I need advice on making a concrete pad

[Looking4new]

I want to make a concrete pad, 12' long by 8' wide. This pad is only for a patio.
We have bad weeds, strangler vine and the goobermint outlawed Round Up and other effective weed killers.
Can I have some advice on how to construct this? To the best of my knowledge the soil has not been disturbed since 1994. I do know it is mostly clay.

 

[shui5612]

What I would do... is dig down a foot... put down a geotextile.. backfill say 8-12 inches of 0-1/2" gravel and compact it ( it is better to go small layers at a time and compact, making sure the gravel is humid), once it is back filled, then dig the sides out abit so that the concrete is thicker on the edges and also makes a sort of "teeth" to hold the gravel cushion.
Then add rebar... being a patio I would go about 16 inch on center with 1/2inch rebar.. with the perimeter in said rebar too. I am not a big fan of the chicken mesh style of rebar...

And then I would pour a minimum of 6 inches of concrete on center. (The edges should be 8-10 inches thick in this scenario).

Most people will probably say it's overkill on the thickness and rebar, but I personally rather build it once, with a good and thick concrete than to build it too thin and having to redo it five or ten years later...

Using this method you should be fine from frost and weeds...

Hope this helps..

 

[Larry Caldwell]

Rebar is a good idea. I don't live in an area where frost heave is a problem, but I suspect it is a problem in North Ontario, so you need it to hang together as it rises and falls with the seasons. Of course you have to wait until freeze danger is past, or it will ruin the concrete.

 

[Billrog]

Hard to give sound advice not knowing the area( elevation) surrounding the patio location or moisture content of the clay. My approach would probably not dig it out and create a reservoir for water under the slab with gravel for water to accumulate. Just level it off pour a 4" slab the is reinforced with a 24" grid of 1/2" rebar.

 

[frugalangler]

Not knowing much at the time, I poured a 12X16 slab by just scraping the sod off (undisturbed area), forming it about 4" a little proud of the scraped surface, but about even with the top of the existing sod, and re-bar on 12" centers, and surprisingly it's lasted nearly 20 years with no cracking or settling, but the patio that was 'professionally installed' with excavation and gravel packed and re-bar'd is cracking and settling after only 5 years, I truly believe there is something to be said about not creating a reservoir for water to collect in under a slab.

 

[shui5612]

My approach would probably not dig it out and create a reservoir for water under the slab with gravel for water to accumulate. Just level it off pour a 4" slab the is reinforced with a 24" grid of 1/2" rebar.

While I mostly agree that we don't know the moisture content to give a better piece of advice, leveling off a piece of earth renders it uncompacted which is horrible for concrete because it puts it under stress when an air pocket occurs.

In the case that water is an issue it would be better to add a drain to my design, which isn't really needed because 0-1/2inch gravel when compacted is practically impervious to water.
Essentially anything bigger than 1/2-3/4inch Gravel becomes a substrate that allows water to flow through it or create a "bassin". While anything smaller acts like concrete..

The issue with building on existing terrain is that you don't know what is underneath it, and the bearing load of the soil which may change from location to location. Hence my recommendation to create a uniform platform which would have a "known" load bearing soil.

As far as thickness goes, 4 inches is usually sufficient for a patio but at the price of an extra half meter cube of concrete, for this project, I would pour off some extra concrete just for long term longevity. (An 8 by 12 platform at 4inches thick or 5 average would require 1cu. Meter of concrete while 6 inches on center with the sides thicker would require 1.5cu. Meter.

Lastly, I'm unsure whether OP plans on mixing concrete bags or getting a ready-mix truck to deliver the concrete; but I would recommend getting a truck on concrete since it will be a more even batch, which you could order in something like 27MPa. If the area is subjected to below freezing temperatures, then request 4% of entrained air. Which is essentially tiny microbubbles that allow for water to expand when it is freezing, this would prevent surface cracking.
It also makes more sense to order 2cu. Meter than 1.1-1.3cu.m when ordering a truck ...

(1cu.m is 1.3 cu.yards
2cu.m is around 2.6cu.yards of concrete, meaning if they pour 4inches they would need around 1.5cu.y ordered)

Hope this helps...

 

[zzvyb6]

A rocket scientist would not bother with a level concrete approach, but rather use leveling feet to fine tune the machinery once it's in place and for future adjustments. If it's that critical, buy a cast iron industrial bed plate and set it according to your specs.

 

[Industrial Toys]

I poured such pads with mesh and rebar around the perimeter and maybe criss crossed, with good results. Make sure the mesh and rebar are pulled up into the mix. I can't believe how many (supposedly) pros don't bother to do this! It's a lot of work, out of sight, out of mind. Better yet, place the steel on chairs.

 

[Looking4new]

I was thinking dig down about a foot and a half, 3/4" clear stone for 4", then 4" of either blue or pink styrofoam and drive the rebar through the styrofoam for the verticals and lay the horizontals on chairs. Top 2" of concrete has nothing in it. The styrofoam will stop the frost from going deeper. 1" of styrofoam is equal to 1' of dirt cover. Frost here goes a minimum of 4' deep.

I am thinking of the bagged concrete mix, mainly because of the added expense of a concrete truck coming from either Huntsville or from North Bay. I am almost exactly half way between the two towns.
That also gives me an excuse to buy a mixer for the tractor.

 

[4get gto]

That also gives me an excuse to buy a mixer for the tractor.

You would think our wives would be on to us by now. :thumbsup: