Hot Rod

hot_rod

going to be the finished floor?

You will want to assure you have enough joist strength below to carry that weight and not allow much flexing. A yard of concrete weighs about 4000 lbs. Any movement would encourage cracking.

I've noticed builders beefing up the floor joists under floors with large ceramic tile, to help prevent flexing and cracking. JLC had an article on joist deflection and tile floors a while back. it explained allowable joist deflection and how the numbers work.

I've always been told you want a slip sheet to prevent the concrete from sticking to the wood. I used basic 6 mil plastic for that on one or our first homes. It also keeps the wood from absorbing all the water from the concrete.

Fiber mesh may help at least 1-1/2 lb per yard.

Shrinkage is the main cause of cracking, proper mix ratio, not adding water will help. W.R. Grace and others offer shrink reduction admixtures.

Maybe some control joints? Outside corners are prone to cracks. It's pretty hard to guarantee a crack proof concrete job. But you sure don't want a crack and movement.

hr

greg_47

Thick slab over a sub floor

Could you offer any insight on applying a 3.5" concrete slab over a radiant subfloor to minimize cracking? i.e. bond breaker, mesh, special mix, control joints?

greg_47

thanks for your reply Hot Rod. It will be the finished floor and probably stained and stamped. I will use the bond breaker as you recomend and ask about fiber mesh from the plant. We will also oil the bottom plates. I am not familiar with you maximum deflection reference. Where might I look it up.

ps tell you wife her books have been a great resource.

thanks greg

hot_rod

this link

explains a bit about deflection

www.awc.org/technical/spantables/tutorial.htm

The article I remember was in either Fine Homebuilding or JLC. It explained how some builders are beefing up floor joists under areas with large tiles to limit movement and tile cracking.

Most lumber yards or TJI reps could help make sure you have adequate joist for the load and spans you have. nothing worse than a bouncy floor. Tough to fix after the fact also.

Remember the potential for any concrete slab to crack is very real. Make sure everyone is comfortable with that potential.

hr

Longtime Lurker

This may help

http://www.toolsofthetrade.net/industry-news.asp?sectionID=1537&articleID=511656