Casting Large Concrete Pavers

[naveenreddyla]

I am about to start a project where I plan to cast a number of large concrete pavers in a mold for a patio and a walkway. The pavers will be 2" thick and vary from 2'x4' to 2x8'. Each paver will have embedded 3/8" rebar and remesh for reinforcement. I also plan to use some synthethic fibers mixed into the concrete as well to prevent cracking. I am debating if water reducer would be needed.

I am using the bagged Quikrete you can get at the Home Depot. My issue is to whether to use the regular or the high early concrete. Faster cure time would help me get the project completed faster. However, I am worried that the high early, since it cures faster might be weaker and more susceptible to cracking. My biggest concern is that cosmetically the pavers look good for as long as possible.

Does anyone have any experience with this process, or have any suggestions that would help produce a casting with maximum longevity?

Thanks,

Naveen

[Joseph Meehan]

[e-mail] wrote: The pavers will

I would suggest that a 2'x8' paver only 2" thick is not likely to last long without cracking unless you have an outstanding foundation and very light traffic.

[William.Deans]

Greetings,

If I was going to make something 2'x8' and only 2" thick that I wanted cured quickly I would look at making it like ferro cement and NOT like poured concrete. Ferro cement is commonly used to make boats and water tanks which have strong thin walls. The process basically involves tacking rebar together and then coating it with as many layers (5 or more on each side offset from each other) of chicken wire (attached to the rebar with hog rings) as you can manage to squeeze high cement mortor through (1 part cement 2 parts fine sharp sand). You can then steam cure the slab for 24 hours and it should be ready to put into immediate service.

Hope this helps, William

[Jim Yanik]

"Joseph Meehan" wrote in news:464e3a99$0$16717$ [e-mail]:

I would not refer to a 2'x4' or larger concrete slab as a "paver". He might as well just cast his walkway in place. Just moving such a piece of concrete,rebar or none,is liable to crack it.

[DanG]

2'x8'x2" will weigh about 400 pounds and will require about 2.66 cubic feet of concrete which will require 5 80 pound sacks of premixed concrete (make sure you are not pricing or considering the 40# or 60# bags).

Anything made of concrete that is long and thin will probably crack.

Rebar should have 1 1/2" of cover. You will have less than 3/4".

Regular sacked concrete is about 2500 # concrete, the minimum allowed by code for foundations. Prestress concrete runs up towards 6000#. Most commercial concrete is 3500 with air entrainment.

There are special processes to create what you are after, but they require exotic concrete blends, special procedures, and technical curing. You can probably produce them if you do a bit more research on what will be required.

[naveenreddyla]

Thanks for the responses so far. I would not have thought of idea that I am proposing, other than I have seen this type of work done in a number of homes here in Los Angeles - albeit expensive ones I have seen on architectural tours. I could not afford the cost of having some one else fabricate custom concrete forms and then have them installed. The only reason I am attempting something like this is that is looks fantastic when the paver/slabs are in place and some sort of ornamental grass such as dymondia is alowed to grow in between the spaces.

Basically the site on which I will be installing these paver/slabs is very hard clay with little give. I expect there to be normal to light foot traffic. The idea with the 2" thickness was to limit the weight so that I could physically install the slab. Additionally, with the paver/slabs that I have seen, they all appeared to be roughly 2" in thickness. I actually cast one that was 2' x4' and was able, without too much trouble to pick it up and move it into place.

I was hoping to solve the potential cracking issue by either adding a little thickness to the paver/slab, using a littl more rebar, fiber or simply curing the concrete wet for 30 days.

Naveen

[William.Deans]

Greetings,

Have you considered pouring a flat concrete slab to go under your ornate concrete? This might sound crazy but when it cracks you might wish you had.

