Driveway Base: is 12" Crushed Stone Overkill?

[keeney]

Driveway Base: is 12\" Crushed Stone Overkill?

I am working on putting in a concrete driveway on my new house construction. Since I am going to be spending big $$$ on a concrete drive, probably with some expensive decorative features for the wife like added color and perhaps some edging or texture, I want to make sure it will last a LONG time.

I have clay soil, but the drainage is good as the whole driveway is mostly on a slope and/or slopes away from the house. The driveway will not have any low spots that would collect water.

The specs I am asking my contractors for are:

5 inch minimum thickness
6 inch thick at all edges and where it abuts curb and garage.
4000 PSI, fiber-reinforced, air-entrained concrete.
#4 (1/2") rebar 18" OC, both directions throughout.
12" MN-DOT class 5 (1"-minus) crushed limestone base compacted in 4" lifts.

The contractors I have spoken with have proposed instead 36" OC for the rebar and only 4" or 6" crushed rock or sand base. They claim what I want is over-kill.

I am concerned about using sand base because it may wash out from under the concrete.

I am concerned about skimping on the base due the the clay soil and MN winter frost heave as well as the fact that much of the driveway is built over the back-filled trench from the water hookup (compacted, but not 100-year settled soil).

I like the idea of lots of rebar. The fiber-reinforcing may prevent some of the cosmetic cracks, but once a structural crack appears, there is nothing like a solid chunck of steel to hold it together.

- Rick

 

[nobull1]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Don't have a good opinion for the base but why not 5000 psi with air.All our city sidewalks spec 5000 and I think it is only a couple dollars more a yard.Not really a big deal in the overall price but 25% stronger.

 

[Poppa]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Keeney, I can't speak to whether this is overkill or not but I can say it looks like you have done your homework and you have come up with something you feel good about. That should be enough. Your the one who is going to have to live with the end result so do what you feel is right. Imagine five or ten years from now and something has gone wrong and all you can think about is "If I just would have doen it the way I wanted to..."

Don't let that happen.

Mike

 

[ljh2]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

keeney,
Having grown up in northern MN, I am familiar with the frost heave issues... especially with clay soils there. While a 12" base may be overkill, I don't think you will regret it. I think your spec's for rebar are spot-on. Your concrete guys might think that it is more than they would do, but YOU are the one footing the bill - AND YOU are the one who has to live with this thing on down the line. Are they going to fix it 5 years down the line after a particularly wet fall and an erratic winter break it to bits... I doubt it. Go with your spec's and sleep well at night. /forums/images/graemlins/smile.gif

 

[tawilson]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

I don't know about driveways, but I have been involved in installing equipment pads outdoors, and the engineers always ask for 12" of base, so go I say go with your gut. Besides, it's your dollar.

 

[Junkman]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

The only thing that I would like to see under the surface that would be in addition to what you have proposed is a drainage pipe or two. I have seen them build concrete roads in PA on I-80 and they always pour the concrete in sections with a space between each section. Then they go back and pour the section that was left blank later on. I assume that it is done this way to make sure that each section cures completely and independently of the one next to it. They do have the re-rod connecting the sections together from what I remember. I have not been a fan of concrete in freezing weather, but I have seen it done successfully in some areas. Personally, I prefer asphalt because it doesn't have the inherent problems of concrete, but everyone has their preferences. I would check with the state highway department and find out what their specification is for concrete in highways and spec that for the mix that you want for your driveway. The person that did my asphalt driveway uses the state mix for asphalt and his works holds up better than any others in the area. It cost more, but it is worth it in the long run. Also consider putting some PVC pipes under the drive in the event that you might want to put water or electric line across it sometime in the future. Try to plan for any future landscape changes whenever possible. Also, consider a slight pitch to one side or the other for water run off, especially in the winter. Speaking from experience, I would put it up a little higher than the lawn for plowing purposes. Saves you a lot of work repairing the sod in the spring.

 

[B7510]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

The comment one person made about air is right on. Be certain that you get "air entrained concrete" and also don't let the contractor add water to the mix. The lower the water/cement ratio the better. If you pay for a high grade mix (4,000 or 5,000 psi) and then add more water, you just wasted your good money and ended up with lower strength concrete. The best thing to do is hire someone who has done drives in your area that still look good after 5 or 10 years. There are a lot of variables. Take advantage of someone else's experience if you can.

Junkman,
Concrete shrinks as it cures. I suspect that they leave the gaps in the road pavement to allow the shrinkage to occur in the first set of slabs, then when the gap is poured the net shrikage is a lot less.

Re-rod doesn't keep the concrete from shinking it just distributes the cracks. You end up with a thousand cracks that are 0.001" each instead of a single crack that is an inch wide.

