Because time is not the same as energy. A sample soaked at room temperature for a month will still have trapped air in pores smaller than 0.1 microns. Boiling forces water into those nano-pores. ASTM C642 is deliberately aggressive. It measures permeable voids—the voids that actually connect to the surface and can transport water.
The trouble is that water, chlorides, and sulfates travel through these voids. Once they reach the reinforcing steel, the bridge dies from the inside.
The sample is placed in a ventilated oven at 110 ± 5°C (230 ± 9°F) for at least 24 hours, or until it stops losing weight. This is the – the skeleton of the concrete, stripped of all free water. Step 2: The Flood (Saturation) Now comes the counter-intuitive part. To find the voids, you must fill them. The dry sample is placed in a water bath at 21 ± 2°C (70 ± 4°F) for at least 48 hours. But that’s not enough to reach full saturation. Air gets trapped in fine pores. astm c642 density absorption voids in hardened concrete
[ Voids = [(B-A)/(B-C)] \times 100 ] What it means: This is the headline metric. It includes all voids that can be filled with water under boiling conditions—capillary pores, entrained air bubbles, and even small cracks. For good-quality structural concrete, this value is often between 12% and 18%. For the failed bridge deck? It was 24%. The Plot Twist: What Boiling Reveals That Soaking Cannot The junior engineer asked a smart question: "Why boil? Why not just soak it for a week?"
By J. Carter, Materials Field Engineer
The technician then weighs the sample submerged in water (using a wire suspending it from the scale). This gives the . Then, they remove it, gently blot it with a damp towel, and weigh it again in air. This is the Saturated Surface-Dry Mass (SSD) (B) . Step 3: The Arithmetic (Calculations) With three numbers—A (dry), B (SSD), and C (submerged)—the hidden geography of the concrete is revealed.
[ D_dry = [A / (B-C)] \times \rho_water ] What it means: The mass of solid concrete per unit volume, including pores. A low dry density might indicate lightweight aggregate or excessive voids. Because time is not the same as energy
Note: Values >20% permeable voids generally indicate poor-quality concrete or a mix design error.