Would it come as a kit. Perfforated cardboard sheets that a user assembles and then poors concrete into? Then you could design a variety of products that could be sold inexpensively at Target in the garden section. I’m thinking of the inetlocking CNC’d plywood furniture they had there last year.
Yes, an simple interlocking cardboard mold is the ‘packaged’ part of the product that the consumer would theoretically purchase, along with their castable material (concrete, plaster?, DIY composite?). The idea is to have the mold packed flat as sheets of either perforated or pre-cut parts that would be folded and assembled into the mold. There are definately other furniture items that can be made the same way - pretty much anything that could be an extruded shape - tables, lounge chair, chaise, bench, etc. Alternately, several of the chairs could be placed next to each other for the bench feel.
I’ll see what I can come up with re: arms, multiple parts/styles, and interlocking peices. That may ultimately take the concept away from the inexpensive aspect and/or deter people to undertake the project because of the complexity or time needed. I’ll definately sketch some things out, as I’ve already thought of a few ideas… 
Regarding the posted sketch – totally gorgeous, and very elegant solution. As a former engineer, though, I gotta point out the structural issue inherent in the design. Casting a flat, thin slab in concrete is generally a bad idea unless it’s reinforced. Unless you’re expecting the DIYer to purchase, bend and place rebar in the mold before pouring, you’ll have cracking and probably failure on the seat slab pretty quickly.
It might help to arch the bottom?
hitch, thanks for the input. The last ‘sketch’ I posted has the thickness at 1.5" but I will do a bit more research on the cement itself to see what should be done. The image really does not show the scale of the chair, which is 18" wide/deep with an 18" seat height.
I have seen other pieces of furniture (tables, countertops) that appeared to be of similar thickness, but they very well could have been reinforced in some way. I would prefer to keep the cross section thin, but adding ribbing may bring the mold into a more complex form than can be made with cardboard, and asking the end user to add rebar is too much.
Perhaps I can find a better (i.e. DIY-type) of reinforcement that is both inexpensive and requires minimal to no tools. One Idea that comes to mind is adding some steel pipe and elbow joints using off the shelf parts from the hardware store, but ultimately, I may need to re-think either the thickness or overall shape to keep the chair as simple as possible.
if the span of the seat is going to be a problem, could you not do a slatted seat, made just with steel rods or flats, rather than using the steel to reinforce the concrete? (you will have to worry about joints, tho) embedded reinforcement needs to be a minimum of 50mm away from the outer atmosphere which means that the seat slab would have to be at least 100mm thick (reinforcement in the middle, and 50mm of concrete on either side) this is for buildings tho, i dont know whether different rules would apply for simple garden furniture. the concrete uprights on your chair wouldnt be a problem as concrete is good in compression
Well, the thing with concrete is, making it thicker won’t necessarily help – its strength to weight ratio in tension is abysmal, which is why almost all large concrete structures have steel reinforcement (and why very few of them are slab shaped). It does have two great advantages though: it’s very strong in compression (as bluegrrl pointed out) and it can be molded in any shape.
I think if you had an angled or arched connection between the legs and the seat, that would avoid a lot of tension cracking problems in the seat slab – check out Gaudi’s columns on the Sagrada Familia, or even Gothic cathedral arches and buttresses. Great examples of turning a structural necessity into something beautiful. Just my own prejudice…
How about something like this?
Not quite the minimalist masterpiece of the previous sketch, but I think it might hold up a bit better. You could probably pull it off with a 5 part mold. Or even 3, if you don’t mind them being big and weird.
well here’s my sketch - not particularly attractive, but made of flat interlocking pieces. the seat is supported by a cross underneath it - don’t know if it will work or not - but if it does, you wouldnt have to use arches like in hitch’s previous sketch
I like the fact that that can be disassembled. I also like the idea of building your own chair from several parts–it would also allow you to “design” many different chairs from just a few different pieces. As a consumer I know I would personally love to see a kit that lets me build a chaise longe, upright chairs or a bench from a standard set of pieces. The more jobs something can do, the greater percieved value.
Would concrete be light enough to move around? Would it be comfortable to sit on?
I’ve been thinking about the problems with my last posted sketch - mostly, that the flat plane would need reinforcement of some type. Fiber-reinforced concrete is widely available and costs no more than plain ‘just add water’ mixes; probably not enough reinforcement to prevent failure of the chair, however. I’ve wondered if some type of reinforcement could be included with the flat-packed mold - possibly wire mesh or molded plastic. Unfortunately, the more complex the chair gets, the higher the cost will be; exactly what I’m trying to minimize to make this more appealing than the plastic/metal chairs on the market.
The estimated cost right now is:
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$5.00 for the cardboard mold ‘kit’
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$10.00 for two 60 lb bags of concrete
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aprox. $15.00 total, plus taxes
And yes, the thing would weight around 120 lbs as shown in the last posted sketch. Moveable - yes - but it would require 2 people. Think of the chairs more as a semi-permanent installation, which adds to the idea that this chair is a non-disposable alternative.
Multiple parts is definately do-able, as are styles, although i think it will be difficult to get very many styles with simple shapes and forms, but I’ll try to post my solution tomorrow.
Pretty cool stuff. Thanks for the link ykh.
Unfortunately, its about as extreme of a DIY product/project as the rock chair that I began with. 
I wonder if bamboo could be made to bend like that? I’m not too familiar with all of the species out there, so it may be possible. With its incredibly fast growth rate, you could have a piece of furniture ‘grown’ in much less time.
well, even if you can’t grow a bamboo chair, you could include bamboo slats to use as the seat or the backrest of your diy-chair-pack
would consumers trade portability for something else? second thought - Chinese “chair” raises question: what if chair is purposely immovable? park benches arent mobile. or big flat-topped stones placed on landscaped grounds. portability issue raised w concrete. but mainly as problem. is it a feature worth giving up? if object is natural. not man-made like concrete. is the feature less important? do we want to protect garden furniture bc of experiencess w metal? rust? and w UV-plastic problem? where is the trade-off? isnt NOT worrying and NOT storing garden furniture an advantage? in contrast to portability?
The concrete garden chair - a study for a possible DIY alternative to traditional garden furniture.
With the rising popularity of DIY projects and the TV shows that inspire people to tackle these projects, an inexpensive, long-lasting chair that the consumer can build themselves for the same as (or less than) the cost of cheap plastic chairs makes sense. The chosen material, concrete, offers some great possibilities for customization (dyes and stains) as well as the use of natural and sythetic waste materials for reinforcement while still remaining inexpensive and simple enough for most to undertake.
The chair presented is designed to simplify the entire process as much as possible, making it more appealing to possible consumers. The flat-packed cardboard mold could be sold through any store, but ideally through those that retail the two bags of fiber-reinforced concrete mix required. ‘Allowing’ the consumer to apply their own labor on-site removes a great deal of the costs involved in manufacturing and transporting such a product. This means that the consumer can purchase a higher quality, extremely durable product for roughly the same amount of money as other inexpensive garden furniture
389 words??? this your final? might re-read guidelines. let images do more of the talking.
Corrected to meet guidelines. 