Return to main page.
This is the procedure to construct the ¼ scale model. It is intended to be the same procedure as used to construct the full scale platform, but scaling up usually introduces some new problems. This will be partially debugged on a 4 foot 'dollhouse' model before we attempt the 12 foot version.
One objective is to keep this low-tech enough to be practical to produce in a third-world, poorly industrilized environment, and cheap enough to appeal to low-cost housing and small business structures. In this, it has been only partially successful. The equipment and skills required are modest. The bulk materials are low-tech commodoties worldwide. But some of the materials are only made in well-industrialized contries, eg polyethylene sheet, mylar packing tape. Fortunately, these are relatively compact and inexpensive to transport, and relatively small quantities are required.
The platform (also referred to here as module or tile) is cast literally in a hole in the ground, lined with polyethylene sheet, a small amount of steel remesh, and some (reusable) plywood wall spacers. The casting is done in several stages, resulting in a structure with a skin of normal density concrete, and an interior consisting of a honeycomb of hexagonal rooms. The interior is cast from lightweight cellular concrete. This material is used to raise bouyancy, reduce materials cost, and permit the use of a novel casting technique that is cheaper than conventional mold-building.
This is called 'neutral bouyancy water-bag casting'. First, a layer of normal concrete is poured to form the skin of the underside of the tile. Then a measured amount of cellular concrete is poured over of it. The cellular concrete is formulated to have the same density as water, by controling the volume of air/detergent foam entrained, so it floats on top of the higher density concrete. Next, an array of plastic bags (made from 6 mil poly sheet) is placed on top of the fresh cellular concrete layer and filled with a measured amount of water. Since the water has the same density as the cellular concrete, the bags become spheres suspended in the concrete. When elastic balls are close-packed, they form a honeycomb of hexagonal cells. The walls of this honeycomb are concrete. When the concrete has set, spacers are removed from the perimeter, making room for a layer of normal concrete that will form the skin of the outer walls. This space is filled until it overflows and forms a layer of normal concrete on top of the honeycomb of cellular concrete and water bags. This will be the skin of the top surface. When the assembly has partially cured (5-7 days) the bags are punctured and pumped out. Now we have a small floating building with lots of unfinished interior space.
There are a few more details to take care of:
The outside must be painted with a sealant to prevent the slow 'weeping' of water vapor through the concrete.
The outer skin is reinforced with remesh (a welded-wire fence material) to improve strength and control cracking.
Each waterbag is confined in a hexagonal cylinder of wire remesh to improve strength, and prevent the bags from touching. Otherwise there would be holes between cells and the corners rounded. While this is actually optional for most of the cells, a few must remain closed to 'sink-proof' the structure, and most rooms will require some 'closed' walls.
Pre-make hex wire fences, air bag, water bags. (need to expand details).
seal bags with tape or heat?
Dig hex pit. clear out rocks, sharp items. tamp down dirt. Important that floor be firm and level. Must be located within 10-15 feet of the water and uphill.
Line walls with 2-piece plywood spacers. They are pinned together at corners with carriage bolts.
Lay airbag on floor. Lead inflation hose up outside of plywood spacers.
Line walls and floor with more 6 mil poly sheet. Leave enough extra vertical wall liner for normal cement surface layer.
Crude seal joins with mylar tape. tack liner to plywood at top with a few taped spots.
Lay wire remesh out on poly sheet floor, raised on small concrete 'chairs'.
Place hex fences. wire together at vertices with scraps of wire fence. Wire to remesh at floor. Use vertex spacer jig while wiring to maintain consistent cell wall thickness.
wire top remesh to tops of hex fences.
place water bags in each hex fence, tape to top of fence.
Pour normal concrete floor.
Pour cellular concrete. place only the calculated amount. Use a baffle to prevent disturbing the normal density layer.
Fill the water bags. The cellular concrete should rise between the fences in a honeycomb formation. Fill to exact calculated height of cell walls, then seal-off fill openings with tape.
Wait for concrete to set 4-7 days. Keep top surface wet.
Remove inner piece of plywood wall spacers.
Insert outer wall remesh. How to hold it in place and clear of outer surface?
Pour normal density concrete into outer wall space. Continue until cell walls are covered to the specified surface layer thickness, just below tops of hex fence.
Wait for concrete to set 4-7 days. Keep top surface wet. Will shrinkage around uncompressable water bags cause cracking?
Drill into top of cells and pump out water. Force paperwad plug deep into holes and fill with normal concrete.
Remove plywood outer walls.