Subbase for Concrete Patio

[ndbwhunter]

I'm planning on doing a stamped patio in my backyard, and can't figure out the right material to use for the subbase. The existing soil is a clay/sand mix and will provide an excellent base for the concrete, but I'll need to bring in some fill to get a few areas up closer to grade. Once the fill is compacted, I'd had planned on using 6-10" of class 5 or gravel to make a good, solid subbase on which to pour the concrete. Do I have the right idea here, or am I way off base?

Will a mixture of course rock (1/2-1"), sand, and clay be suitable for a subbase? Is this essentially what "class 5" is? I have access to a gravel pit near my house and will save a ton of money if I'm able to go that route. If I'm able to use this mixture for the subbase, what type of compactor should I use? My research is telling me that a vibrating plate compactor will be the best for this gravel mixture. Can anyone with experience tell me otherwise?

Keep in mind, my concrete experience is limited to quikrete and google searches, so any information will be greatly appreciated. A friend of mine will be doing the pouring and stamping, but I want to get most of the prep work done before he comes to town.

 

[Big Iron]

You must be landing airplanes on that patio! I'd recommend 4" of compacted gravel at a minimum. Pitrun sand will also work beautifully for fill material, but it's always nice to have uniformity with your subgrade (recommend getting similar fill material to existing). Plate tamps/ jumping jacks will work fine for compacting if you are using gravel. Make sure you add some moisture when packing in the material, and before you place the concrete.

Most concrete outfits will fine tune the grade before they set forms. Make sure your buddy cuts that concrete the day after placing to prevent cracking in the wrong locations! Good Luck

 

[ndbwhunter]

I spoke with a few contractors that are more familiar with the gravel pits around here. They indicated that the clay content is usually too high to use as a suitable base. One recommended using crushed concrete in place of class 5. I was really hoping to avoid any additional costs, but it needs to be done right. I just wish I knew if the material that I have access to would work.

 

[Kurtr]

If it passes for dot class 5 it is fine but most just call for engineered fill. Which is just screened 1 inch minus with less than 10% retained on the 200 sieve. But it's your money and if you want to spend it on high dollar aggregate under your concrete go for it.

 

[ndbwhunter]

If it passes for dot class 5 it is fine but most just call for engineered fill. Which is just screened 1 inch minus with less than 10% retained on the 200 sieve. But it's your money and if you want to spend it on high dollar aggregate under your concrete go for it.

I just want to make sure it's done right. I'm not sure what "right" is, which is why I'm asking for your opinions. The issue that I'm having now is that I don't know if the stuff in the pit could be considered class 5.

 

[Obi-Wan]

If your buddy has the experience with stamped concrete why are you not asking him for advise?

I prefer to stay away from sand on exterior concrete because it has a tendency to wash out. I feel the crushed concrete is the way to go if possible but can add expense and you can pack it with a plate compactor, I assume this patio is not that large and should not be a terrible expense. Make sure you get all the vegetation & top soil from below the slab area prior to placing the fill.

The concrete will determine when it needs to be cut, many times cutting the next day is to late and the stress cracks have already formed. Multiple factors go into the curing rate of concrete from the mix design, slab thickness, wind, sun, shade, temperature, soil moisture content. Cut the slab as soon as you can get on it with a soft cut saw and does not spall, cuts should be 1/3 the thickness of the slab

Are you going to install rebar? #4 bar 24" OC will add approximately $ .25/sq ft and while structurally it is not need it will help with elevation separation when the concrete cracks, and it will

 

[ndbwhunter]

If your buddy has the experience with stamped concrete why are you not asking him for advise?

I prefer to stay away from sand on exterior concrete because it has a tendency to wash out. I feel the crushed concrete is the way to go if possible but can add expense and you can pack it with a plate compactor, I assume this patio is not that large and should not be a terrible expense. Make sure you get all the vegetation & top soil from below the slab area prior to placing the fill.

