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Variations in samples of concrete and concrete making materials make it difficult to obtain samples with the assurance that they are completely representative of the source or production of these materials. Test results are more reliable as the number and size of samples increase, but additional samples or samples of greater size increase the cost of testing. It is necessary, therefore, to establish the accuracy desired in each case commensurate with funds and facilities available. It may be better to make no tests than to make tests with poor samples which do not portray the actual properties of materials. An engineer or architect who must rely on samples or tests that do not accurately represent materials or structures could probably make more objective decisions if there were no samples or tests available. He could at least make conservative assumptions rather than rely on fallacious information. For convenience in discussing statistical considerations, tests for concrete and concrete materials are separated into one or more of the following general categories : 1. Acceptance Tests—Tests which demonstrate that the material in question will meet specific requirements of the work as specified. Examples: Grading of aggregate, soundness of aggregate, slump, and air content of concrete. Performance tests may also be used as acceptance tests. These tests demonstrate how a given material will perform in concrete under field or simulated conditions. Examples: Freezing and thawing of concrete prisms, expansion of concrete prisms due to alkali reactivity, strength of concrete, drying shrinkage, and rate of hardening. 2. Construction Control Tests—Construction control tests are made at intervals throughout construction of a project. These tests not only provide a check on the performance of materials and may be used for acceptance or rejection of the work, but they also measure the uniformity of concrete produced. The strength of 6 by 12 cylinders is universally accepted as a standard of control in the United States, since most properties of concrete and concrete materials, as well as the influence of testing and construction practices, are reflected in the strength of concrete cylinders. 3. Research Procedures—Tests which are conducted in the laboratory or field to establish relationships among the variables which influence the properties of concrete. Examples: Relationship between strength and water-cement ratio, influence of admixtures on the properties of concrete. Rate of hardening at various temperatures.
Cordon, W. A.
Associate professor of civil engineering, Utah State University, Logan, Utah