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Home driveway question
2

Home driveway question

Home driveway question

(OP)
I need an expert's advice on my home driveway situation. My sewer drain pipe runs under the concrete driveway. After failing to remove the blockage in the sewer pipe, he is going to cut a hole in the driveway and do the necessary repair. He recommends a new driveway afterward. This is why. My concrete driveway is cracked into several large pieces (25 year old house). One piece is noticeably sunk. According to my plumber, the piece sank under vehicle load, moving the soil underneath, which in turn damaged the sewer pipe.

Here is my question. Did the sinking concrete move the soil underneath? Or did uneven settling of soil cause concrete pad cracking and sinking? If the former, I will need to get a new driveway. If the latter, I may not need a new one.

I live in Dallas. We have an expanding clay soil. As it goes through dry/wet periods, it can cause structural damage to homes, they say. Thank you.

RE: Home driveway question

This is a bit of a chicken or egg syndrome....

Most likely shrinkage cracks occurred in your slabs long ago. As time progressed the cracks widened. As they did so, they lost a lot if not all of their load transfer capability as wheels roll across the crack. This causes the slab to deflect differentially under the load. It also allows more water to get into the underlying soil. Both of these can cause soil displacement and depending on the depth of the pipe, can damage the pipe.

I would recommend replacing the driveway, making sure that properly spaced control joints are cut the same day as concrete placement. The contractor will balk at this but make him do it.

The slab should be consistent in thickness with a minimum thickness of 6 inches. The tolerance should be -1/4 inch, +3/8 inch. Wet cure the concrete for 7 or more days before putting load on it. Use 4000 psi concrete. You need the durability of the stronger concrete. Lower strength will do for the loading but will be less durable.

RE: Home driveway question

Or, the sewer pipe failed after 25 years and the soil above and around the pipe is washing away into the pipe causing the driveway to settle. Fix the pipe and the section of driveway above the pipe.

www.PeirceEngineering.com

RE: Home driveway question

Living in an area with expansible clays must be difficult. Thus, the recommendations of Ron might be considered over doing it, yet that high expand-shrinkage potential soil must be a constant headache. Hopefully the new sewer pipe is tough stuff and also can be affected by that soil situation.

RE: Home driveway question

I am here now. Made a mistake on posting :)

I think it is time to let go the old drive way, and provide a new. The linked paper discuss the design and construction of pavement over expansive clay. It's worth reading and take notes. Link

RE: Home driveway question

I agree with Ron and retired13. Multiple cracks and failures like that indicate a subbase and bearing soil problem, and replacing the driveway wholesale will probably be the best bet in the long run.

You'll want good new embankment or subbase material (at least 6" if you're doing a 6" slab) to help mitigate some of the expansive clays issue. 4000 PSI concrete is likely good; we use a 4500 PSI standard for concrete pavement on our highways in CO but I don't think you need the same strength. Could consider tooled joints versus sawn joints (tooled grooves 1/4 slab depth deep x 1/8" wide) without any expansion joints if the driveway is shorter than a couple hundred feet.

RE: Home driveway question

I noticed no one has mentioned reinforcement. Was it just assumed that the standard welded mesh would be in the new slab, or is there some reason why it shouldn't be? I'd consider it a small expense for a big gain in ability of the slab to handle movement of the subgrade due to the expansive clay.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

Posting some pictures of your existing driveway may help with the advice you are given. "Cracked into several large pieces" doesn't sound like a replacement is warranted to me, but what I am envisioning could be quite different than what is actually there.

The performance of the driveway (or any slab on grade) is very dependent on the subgrade preparation. In residential construction it is very common for driveways to move and crack due to poor subgrade preparation, expanding soils just compound this problem. Residential driveways and slabs on grade are usually unreinforced or minimally reinforced, so they will not bridge and distribute loads over any soft spots in the subgrade. All of this will lead to cracking and settlement you are seeing.

How deep below your driveway is the sewer pipe? Is it right below the concrete so it is depending on the driveway to essentially "bridge" over it? Has soil started eroding under the concrete due to water infiltration (from rain or a broken sewer pipe)?

If the cracks don't bother you and water infiltration has not been an issue, I see no reason to replace the entire driveway. I would still recommend sealing all of the cracks to help prevent moisture intrusion under the slab.


Quote (Ron)

making sure that properly spaced control joints are cut the same day as concrete placement. The contractor will balk at this but make him do it
Ron, do you see contractors having issues with this and not doing it in your area? I would be quickly finding a new contractor if this were an issue for them in my area.

RE: Home driveway question

This old time engineer that has dealt with both rigid and flexible pavements disagree somewhat the statement "The performance of the driveway (or any slab on grade) is very dependent on the subgrade preparation". Fortunately rigid pavements can span a lot of poor subgrade. Thickness depends more on loads and frequency. Add another inch and amazing different life. As to reinforcing using the packaged wire placed in the ready mix load is an easy way and it can produce wonders.

RE: Home driveway question

You are correct oldestguy. In my statement I was specifically thinking about nominal 4" SOG that are typically found in residential construction for driveways, garage floors, basement floors, etc.

RE: Home driveway question

I think good subgrade preparation and quality pavement with adequate thickness are hand in hand twins headed for success. Somehow I've never specified any concrete slab less than 6" thick w/wo reinforcement. I guess that's why my late boss had quit to ask me handle SOG design jobs:)

RE: Home driveway question

I had read somewhere when investigating paving my driveway with concrete (don't remember where at the moment) that a 4" reinforced slab (typical 6x6 mesh or fiber reinforced concrete) would be as good or better than 6" of plain concrete. I don't know how accurate that is, but that would seem to indicate reinforced would be a significant improvement, especially where the subgrade is questionable. At less than a quarter per square foot (for the mesh - around here it's similar for adding poly fibers), I'd not hesitate to throw it in, if opting to replace the driveway.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

BridgeSmith, I agree, just make sure you are there when they are placing the concrete to make sure the wire mesh ends up located in the middle of the slab and not on the ground as is often the case.

