ACI - PRC-214.4
Obtaining Cores and Interpreting Core Compressive Strength Results—Guide
|Publication Date:||1 May 2021|
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. Distinct procedures are presented for two cases:
1) To assess whether concrete strength in a new structure complies with strength-based acceptance criteria
2) To determine an in-place concrete strength that is equivalent to the specified concrete compressive strength fc′ to be directly substituted into conventional strength equa tions with customary strength reduction factors for strength evaluation of an existing structure
It is inappropriate to use procedures in Chapter 9 for determining equivalent specified concrete strength to assess whether suspect concrete in a new structure complies with strength-based acceptance criteria.
The order of contents parallels the logical seque nce 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 technique s that should result in high-quality, undamaged, representative test specimens. Chapter 6 describes procedures for testing cores and detecting outliers by inspection of load-machine displacement curves or using statistical tests from ASTM E178 . Chapter 7 summarizes the subsequent analysis of strength test data, including use of AS TM C42/C42M precision statements, research findings concerning accuracy of empirically derived core strength correction factors, and statistical analysis technique s. Chapter 8 briefly elaborates on criteria presented in ACI 301 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 concrete strength fc′ and can be directly substituted into conventional strength equa tions with customary strength reduction factors for strength evaluation of an existing structure.
Example calculations are presented in the appendix for:
a) Outlier identification in accordance with ASTM E178 criteria
b) Determining whether a difference in mean strengths of cores from beams and columns is statistically significant
c) Applying minimum strength acceptance criteria for samples with more than three cores
d) Computing the equivalent specified strength using the two approaches presented in Chapter 9