POUR NEW CONCRETE OVER OLD?

[npalen]

I understand that the plastic will prevent moisture transfer but what exactly am I trying to prevent by using it?

 

[Pipestone Kid]

I started in construction in 1950 and have poured more concrete than I care to remember. I have only five words to say--DO WHAT GARY FOWLER SAID!!!!

 

[Deere Dude]

. I have only five words to say--DO WHAT GARY FOWLER SAID!!!!

What he said.

 

[Egon]

I understand that the plastic will prevent moisture transfer but what exactly am I trying to prevent by using it?

Plastic allows movement and acts as a cheap separation membrane.

Sorta like Ditra when laying tile.

 

[mehig]

Personally,GARY FOWLER I'm confident you know what you're talking about, especially seeing that you're familiar with the correct industry terminology. "PLACING CONCRETE" is the proper term....because if you're "POURING CONCRETE" which is what the majority "DO" it's had too much water added. The installer will often add water on site so it's easier to push around. But the extra water lowers the PSI of the mix and takes longer to set up. There's no point in making it harder than it needs to be. Knowing that water is so often added on site is why I recommended 5000 psi concrete.

 

[Pipestone Kid]

Personally,GARY FOWLER I'm confident you know what you're talking about, especially seeing that you're familiar with the correct industry terminology. "PLACING CONCRETE" is the proper term....because if you're "POURING CONCRETE" which is what the majority "DO" it's had too much water added. The installer will often add water on site so it's easier to push around. But the extra water lowers the PSI of the mix and takes longer to sit up. There's no point in making it harder than it needs to be. Knowing that water is so often added on site is why I recommended 5000 psi concrete.

As long as we are going to get hung up on proper terminology, instead of results, I have never seen concrete "sit up"

 

[mehig]

As long as we are going to get hung up on proper terminology, instead of results, I have never seen concrete "sit up"

I guess a person would be waiting a while....

 

[Gary Fowler]

Well at least till a major earthquake hit and then it may "sit" up or down depending on whether it is elevated slab or surface.

 

[smiley]

I haven't seen any mention of freezing temperatures. That's one of the biggest enemies of concrete there is. If the ground freezes unevenly and heaves, it can shadow through and break apart on the block lines but with fiber, rebar or wire mesh and 40 years of settling, not likely. I agree with Gary including the 3000psi. We use 5000# on bridges and structural applications.

 

[Gary Fowler]

I understand that the plastic will prevent moisture transfer but what exactly am I trying to prevent by using it?

Two things. First concrete is porous and will wick up moisture from the ground and deposit it in the form of "sweat" on the surface when the outside temperature changes from very cold to warm and moist. The concrete holds both heat and cold(lack of heat technically as there is no such thing as cold ) and takes a while to stabilize so warm moist air condenses on the surface and it sweats. A moisture barrier will keep some of this down. Second, it acts as a separator so the new concrete doesn't bond to the old concrete and allows the new slab to float.