Given rubble masonry’s terrible reputation, you’d think I have loads of pictures of it failing, but I don’t. I have pictures of it in bad condition, I have pictures of incipient failure and I have pictures of it doing what it does, which is erode and crack peculiarly. But I have few pictures of it collapsing or otherwise giving up.
The picture above gives a hint as to why. There’s a rubble wall above grade, even more exposed to weathering and changing loads than the usual rubble foundation walls. It’s got a big crack running through it, obviously, but the crack hasn’t moved much and there are really no other signs of distress. Maybe I’m thinking about it backwards: maybe, instead of assuming that this terrible-looking masonry is going to fail, I should be asking how it’s working.
We assume that unreinforced masonry is brittle and can carry very little tension, with the allowable tension typically controlled by the strength of the mortar. The defining characteristics of rubble are that (a) the masonry units are irregularly shaped and therefore (b) the mortar joints are thicker than normal. In terms of structural capacity, the strength in compression or tension is controlled by the material properties, the same as for a brick or ashlar stone wall. The rubble is more porous because of the larger percentage of mortar, but that does not in itself create a structural problem. So if the stone is reasonably strong (schist is commonly used for rubble although other stones are used) and the mortar is reasonably strong (natural cement was common, which is stronger than lime mortar) and the mortar adheres well to the stone (sometimes it does and sometimes it doesn’t, depending on the exact materials present), then why shouldn’t rubble perform well?
In structural engineering, at least, an ugly appearance is not indicative of worth.
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