Fantasy concrete construction

I can't quite picture this.

Are the pre-cast concrete walls being set into the asphalt and being held together by the pillars at the corners? If so, how are they attached to the foundation slab?

I see a few problems with this.

I don't think asphalt would be a good enough water barrier.

The integrity of the structure relies on the strength of the rebar at the corners, and that's not going to be enough to hold those heavy concrete forms.

Speaking of which, how are you going to manipulate those huge concrete panels?

Concrete has been used to make buildings in similar ways to this: look up prefabs, or prefabricated concrete buildings. You might find some helpful pointers that way.

You might also search tilt slab construction.

Not a builder, but I'd think that 3" is nowhere near deep enough to support concrete walls. If we were talking 3', that might be more like it, but even then, I'd have thought there'd be an issue.

AwP_writer said:

Oops, I meant 3', I'm tired and getting sloppy. How deep should it be?

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Shades of Spinal Tap!

This reminds me somewhat of the construction of houses in Arab countries. The construction is different because they have to withstand the hot climate and also because there's a lot of limestone, which makes the raw materials for concrete easy to come by. You have the foundations first, and they dig deep for the foundations (way more than 3 feet deep, more like 5-6 metres or so (sorry I don't do imperial*), not sure what the foundations are made of (probably concrete, but can't say for sure). To construct the walls, you make steel-reinforced concrete pillars at the corners and set points within the building (the bigger it is the more you need). You have wooden boards to shape the pillars and the concrete is poured in around the steel supports and left to set. When the concrete is set, the wooden boards are removed and concrete bricks are used to construct the rest of the walls in between the pillars.

*apart from miles. And yes I'm eternally miffed that my sat nav doesn't have a setting for giving distances in miles and metres.

The thing that strikes me about your suggested construction: 1. what's happening about the foundation? I'm not sure what they do about foundations in western prehabs, but the building is weakened and prone to subsidence (and just plain old falling down) without a decent one. 2. Concrete on its own isn't that strong. A steel structure inside the concrete strengthens it.

Another thing that strikes me: the answer to your query depends massively on how big the buildings are and how permanently you want them to be there. You can make a decent cottage out of wattle and daub (which is basically mud and sticks) if you don't need it to last forever, however if you try to build a skyscraper out of wattle and daub you're going to have a bad time.

Wattle and daub is surprisingly strong though... some historical buildings in the UK are made of it and still standing.

What kind of weather it has to withstand is another question. British houses are better at withstanding extreme weather (like tornadoes and hurricanes) than many American houses, because they're constructed of two layers of brick, often with insulation in between. They're constructed like this because they have to withstand the cold and the rain, but it so happens they also withstand extreme winds better, and being battered with debris that gets thrown at high speeds by extreme winds. But they do sustain some damage and wouldn't withstand an F5 tornado. If you live in an area prone to extreme tornadoes then you are probably better off having a decent bunker/underground shelter and rebuilding your house out of cheap materials each time.

BTW I'm not an expert. I won't know the answers even if you give the above information. But there are some different factors to think about when it comes to considering what building methods and materials would be used in a fantasy setting. For me, matching it with the climate and other local variables (e.g. availability of raw materials) is more important than the precise details of how they're built. For a blatantly obvious example - an igloo makes total sense if you live in the Arctic; ice is everywhere and it's a great material for insulation and the average temperature is cold enough that it won't melt. But an igloo in London would make no sense.


For waterproofing, damp proof course is used in the UK. It's a layer of plastic (I don't know more specifically what it's made of, e.g. what type of plastic or if its a composite) just above the foundations but below the ground floor. This stops the damp from rising up through the walls, which causes mould and other nasty problems. I'm not sure what they used before plastic existed but that would be something you could look up.

Helix said:

Shades of Spinal Tap!

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And oh, how they danced... the little people of Stonehenge...

We recently put a new concrete floor in one of our outbuildings, which isn't structural: but it has to support a lot of heavy machinery and a forklift carrying pallets of wood which weigh about a tonne each. That floor was laid onto a weathered shale surface which was pretty solid, with about six inches of hardcore then another six inches, at least, of concrete.

I used to live in a brick-built house which had been built with no foundations as such: it was on a float of concrete which was about nine inches deep, though, which covered the entire footprint of the house.

