Thursday, June 21, 2007

How can water get through concrete?

Concrete is naturally porous - typically 10 to 18 percent air (extremes 2-60%). To make concrete "workable," a water/cement ratio of 0.45-0.50 is used but cement only needs 0.25 w/c ratio for hydration. As concrete cures, the surplus water escapes to its surface where it evaporates. This "bleeding water" leaves a network of tiny capillaries (pores).

We cannot see the pores. Their size ranges from 3 nm (millionths of a mm) to 0.1 mm (the diameter of a human hair). The median is about 1 micron (1,000 nm) but a water molecule is 3,000 times smaller (0.28 nm). By all logic, concrete should leak like a sieve!

So, what holds the water back? Not the concrete but the physics of water itself. Although not as viscous as molasses, it takes pressure to push water through dry microscopic pores. Surface tension holds the "blob" of water inside the dry pore until its surface gets wet. But add a little water pressure during occasional rainstorms or condensation on the interior and after a while, the pores get wet and the concrete starts leaking. Concrete is hygroscopic - it attracts water by sorption and once wet, the pores draw in groundwater by wicking action (capillary suction).

But nothing holds back gases - invisible water vapor and radon gas flow right through the pores!

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