C642 measures the total permeable void volume. A sample with 6% entrained air plus 1% entrapped air might show 7% voids—which looks fine. But if those 6% are disconnected (good) and the 1% is a single large honeycomb channel (bad), the concrete will still leak. C642 gives you the quantity of voids; experience and microscopy give you the quality .
Here is where the story gets subtle. ASTM C642 cannot tell the difference between good voids and bad voids. astm c642 density absorption voids in hardened concrete
The internal structure of concrete significantly impacts its performance. ASTM C642 allows engineers to: C642 measures the total permeable void volume