Which of the following governs the collection of concrete samples for testing?

The collection of concrete samples for testing is governed by ASTM C172. This standard provides detailed procedures for obtaining concrete samples from a batch before it is placed in forms. It emphasizes the importance of collecting samples in a way that ensures the specimens are representative of the whole batch, addressing aspects such as the timing of sampling, the tools required, and the necessary conditions to prevent contamination or loss of material.

The significance of this standard lies in its role in ensuring the integrity and reliability of test results. For instance, it specifies that samples should be taken at various points in the delivery process and describes the minimum quantity needed for reliable testing. This ensures that the samples will accurately reflect the properties of the concrete that will be used in the actual construction.

In contrast, the other standards mentioned do not focus on the sampling process specifically. ASTM C33 deals with concrete aggregates, ASTM C31 outlines procedures for making and curing concrete test specimens in the field, and ASTM C138 concerns the density (unit weight) and air content of freshly mixed concrete. Each of these serves its own purpose but does not directly govern the sampling process as ASTM C172 does.