Dug trench in basement to replace pipes, now pouring concrete back. Do I need gravel or sand underneath?

Question

I had to cut out some concrete and dig up my old drain pipes in my basement as they were old cast iron and falling apart. I have a trench about 24 inches wide and 15 feet long as well as another area for a new bathroom about 5' by 5' that I am ready to fill back in. I will backfill with the dirt I removed from it in the first place and want to pour 4" of concrete. Do I need to put concrete and/or gravel UNDERNEATH the concrete, or can I pour the concrete directly into the backfilled trenches?

Also, do I need to put anything on the existing concrete edges of the trenches I dug to aid in sealing the new concrete to the old?

Sorry there was a type-o above... I meant do I need to add SAND and /or GRAVEL underneath the concrete I will be pouring, or can I pour directly onto the backfilled soil?

Answer 1

Actually, it is good practice to back fill around the pipes with gravel. The old soil will have air pockets and will settle under the new slab, leaving a void between the settled soil and new concrete. Gravel will settle less, and you can tamp it down with a hand tamper or just drop a cinder block on it a few times to tighten the gravel up.

You don't need any sealant/bonding agent between old and new concrete, but you'll want to wet the old concrete down so it doesn't suck the water out of the new too quickly (the edges, that is). There will always be a crack left afterwards, so expect it.

If there was poly under the old slab, put some under the new section. If not, then don't bother. Poly is supposed to retard water vapor from coming up through the slab, but if there is none under the old slab, it won't do anything.

Answer 2

Any time the bed for concrete is not laid properly you are going to eventually have some kind of problem, usually with cracks. When they poured the concrete, did they use mesh or other metal like rebar for stability? It would appear not, since the metal should have given enough tensile strength that you would not incur cracking along where the pipe is. Fact is the pipe should have been buried at least six inches below the compact for the concrete. Your local buildings and safety department may be able to tell you if there is a code that specifies how concrete driveways should be constructed. However, do not discount the fact that some contractors do short on the bags/load cement to same costs. That could also be your problem. Barring being able to find a buildings and safety department write to your state attorney general and explain, send pictures, and ask that the entire package be forwarded to the correct offices. Get ready for a long wait though.

Answer 3

Use sand only--but make sure it is compacted good. Water it down and don't be in a hurry. Just make sure that the pipes are covered with sand so that the concrete does not adhere to them (the next guy might hate you!). You can buy a hand compactor or just make one with a piece of plywood and a 2x4. I am assuming that you are going to cover the floor with something so if it does crack it won't be a big deal--if there is adequate drainage under the home no settling should take place. The concrete will seal fine to the existing trench edges. Just make sure that the sand is settled and there will be no worries--sound like the hard part is over anyway!

Answer 4

I should add to these answers, make sure that your pipes have the appropiate amout of fall, i would suggest you use a small torpedo level, about 6 -10 inches long. Make sure you got at least a 1/4 inch per foot of fall. Sometimes when putting the soil/concrete back, the weight can shift the pipes down and cause a "backfall" in the new piping. I prefer to use some bent rebar over the pipes, pound them into the ground to ensure they wont budge when replacing the floor. Fall, or slope, is important, putting the pipes back in "backwards" could be worse than the old pipes you dug up the floor to remove in the first place.

Answer 5

Concrete Trench

Answer 6

Gravel with an average aggregate size of 3/4". Some of the concrete you removed can also be used to decrease the volume and the amount of concrete you need.

Answer 7

yes, use gravel. the only time you dont need gravel under concrete is when you are pouring footings for a house, because the concrete footings needs to rest on the virgin soil. Otherwise, use gravel.

Answer 8

Yes you need to put down a layer of poly over the sub-grade before you pour in the concrete. You should not need any sealant around the broken edges.

Answer 9

sand and gravel makes a better base for the concrete amd won't give away, creating hollows which would allow the concrete to crack.

Answer 10

Sand or gravel should be good, just make sure to compact it down before adding cement.