 

[nobull1]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Your right about the water in concrete the guy's that place the concrete like to get the slump up for ease of placing.A little extra water makes their job a lot easier and your concrete a lot weaker.I would spec 3 or 4" max and see what they have to say.They also don't like air as it can make finishing a lot harder.I would like to see the slips from the concrete supplier to know I got what I wanted otherwise there is no way to know if you even got what you paid for.

 

[Darren]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

You didn't mention how long the driveway was. The rods sticking out of the highway slabs that one poster mentioned aren't rebar. They're dowel rods. The rods maintain alignment of the slabs across the expansion joints. Without proper expansion joints, the slabs can be forced up and break on the ends during very hot weather. Without the dowels you may see differential settlement of the slabs. Since you're not running heavy vehicles on your driveway at highway speeds, any settlement should be slight. The base you're planning would minimize that. Just make sure that the drainage is such that no water can pool under the slab and freeze during the winter.

Other than that, the base thickness you're planning should minimize frost heave. If the contractor does use dowels he has to install them so the concrete can move on the rod. Otherwise they can lock up and shatter the concrete.

If you're planning on using salt in the winter look into using the epoxy coated rebar. Those are the green bars you see used in construction especially in bridge decks. That will help prevent salt from rusting the rebar and cracking the concrete later. No salt no worry. If you're heavy into salt and you absolutely don't want the concrete to spall check with Carpenter Steel for stainless rebar. The stuff has a much higher yield than regular bar meaning that you can go to a smaller size, get the same strength as a larger carbon steel rebar and minimize the extra cost of stainless.

 

[Egon]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Have you had frost heave on the driveway area before? If so the soil in the heaving area will have to be excavated and replaced with material that does not allow for capillary action for water from below. It doesn't matter what you do on the surface the frost heaves/ ice lenzes will continue as long as there is subsoil moisture and capillary action. The frost lenzes start from the bottom and keep adding to the bottom.

The 12 inch subbase may be overkill but will certainly be for the better.

Insure the subbase [clay] is packed and graded so there will be no water collecting and it will stay dry.

Note: all information is from 40 years ago and I have a poor memory.

Egon

 

[Soundguy]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

12" compacted base is what we put under asphalt roads.. in many of the cities and municipalities.... trhen say 2.5" of asphalt...

So yes.. you are on the 'heavy' side... your money though.

Soundguy

 

[keeney]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

The driveway is only about 2500 sq-ft. 12" of base is about 90 yards or 135 tons. At $11 per ton delivered, its about $1500. Cutting it in half to only a 6" base would save $750 in stone and 45 yards of excavating work.


So am I better off putting that money into:

1. More base stone.

2. More concrete thickness

3. More steel (rebar).

- Rick

 

[djradz]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Make sure with all this stone that you're not merely creating a place for the moisture from the surrounding soil and drive itself to collect - sort of like a big septic drain field under your drive. I live in south central lower Michigan with lots of clay and we've gone to using more sand and sandy compactible fill rather than stone. Possilby if you provide drainage of the base, off to a lower area, you'll be OK.

 

[keeney]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

I am looking at Mn-DOT class-5 which is comprised of 1-inch minus crushed limestone. My understanding is that this includes a range of stone sizes under 1 inch including "fines" which should act to fill the spaces between the stones and lock together when compacted.

I don't like the idea of using pure sand because if there is water running through a crack or along the edge of the pavement, it would wash out the sand, undermining the support.

 

[PaulK]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Landscape fabric is another cheap insurance policy. Here in northern NJ, my friend's 100 year old farm driveway of crushed stone is at least 12" deep and still has clay contamination from below each spring. My extended driveway - 12ft. x 80ft. has landscape fabric below 8" of QP [3/4"+1/4"+stone dust] and is perfectly clean down to the fabric after 4 years. The fabric is about $20 for a 4'x100 roll. It's a no-brainer for all of my drainage applications.
Paul

 

[polecat]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Nobull and the junkster r spot on ..go with the 5000psi and the drainage pipes r a must >>>>i've been in in charge of many a substation building and the biggest quandries r the drainage and the strength of the concrete..psi wise..what size rebar is a biggy!!!and in what form or should i say how deep r your drainage pipes going 2 b!!!..maybe 3/4 rebar next to the joints of each form...it only takes one bad year with nosnow(and i've spent a winter here and there inMN...cold,cold,cold..and no snow can bust up the best of intentions..we alsays use 2' pieces of 3/4 rebar on our joints and have yet 2 ever see a crack form...ever !!!and i'm from maine...2boot..it jist makes a driveway like the one u r building into a true floating slab ..especially in areas like mine, yours, and nobulls....other than that u can never overkill a drive..especeially seein as how our frost level was almost 6' this yaer!!!!!!!!!!!
BEST OF LUCK
TODD

