Vapor Barrior under Concrete Slab??

[Steve]

There are some local code around here (Mason Co. WA) that call for vapor barrior under concrete slabs. And if the space is to be heated, they require

2" of foam insulation under the slab or insulate the overhead or walls. (in other words, so many 'R's of insulation for energy efficiency.)

My carport slab area wasn't large enough to require a permit and I wasn't going to heat the open area. However I did put a 6 mil vapor barrier under the slab. Thought it might help keep the the concrete from sweating..

I think that might have been a mistake.. When I toweled and finished the surface, I had problems getting the cement/sand to the surface and now, after full cure, the surface is stull soft and I can scratch it with my finger nail or if I drop anything on it, it chips or leaves a mark.

I have heard that a local cement finisher won't take a job if there is going to be vapor barrior under the slab. I don't know his reasoning..

I'm not sure of the mechanics/chemistry in cement finishing but I have done a few jobs in the past and never experienced anything like this..

BTW. the humidity this time of year is around 100% but I'm pouring under a shed roof.

I mixed my own cement in a 3.5 cu/ft mixer and it was 1 cement, 2 sand and 3 rock (5/8"). I'll admit my mixes were fairly wet (not sloppy wet) since I was working by myself and didn't want it to set to fast.

I did four seperate pours for my 240 sq/ft carport and all four have the same result.

Anyone have any ideas on what is going wrong here.. ??

I have now poured 240 sq/ft of a 1100 sq/ft total slab area. (yah! I know, I should call in a redi-mix for an area that large, but I'm working alone. I got plenty of time).

Steve

[Joseph Meehan]

Let's start with your misunderstanding about concrete. Concrete cures, it does not dry. Your problem has to do with that extra water you added and the only fix is to remove it all and start over. It will never be strong. The mix ratios are very important. Your only chance relates to the fact you did not say how long it has been since you mixed the concrete.

The vapor barrier had nothing to do with the problem.

[someone]

Let me get this straight. You are required to put 2" foam board under the concrete slab? If this is correct, I wonder how long it is before the whole slab turns into broken pieces. The weight of a vehicle alone would be enough to compress the foam, and crack the concrete. I wonder how much the foam manufacturers paid to force this rediculous law. Glad I dont live there !!!!

[Java Man (Espressopithecus)]

Our basement slab meets local codes. The slab was poured on a crushed rock bed under 6 mil poly sealed to the foundation walls. No problems finishing or curing.

Rick

[jim]

you made a too wet mix.. thats is why its not hardening correctly and does not have any strength... tear it up and call in a redi=mix truck with a load of cement and pour it this way... dont let them add water to the mix.. they want to add water to get the concrete to flow faster so they can get out of there and do another load(each load is more money for the truck driver..., dont let them add more water to the mix...)...

[raven]

Too much water in your mix. Cement doesn't dry, it sets. Idealy the water you put in is just enough to combine into the dry cement and all be consumed into the final product with no evaporation. the vapor barrier and the 100% humidity are things that help keep the water from escaping from the mix before it can be bound up into the hardened cement.

So, there's nothing you can do short of tearing it all out and startting over, *but* all may not be lost. Your mix may be taking a long time to cure and may well cure stronger over the course of a week or month. Wait and see and next time don't add so much water.

John