What causes the surface of old bricks to erode away into sandy powder?
Wednesday, June 27, 2018
The severely eroded bricks on this pier under a 90-year old house in Gainesville look like they have been sand blasted, but the culprit is actually a phenomenon called “rising damp.” Moisture rises up from the ground through porous building materials like this soft brick, which was probably under-fired when manufactured, by capillary action. Freezing temperatures and the salt crystals in the saturated brick cause it to disintegrate over time.
Rising damp is different from freeze fractures, which can happen in a denser brick, where water only gets into cracks in the surface, but expands upon freezing to cause spalling off of ragged pieces. Bricks become more water-permeable as they age, and may experience a combination of rising damp and freeze fractures if in ground contact at a wet location.
Ocala block can also be attacked by rising damp, as shown in the photo below of a retaining wall, which here appears to be working in combination with freeze fractures. Notice the channel in the bottom of the wall where it is more deeply eaten away. Ocala block is more susceptible to rising damp because it is more porous than brick and, unlike other concrete blocks, it is often used exposed to the weather without a stucco coating or paint. See our blog post What is "Ocala" block? for more that this building block that is unique to an earlier era of our part of Florida.
Rising damp has been recognized by builders in Europe for literally hundreds of years, and the standard remedy has long been a non-porous barrier Damp Proof Course (abbreviated to DPC) laid into the base of the wall. According to the British textbook Understanding Housing Defects, “The Public Health Act of 1875 made DPCs compulsory for new houses. A variety of DPCs were installed, including lead-cored felt, bituminous felt, two courses of dense (engineering) bricks, or stones laid in cement mortar to break joints."
Also, see our other posts How do I recognize serious structural problems in a house? and What are the places to look for structural cracks in a house?
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To learn more about exterior walls and structures, see these other blog posts:
How To Look At A House
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