Compaction of hardcore under slab

[WWilts]

Limestone spoil has some sandy soil in it (negligible clay). Meant to hold up 100mm slab. 

 

Some compaction of the limestone done. How to test if compaction was sufficient? Tried hammering in two improvised penetrometers: a) 4mm diameter metal b) 45mm square wood. Quite different results. What test would be sensible?

PS Does the surface need to be near-level before the slab is poured?

 

 

[markc]

Good rule of thumb is stamping your foot, if you leave a decent print then could do with more compaction. A stake can still be hammered into compacted ground.

The idea is to get rid of any voids and prevent further settlement.

[WWilts]

Tks.

 

How important is it to level out any undulations in the hardcore before putting down blinding & then DPM?

[markc]

1 hour ago, WWilts said:

Tks.

 

How important is it to level out any undulations in the hardcore before putting down blinding & then DPM?

Needs to be reasonably level but the blinding is there to level it up

[saveasteading]

That isn't too good. The stone seems lumpy and not especially well graded (finer stuff that fills the gaps all the way through.)

Also too rough on the surface, so you should spread fines on it, sand or stone dust.

The trouble you then get is that it might be rather thick and will take foot prints.

 

Why does a smooth finish matter? 1. your dpm will puncture when you walk on it or pour the concrete, and might have air pockets left at high stones.

2. all concrete shrinks, and should be allowed to. With this surface it will not move where rough, and cracking is likely.

 

Difficult to tell the scale. What size would you say the biggest stones are, roughly?

How big is the floor area?

 

As to strength, I can't imagine you will ever fall through it, but there might be  chance of some movement and cracking  over many years. as the stone seems to be so variable

[WWilts]

15 minutes ago, saveasteading said:

Difficult to tell the scale

Blocks in pic are 215x440mm.
Largest stones at least 100mm.

Scale as in pic of foundation masonry
 

On top of this stone, builder intends scalpings (compressed) topped by blinding sand. Top of floor screed will be at top of visible block. 

From screed surface down:
65mm screed
100mm insulation
100mm concrete

DPM

 

Top of underslab drain opening is about 280 mm below top of floor screed

[saveasteading]

1 minute ago, WWilts said:

scalpings (compressed) topped by blinding sand

No problem then. The only issue I see is that the very big stones sticking up  might rock, so give them a bash before the scalpings

[ETC]

Put the DPM above the floor slab.

[saveasteading]

my own opinion is that 50mm of sand is too much as it is difficult to keep it from kicking up, 40 scalpings and 10 sand good, which may be what you have in mind anyway.

[WWilts]

2 hours ago, ETC said:

Put the DPM above the floor slab.

Is there some reason why not everyone does that? 

[ETC]

Easier to do once the roof is on and windows in. No mucking about trying to keep the DPM clean and tear free plus it’s easier to connect the DPC in the walls to the DPM in the floor.

[WWilts]

1 hour ago, ETC said:

Easier to do

Yes, but found this:

"Typically, a DPM is a polyethylene sheet laid under a concrete slab to prevent the concrete from gaining moisture through capillary action"

 

and this, re sulphates + limestone

https://www.constructionnews.co.uk/archive/limestone-ban-targets-sulphates-28-05-1998/

Have limestone, cannot exclude presence of sulphates in the small amount of other soil

Edited September 11, 2021 by WWilts

[PeterW]

That’s from 1998. 
 

I’d go with the advice above and move on 

[WWilts]

1 hour ago, PeterW said:

I’d go with the advice above

DPM over slab?
Or under slab?

[tonyshouse]

I hate blinding, I live vb in floors 

[Conor]

That's too rough. You need a layer of crusher run or fines compacted over it. Hire a roller for a day. Much more fun.

[PeterW]

15 hours ago, ETC said:

Put the DPM above the floor slab.

+1 to this. Put one below if you want but it will be sacrificial

 

other way is use a 25mm layer of Jablite / EPS70 on top of the stone and blinding then do the DPM - protects and provides insulation at the same time. 

[saveasteading]

Agreed re contaminants/chemicals attacking the concrete.

In some regions/ situations it is even necessary to put dpm between the ground and footings.

With the dpm below, the concrete will forever be dry and inert.

 

Another reason for the dpm being under is so that the concrete pour is clean. Sand and stone should not mix with it and diminish the concrete quality.

Also, delivered concrete has  a precise mix of ingredients, including water. If poured onto a dry stone or sand surface it will lose water and the mix is changed.

 

And then there is allowing the concrete to slip on the surface while it cures and shrinks, reducing cracking

 

This matters a lot with a commercial building.

A lot of it doesn't matter too much with a house, where loads are low, and there is a layer of insulation, then  screeds and floor coverings. ie an inferior job gets by , usually.

But why not do it as well as possible?

[WWilts]

Sulphate resistant concrete* was recommended by geologist for a site within about a mile, which (unlike our site) has clay. But water can probably carry sulphates to our site. So we don't dare omit the DPM below the (ordinary) concrete slab. Sulphates swung it, together with the issues highlighted by some. 

 

Sulphates might be a problem for others, especially those excavating deep for basements. For those interested in the technical details re sulphates & soils, & the damage possible in the presence of water, there's a detailed (free) report. Here. Lots of big words. Pg 70 or thereabouts was a sufficient dose for me.

 

*"Sulphate Design Class DS-5; Aggressive Chemical 
Environment for Concrete (ACEC) Class AC-4s"

[WWilts]

22 hours ago, PeterW said:

25mm layer of Jablite / EPS70 on top of the stone and blinding

Great idea. Will help cushion the underslab drain too, at its highest point. Is the blinding still necessary if EPS used?

[PeterW]

Not really as long as it’s flattish