Concrete floor on slope

Slope -- how much -- I mean what angle to horizontal?

If it is less than 30° do not even worry-- will cling to existing compact earth--"-Angle of Repose due to friction"

If it is say 30>45° --- Anchor the concrete to soil below.How? One way will be to drive twisted steel scrap rods Dia 16mm>20 mm into soil below say 60 cm down at 60x60 cm grids-- with eyeletted top ends jutting out 5 cm to meet concrete.

Thus readied ---pour the 8> 10 cm concrete -pretty stiff --such that it does not flow down slope. No reinforcing bars needed here(assuming well compacted original soil).

If slope exceeds 45°---plant flowers intead of concrete.

Thanks!!!

Can you post a sectional sketch ? If you intend to have concrete lying on a slope of compacted soil , it will move without retaining ground anchors etc. Your intent is not clear to me , but there are a lot of factors to consider (ground type , compaction , water table ....etc ). An unstable raft of concrete could be a serious danger even if it is just a patio. Angle of friction will tell you how a pile of material will slope naturally , but once you begin loading it , everything changes.

Here in California we would approach this as a slab on top of a retained slope. In other words, a retaining wall would be built. The footing/retaining wall could be integrated with the slab pour. If the slope were moderate a compacted soil bench could be built then the slab poured on top of that. You would need 98% compaction. xdesign@pacbell.net

The easiest way to keep a slab from sliding down a slope is to cut the slope down and make it flat under the slab. Then dig footings two foot square in all four corners down to below any frost line you might have. Once the footings are in, you can clean up and relevel your base and pour your slab. The big thing is to have your base level and compacted. The best base for under concrete is sand. Rake it out smooth, then hose it down with a garden hose till water stands in your forms.Then as the water seeps away you will see where you need more sand.Look at the beaches where tidal waters compact the sand hard enough to drive on. The base being solid is a must if you don't want any cracks in your finished patio a year or so down the line.

400mil wide post holes 900mil deep along top and bottom edge with steel ties tied to you reo in the slab and pour all together as one.

alf arnold Be

Having some basic knowledge regarding construction, I wanted to add my .02-cents. Since the slap will be outside, your biggest consideration is climate and rainfall eroding the soil away from under the slab. This condition makes the requirement for footers (mentioned above) all important.

On flat soil, the soil is generally dug out from 6 to 10 inches or so below grade, depending on soil conditions and frost line. For a patio, the soil condition is more critical than the frost line. Anyway, once an area has been removed it is filled in with large gravel, then smaller gravel, then finally the concrete. The gravel allows water to flow between the pad and the soil minimizing the affect of erosion and reduces the effects of the water expanding as it freezes in the winter. Note that even on basic low-load pads, such as side walks, the use of large and small gravel. Additionally, since the pad (even on flat soil) is dug into the soil, the surrounding soil helps contain it and retain it's original position.

On sloped soil, the recessed nature of the pad into the surrounding soil is still important but not feasible sometimes due to the amount of slop or the soil conditions or even how much the surrounding slope will redirect water runoff under the slab. Although sand is good for a few applications, it is not the best for sloped applications. I've seen too many sand foundations wash away over the course of a few years, especially if the ground has a lot of clay which limits saturation and promotes run off. If the slope it too steep the sand will be eroded away due to sands inherent nature of not being able to be compacted enough... it's too porous, and ultimately seeps into the original soil. Also, due to its nature, the amount of its mass compared to the amount of the air pockets around it is too balanced thus providing limited resistance to expansion when it is saturated and then freezes.

Based on the limited information supplied regarding your situation, I would suggest various sized gravel and footers. But how large and deep they are will be dependant on the slope, the size of the pad, the loading on the pad, soil conditions and average climate conditions of your area.

If the slope is such that you are worried about the pad sliding in the least, and your intended propose for the slap is as a patio, presumably recreational activities for people, I would spend the little extra for a professional to come take a look at it. In the least, contact a local technical college or university that has a construction, survey, or architectural design program and see if you could have a student look at it as an assignment which would ultimately be approved by the professor whom hopefully was once certified in the field. Also, your local building codes are probably going to have strict requirements for any structure that is built on a slope and is occupied by people.

If the slope is over a certain percentage, then most likely a retaining wall (mentioned above) will be required.

In summary, the fact that people will be on it and you think there is enough potential for sliding that prompted you to ask the question, would be enough reason for me to go the extra step and get the footers professionally designed and installed. Once the footers are installed, if you still want to pour the main pad yourself, you can try it. However, a professional installation regarding this aspect will also minimize the chance of cracking since they will know what type of admixtures are needed on the day of the pour to insure proper workability and curing rates. Not to mention they will give you a nice finish, and insure they don't trowel the surface too much, ultimately settling the aggregate lower which will result in the top layer chipping and pitting over time.

Also, be cautious of anyone that is providing you dimensions and firm procedures on this thread without knowing all of your details. Since you posted as an anonymous guest, we don't even know what part of the world you live in, what your actual slope is, how big you want your pad, your soil/climate conditions, etc. Not that they are not knowledgeable or don't know what they're talking about, but construction is dynamic and what was perfect for them may not work for you.

All the above is true.

A couple of additional points that may help:

Make sure that the concrete is not too wet. It should be a little drier than normal.

Also putting a metal mesh (typically 3" or 4" square holes) or rebar in the middle of the concrete form will stabalize the concrete and add strength

Yes... thank you Techno for your mention of metal reinforcement. Sometimes the obvious goes without mention so I didn't focus on that aspect.

The footers will contain steel rebar and the pad will contain a metal grid. If you're pouring the pad yourself, set the metal grate over the final level of gravel. As you pour, lift the grate up into the concrete about halfway.

Though the rebar and grating will not necessarily prevent cracking (proper concrete consistency, admixtures, and curing will prevent that) they will prevent separation once cracks do form. A hairline crack, held tightly together via metal reinforcement, still maintains a high level of structural rigidity. Whereas a crack that separates pretty much provides none.

Also, just to clarify a previous point regarding excessive trowling (not sure of proper tense usage or spelling.) If you pour your own concrete, the process of trowling the surface to get it smooth, level, and apply an aesthetic finish can be over done. As you move the trowel back and forth you draw water and Portland Cement to the surface and likewise settle the aggregate to the bottom. If you've ever noticed a concrete surface that is chipping, it is due to over trowling creating pools of diluted Portland Cement at the surface.

To the original poster, some before and after pictures would be a nice add to this thread.

Right here we go, the site for the slab must be dug out and the back of the slope where there is the maximum depth put a concrete retaining wall to a depth of two feet greater than the soil surface. this should be allowed to fully set before any further work. It should extend to 12" 300mm above the top of the original soil level at a minimum. Three feet is ideal, it should be at least 9" to 12" thick re-enforced with 3/4" re bar. Now compact the soil and add 4" to 6" of hard core that is old concrete or brick rubble NO sand. maybe course gravel but not too much. The slab should be from 6" to 12" deep depending on how big it is, you should add wooden cross pieces every 8 to 10 feet to allow for movement, you can of course add steel re-enforcement as well. The bigger the slab the thicker and the more re-enforcement. If you are sloping the slab its self then pour in footings top and bottom but put in drainage tubes to let any water make its way under with out washing away the soil from under the slab. Pass the porous pipes all the way from front to back. I guess I must have left something out but it is not rather late and I am sure somebody will fill in for me.