RE: Home driveway question

I don't against reinforcing. But I won't use 4" even with wire mesh, for inadequate cover in my opinion. I usually use #4 bars for light duty SOG, so you can see why I wanted 6" thick slab.

RE: Home driveway question

Fibermesh is a lower cost, labor-saving option for concrete reinforcement. Unlike rebar which must be set precisely before concrete can be poured, Fibermesh pours with the concrete saving time, as well. Overall, according to Propex Concrete Solutions (which manufactures Fibermesh), Fibermesh can be used in residential, commercial or industrial applications as the primary form of reinforcement without a rebar skeleton.

RE: Home driveway question

On a typical driveway, I agree with the 6" minimum thickness being prudent. With expansive clay (or any clay) in the subgrade, I'd say 6" thickness is a necessity.

In my case, I have a well-compacted in-situ sand-gravel base that I planned on keeping undisturbed, in a location inaccessible to anything larger than a full-sized pickup. I considered a 4" slab with carefully placed reinforcing mesh, and probably poly fiber, as well for cheap insurance (local ready-mix suppliers charge $10/CY to add fibers).

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

The reason I don't like mesh too much is construction concern. The rebars usually set tightly in place throughout the concrete pour, while the mesh, more often than not, will sink to the bottom due to weight of the wet concrete if attention is lacking. I often ask my foreman, where is my mesh? "Yes sir, here it is", a big smile and two muddy hands :)

RE: Home driveway question

Quote (oldestguy)

Fibermesh is a lower cost, labor-saving option for concrete reinforcement.

OG...this might be a first....I disagree with you. I don't consider fiber in concrete, whether steel, glass or polypropylene, to be "reinforcement". Fiber is a mix enhancement that may increase compressive and tensile strength properties of the concrete, but doesn't truly reinforce it in the same manner as rebar or welded wire fabric.

RE: Home driveway question

Ron; I partly disagree.
Any attempt at "reinforcement" is better than none. For a case where normally none is needed, BUT the site is crummy, it is an easy way to improve things. I can only Point to my limited observations of a few such uses where I'd expect cracks and faulted sections to be plenty, and there are NONE.

RE: Home driveway question

Quote (retired13)

...while the mesh, more often than not, will sink to the bottom due to weight of the wet concrete if attention is lacking.

I agree, but when properly placed and supported, it does its job quite well. Without proper support, bars end up on the ground instead of in the slab, too. QC is important either way.

In my case, I plan on setting the mesh myself, so it will be, and will stay, right where it's supposed to be.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

I heard this from a steel fiber salesman, so maybe I should serve it with a side of salt, but since polypropylene's mechanical properties are so much lower than concrete's, it's more of a shrinkage crack reducer than true reinforcement. Once the concrete cures, the poly doesn't do much.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/

RE: Home driveway question

AC - that is true for all fiber reinforcement, whether steel or poly. The random disbursement does well to limit the width of random cracking due to shrinkage (especially during curing). That same random disbursement, however, renders it useless as "true" reinforcement. Fortunately for a slab on grade or other light duty pavements (residential driveways, sidewalks, etc.) they're typically designed as plain (unreinforced) concrete and the mesh or fibers are there for the sole purpose of limiting shrinkage cracks.

For heavy use slabs/pavements where the concrete needs some flexural capacity beyond it's modulus of rupture, you need continuous bars whether you have fiber or not.

RE: Home driveway question

The fibers should reduce the size of flexural cracks, as well. According to The Constructor website, even steel fibers don't add flexural strength. Even the sites selling the steel fibers mention only smaller cracks, increased impact resistance, and increased shear strength, but not increased flexural strength. They also say to expect some rust staining with steel fibers for outdoor or wet locations.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

How would they reduce the size of flexural cracks without increasing strength? The crack width is a function of the deflection, and the deflection is proportional to the effective moment of inertia, and so is strength. Rebar increases the strength (and also post cracking stiffness) by increasing the transformed moment of inertia. For fibers to reduce those cracks, wouldn't they also need to increase it and thereby contribute to the flexural strength?

To be clear, I agree they don't add to the strength - I also don't think they reduce flexural cracks.

RE: Home driveway question

The combined width of the flexural cracks are probably the same, but the size of the individual cracks is reduced. The result is narrower cracks at closer spacing, just as closer spacing of reinforcing bars is supposed to reduce crack width, even if the overall area of steel is the same (I assume ACI has "crack control" provisions similar to AASHTO?).

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Home driveway question

Ahh...I see. Thanks.

RE: Home driveway question

Gee, what fun can be raised here by adding to the subject matter.

RE: Home driveway question

Fiber in concrete, an excellent topic for its own thread!

RE: Home driveway question

If you look at Dallas businesses, you'll find dozens, maybe 200 businesses doing expansive clay remedial work. That says a lot. Check your house for damage while you are on this topic. Having said this, my take would be to check the soil beneath the driveway. No sense spending money for the same result. It could be other things besides expanding clay: poor subgrade, poor compaction, driveway to thin, grading and poor run off. For a look at your expansive clay situation, go to https://websoilsurvey.sc.egov.usda.gov/App/HomePag... and plug in your address.
Good luck.
oldinspector

RE: Home driveway question

Residential slabs on grade in S.Cal founded on expansive clay are frequently prestressed for crack control and to help bridge the bad spots.

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