Back to the failed bridge. The cores showed:

| Property | Typical Value | | :--- | :--- | | Dry Density | 125 – 155 lb/ft³ (2000 – 2480 kg/m³) | | Absorption | 4 – 8% | | Permeable Voids | 12 – 18% |

The junior engineer asked a smart question: "Why boil? Why not just soak it for a week?"

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 .