Concrete/Rebar question

[Matt Meiser]

I need to replace a 3x5 section of sidewalk near our Bilco door. I our area, the precast steps are bolted to the foundation wall, then the foundation is backfilled. They often don't get good compaction in the clay soil and eventually a gap opens under the steps. One segment of sidewalk started falling into that gap. I had some other concrete lifted and the company recommended replacing that section.

I broke up the existing concrete and used a lot of the little pieces to fill the gap as best I could. I'll be dumping some sand as well to fill the smaller gaps. I KNOW I did not completely fill the void--he said that would be impossible short of using a ton of very runny cement mix. He suggested pinning the new concrete to the stair casting and the adjacent good sidewalk with rebar because it will drop again. I'm also planning to use some rebar in the slab so I can basically have a bridge when, not if, it drops again. I'm wondering what, if anything I need to use to "glue" the rebar in place once I drill the holes in the existing concrete? I think my options are basically hydraulic cement or epoxy. Or should I just install it loose and let the newly poured concrete hold it in place?

[Jerry Thompson]

The guy that joined a raised slab to our original slab used a tube of adhesive that can be purchased at a BORG. It was a tube and was used in a caulking gun. I do not know the name of the product but all is still good after a long time.

[Matt Meiser]

Yes, they sell epoxy that way--$25 a tube at our Lowes IIRC. I'm having trouble coming to grips with a $25 tube of "caulk" :D

[Charlie Velasquez]

First, they make an epoxy specifically for anchoring in concrete. This is one brand from Lowes.

http://www.lowes.com/pd_354677-286-862031_0__?productId=3416896 (http://www.lowes.com/pd_354677-286-862031_0__?productId=3416896) (See you already found that)

Second, I am surprised they recommended, and it seems a common construction practice in your area, to connect something that will heave with the weather to something on a foundation and footers. I would be tempted to let it float.

Third, Flowable Fill.
Just used some a couple of weeks ago. One of our school buildings has a downspout connected to a 4" drain that is buried under a sidewalk and then runs underground adjacent to 20' fall of concrete steps. Joint had separated 5' past sidewalk which washed out that area that created a 3' hole mostly under the steps. That hole filled with water and washed out under the steps all the way down. Dirt above fell into that hole and washed away. Had a hole under the steps and sidewalk about 25 feet long and 18" in diameter all the way back to the downspout.
Our maintenance staff mixed their own flowable fill, started at the top and just started pouring. Dammed the places it seeped out as we went along. Had to stop after a while to let that cure as hydrostatic pressure was too great at the bottom. Back in the pm, let cure, then finished the next day. Took about 3,500 pounds of sand and about 100 pounds of portland cement. $11 for the cement, $20 for the sand.
Minus the curing time, it only took 2 people about 3 hours to backfill the hole.
You shouldn't have that much.

Not that hard and may solve your settling problem.

[Stan Calow]

Is there a reason not to have a mudjacker fill the void? I am looking at similar problem.

[Judson Green]

Second, I am surprised they recommended, and it seems a common construction practice in your area, to connect something that will heave with the weather to something on a foundation and footers. I would be tempted to let it float.



I'm surprised too. Feel it could lead to other problems possible with your foundation and that could get pricy.

[Matt Meiser]

He said they have seen where they pump mud in there for hours and never fill it. I'd be a little worried about it getting into the footing drain too.

I'd be pinning a new 3x5 slab to the step casting, and to a maybe 4x10 slab on the other side of that so its not a huge area--still enough to worry about?

[steven c newman]

Used to do a LOT of concrete foundations and such.

Hilti has a two tube gun of epoxy made for this.

Called doweled joints. We'd also add a expansion strip between the two halves of the joint. I'd also tie the dowels to the rebar in the new slab, with a couple wire ties. makes a mechanical joint between them.

Have also used a smooth rod. A red grease is smeared on the entire rod. Concrete won't stick with the grease there. Dowels can move and the slabs expand or shrink.

Void fillers? Pea Gravel. it will find it's own "level" as it will not compact. Won't move much, either. We used it to "protect" pipes and other utilities running under slabs or footers.

Dowels at both ends, connected to the rebar in the new slab. Pea gravel to fill the voids. then add the new 'crete.

[Phil Thien]

"Concrete that floats should never be attached to concrete that don't."

[Matt Meiser]

But what about concrete that sinks :D

[Charlie Velasquez]

I'd be pinning a new 3x5 slab to the step casting, and to a maybe 4x10 slab on the other side of that so its not a huge area--still enough to worry about?really don't think it matters how big your new section is. The ground in Michigan is going to heave and anything on top of it is going up with it. Something is going to give.

If your anchoring points are stronger than the re-rod, then you will have bent re-rod dowels that will keep your section higher than the soil when the soil goes back down in the spring (unless the weight of your section is sufficient to rebend the re-rod, in which case after a couple of seasons I would expect the re-rod to fail and the net result would be not pinning it).

If the re-rod is stronger than your anchoring points, then the anchoring spots will fail. You said your steps were cast, so I'm guessing the steps will fail before the slab.

I don't see any flaws in my thinking, but, I could be completely wrong, too....