 

[jimgerken]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

Hi Keeney. I would alter what you are thinking ONLY SLIGHTLY: The base you are requesting is a good idea. Keep it 12 inches, but add some packing in layers when it is put down. I am not sure how to specify this, maybe others can add that. Next, I would make the edges even thicker than 6", maybe 8 inches, in at least a foot. This is where the stress can attack an otherwise good "system". I always specify 4000# also, but if 5000# is only a couple more dollars per yard, go for it. Lastly, 1/2" rebar on 24 inch centers is enough steel, and it MUST be set up 1/3 of the slab thickness from the bottom on risers or bits of broken concrete block, and wired together at every crossing. Overlaps can be wired together and should overlap 18" or so. Extra rebar can be used at the edge, below the main rebar grid (since the concrete is thicker here, its deeper), suspended by more tie wire or set on blocks again. If I was doing this, I would hire/perform the excavation, have the fil brought in in layers, packing as it was spread, then do all the steel installation myself. It is time consuming but you can do a great job yourself and typically you can't get a contractor to take you seriously on the details. Then hire a finishing team to do the concrete. Work directly with the concrete mixing company to buy what you want, that way you are paying and you know what you bought. This is the cheapest way to get flat work done. Get the finishers bid in dollars per hour, and shop a bit. The concrete cure rate will drive him along, you won't have to. Specify the finish you want (broom, whatever), and watch it is done this way. That, take pictures and get beers will be your main jobs on crete day.

 

[Currmudgeon]

Re: Driveway Base: is 12\" Crushed Stone Overkill?

First, provide a good subgrade under your stone. Cut out and replace anything that is not sound clay. Grade your clay subgrade and proof roll it with a loaded dumptruck. Any places that move need to either be undercut and replaced with aggregate, or compacted more, or both. Put your effort into providing a good subgrade.

Pay particular attention to proof rolling that old trench. You may need to undercut that.

I generally place concrete pavements on a 6-inch compacted crushed aggregate base. Use State DOT road stone. Concrete pavement isn't as sensitive to base thickness as is asphalt, because it is a rigid pavement. The base does allow you to have a clean, compact surface. Compact the base to 95%, if you have a way of measuring it. Watch the moisture content in the aggregate, some compact well in only a narrow range of moisture levels. Proof roll your base with a loaded dump truck.

If you insist on the 12 inch base, place it in two lifts and compact and proofroll each lift.

Depending on your drainage, you might consider a porous base, using open graded stone. This will allow you to put drain tile along each edge and extract any water that gets into the base course. You'll definitely want to use geofabric with a porous base, and it's not a bad idea with a normal base on clay.

Watch your drainage. You want to get the water away from your road. Put in roadside ditches and drain them to somewhere, if possible. This will do more than anything else to keep water out of your base. A saturated base is probably the biggest cause of road failure.

Put some cross grade on your slab if you can, to get the water off the road. You can either crown it or slope to one side. I generally call for a 2% (1/4 inch / ft) cross slope on roads. You can go as flat as 0.5% slope on concrete, if you can control the grades that accurately. Anything flatter, and you will have birdbaths.

4,000 or 5,000 psi mud with air entrainment is what you want. I concur with not allowing the addition of extra water to help with placement. That weakens the mix. A five inch slab is generally adequate for cars. A seven inch slab will generally carry semis. I like the thickened edge slab, as the edges are the weak bit. You might add an inch to your edges.

You might consider welded wire mesh instead of rebar. You may be able to get the same weight of metal if you pick the right mesh. The biggest value of reinforcing is to eliminate shrinkage cracks. It doesn't do much for the structural strength of a slab, as the slab is pretty thin. But, as you say, it holds the pieces together.

Be sure you put in control joints as soon as you physically can. The joints should be spaced at maximum intervals of thickness/24 (two feet per inch of thickness) with the panels as square as practical. If you're going to saw cut the joints, the saw should be on the slab as soon as the concrete will cut without ravelling. If you wait until tomorrow to saw cut, much of the cracking has already occurred. Tooled joints, of course, are put in while the concrete is still soft, and are fine if they are deep enough. I usually think about cuts t/4 in depth.

Visqueen the surface or apply curing compound to ****** moisture loss. Concrete cures by chemical hydration, not by drying. It needs to be kept wet. Once it dries out, it stops hydrating.

Concrete reaches about 2/3 of design strength in 7 days and full design strength in 28 days.