Foundations depend on the ground you're building on, really. A trench half-filled with hardcore is all you need, then you pour concrete in on top. If you're building onto loose or weak ground you need more than if you're building onto bedrock.

AwP_writer said:

I know this isn't how things are done, but I want to know if it realistically could be done this way. You have a concrete slab as a foundation, around the edges are trenches. You put a thick layer of asphalt in the trench for waterproofing. You have pre-cast concrete slabs with holes for doors and windows already cast in, and rebar sticking out the sides. You put the slab in the trench, and at the corners where the rebars are crisscrossing you put up forms made of wood or something and cast a pillar enclosing the edges of the walls and the rebar ends. Once the pillar is done drying you take away the pillar form, and voilà the walls are done. Would that work? Would it almost work with some modifications? Any major issues I'm not considering?

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I see bridges and highway ramps made with huge premade concrete tiles. Look into that as a starting point.

Your two basic options for foundations are a) slab foundations, and b) trench footings. In the case of a slab foundation, it's just that. A slab, preferably eight or more inches thick, with a double mat of rebar, sitting on prepared subgrade. It's structural the whole way across, and the walls are cast, or placed, on the edges, doweled in to the slab. With a trench footing, you have an excavated trench, on undisturbed native or engineered subgrade, three or more feet deep, where you pour a keyed footing with the base wider than the wall foundation. The floor is poured separately and is not part of the foundation structure.

Which you use is a function of size and soil conditions. Economics favor trench footings for larger buildings, and slab foundations for smaller. It is difficult for me to visualize exactly what kind of foundation you (OP) are creating, and where the asphalt comes in to play. It sounds like some sort of, but not quite a pier foundation. What is the function and size of the building in question? What dictates the need for a unique means of construction?

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Your two basic options for foundations are a) slab foundations, and b) trench footings. In the case of a slab foundation, it's just that. A slab, preferably eight or more inches thick, with a double mat of rebar, sitting on prepared subgrade. It's structural the whole way across, and the walls are cast, or placed, on the edges, doweled in to the slab. With a trench footing, you have an excavated trench, on undisturbed native or engineered subgrade, three or more feet deep, where you pour a keyed footing along the perimeter of the building with the base wider than the wall foundation. The floor is poured separately and is not part of the foundation structure.

Which you use is a function of size and soil conditions. Economics favor trench footings for larger buildings, and slab foundations for smaller. It is difficult for me to visualize exactly what kind of foundation you (OP) are creating, and where the asphalt comes in to play. It sounds like some sort of, but not quite a pier foundation. What is the function and size of the building in question? What dictates the need for a unique means of construction?

I'm not sure what technology is available in your cities, but handling giant slabs would require a lot of heavy equipment, or a pharaoh's tomb worth of labourers. If you want the advantages of modular construction, why not go with that classic 'brick construction'? Standard bricks, concrete blocks, adobe blocks, etc. Easier and simpler to repair, adjustable in size to fit various locations, and can be done by semi-skilled labour without massive equipment. Heck, you can do igloo-style construction if you want (and domes are sturdy, if you're expecting attack.)
My objection to your slot and panel construction is the slots. Every time I try to visualize it, the slots just seem like the 'snap off here' lines on a chocolate bar - an inherent weakness. Unless your foundations are so deep and solid that the slot, in proportion, is merely a scoring of the surface.

Hardcore is compacted rubble

Hard core is rubble, as waylander has explained.

You could consider alternatives to concrete. Limecrete is a very traditional building material with similar but different properties to concrete. Making cement requires kilns at very high temperatures, which might not be feasible in your world, for example; limecrete is harder to work with but is breathable so one doesn't use damp proof barriers in the same way as one does when using concrete.

As you've described it, I think it's the edges of the foundation that would snap off, unless the depth of the foundation was great enough to overcome the weight/pressure of the wall panels. The panels should be alright once they are in place and secured from wobbling. (If they wobble, again, the risk of breaking at the point where the movement of the panel is stopped by the bracing of the slot.)
I don't think 'wider' foundations are going to change this much. Thicker and deeper. Wider will only increase the chances that more pressure will be put on the edges.

Why does your structure need to be made of concrete? How does that matter for the story?

caw

AwP_writer said:

Are you thinking insanely deep, or just on the deeper than average side? If it's just a little deeper than average, that should be fine to do and not too much of a problem for them.

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I honestly don't know specifics, I think you'd need an engineer or a contractor to do calculations.