(left, mosque) Bernd Fitzner; (right, Worcester College wall) Heather Viles

Drip, drip. Light bands reveal salt damage in walls at a mosque in Cairo (left) and at the University of Oxford in the United Kingdom.

How Tiny Drips Can Crumble a Building

It doesn't take a flood to destroy a building. Mere moisture over many years can do the same. Now, materials scientists have found a way to predict how moisture works its way through a given building and location, something that should lead to better assessments of the health of historic structures.

Any building made of porous stone or brick is vulnerable to destruction by moisture. Groundwater rising up through the material via capillary action can weaken chemical bonds, and the salt it carries will eventually crystallize and expand, cracking rock and mortar. Over 100 years' time, even slightly salty groundwater can transport as much as 4.2 kilograms of salt through 1 meter of wall.

Until now, however, no one had developed a way to gauge exactly how water behaves inside a masonry wall, or how climate can affect its movement. So for 3 years, materials scientists Christopher Hall and Andrea Hamilton of the University of Edinburgh in the United Kingdom and colleagues studied two centuries-old stone walls: one a freestanding wall at the University of Oxford in the United Kingdom and the other part of a mosque in Cairo. They then developed a mathematical model to explain where, how far, and how much water wicks upward through the tiny channels, or capillaries, within the walls.

The team found that, depending on humidity and ground moisture, stone and mortar can transport more than a liter of water a day. It's a "largely invisible process," says Hall. But over the long term, "the total amounts of water flowing through structures can be huge," he says, particularly in historic buildings that are hundreds of years old, where water has begun to hollow out the stones.

The researchers also discovered that the way moisture travels through stone is largely determined by local climate—a factor called the potential evaporation (PE) of the atmosphere, which combines temperature and humidity over time. As the team reports online today in the Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, the average PE in Oxford is about half of that in Cairo. That means more evaporation occurred at the Cairo site, which should have caused water to leach out of the walls before it could rise very high. But because the Cairo wall is much thicker than the Oxford wall, it takes water longer to leach out, and thus the salt water actually rises higher in Egypt than it did in the United Kingdom (see photo). Either way, it's bad news for the buildings, as salty water in walls will eventually destroy them.

The paper "makes excellent predictions about a complicated and important problem," says materials scientist George Scherer of Princeton University. It shows that the amount of water and salt are "strongly dependent on the local climate." Therefore, warming temperatures could "enhance the deterioration of monuments by accelerating the rate of evaporation."

This item has been corrected. It originally said that water rises through stone via osmosis. Capillary action is responsible for the rise.

Posted in Math, Technology