What is the vacuum applied to flasks for GMM?

The correct choice indicates that the vacuum applied to flasks for obtaining the specific Gravity of Maximum Density (GMM) is set at 30 +/- 2 mm. This vacuum level is critical because it ensures that air bubbles trapped in the asphalt mixture are effectively removed, which allows for a more precise measurement of the maximum specific gravity (Gmm) of the mixture.

A vacuum level of 30 mm creates sufficient suction to draw out air without being excessively high, which could risk damaging the sample or flask. By adhering to this vacuum range, the accuracy of the GMM tests is maintained, leading to reliable results for subsequent calculations related to mix design and quality control.

The other options suggest different vacuum levels, which do not align with standardized practices for achieving optimal air removal in the testing process aimed at determining GMM. Using incorrect vacuum levels could result in inaccurate specific gravity measurements, thereby adversely affecting quality assessments and mix design decisions.