Civil Engineering Association

ACI 214.4R-10: Guide for Obtaining Cores and Interpreting Compressive Strength Results

This guide summarizes current practices for obtaining cores and interpreting core compressive strength test results in light of past and current research findings. Many of these findings are based on older references as the research has reached a mature state. Parallel procedures are presented for cases where cores are obtained to assess whether concrete strength in a new structure complies with strength-based acceptance criteria, and to determine a value based on the actual in-place concrete strength equivalent to the specified compressive strength fc′. The latter can be directly substituted into conventional strength equations with customary strength reduction factors for strength evaluation of an existing structure. It is inappropriate to use procedures for determining the equivalent specified concrete strength to assess whether concrete strength in a new structure complies with strength-based acceptance criteria.

The order of contents parallels the logical sequence of activities in a typical core-test investigation. Chapter 3 describes how bleeding, consolidation, curing, and microcracking affect in-place concrete strength in structures so the investigator can account for this strength variation when planning the testing program. Chapter 4 identifies preferred sample locations and provides guidance on the number of specimens that should be obtained. Chapter 5 summarizes coring techniques that should result in undamaged, representative test specimens. Chapter 6 describes procedures for testing cores and detecting "outliers" by inspection of loadmachine displacement curves or using statistical tests from ASTM E178. Chapter 7 summarizes the subsequent analysis of strength test data including use of ASTM C42/42M precision statements that quantify expected variability of properly conducted tests for a sample of homogeneous material, research findings concerning accuracy of empirically derived core strength correction factors, and statistical analysis techniques that can determine if the data can be grouped into unique categories. Chapter 8 briefly elaborates on criteria presented in ACI 318 for using core test results to investigate low-strength cylinder test results in new construction.

Chapter 9 presents two methods for estimating the lower tenth-percentile value of in-place concrete strength using core test data to quantify in-place strength. This value is equivalent to the specified compressive strength fc′ and can be directly substituted into conventional strength equations with customary strength reduction factors for strength evaluation of an existing structure.

Example calculations are presented in an appendix for:

• Outlier identification in accordance with ASTM E178 criteria;

• Determining whether a difference in mean strengths of cores from beams and columns is statistically significant; and

• Computing the equivalent specified strength using the two approaches presented in Chapter 9.

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.