How much stronger does structural fiber really make concrete?

Question

I am in the process of a crazy project. I am pouring a ceiling/roof over my 12'x12' basement entrance. I know, I know, your crazy! Oh well, I'm doing it anyway. I am peaking the ceiling so the center is much lighter than the outside edges (the outside edges will be sitting on solid, poured concrete walls 10" thick and will be about 16"-18" thick). I am framing out a void in the form to accomidate a 74"x52" steel door. I will also be using ALOT of 5/8" rebar ($300 worth) running from side to side both ways. Just so you know, I am bracing the bottom up with an insane amount of 2x4's and 4x4's... Anyhoo, my question is how much stronger will the structural fiber really make the concrete? My main concern is the center cracking along the peaked ceiling and I think thats what the fiber is basically for... The crete will hold without the fiber but I'm using it just for added piece of mind. Any expert input would be greatly appreciated. Thanx.

Answer 1

I told you the last time you asked this that I didn't think it was a good idea.  I hope you have at least talked to an engineer about it.  In any case, with all the rebar you plan to use, fiber is unnecessary.  You may have a bigger problem with the concrete not being strong enough to carry its own weight as well as the weight of the rebar.  You seem determined to make this pour, so I will offer you one more piece of advice.  Let the concrete set for 7 days before you strip the forms, then once you do, immediately place shores under it about every 5 feet or so apart.  Leave these in place for 21 more days.  Concrete reaches 95% strength in 28 days.  Hopefully this works for you.  Good luck.

Answer 2

perhaps this could help

http://www.elastoplastic.com/pdfs/Pressr...

Answer 3

What is most important is that the concrete is cured properly.  This will do more than anything to give strength to the concrete.  Once you have poured it and it has set up, keep it wet for at least 10 days.  Concrete will attain 80% of its maximum hardness is 3 days, but will keep curing and getting harder for years if not allowed to dry out.  If you can keep it wet, perhaps by hosing it down and covering it in plastic, and then not letting it dry out for a month you will be in pretty good shape and have concrete like iron.

If it dries out the curing process is killed.  The fiber is mainly used for strengthening thin layers of concrete, like 1/4" or so..

You might consider creating an arch, or curved underside instead of a straight peak. this may transmit the weight of the structure better out and down towards the sidewalls.  Just make sure your sidewalls are strong enough to support the sideways pressure.  This is why Gothic cathedrals used flying buttresses to carry the weight of the vaulted roof downwards and outwards.  If your sidewalls are not strong enough to hold the outward force they may both end up tipping outwards.  If you make an arched roof you do not need it to be anywhere near 18" thick, you could probably get away with half that, but make it an even thickness all they way across.