Hope this helps, William

[Don Young]

I don't think 2" is nearly thick enough for pavers/slabs that large. I would make a good foundation and pour 4" sections in forms where I wanted them. Neither rebar, fiber, or long curing will prevent cracking of a section like you propose, even if you do not move it. Be aware that fiber will probably prevent a smooth surface and rebar would be very difficult to keep within such a thin section over 4 or 8 feet. The rebar would also be likely to rust and cause the concrete to split because of inadequate concrete coverage.

Overall, my personal opinion is that this is not a good idea at all.

Don Young

[** Frank **]

Actually I've seen a concrete slab something like 2'x6'x2" supported at both ends by cinder blocks with three guys jumping on it in the middle. It actually bend about 4" in the middle and spring back to original shape when those guys got off - no cracks and seems to be indestructible. Flexible concrete! Don't remember the composition but I believe it had lots of fiber mixed with the concrete.

Like someone said your small one is about 200 lbs (2.7 x 60 lb bags plus rebars plus water) and the large one about 400 lbs (5.3 x 60 lb bags plus rebars plus water) and will crack without some very special treatment. Also unlikely you could lift these slabs by yourself like pavers all day long without screwing your back.

I'm doing pavers myself but its more like 1' x 2' x 3". I'm using a concrete vibrator, no rebars.

[Ivan Vegvary]

Agreed!

Ivan Vegvary

[Ivan Vegvary]

"very hard clay with little give"

Clay is one of the worst sub-bases available. Very expansive when wet. Gauranteed to heave and crack your pavers.

Ivan Vegvary

[Norminn]

I know next to nothing about concrete, other than a few tips learned when speaking with contractors when my hubby was building manager for our condo. If I was pondering a project such as yours, I would use smaller pavers and no rebar. I'm thinking that the thickness of the concrete over the rebar will be only about 7/8" on either side. Seems likely you would end up with a lot of burst concrete from the rebar rusting and no surface strength along the rebar. I like pavers with grass as you describe, but I would be inclined to use smaller pavers and forget the rebar.

[RicodJour]

Sorry to burst your bubble, but with a response like that you obviously know far more about concrete than you give yourself credit for.

R

[naveenreddyla]

Pouring a concrete foundation is out, since my site conditions won't really allow for it. I can lay a subbase of sand or pea gravel if that would hopeflly preventing the clay from heaving.

Given the responses I am willing to pour thicker paver slabs in place, given that most who have responded think that this would prevent cracking. What is the mimimum thickness - 3", 4"? If I pour to this thickness, can I avoid intalling a subbase, since that would entail a great deal more work?

With regard to the thickness of the concrete being 2" liable to break from the movement and rusting of the rebar, how is this problem avoided with concrete countertops, given that these are generally cast pretty thin and poured in long sections with rebar and remesh from what I have seen? Obviously countertops are subject to less tensile pressure than something being stepped on, but are subject to moisture.

One of the other options I ran across was a company here in CA called Rapid Set

They make high performance concrete which is great deal more expensive than the bagged Quickrete. It costs about $12 a 60lb bag. I weirdly enough ran into a rep yesterday in the Home Depot who said that if used properly, the stuff is guaranteed not to crack since it is primarly used for roads and other high performance situations. It sets in 15 minutes, but apparently this time can be extended with a set control substance you can by from them. I am going to call them on Monday and see how this could be used, and what adjustments would have be made for my specific situation. Just curious, does anyone have experience with a product such as this?

Naveen

[DanG]

Concrete does not like to be long and thin no matter how thick it is. As a general rule the length should never exceed 1.5 of the width. You will notice joints tooled or cut into exterior concrete roads, sidewalks, driveways, etc. If the concrete is cracked, try to visualize why. You are bucking an uphill battle other than this next bit.

I was quite surprised by an article in one of my magazines. Here is quite an interesting article from the March issue of Concrete Construction magazine:

If I had read it elsewhere I would have been a doubting Thomas. It may well give you some of the information you seek.

Concrete countertops are interior and sealed, hopefully the reinforcement won't start to rust. It is the expansion of the rust that destroys the concrete.

