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