I live in an area known for expansive clay soil and with a recent and currently ongoing drought, conventional slab foundations are suffering the effects.
I have read about precast concrete foundations like those from superior walls designed for basement type foundations, but we don’t build basements in this area.
I wanted to put the idea out there for discussion about precast concrete slabs. One concept I found was from a company called Cell Blocks, Inc which are used for cell towers and mechanical buildings. http://www.cellblocksinc.com/
In this commercial application, the blocks are 7’x7’x2′ and are interlocked together and post-tensioned to form a single mass slab.
My thought is that precase slab modules can be brought on site and set on a leveled compacted gravel base, post tensioned and framing can be begin immediately.
The compacted gravel base would isolate the floating slab mass from the expansive soil and the precast system would remove doing formwork, pouring concrete and waiting for curing.
While a 2 foot thick slab may be overkill, the concept could be applied to say a 12 inch thick slab set on a 12 inch compacted gravel base.
Further research as shown the precast has also been used to create highway and air port runway sections so clearly, strength and durability are not an issue given the abuse those applications put on the concrete.
I know this is out of the box thinking so please think forward a bit as you give your thoughts.
Replies
I suppse
I suppose it could work - the only caveat would be chases for utilites, most important plumbing drains.
Terry
Terry
Where I am in Texas, we have to build above the 100 yard flood plain so the thought was rather than building on piers that raise the first finished floor to 60-72 inches, just take the first finished floor up to 120-132 inches and use the wasted space below as a garage - actually would be a rather large garage but who would complain about that?
In that way, the plumbing and other utliities would run below the first finished sub floor and the only through slab penetration would be the main sewer connection which could even go through a sidewall and connect through a sewer connection immediately next to the slab requiring no slab penetrations. It would be kinda of like a basement built on grade.
While 7x7x2 blocks weigh close to 15,000 lbs each,, a smaller 4x4x1 block would be much more managable weighing in at say 2400 pounds per block.
I'd suggest you try to find some industry association you could consult. There must be a "Precast Concrete Association" or some such.
Dan,
The precast concreate association are on my list to contact, but my thought by asking the question here is to get opinion from people without a vested interest in precast concrete who would very likely say "no problem, yes could be done" vs people in the trades who can view it from a different perspective and may think of it as a time and labor savings with no formwork, concrete pours, stripping forms and waiting for curing.
As a structural engineer, I do a lot of commercial work throughout Texas and am very familiar with the expansive soils issue. I also do a lot of work with precast/prestressed hollow-core concrete panels. The panels are 4 feet wide and come in varying lengths from 10 - 45 feet. Thickness varies from 6" to 12" depending on load requirements. As the name implies, the panels are formed with hollow cores running along their length to limit their weight. We typcally see this type of material used in hotel and multi-family construction... where it allows for clear floor spans from exterior walls to center corridor walls. That's not to say it couldn't be done for single family residential projects, but I believe it would be cost-inhibitive. You are correct, you would save on labor and time associated with formwork, but there would be added costs for transporting the panels to the site and having a crane on site for installation. The main advantage to using these panels is speed of construction, which isn't necessarily a priority in residential construction like it is in commercial.
Regarding the expansive soils, I would support the ground level panels on grade beams that rest on a deep foundation system (drilled piers). I would provide a void below the panels and finish grade to allow for soil movement. This again, would be extremely expensive for residential construction. I belive you're better off going with an engineered slab and/or floating walls.