ACI Concrete Field Testing Technician - Grade I Practice Exam

Which types of aggregates cannot be used with the pressure method for determining air content?

1. Fine, medium, and coarse aggregates

2. Lightweight, air-cooled blast furnace slag, or high porosity aggregates

3. Concrete, asphalt, and gravel

4. Natural, manufactured, and recycled aggregates

The correct answer is: Lightweight, air-cooled blast furnace slag, or high porosity aggregates

The correct choice highlights the specific types of aggregates that are not suitable for the pressure method used to determine air content in concrete mixes. The pressure method relies on the property that air voids in the concrete mix can be compressed, allowing for the measurement of the air content based on changes in pressure. However, lightweight aggregates, air-cooled blast furnace slag, and high porosity aggregates tend to have a significantly higher absorption of water and are more susceptible to the effects of water saturation. This can lead to inaccurate readings of air content, as the inclusions in these types of aggregates may interfere with the pressure measurement system. In contrast, other aggregate types like fine, medium, and coarse aggregates, natural, manufactured, and recycled aggregates typically possess consistent density and porosity characteristics that do not adversely affect the pressure measurement methodology. Concrete, asphalt, and gravel are also standard materials that can work in various testing conditions and wouldn’t present the specific complications seen with lightweight and high porosity aggregates. Therefore, the distinction made in the correct answer is important for ensuring accuracy in testing air content in concrete.