POUR NEW CONCRETE OVER OLD?

[npalen]

Needing some opinions on how to proceed with this project. Pixs show the 40 year old 9'x12' patio made up of 3' squares outlined with redwood. The squares were originally poured at two different depths to form a checkerboard pattern with loose decorative stone (since removed) placed in the squares at the lower depth. The redwood liners gradually rotted away and let moisture get under the squares resulting in very uneven settling. What I'm wanting to do is pour a new 9'x12' slab over the top to where it will be 3"-4" higher than the surrounding concrete.
My question is this: Can I do it without removing the old concrete but leveling with sand prior to pouring the new concrete? Has the weight of the settling concrete and the moisture combined to make a reasonably good base? The new slab will support a gazebo or pergola. The slab was originally poured over unpacked soil. Any and all suggestions are appreciated.

 

[Don87]

I'm not a 'concrete guru', but I would just clean the slabs, wet them good, then pour the new concrete over them(if you wet them first the new concrete will adhere to the slabs. I would not level with sand.

 

[newbury]

Go to the quickcrete site, they have a LOT of info.

I suspect it would be best to tamp sand down into the lower areas, get everything level, add some mesh with support to keep it an inch off the old surface then pour.

 

[Localmotion]

Go to the quickcrete site, they have a LOT of info.

I suspect it would be best to tamp sand down into the lower areas, get everything level, add some mesh with support to keep it an inch off the old surface then pour.

Exactly what I was about to say - especially the mesh. Also I'd suggest tie the mesh to the original base slab using rebar and maybe use fine quarry dust rather than sand - less likely to mix with the concrete when you pour it...

 

[mehig]

If you're slab is framed with 2x4's that will producea 3.5" slab, which is adequate thickness, provided you use 5000psi concrete for your pour. In my area, typically basement walls use 3500psi. 5000 psi will cost a little more but it's well worth it because it's less likely to crack. From looking at your pictures I'd say your good to go, just clean the loose debri away. It would not be a bad idea for the new slab to over hang the old work. Dig a trench/ditch around the inside perimeter of the framing so the concrete goes several inches below the grade of the existing soil to deter water from getting beneath and between the old work and the new work. Sometimes on a patio you may want pour the slab higher against the house to direct water to the outside edge of the patio. Use rebar(3/8"ok 1/2" overkill), bend the corners and wire up on 2 or 3 foot squares. Concrete wire is what is usually used for flat work like this but you would have to buy a whole roll at nearly $200 and use less than half the roll as opposed to $100 worth or rebar. Be sure to broom finish the surface for a non slip surface. Most importantly....use some sealing product within 8 hours for best results...follow the directions on the 5 gallon bucket. Use sealer yearly for optimum longevity

 

[SPIKER]

clean the old concrete good using some muratic acid and add some bonding agent after rinsing well. follow the direction on the bonding agents bottle cost less than 20 bucks for your site size. You can fill in the rotted away areas with sand and or fine gravel packed into the seems to bring it up to level.

Mark

 

[bigtiller]

It's a small area with 12 little square chunks of concrete. I would pick up one edge of the little squares, roll them out of the way and start all over with a new compacted base. Pouring new over old seldom gets the results we were looking for. The cracks and joints transfer up into the new in just a few seasons unless there is more than just a couple inches of sand between the two.

You could use the old squares to make a sidewalk, or sell them on Craigs List.

 

[Depmandog]

I agree with the last poster. Tear out and start fresh. Make sure you compact well prior to pouring.

Ever see a pot holed road, and the county goes cheap and patches the pot holes and then puts a thin overlay down. Looks nice for a while but It doesn't take long for those pot holes to reappear.

No bigger an area than you have...pull the pavers! If not I truly suspect you will be sorry in the long run.

 

[Rock knocker]

Agree.

The old will telegraph right thru the new.

 

[npalen]

OP here. I'm not against taking out the old concrete except I remember rebar being used in the original pour so it will take a bit more effort. May have to rent a jackhammer. I've been trying to convince myself that the 40 years of settling with moisture present would have compacted the underlying soil but probably need to rent a compactor also. Would one of these do the job?
Northern Industrial Single-Direction Plate Compactor with Honda Engine | Compaction Equipment| Northern Tool + Equipment

 

[whistlepig]

OP here. I'm not against taking out the old concrete except I remember rebar being used in the original pour so it will take a bit more effort. May have to rent a jackhammer. I've been trying to convince myself that the 40 years of settling with moisture present would have compacted the underlying soil but probably need to rent a compactor also. Would one of these do the job?
Northern Industrial Single-Direction Plate Compactor with Honda Engine | Compaction Equipment| Northern Tool + Equipment

What kind of base are you compacting under the concrete to be poured?

 

[Carl Bert]

<img src="http://www.tractorbynet.com/forums/attachment.php?attachmentid=299915"/><img src="http://www.tractorbynet.com/forums/attachment.php?attachmentid=299916"/>

Needing some opinions on how to proceed with this project. Pixs show the 40 year old 9'x12' patio made up of 3' squares outlined with redwood. The squares were originally poured at two different depths to form a checkerboard pattern with loose decorative stone (since removed) placed in the squares at the lower depth. The redwood liners gradually rotted away and let moisture get under the squares resulting in very uneven settling. What I'm wanting to do is pour a new 9'x12' slab over the top to where it will be 3"-4" higher than the surrounding concrete.
My question is this: Can I do it without removing the old concrete but leveling with sand prior to pouring the new concrete? Has the weight of the settling concrete and the moisture combined to make a reasonably good base? The new slab will support a gazebo or pergola. The slab was originally poured over unpacked soil. Any and all suggestions are appreciated.

