Home driveway question 2

[knowlittle]

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.

 

[BridgeSmith]

...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

 

[ACtrafficengr]

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/

 

[phamENG]

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.

 

[BridgeSmith]

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

 

[phamENG]

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.

 

[BridgeSmith]

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

 

[phamENG]

Ahh...I see. Thanks.

 

[oldestguy]

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

 

[Compositepro]

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

 

[oldinspector]

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/HomePage.htm

and plug in your address.
Good luck.
oldinspector

 

[BUGGAR]

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

 

[IRstuff]

In Yorba Linda, the Toll Brothers houses used post-tensioned slabs.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm