Unclear Boundaries

That beautiful concrete is the undercroft, for want of a better term, at the London Bridge rail station. The station has a top-heavy configuration, with the entrances at street level and the tracks above. This makes sense for a constrained-site urban station, and London Bridge’s operational set-up reminded me of Jamaica Station.

The concrete groin vaults are carrying heavy (train) loads in the same manner that masonry vaults have carried heavy loads for thousands of years. Which leads us to a philosophical question about analysis: what’s the difference between a concrete structure and a masonry one? I don’t know if the concrete vaults are reinforced normally (for today’s codes), lightly, or not at all, and I suspect it doesn’t matter much. Groin vaults are both strong and quite rigid, so even if the concrete is reinforced for bending, it may never deflect enough to crack, and if the concrete hasn’t cracked then the reinforcing is doing little work. (I didn’t see any cracks but I also didn’t stick my nose up against the concrete looking for them.)

If the vaults are carrying the loads above in pure compression then they are working, structurally, in exactly the same manner that masonry vaults do. So what’s the difference between concrete and masonry? The fact that one is placed as a semi-liquid that cures while the other is placed as discrete blocks? Even that distinction is blurry, since the blocks are (almost always) set up in mortar that is a semi-liquid that cures in place. Both are, under current codes, usually reinforced with steel rods; both were, in the past, usually not reinforced. The materials are usually different but they don’t have to be: concrete blocks are masonry, and concrete can be made with the same cements used in masonry. This is probably a good place to note that when reinforced concrete developed as a structural material in the late 1800s and early 1900s, the laborers who worked on it first were recruited from the ranks of masons.

Perhaps the difference is in intent. If we are designing structures that rely on compression and can therefore be built in wet concrete or masonry blocks, then we’re talking about “masonry”. If we are designing structures that rely on composite action between steel reinforcing and a brittle surrounding matrix (of wet-placed concrete or masonry blocks and grout), then we’re talking about concrete. This is a semantic game, but not without historical precedent: Roman concrete structures were basically brick or stone masonry with concrete cores, and used traditional masonry forms. Perhaps the point is that the difference between masonry and concrete is smaller than our material-based engineering education and codes would have us believe.

In any case, look at those beautiful vaults. Their shape is perfect, and the board-form marks add just enough texture.