Sorry to disagree with some of the other posts, but if the original squares are set in unpacked soil, as you put it, you need to pick up the old, dig down enough to put 6" of gravel and as long as its not being used as a driveway, you can use 4" of concrete. You can either use two layers of rollout wire or just order the concrete with fiber strands. Personally I use both, but some claim that's overkill. As far as what concrete to order, 5000 psi is a better choice for supporting more weight, but it actually is more prone to cracking than 3500 psi.

So if its a walkway, go with 4" of 3500 psi concrete over 6" of compact gravel. If its a driveway, then I would go with 6" of 5000 psi concrete over 6" of compact gravel. I would use double wire and fiber strands in either

Just to mention, whatever you decide, you NEED to compact the gravel ( preferably with a gas powered compactor ). Reason being, once the concrete dries and the loose soil and gravel settle, any voids under the concrete is where the concrete is more likely to crack. Hope this helps.

 

[npalen]

What kind of base are you compacting under the concrete to be poured?

Around this part of the country, I've never seen anything other than river sand used under concrete. When asking about compacting, I was referring to the soil which I believe is a type of loam and it is the "native" soil, not fill.

 

[the old grind]

I would think if re-bar spanned any of the old joints you could grind or torch 'em thru' where wood was cleared out, but that puts me in the 'pull 'em' camp so far, huh? If your soil drains well a compacted gravel base would raise you up and allow sloping your forms to direct runoff that wind blows under the gazebo. (1" in 10' ought to be close enough & go diagonal if that's best.) Water could collect in a porous base. Here in MI it'd freeze with unpleasant possibilities. Ground freezing might not be a regular concern in your part of KS. If you have hardpan that puddles your yard when it rains I'd stay with 'native' soil, adding as needed to raise/grade for the forms. No reason I know of that a layer of vis-queen wouldn't be enough over that to minimize contamination in such a scenario. It's code here even over sand/gravel.

If you choose to leave the blocks, could you drop in a few rebars supported midway up in the grooves wherever they'd drop in easily? (... if they'd span few blocks, and old ones weren't cut or overly difficult to go around....) Suppose you bag-creted them in place during a pre-leveling step. IMO, whether the block pattern might transfer to cracking above isn't predictable. Be sure to etch and rinse gently for adhesion of the pour, with no brooming/brushing that might loosen & leave dust or excess grainy stuff that sets you back. Pre-wet, but don't flood when you're ready to pour.

Once your slab has firmed up (hour or two?), try keep it cool & wet, .. for days if convenient. Slower drying leads to a stronger set than letting it sun/air dry too quickly. btw: due to the thickness at the bottom (you might be surprised) and that it only can evaporate thru' the compact downriver side, tour-guides say that concrete deep under the Hoover Dam is still hardening, and that it's a good thing. Anyway, I'd start over. Seems it'd mean a bit more time/effort invested, but not so much the total co$t. JMHO

 

[mjncad]

Sure you can "cap" the old concrete with new; but I wouldn't for the reasons already mentioned. I'd use #3 rebar 24" on-center each way instead of mesh as it's too easy to trample the mesh to the bottom of the slab during the pour, which will make the mesh useless.

The plate compactor or a "jumping jack" compactor should work. In any case, compact the crap out of the native soil, especially if you disturb it, and then compact any rock that you may place as a sub-base course.

3,500 PSI concrete should be just fine for a nominal 4" patio slab.

 

[Sackett2]

because there is rebar under it AND it has settled for many years; I would just cap it. Many properly laid slaps around here crack anyway (my father-in-laws quote was "Two things you can guarantee with cement is that it will harden and it will crack")

 

[Egon]

Spend money and do it right? Or, cover the existing slab with one inch of sand, lay down poly and form up for a full four inches of concrete. Ensure the perimeter edge concrete goes down to the old but separated with poly. Lay in some rebar around the perimeter. If you cut the new pad mke sure the cuts are above the joints in the old concrete.

An alternative is to use sand over the old pad and use concrete pavers.:thumbsup:

? Consider stain if you pour concrete?

 

[flusher]

OP here. I'm not against taking out the old concrete except I remember rebar being used in the original pour so it will take a bit more effort. May have to rent a jackhammer. I've been trying to convince myself that the 40 years of settling with moisture present would have compacted the underlying soil but probably need to rent a compactor also. Would one of these do the job?
Northern Industrial Single-Direction Plate Compactor with Honda Engine | Compaction Equipment| Northern Tool + Equipment

Jackhammer? I'd use a concrete saw if you decide to start from scratch.

Good luck.

 

[Gary Fowler]

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.

 

[brin]

OP here. I'm not against taking out the old concrete except I remember rebar being used in the original pour so it will take a bit more effort. May have to rent a jackhammer. I've been trying to convince myself that the 40 years of settling with moisture present would have compacted the underlying soil but probably need to rent a compactor also. Would one of these do the job?
Northern Industrial Single-Direction Plate Compactor with Honda Engine | Compaction Equipment| Northern Tool + Equipment

I have to agree with those who have said to tamp down the existing squares , fill in the rotted spacers and pour your new concrete...I agree with your thought...why in the world would you want to tear up a 40 yr. old base ? I have done exactly what you are doing and built part of our old house on it...it worked fine for 30 yrs. and it still working fine for the new owner....I don't think you need any compactor either...not after 40 yrs....

I just suggest you don't try to over think this project....I have done exactly what you are contemplating more than a couple of times and it has worked fine with great results....:thumbsup::2cents: Unless I'm wrong.....but I'm not...LOL