Straw Bales as Concrete Void Form for Garage Floor (2025 Update)

I think that by the time you buy,transport,and lug around that much hay you would be better off using concrete with extra rebar. tonyg

---------- > Does anybody have any thoughts about using straw bales as a "void

I've seen it used for walls but not floors. If you want to save just backfill with old material.

- Wpg Man -

- Nehmo - Crushed stone of the type you use under concrete in Kansas City runs about US$25/yard^3 delivered. It's about the cheapest thing there is.

Wpg;

Straw-bale-formed waffle slabs have been used on a number straw bale homes,mostly in Quebec, so it is something that has been done. The intention was that the straw bales would serve as insulation, as an alternative to foamed plastic insulations.

There have also been forensic studies done on the above slabs (when they were about 10 years old) and as you would expect, the straw did show signs of decay so you should not have any expectations for the straw be there permanently.

Without doing any number-crunching, my wild-ass guesstimate would be that the slab between the ribs would need to be at least 200 mm thick, with reinforcement designed for 2-way action (ie Lots of engineering calc time + lots of time placing reinforcement)

I also think that the bottom line would be that an unreinforced thinner slab, (non-waffle) cast on compacted crushed stone fill would likely be more economical and just as serviceable.

Thanks for the reply, and thanks to all others that replied (even if you think I'm crazy, he he).

Your description of a "waffle" was just what I had imagined. If you were able to look up (from below) it would look like a "coffered" ceiling.

Yes, the biggest concern is the strength of the slab between the beams. Of course that would require an engineer. However, I can't imagine a problem with an 8x8 ft slab. I've seen lots of untensioned

As for gravel. A 24x24 garage pad with 2 feet of gravel requires approx $1000 of gravel. If you don't want to break your back tamping every 6" of the stuff, it's gonna cost another grand for the labour. The hay bales would cost almost nothing (less than 100 bux, or you could scrounge some for nothing). I think the labour is less with the bales (still a bit of work though). Even if the cost of gravel were "zero", I hate the results because any settling or frost just cracks the slab. I want that sucker so strong that you could flip it over in one piece.

Mark my words, I will start a revolution in the construction industry. Soon we will see office towers constructed with the "hay bale" method!

----SNIP

What ever happened to a good'ol back fill? I'm pouring another slab on block and throw anything solid in the hole. Old lollie poles,scrap block rocks ect.

How much rebar in the (20" deep?) beams and 8'x8' slab, if it supports a 4,000 pound car? How thick would the slab be?

Nick

I know you've had a load of replies, but I have to agree that incorporating something that decays under concrete is a really bad idea! Generally when pouring concrete you try to remove any organic matter, not add it!

To be honest i am not sure what you are trying to construct, but how about something that doesn't degrade like polystyrene (plus don't forget some mesh reinforecement!

Marc

replying to Wpg Man, Don wrote: I think the straw is amazing. My dad used to use bales of straw to dummy up porches for a concrete pour. Long as it has lots of rebar in it he said it was good to go.

I'm seeing these all over now, and they are calling them Wpg-slabs. Seriously, though, as a structural engineer, I wanted to flesh this out, as it is a primary return on google. These are called waffle slabs, and they have been used for this application all along. They usually use something like a plastic bucket as a form