, p. 12

9.3—Percentage of in-place strengths less than fc′, p. 13

9.4—Methods to estimate the equivalent specified strength, p. 13

CHAPTER 10—REFERENCES, p. 15

Authored documents, p. 15

APPENDIX—EXAMPLE CALCULATIONS, p. 17

A1—Outlier identification in accordance with ASTM E178 criteria, p. 17

A2—Student’s t test for significance of difference between observed average values (Larsen and Marx 2017), p. 17

A3—Acceptable minimum strength criteria for samples of more than three cores, p. 18

A4—Equivalent specified strength by tolerance factor approach (Hindo and Bergstrom 1985), p. 20

A5—Equivalent specified strength by alternate approach (Bartlett and MacGregor 1995), p. 20

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