The concrete will determine when it needs to be cut, many times cutting the next day is to late and the stress cracks have already formed. Multiple factors go into the curing rate of concrete from the mix design, slab thickness, wind, sun, shade, temperature, soil moisture content. Cut the slab as soon as you can get on it with a soft cut saw and does not spall, cuts should be 1/3 the thickness of the slab

Are you going to install rebar? #4 bar 24" OC will add approximately $ .25/sq ft and while structurally it is not need it will help with elevation separation when the concrete cracks, and it will

I did ask him for advice, and he recommended that I go with class 5 or crushed concrete. My biggest concern wasn't necessarily what to use, but rather what not to use. The gravel pit near my house is loaded with material, and I have no way of knowing if the stuff I'm getting from there is actually class 5 or even ND DOT approved.

I will only need about 9 yards of crushed concrete, so I guess it's better to be safe than sorry. I wouldn't do any concrete without rebar, so that's already been purchased.

Is there anywhere in Mandan/Bismarck, other than knife river or strata, that has crushed concrete?

 

[Kurtr]

a plate packer wont do it if we are talking about doing it right a jumping jack will be needed and the right amount of water so as it does not go over the curve but then a proctor would be the only way to know it then getting density shot would be needed as any thing under 98% would not be acceptable........ that is utterly ridiculous your building a patio and not a sky scraper me personally i put fill gravel under my new house my garage floor and then base course ( in ND that is class 5) under my drive way because if water gets under there some way i dont want it to sit i want it to drain. With the rebar in it the biggest factor is going to be your concrete guy and sawing it at the right time and making sure he does not slop it up and you get a bunch of shrinkage. Plate packers make it look nice but do little in the way of packing any thing but the top two inches and i have dont a whole bunch of density testing that has proven that as fact. Is the crushed concrete blended with gravel to make it pack better or is just straight recycle? Who owns the pit by your house i am sure they have production reports that can be found.

 

[LBrandt]

Money spent now is well spent for the future.

 

[Big Iron]

Since were getting this technical better make sure we have enough fractured faces in the material. Request crushing reports from the supplier...

Good grief, this is for a patio guys.

 

[ndbwhunter]

a plate packer wont do it if we are talking about doing it right a jumping jack will be needed and the right amount of water so as it does not go over the curve but then a proctor would be the only way to know it then getting density shot would be needed as any thing under 98% would not be acceptable........ that is utterly ridiculous your building a patio and not a sky scraper me personally i put fill gravel under my new house my garage floor and then base course ( in ND that is class 5) under my drive way because if water gets under there some way i dont want it to sit i want it to drain. With the rebar in it the biggest factor is going to be your concrete guy and sawing it at the right time and making sure he does not slop it up and you get a bunch of shrinkage. Plate packers make it look nice but do little in the way of packing any thing but the top two inches and i have dont a whole bunch of density testing that has proven that as fact. Is the crushed concrete blended with gravel to make it pack better or is just straight recycle? Who owns the pit by your house i am sure they have production reports that can be found.

The difference in price between class 5 and crushed concrete is $0.80 per ton. I haven't asked about the composition of the crushed concrete yet, but they did say that it compacts very well. I will find out beforehand, but for now, I'm assuming that it's mixed with gravel. I'll do some searching for the production reports for that particular pit. It's not longer listed on the ND DOT page, but I'm guessing they would have access to older reports.

In my initial post, I mentioned using 6-10" of class 5 or gravel. I understand that it would have been overkill, but the material was free to me. Now that I'll be paying for material, I'm planning on leveling everything, and using a 4" base. There is no organic matter in this area, so I'll just have to bring in fill, level and pack, before adding the crushed concrete base.

- - - Updated - - -

I'm learning as I go, so all of this information is very helpful!

 

[Kurtr]

here is how i will do my patio 1. scrape top soil 2. place fill gravel and pack it roughly 4-6 " depending on top soil depth. 3. set forms place rebar 2' oc on 2" chairs 3. pour concrete. the thing you are trying to avoid is the native soil getting wet as that is what expands and contracts. Thats why you want to avoid stuff with high clay content (P.I.) . Think of it like water we all know what happens when the beer can freezes. For .80 per ton if it makes it feel good i would use the crushed concrete its not that much more. What are they getting for that in bismark these days?

 

[Kentucky Windage]

Tip

Whatever native soil you dig/ disturb, it needs to be removed. If you dug down 6" when you intended on only 4", don't put the 2" back. Use your crushed concrete or class 5.

