To extract a core sample, the coring rig is placed at the desired location and the barrel-cooling water is turned on, as show in Figure 4-21. Note that dry ice placed in a modified 5-gal. bucket can be used to cool the core barrel if samples at in situ moisture content are desired. This is particularly advisable if any of the HMA layers to be cored are suspected of being stripping-susceptible since a wet-coring process may severely damage these layers in the coring process.

The barrel is spun at about 500 rpm and gradually lowered through the asphalt layer. If the base is stabilized, the operator may want to cut through it as well to obtain an intact sample.

After the barrel has cut to the desired depth, it is retracted while still spinning. Then the core barrel is stopped and the location of the core is observed. If the core has twisted off and is now lodged in the barrel, the barrel is struck lightly with a mallet to loosen the core. If the core is still in its original position, metal shims (or screwdrivers) are used to rock the core back and forth until it breaks free from the base or subgrade. Then the core can be lifted out with bent welding rods or loops of thin wire.

Normally, the core diameter is either 4 in. or 6 in.; 6-in. cores are needed if further laboratory evaluation using Hamburg or the Overlay tester is desired. Figure 4-22 shows the extracted core samples with a 4-in. diameter.