Guide for Obtaining Cores and Interpreting Compressive Strength Results
|Publication Date:||1 June 2010|
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.