ACI 214-77 PDF

The purposes of strength tests of concrete are to determine compliance with a strength specifica- tion and to measure the variability of concrete. Concrete, being a hardened mass of heterogeneous materials, is subject to the influence of numerous variables. Characteristics of each of the ingredi- ents of concrete, depending on their variability, may cause variations in strength of concrete. Variations may also be introduced by practices used in proportioning, mixing, transporting, plac- ing, and curing. In addition to the variations which exist in concrete itself, test strength variations will also be introduced by the fabrication, testing, and treatment of test specimens.

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The purposes of strength tests of concrete are to determine compliance with a strength specifica- tion and to measure the variability of concrete. Concrete, being a hardened mass of heterogeneous materials, is subject to the influence of numerous variables.

Characteristics of each of the ingredi- ents of concrete, depending on their variability, may cause variations in strength of concrete. Variations may also be introduced by practices used in proportioning, mixing, transporting, plac- ing, and curing. In addition to the variations which exist in concrete itself, test strength variations will also be introduced by the fabrication, testing, and treatment of test specimens.

Variations in the strength of concrete must be accepted, but con- crete of adequate quality can be produced with confidence if proper control is maintained, test results are properly interpreted, and their limi- tations are considered. Proper control is achieved by the use of satis- factory materials, correct batching and mixing of these materials, correct batching and mixing of sired quality, and good practices in transporting, placing, curing, and testing. Although the com- plex nature of concrete precludes complete homogeneity, excessive variation of concrete strength signifies inadequate concrete controL Improvement in control may permit a reduction in the cost of concrete since the average strength can be brought closer to specification require- ments.

Strength is not necessarily the most critical fac- tor in proportioning concrete mixes since other factors, such as durability, may impose lower water-cement ratios than are required to meet strength requirements. In such cases, strength will of necessity be in excess of structural de- mands. Nevertheless, strength tests are valuable in such circumstances since, with established mix proportions, variations in strength are indi- cative of variations in other properties.

Test specimens indicate the potential rather than the actual strength of the concrete in a struc- ture. To be meaningful, conclusions on strength of concrete must be derived from a pattern of tests from which the characteristics of the concrete can be estimated with reasonable accuracy. Insuf- ficient tests will result in unreliable conclusions.

Statistical procedures provide tools of consider- able value in evaluating results of strength tests and information derived from such procedures is also of value in refining design criteria and speci- fications.

This report briefly discusses variations that occur in the strength of concrete, and presents statistical procedures that are useful in the inter- pretation of these variations with respect to re- quired criteria and specifications.

For these sta- tistical procedures to be valid, the data must be derived from samples obtained by means of a random sampling plan designed to reduce the possibility that choice will be exercised by the sampler.

To insure this condition, the choice must be made by some objective mechanism such as a table of random numbers. If sample batches are selected by the sampler on the basis of his own judgment, biases are likely to be introduced that will invalidate results analyzed by the procedures presented here.

Reference 1 contains a discussion of random sampling and a useful short table of random numbers. Additional information on the meaning and use of this recommended practice is given in Realism in the Application of ACI Standard Y, in Although the information given was based on ACI , most of it is still relevant.

Differences in strength can be traced to two fundamentally different sources as shown in Table 2.

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ACI 214-77

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Recommended practice for evaluation of strength test results of concrete (ACI 214-77)

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