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.

 

[Egon]

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.

If the OP had frost heaving it would have showed up. If there is frost heaving the concrete is gonna lift and it will break regardless of rebar/wire/fibreglass.

 

[Don87]

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.

Maybe the OP won't ask so I will.

Doesn't one want the new to bond to the old in this application?

 

[SPIKER]

Follow Gary Fowlers post on PG2 and mine on PG1 for a patio. fiber mesh is good & if you wanted some of the wire but not required.

clean out any of the left over wood and pack the joints with compact-able sand/or decomposed granite AFTER cleaning & etching the OLD concrete. be sure to a day or so prior treat the concrete with some bonding agent to tie the OLD to the NEW concrete.

Mark

 

[Rock knocker]

My resume is not unlike Gary Fowlers, and I would say the last thing you want to do is bond the old to the new. The old, now rotted and gone, cedar screed strips will act like a perfect stress risor and could easily lead to cracks at every location.

In my opinion, it would be better to coat the entire existing surface with plastic and 1/2" sand as a decoloment surface, and pour 3" vs 3.5" of new concrete. The last thing you want is a bond between new and old.

 

[Carl Bert]

I have 30 years of experience as a Quality Manager on heavy construction projects. I have worked with engineers and inspected placement of concrete for many years on construction sites for foundations for buildings and equipment, slabs for sidewalks etc. I can tell you that if you are putting in a 3 3/4" slab using 2x4 forms, over the top of the existing patio block, you wont have any problems with cracking due to the blocks underneath. Your wont get a more compacted foundation for your slab than what you have now. I disagree completely with the folks wanting to rip it out and disturb the soil under a concrete patio that has settled for years. It will be well compacted and much better than any sand or rock that you can compact. Only if it is not well drained would I think of disturbing the existing work. YOU do need to put in a full 3+ inches of concrete and since you need minimum of 2" of concrete over any embedded steel, you cant put more than one layer of anything in the concrete with less than 4" thickness. If you can get it, I would get the concrete with the nylon fibers in the mix and just leave all the wire, rebar etc out of the concrete. It works better than any other concrete to prevent cracking and it adds strength to the concrete by allowing it to expand and contract with temperature changes much better than with steel reinforcement. It is a patio and as such not subject to heavy loading so 3000# concrete is all you need. The more cement you put in the mix to gain higher compressive strength, the hotter it is going to get when curing and more likely to form cracks.
The only problem with nylon fiber is the whiskers that come up on the surface when power trowelling it and burn off leaving little burned spots on the surface. You probably want to just broom finish it if outside as a power trowel finish will be slick to walk on so the little whiskers wont be an issue.

I appreciate what you're saying, but how do we know that the existing concrete slabs are not being heaved up each winter due to frost and settling back down. I'm on an iPad, so I can't see where the OP is from, but if frost is a concern, why would you leave the existing concrete slabs down, which are sitting on compacted soil with no drainage?

 

[npalen]

OP here. Yes, we definitely have frost heave here in Central Kansas. I believe the frost line is generally considered to be 30" below grade.

 

[mjaybee]

I've been in the concrete industry for 25 years. Just poured a 12 yd. patio off the back of my house and a 7 yd. off the front. Some of it was over existing concrete. I like rebar (it's a CA thing) to hold the concrete together so when it cracks (and it will) it won't lift or separate (hey, wasn't that a women's bra? but I digress). I didn't use any plastic sheeting as you don't need a moisture barrier outside, and put an inch of sand over the existing concrete so cracks wouldn't come threw from the old. I've never seen plasticmesh for concrete but have used fibermesh (fiberglass strands) many times. Have seen , but never used steelmesh (small metal wire strands, nasty stuff) used by military for tank pads. Mesh is used sometimes in place of #9 wire mesh or rebar. The wood is only thing that would bother me as it will continue to rot and cause voids. I would probably try to take it out, if possible. As for the frost, I got nuthing, Ca and all. Good luck.

 

[smiley]

If the OP had frost heaving it would have showed up. If there is frost heaving the concrete is gonna lift and it will break regardless of rebar/wire/fibreglass.

The reinforcement spreads the stress over a wider area and can crack but except in extreme cases won't break apart and generally settles back in after frost is gone.

 

[npalen]

Guys, I really appreciate all of your comments and I understand that there will be a diversity of opinions on a subject like this.
I realize that ultimately I will have to make the decision on how to proceed but, again, thanks.

 

[Rock knocker]

Your problem is that all the other opinions expressed here are in one way or anther, correct