What ASTM method must be used to obtain the concrete sample?

The ASTM method C172 is specifically designed for sampling fresh concrete. This method outlines the procedures for obtaining representative concrete samples before testing, ensuring that the samples reflect the characteristics of the entire batch of concrete. It includes detailed guidelines on how to collect samples during the mixing process, how to handle and transport these samples to the testing location, and the importance of sampling different portions of the batch to avoid bias.

The other methods mentioned do not pertain to the sampling of concrete. For instance, C29 is focused on determining the bulk density of aggregates, C231 is related to the measurement of air content in freshly mixed concrete, and C9 is not a recognized method within the context of concrete testing. Thus, C172 is the correct choice for obtaining concrete samples, as it directly relates to the proper sampling procedures outlined in the ASTM standards.