ACI Certification Practice Exam

Which method is used to determine the air content of concrete containing lightweight aggregate?

1. Water displacement method

2. Pressure method

3. Volumetric method

4. Mass loss method

The correct answer is: Pressure method

The pressure method is specifically designed to measure the air content in concrete, including those formulations using lightweight aggregate. This technique involves using a pressure chamber where a known volume of concrete is subjected to pressure. The change in pressure, resulting from the trapped air bubbles within the concrete, allows for the calculation of the air content. The advantage of the pressure method is its accuracy and reliability, particularly when dealing with the unique characteristics of lightweight aggregates, which can affect the measurement of air content differently than conventional aggregates. Other methods may not be as suitable for lightweight aggregates. For instance, the water displacement method may not account for the differences in buoyancy in lightweight aggregates effectively. The volumetric method, while a standard practice in some scenarios, may not provide the precision needed due to the inherent variations in the density and voids of lightweight aggregates. The mass loss method is typically not applicable for assessing air content. Therefore, the pressure method stands out as the most effective choice for this particular scenario.