What is a key consideration regarding air content for air-entrained concrete?

Air content is a critical factor in air-entrained concrete, as it significantly influences the material's performance, durability, and resistance to freeze-thaw cycles. The correct choice highlights that air content can be adjusted on-site as needed. This flexibility allows concrete producers and contractors to make necessary modifications to achieve the desired performance characteristics based on environmental conditions and project specifications.

Air-entrained concrete relies on the addition of air-entraining agents during mixing, which creates tiny air bubbles that help protect the concrete from damage caused by water freezing and expanding in the pores. If the air content measured in the fresh concrete does not meet the specifications, adjustments can be made, such as incorporating additional air-entraining admixtures or modifying the mixing process. This adaptability is crucial for meeting the concrete's performance requirements throughout the construction process.

In contrast, the other options involve aspects of air content management that do not align with standard practices. For instance, while air content may need to be monitored before or after mixing, it is not a set requirement to sample before placement. Sampling may also not ensure that the air content remains below a specific percentage; rather, the focus is on achieving the optimal air content rather than a strict upper limit. Sampling after mixing is common