scope:

INTRODUCTION

This document provides an introduction to the evaluation of concrete strength tests. The procedures described are applicable to the compressive-strength test results required by ACI 301, ACI 318, and other similar specifications and codes. The statistical concepts described are applicable for analysis of other common concrete test results including flexural strength, slump, air content, and density.

Most construction projects in the United States and Canada require routine sampling and fabrication of standard molded cylinders. These cylinders are usually cast from samples of concrete taken from the discharge of a truck or a batch of concrete and molded, cured, and tested under standardized procedures. The results represent the potential strength of the concrete rather than the actual strength of the concrete in the structure.

Inevitably, strength test results vary. Variations in measured strength may originate from any of the following sources:

Batch-to-batch variations of the proportions and characteristics of the constituent materials in the concrete, the production, delivery, and handling process, and climatic conditions; and

Variations in the sampling, specimen preparation, curing, and testing procedures (within-test)

Conclusions regarding the strength of concrete can only be derived from a series of tests. The characteristics of concrete strength can be estimated with reasonable accuracy only when an adequate number of tests are conducted, strictly in accordance with standard practices and test methods.

Statistical procedures provide tools of considerable value when evaluating the results of strength tests. Information derived from such procedures is also valuable in refining design criteria and specifications. This report discusses variations that occur in the strength of concrete and presents statistical procedures that are useful in the interpretation of these variations with respect to specified testing and acceptance criteria.

For the statistical procedures described in this report to be valid, the data should be derived from samples obtained by means of a random sampling plan designed to reduce the possibility that selection will be exercised by the sampler. Random sampling means that each possible sample has an equal chance of being selected. To ensure this condition, the selection should be made by some objective mechanism such as a table of random numbers. If sample batches are selected on the basis of judgement by the sampler, biases are likely to be introduced that will invalidate the analysis using the procedures presented here. Natrella (1963) and ASTM D 3665 provide a discussion of random sampling and a useful short table of random numbers.

This report begins with a discussion of the sources of variability in concrete as produced, mixed, and transported, and the additional variability of samples obtained from the concrete as delivered and tested. The report then describes the statistical tools used to evaluate the variability of concrete and determine compliance with a given specification, including both random variation and variation due to assignable causes. Statistically based specifications are also reviewed.