 

[jer79]

typically we avoid having clay/ black dirt under a slab ( I guess,we wont add any that isn't already there). we use gravel to get to desired height. reason being clay and black dirt are kind of unpredictable in the future.. wet years it swells, dry years it shrinks. on a basement for example we backfill outside the walls with clay, but inside where the basement floor will be there is no clay added back in and compacted... all gravel

 

[AR-15]

Remember after the concrete dries you can't fix what you did under the slab, looks like CL5 would be fine, but crushed concrete with the crusher dust in it is some very good stuff

 

[Kurtr]

Are you saying they blend crusher dust which is a by product of crushing operations or the dust from recycling concrete. Does any one know why free draining material is specified for pretty much all commercial concrete slabs?

 

[ndbwhunter]

here is how i will do my patio 1. scrape top soil 2. place fill gravel and pack it roughly 4-6 " depending on top soil depth. 3. set forms place rebar 2' oc on 2" chairs 3. pour concrete. the thing you are trying to avoid is the native soil getting wet as that is what expands and contracts. Thats why you want to avoid stuff with high clay content (P.I.) . Think of it like water we all know what happens when the beer can freezes. For .80 per ton if it makes it feel good i would use the crushed concrete its not that much more. What are they getting for that in bismark these days?

It's selling for roughly $16 a yard, but $23 a yard delivered to my place.

This is new construction, so there is no top soil to remove. In fact, I'll have to add some fill to get it level before adding the concrete. Is this going to be an issue?

The native soil is very hard and has not been disturbed (except for the area that was backfilled around the house). The front 1/2 (away from the house) needs to be brought up about 6-8" to level the area. I guess I could leave it as is and fill the entire thing with crushed concrete, but that would nearly double the amount needed.

My plan is to add fill in 2" lifts and pack. If I used a skid steer to add that fill, would that be enough to pack it? I'd still wet the area and pack it again before adding the crushed concrete.

 

[sierra1995]

Are you saying they blend crusher dust which is a by product of crushing operations or the dust from recycling concrete. Does any one know why free draining material is specified for pretty much all commercial concrete slabs?

it's a by product of the crushing operations. Crushed concrete makes a very good surfacing material for driveways and other gravel applications.

You want to drain any moisture away to prevent heaving, same reason why most roadways are built on gravel and not clay.

But as far as the patio is goes, i would not recommend placing new concrete over crushed concrete. The reason is that the new concrete will bond to the crushed, and will re-activate any cement left over in the crushed concrete. You'll end up with a slab twice the thickness, which may sound good, but it will crack differently than a standard 4" slab on gravel.

Just go with the pit run gravel as your base. If you want to spend a bit more money, put some heavy duty fabric down to separate your clay from the gravel. Also, I would just use Macro Fiber polypropylene (Mac Matrix) reinforced concrete, not steel. It's only a patio and the macro fibers will hold any uncontrolled cracks as good as or better than steel. Whatever you do, avoid welded wire sheets at all cost!

 

[PAIN]

Fellas, let's not overthink this. Provided the existing soil type and minimal loading on the concrete, anything over 4" under the concrete is a waste of money. Have the clean Class 5 hauled in and don't look back.

 

[Kurtr]

it's a by product of the crushing operations. Crushed concrete makes a very good surfacing material for driveways and other gravel applications.

You want to drain any moisture away to prevent heaving, same reason why most roadways are built on gravel and not clay.

But as far as the patio is goes, i would not recommend placing new concrete over crushed concrete. The reason is that the new concrete will bond to the crushed, and will re-activate any cement left over in the crushed concrete. You'll end up with a slab twice the thickness, which may sound good, but it will crack differently than a standard 4" slab on gravel.

Just go with the pit run gravel as your base. If you want to spend a bit more money, put some heavy duty fabric down to separate your clay from the gravel. Also, I would just use Macro Fiber polypropylene (Mac Matrix) reinforced concrete, not steel. It's only a patio and the macro fibers will hold any uncontrolled cracks as good as or better than steel. Whatever you do, avoid welded wire sheets at all cost!

i know what crusher dust is having made a couple hundred asphalt mix designs and doing production reports on crushers. I was wondering if they are blending it with cd or fill gravel.

Fellas, let's not overthink this. Provided the existing soil type and minimal loading on the concrete, anything over 4" under the concrete is a waste of money. Have the clean Class 5 hauled in and don't look back.

I agree 100%