Blinding / slip membrane for external ground slab?

[kxi]

We're laying about 150m2 of concrete farm roadway, and the SE's original spec called for:

- 200mm concrete

- 50mm concrete blinding

- Compacted subbase (crushed concrete)

(over clay / hoggin)

 

There are no walls or anything on this - just a roadway with falls & drains. It's also reinforced, but that's another story.

 

We (now) don't see the point of the concrete blinding.

 

As I understand it, blinding is supposed to:

1. provide a clean and level surface to work off - but the compacted subbase already does this

2. prevent punctures if using a DPM - which we are not using since it's an external slab

3. prevents water draining out of the bottom of the slab when poured - which might dry it out too quickly when curing

 

So, only item 3 is really needed here? 

It seems to us (naively) that sheets of polythene would do the work of item 3 more cheaply, quickly and effectively than any blinding?

In addition, the plastic sheet would act as a slip membrane allowing the new slab to move a little while curing and so reduce the risk of cracking during curing?

 

Any thoughts on just using a plastic sheet instead of the concrete blinding? Or nothing at all?

Edited July 1, 2021 by kxi

[PeterW]

What lengths are you laying this in as over about 5-6m then it will crack. If you do it “properly” then you need to put expansion gaps in between the slabs, if you want to do it quicker then I would shutter and lay every other bay, then use a standard DPC as a slip between each bay and lay the second set. 
 

What is the finish specified on the surface ..?

[Russell griffiths]

It will crack, end of. 

Plastic sheet underneath will not stop it cracking, what it will do is stop the concrete drying too quick especially in this weather. 

You will need saw cuts every 5-6m so you can make it crack where you want it to crack, you need to provide a weak spot or it will just crack where it wants. 

Blinding not needed, very heavily consolidated base, then plastic, then mesh as designed then concrete. 

[markc]

+1 to this

[kxi]

Thanks all.

 

Yes, plan is to lay in bays or saw cuts, but need to clarify exact layout. There's lots of falls & facets to accommodate which will likely dictate it.

 

As an aside, the roadway which we tore up was:

- Laid perhaps 1960

- Over the clay with a bit of rubble (amazingly not too much asbestos sheeting)

- 150mm concrete

- No reinforcement

- Laid in roughly 3x3m squarish bays just butted up against each other

 

Pretty much no cracks at all. It does make me wonder. 

Why we tore it up is another story.

[Russell griffiths]

If laid in bays you need to mesh between the bays, or you can get a lip if one moves. 

[PeterW]

Last one we laid was done using 4.8 x 2.4m sheets of mesh from memory and bays were  proabably a foot larger than the mesh. Between each bay were 3 lengths of 1/2”  rebar about 3ft long, half covered with a length of hosepipe to allow one end to move. Think it’s still there and no cracks - must be at least 10 years old now and that’s a farm track taking machinery. 

[SteamyTea]

Can glass fibres (what we used to call choppings) be put into the mix to help reduce shrinkage and the associated cracking.

[dpmiller]

the concrete part of our shared lane was apparently 35N with fibres and no mesh,  it's showing no damage after ten years of farm traffic.

[markc]

1 minute ago, dpmiller said:

the concrete part of our shared lane was apparently 35N with fibres and no mesh,  it's showing no damage after ten years of farm traffic.

Must be a good sub base.

My GF`s drive was done 4 years ago, with fibres and it is has several cracks ... appears around 120 thk at the edges but i suspect much thinner in the middle.

Very little use and she only has a Corsa

[George]

Sounds like an unmodified standard detail.

 

Concrete blinding is great but is a bit OTT for a concrete drive.