I was out for my lunchtime walk the other day and walked past this pair of iron
gates near my office, the frame had been set into the stone base walls and the
rust of the frame has blown the stone away of both sides, almost identical to
each other. On closer inspection, there appears to be no lead in the interface
between the iron and the stone, which may not have helped the situation.
It did remind me of a wall I walk past to get the train
where a gate pintle has been cut off flush with the brickwork and as it has
corroded, and as it has expanded it has started to fracture the wall. See the
photos below.
When iron corrodes increases in volume from its original
size. This expansion creates tremendous pressure within the surrounding masonry
which is completely in flexible and unable to accommodate this change resulting
in the fracturing and failure of the surrounding masonry.
The corrosion process accelerates in the presence of moisture and oxygen. Once
initiated, it becomes a self-perpetuating cycle: cracks created by the
expanding iron allow more moisture to penetrate, which further accelerates the
corrosion process.
I have come across many projects,
especially churches, where Iron cramps have corroded and are causing significant
damage to stonework from within walls and parapets. Iron was used in masonry
construction for centuries, providing structural support and reinforcement to
stone and brickwork walls, typical fittings are:
- Tie rods
- Cramps and dowels joining adjacent stone blocks
- Window and door hardware anchors
- Decorative iron elements embedded in facades
Unfortunately when left uncheck, corroding iron left within a building lead to
failure of the surrounding stone or brickwork and requires invasive measures to
repair the stone and remove the corroding iron. I once had a church wall where
the corroding iron cramp went over 12 inches into the wall, thankfully I had an
experienced stone mason and he was able to remove it, and replace it, with only
minimal damage to the surrounding masonry.
The replacement material of choice is often stainless steel,
due to its non-ferrous properties and strength. This is used for most repairs
these days, although depending on the circumstances of the repair some
engineers worry about changing load paths through a structure with a stronger
dowel or cramp in some instances. Other materials for replacements can be a
traditional material like slate, there are more modern materials which can be
used like Perspex or carbon fibre, although I have not used these yet in a
project, but provided its sympathetic to the repair you are undertaking, and it
provides enough tensile strength, these could be used. If you are working with marble, brass dowels
should be used, although I’m yet to do a project with Marble, one day maybe.
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