Backfill before concrete

[cbs34]

Slab on grade construction. Removed the concrete and cast iron drains. Replaced with dwv. Any tips on backfilling the hole? Any advice on mixing/setting the concrete?

I'm thinking about a pea gravel bed under the pipes to set the grade and then mixing the concrete in a 5 gallon bucket when we're ready to pour.

For what it's worth, the original install seemed to have mortar poured at the cast iron joints, then fairly large rocks on top, then more mortar before the slab was poured. If you look in the photos, you can see one of the rocks we removed.

 

[Twowaxhack]

I think pea gravel would be a waste of time.

Mix up your concrete and pour it.......

We’ve poured in dry bag mix before and mixed it in the hole. It wasn’t load bearing. Works great and lasts, I still work in these houses from time to time.

 

[cbs34]

Great information - thanks for the tip. Did you ever worry about vibrating the concrete to remove air bubbles? One area we have to fill is directly below the shower - does that change anything?

 

[Twowaxhack]

Great information - thanks for the tip. Did you ever worry about vibrating the concrete to remove air bubbles? One area we have to fill is directly below the shower - does that change anything?

We just mixed it good with a hoe and shovel. Put in a little more water than instructions say but not much more. Too much water will make make a weak mix.

We were filling in trenches 18” wide and 4” deep where we replaced all the cast iron under a house slab. We did probably 15-20 in a neighborhood over about 10 yr span. We patched the slabs.

Our fill was red clay sand mix. It was already there from the original slab. We dug it out, replaced the cast iron with pvc, then back filled. Packed it tight with a hand tamp, covered with plastic then poured in our dry bags of premix.

I still work in these houses and they’re no problems with the floors.

 

[cbs34]

I like it - sounds like a great plan to me. Thanks again.

 

[Twowaxhack]

We put 2” of mix in then wet/mixed it. Then immediately put in two more inches and wet it good. Mixed right in the trench

 

[cbs34]

Perfect. Years ago I worked on a Habitat for Humanity project rehabbing a house. Setting posts for a handrail we mixed it in the hole and it seemed to work pretty well.

Twowaxhack - in your post above you mentioned trenches that were 4" deep. Ours are between 11-15" inches just FYI.

 

[Twowaxhack]

Perfect. Years ago I worked on a Habitat for Humanity project rehabbing a house. Setting posts for a handrail we mixed it in the hole and it seemed to work pretty well.

Twowaxhack - in your post above you mentioned trenches that were 4" deep. Ours are between 11-15" inches just FYI.

QuickCrete gets hard! I use a pump up garden sprayer with water to slick it down with a steel trowel.

Yeah ours were too but our concrete was only 4”

 

[cbs34]

QuickCrete gets hard!

Yeah ours were too but our concrete was only 4”

OK that makes sense. I had a senior moment there.

 

[breplum]

Most important: Dowel rebar into all the open edges at say, 12" spacing. That will keep the concrete slab patch from settling, no matter what.

 

[MicEd69]

I have always placed a sand bed in the bottom of the trench to easily establish the correct slope of the pipe. Then I cover the pipe with more sand and if necessary, add fill dirt to bring the level in the trench to no more than 6" to the top of concrete. I add 2" to 3" of 3/4" gravel for a water path to my basement sump, "paint" the sides of the trench concrete with QUIKRETE Masonry Bonding Agent, then 3" to 4" of concrete mixed in a wheelbarrow.

Doweling in rebar is a great idea as breplum suggested, but in a basement location, I doubt that there is enough load to be concerned about.

I may be overly cautious, but the PVC pipe does move slightly based on its temperature. Although this looks like a toilet installation, this may not be a concern in this case. I had a 15-foot run of 2" PVC from a hot tube under my basement slab and had the temperature movement of the PVC pipe at as a major concern. I just like the comfort of having the sand encompassing the pipe, so it is freer to move if it wants to.

 

[Twowaxhack]

Most important: Dowel rebar into all the open edges at say, 12" spacing. That will keep the concrete slab patch from settling, no matter what.

I did that when I added on to my concrete slab. Epoxied rebar horizontal into the existing footing.