Which types of aggregates should not be used when employing the pressure method of air content determination?

When determining air content using the pressure method, the use of lightweight aggregates, air-cooled blast furnace slag, or aggregates with high porosity is discouraged due to their unique physical properties. These materials can retain a significant amount of air within their structure, which can lead to inaccurate readings of the actual air content in the concrete mix.

In applying the pressure method, it is essential that the aggregates do not trap air in a way that would interfere with the measurements. The method relies on accurately measuring the volume of air within a concrete sample, and aggregates that are lightweight or highly porous tend to distort these measurements since they can contribute additional air volume that is not representative of the entrained air in the cement paste.

Thus, including these types of aggregates could skew results and undermine the reliability of the pressure method for determining air content. Proper selection of aggregates is crucial for ensuring accurate testing and maintaining quality control in concrete production.