There are many different acrylic modifiers to add to the concrete to increase flexural strength. If you have little or no experience with concrete I would be concerned with your ability to keep up with the Rapid Set product.

[naveenreddyla]

sidewalks, driveways, etc. If theconcreteis

ofConcreteConstruction magazine:

That article is a very interesting read. Not only for they were able to achieve, but in understanding the underlying forces that cause concrete to crack. I was not aware of the curling aspect.

Just to show that I am not crazy, you can see some pictures of the paver/slabs I am talking about at the this link

Any thoughts on how they are making this work? Or is this just wasted money. Mind you I have seen this done on a number of homes.

Naveen

[Norminn]

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Countertops would never be exposed to the amount of water that outdoor applications are. Rusting rebar is a fairly common concern in Florida, with all the balconies on condos....We have had a number of spots on our condo where rebar had rusted and cracked concrete. This occurred on balconies and second-floor deck in open atrium. Salt is an added issue here, but concrete/stucco has to be painted and maintained to slow or stop the problem. Condo across the street had 13 stories of balconies jack-hammered, repaired and painted a couple of years ago.

I would also be concerned with the 8' length pavers cracking from expansion and contraction. We have sidewalks with extremely long sections and all the sections have cracked, roughly in the center.

With your clay soil and the possibility it can heave or shrink from changes in water content, it seems that consistency would be the major issue. If it rains, it will be evenly wet and expand fairly evenly, I would think. Another reason smaller pavers would make a difference...easier to pull up and re-level if the soil moves.

[sylvan butler]

I've moved 2'x4' several times, and even thinner pieces than the 2" the O.P. is planning to cast.

That said, the 2'x8' will be considerably more interesting... But I think doable.

However, 2" thick is adequate for a walkway on a good base, but I'm skeptical of its use for heavier loads.

sdb

[sylvan butler]

It sounds like you are in way over your head. You need a LOT more study.

Very poor base. Proper base will be done by excavating and putting in a few inches of compacted 3/4" road base, perhaps topped by compacted and leveled sand. Finally layer an inch or so of moist sand screeded to height. Then lay and bed the pavers in that base.

Can you guarantee that? Or is this adjoining or near a driveway where a vehicle might drive onto it? If really only foot traffic then a base of

2" compacted 3/4" road base will probably be enough. (Still need the inch or so of moist sand screeded, into which to bed the pavers.)

Buy pavers. They use lightweight aggregate, air entrainment, etc. to reduce the weight and cost and increase the strength.

Fiber or ferrocement would probably work, maybe remesh (10ga or so) but even 3/8" rebar in a 2" slab is iffy.

Don't forget some retaining method at the edge. Steel is nice, set down about grade (ground level) to about 1/2" above grade.

sdb

[naveenreddyla]

The time of being in over my head is long since past. Two retaining walls, french drains, and 80 cu yard of excavation and removal of dirt and rubbish and I am still going.

I can guarantee that there will be light foot traffic, since this is not a car accessible location. The only other weight would be normal patio items.

As far as the compacted gravel sub base with sand. This was the original plan, until we decided to plant the dymondia or some other ornamental grass between the paver slabs. I figured having a dirt base underneath would be easier to plant with. I was also hoping that the strength of the concrete would be able to overcome any shifting of the earth. I suppose I can some scoop the some of the sand out from the perimeter of each paver/slab once they are put in place, and replace with soil.

Does anyone have a recommended minimum thickness that the paver/slab should be. If I increase to 3.5"-4" thickness will this work with the

3/8" rebar, mesh and fiber? I know this will be quite heavy, but I can reduce the size of the paver/slab to be roughly 2' x 4', or maybe slightly smaller so that I can get them in place.

I am looking to price out how much casting these paver/slabs would be. Though the the rapid set strong concrete sounds like an interesting option, the cost becomes prohibitve quite quickly, and making the effort not worth the time. My initial hunch is that pprecast pave/slabs will be expensive, which is why I decided to undertake this project in the first place.