New Patio Concrete Slab - Slope Question

[cr1973]

Hello all,
I am going to help my brother pour a new 11x33 ft concrete patio. I am curious about slopes for drainage - what % is ideal? 1%?

Thanks

 

[KRSServices]

CR,

I would strive for 1%. The reson is that it will be very difficult to achieve a uniform grade with anything under that slope. This way even if it flattens to .7% or .5%, it will still run. If you start at a flatter grade, it is inevitable that the little "bird baths" will develop and be a nuisance. I just helped my father-in-law build a new patio slab, and 1% was the grade we used.

Also, try to string the perimeters as much as possible for your finishing reference during the pour. Using this to set the form height is invaluable!

KRS Services

http://www.krs-services.com

 

[cr1973]

Thanks for the input. I figured 1% would be a minimum for workign with, especially with our limited/0 experience in actual pouring of concrete.

Should I cure this thing? THere won't be any heavy loads on it. Maybe I should just occassionally spray it down with water the first few days it cures?

 

[Taro]

Keep the surface moist for the first few days. If you let it dry out between spraying, it could cause cracking or dusting. Covering the patio can help prevent rapid moisture loss.

 

[jimbo]

Engineers commonly use 2% for a roadway cross-slope.

It is commonly insufficient, due to settlement.

More than 1% is desirable for a patio. 2% is not too much, if the slope is available. Most people use a slope of 6" in 10' away from a building, and then switch to a lesser slope.

After 37 years in the same house, I wish I had more slope away from the house. (Too late now to change)

 

[cr1973]

Well we are meeting up against a wooden deck which is about 1/3 of his total patio. We are just replacing the existing concrete, and meeting up at the deck. I don't want to have TOO big of a step between the concrete and wooden deck area. I'll have to take some shots and see what works.

 

[jheidt2543]

By way of reference, a handicapped ramp is sloped at 1:12 which is 8.33%. The suggested 6" in 10' is a 5.0%, which I think is a bit steep around the house IMHO. The 1% - 2% should be fine.

Yes, you definitely want to cure the slab. After finishing the slab, wet cure it for 3-7 days by covering it with a 6 mil. clear poly. I would also spray water over the poly. to help keep the temp. down during the summer heat. The clear poly can sometimes leave "fold marks" on the concrete, which should wear off in a few weeks/months. After the 3-7 days use a high solids, non-yellowing cure & seal compound. Firms like Sonoborn, R.W. Meadows, and Master Builders all have good ones sold in gallon containers. Follow their directions!

 

[cr1973]

Ok now what about reinforcement? Some people said I should stick wire mesh in there, some people said to use the fiber concrete, while others said that a simple patio won't need any reinforcement.

 

[boo1]

Tyipcaly we use 1/8" to 1/4" slope per foot.
The wire reinforcement does not eliminate cracks – it simply holds them together. (probem is it usualy is on the bottom)
The fiber will help as micro cracks begin to appear due to shrinkage as water evaporates form the concrete (plastic shrinkage), the cracks intersect with the fibers which block their growth and provide higher tensile strength capacity at this crucial time.

 

[cr1973]

Soooo we are better off using the fiber concrete?
I know subgrade preparation is very important. Could we just tamp the heck out of it with shovels or should we actually rent push-compactor/vibrator?

 

[chicopee]

1% will be fine, but dont forget to put in wire mesh.

 

[dicksewerrat]

You should pay as much attention to the sub base as the slab it self. Rent a compactor. Have you thought about making this in three seperate slabs? or at least control joints?

 

[cr1973]

Sewer-rat,

We though about doing it in 2 pours on different days, but then we'd get hit for a minimum load charge 2 times, so we are going to do it all in one pour.

Definitely will joint the thing, and will probably rent a compactor aswell.

 

[cvg]

reinforcement - wire mesh or fibers is not normally used in residential patio construction. If you do use wire, make sure it is supported otherwise it ends up on the bottom as stated previously and will then be rendered useless. Consolidation of the concrete in and around the reinforcement will be a little more difficult. Make sure you don't get any voids. You can use the fibers, but probably a waste of money. Put the money and effort into the subgrade and into the concrete finishing and curing. Also, if you are worried about cracking or strength, more cement in the mix will also help. Proper leveling and compaction of the subgrade is key and a compactor is recommended.

 

[Taro]

cvg,
Good advice, mostly. I've got to disagree with your statement that "if you are worried about cracking or strength, more cement in the mix will also help". Actually, more cement in the mix typically requires more water to hydrate it and leads to increased shrinkage cracking. I would recommend using the lowest water/cement ratio along with the lowest strength concrete that will do the job. ACI 318 Table 4.2.2 gives maximum w/c ratio and minimum strength requirements for various exposure conditions.

 

[cvg]

I agree, "the lowest strength ... that will do the job". I would not suggest increasing the strength, just using a suitable mix. Often when "do it your selfers" order a concrete truck for a patio or whatever, they don't pay attention to concrete strength and the plant may send an inferior mix, knowing that it won't be questioned. I would recommend a 3,000 psi at 28 day mix would probably be adequate for most patio slabs. If you are supporting a roof, make sure to look at the roof load and design your slab to support it.

 

[boo1]

I use the 3000 psi mix with fiber. To minimize cracking don't add water to the mix on site. After mix has set up, wet down and cover the surface with polly to wet cure for min 3 days (ideal 7 days).

 

[Puckman]

What do you recommend as a sub base for the concrete patio? Is it recommended to pour the concrete directly on dirt?