The Power of Rust
By Robert G. Wilkin, P.E., SECB
We have known for many years that as rust forms on metal installed in masonry or concrete, the rust expands up to seven times the original thickness of the metal. The force of this expansion, if confined, can reach as high as 9,000psi. Contrast this to normal concrete which is only 3,000 or 4000psi in compression and less than 1,000psi in tensile strength. The effect of this rust expansion and its damage to structures is known as “rust jacking.”
In concrete, reinforcing bars (rebar) which are cast too close to the surface are more affected by surface moisture and are more likely to rust. As the rebar rusts, it expands causing the concrete to spall and cracks to form, which we are all familiar with.
Steel lintels over window openings commonly rust as well causing cracks in the mortar and brick above and adjacent to the opening to move upward. Covering the lintel with a good quality flashing helps the steel last longer. Two recent projects of similar age (across the street from one another) are good examples of how effective flashing protection on window lintels limit rust jacking from occurring. Development #1 has a thin copper flashing that protects and keeps the lintels dry to avoid rusting. Only a few minor cracks were noted in Development #1. Development #2, however, had no flashing, resulting in significant rust jacking.
Replacing rusted steel lintels requires a great deal of work. At least three courses of brick must be removed and a new structural assembly installed. We have had good luck in lintel replacement by replacing-in-kind as well as by changing the structure above the window. We have created new lintels with precast concrete, cast in the thickness of the brick. The concrete provides the structure (replacing the steel lintel) as well as the finish (replacing the brick). The assembly still requires the through-wall flashing behind the lintel.
The damage to masonry on a another recent building we are working on due to rust jacking is very interesting. The window lintels were painted when installed in the 1920’s but not protected with flashing. The lintels have rust jacked and bowed downward into the window opening as much as 1/12”, and some have rusted through. The ends of the lintels are built into the masonry and have pushed the brick upward causing significant cracks in the bricks and adjacent limestone. Rust on the upturned leg of the lintel angles also formed and pushed the brick outward as well as upward. This movement can make portions of the masonry unstable by cracking header bricks which are usually installed to bond the veneer brick to be inner wythes.