Concrete and cement questions

[securitynewbie]

Hi guys. I have a question (or two).

Our drainage pipe was damaged, so we had to get a trench dug up. I've fell out with the guys who did the job.

Anyway to cut a long story short I've been left with a hole. Not only that, but they also took out what looked like a mixture of solids and clay from under the drain pipe with an SDS drill chisel, so it has little support.

1. What is the mixture of solids and clay and how do I fill under the drainage pipe to support it again?

2. The original surface was concreted, so I'd like to replace this:
a) What materials do I need to make concrete?
b) Can I use rapid set cement as part of concrete material?
c) I plan to work on the weekend which cold/frosty. Is it worth adding something into the concrete mix to help it cure?

Thanks for your help.

 

[Paulg31]

I’d use something like 10 mm pea gravel to bed the pipe. This should support the pipe and then be back filled to surround it. You can then use your soil above it.

I’d be carful concreting in frosty weather. The general rule is don’t concrete unless it is 3C or rising. You’ll need to insulate it if it’s colder overnight.

 

[securitynewbie]

Cheers Paul.

The pipe is hanging almost midair with nothing underneath it, so how can I put 10mm gravel under it?

Regarding the concrete. It's going to be around 5C-7C during day and around 2-3C at night. This is why I'm thinking to use rapid set cement and some sort of frost protection.

 

[John D v2.0]

If it's above freezing the concrete won't freeze. Even below freezing it's rare the frost gets very deep into the ground. Cement It generates a bit a self heat when curing too. You can always use warm water and cover it if you're worried.

 

[securitynewbie]

Thanks John, that's very helpful.

This brings me on to my other questions:

1. Can I use rapid set cement to make the concrete?

2. Is it worth purchasing a 5l frost protection chemical to add into concrete or is that not needed?

3. What is the mixture of solids and clay and how do I fill under the drainage pipe to support it again?

 

[noseall]

Unless the pipe is in a location that requires unusual support, just fill it entirely with pea gravel.

 

[securitynewbie]

Cheers noseall. The garden is normally used for parking vehicles and the pipe is below.. I need to make sure that the ground is solid enough to drive over. Since the pipe is hanging almost midair, will filling pea shingles under it be strong enough?

Also any help on my other questions would be very much appreciated.

Thanks

 

[noseall]

will filling pea shingles under it be strong enough?

For vehicular traffic - no. Just bang some concrete in there but support the pipe (usually with a brick) as you do. Then chuck some material on top to stop jack getting at the cement.

 

[Ian H]

I would just use pea gravel. Once it's in you can stand on the pipe and it won't move, once backfilled and reinstated you can drive over it.

The pea gravel is free flowing so just pour it in then knock it under the pipe with the blade of you spade.

You can use rapid set if you know you will have it done fairly quick.

 

[tomfe]

Depending how deep the pipe is just pea in the hole will do.
If you do will with concrete I'd wrap some polystyrene around it so any movement won't crack the pipe.

 

[John D v2.0]

Just for clarity, the OP isn't actually s suggesting to concrete the whole hole, if you read the actually questions the concrete is only the top layer.

. What is the mixture of solids and clay and how do I fill under the drainage pipe to support it again?

2. The original surface was concreted, so I'd like to replace this:

Having said that my concern would be cracking of the concrete where the backfill settles. So maybe the pea gravel only then concrete would be better?

 

[securitynewbie]

Cheers everyone for your help. I already have lots of pea shingle (10mm??) left over from a previous job, so I guess I could use that?

John is correct in saying that I'm not looking to concrete the whole hole, just the top few inches.

Correct me if I'm wrong, but can the following method work? :-

• Fill the lowest drop of the hole with a 5-6 inches layer of loose ballast and lightly mixed concrete
• Fill the remaining hole with clay/soil upto the bottom of the drain pipe
• However when I get to the drain pipe, I fill under that with pea gravel.
• Then the rest of the hole with more clay and soil.
• The final layer is then concreted

Here is a picture to better explain:

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