Raft foundations. How does the mesh work in a slab?

[Gus Potter]

Hello all.

 

A Client of mine asked me why I had only put reinforcing mesh in the top of the raft slab on XPS insulation and not in the bottom as they had seen many details where there was steel mesh in the bottom and top of the slab or just mesh in the bottom. One underlying reason for the good question was.."what kind of temporary construction load can I put on the slab? .. as I want to run a machine about on it"

 

The following is a bit of an introduction to the theory on how you can place the reinforcing mesh (laymans terms).. the experience bit I'll leave mostly out for now.

 

Please remember that all raft foundations need to be designed on a case by case basis. Also raft design (particularly on thick insulation) is a dark art in that we design them partly on theory, partly on experience and by adapting standard design guidance to suit the particular site and your (often self builder's) requirements..

 

Please note that non of this covers.. difficult ground, made ground, sloping sites and so on.. there are many types of raft foundations.. I'm just talking about mesh reinforcement here and assuming a fair wind (in terms of ground conditions) below the insulation layer.

 

This design is a essentially a passive raft on XPS insulation on CLAY soil. I use capital letters her as when you look at any ground investigation report the dominant mater is captialised. The soil description is wriiten in order of priority.

 

This particular raft has to take some point loads from a structural frame around the perimeter and a modest loading from the external walls that sit on the edge of the raft. This raft has an edge beam to carry the column loads to spread them about. Below is a rough drawing of the slab away from the edges, I've ommited the mesh, DMP etc for simplicity.

 

In the case above there is a capping layer of 6F5 as the soil contains organic material, a bit of made ground and so on.. we kind of need to dig that out as if not we would have to have a much thickened and more heavily reinforced slab. We are aiming for economy in the round here. The capping layer is there essentially stop excessive settlement caused by say made ground and any organic material decomposing.

 

Forgive the lack of brevity but the concrete slab does not know what it is resting on.. it just wants to know what is holding it up, how squashy it is and how strong it is. What we often like to do is to make sure that the type one and capping layer is stronger in terms of compression strength than the insulation and also settles less than the insulation. In other words the stuff below the insulation is less elastic. This lets us concentrate on how the insulation behaves.

 

Ok lets look at a typical insulation, say an XPS. Now when you look up the manufacture's data tables the will often say our stuff is rated at 300Pa at 10% compression. Now 300 kPa is 30 tonnes a square metre! Now that is a great headline figure!

 

But on a 300mm thickness of insulation that equates to 30mm of downwards movement.. just imagine what that is going to do to your doors and windows, drains, finishes etc. What we do is to say.. how much movement can we live with. Say we can live with 6.0mm of movement when the building is subject to it's full load. The sum to get a reasonable load bearing capacity of the insulation  is: 300 * 6/30 =  60 kPa.. which still is a lot of bang for your buck if the loads are spread about evenly.. which often they are not.

 

Anyway we now know if we put 60 kPa on the insulation how much it will squash down by and from that we can calculate its elasticity.. how squashy it is.

 

On top of this insulation we put a concrete slab. Now plain unreinforced concrete has a small capacity to resist tension.. not a lot but it does do something if we have tension loads spread over a wide area.. it can hang in there! There is a design guide that is used by many of the big industry suppliers of software (TEDDS/ TEKLA for example) to the SE community is called TR34.. Concrete industrial ground floors. In this design guide is a brief mention of concrete slabs on insulation. What many folk like me and other designers / specialist Contractors do (the Americans pioneered this so I use their guides as well) is to take the basic equations and adapt them in a conservative way.

 

To adapt the equations in principle is relatively simple. Below is a simple model of how an unreinforced slab fails away from the edges under a point load.

 

 

One way of appraoching this is to assume that the first crack in the concrete happens in the bottom of the slab, now we are adapting conservatively the standard TR34 rules.  But we need to know what distance "x" is. This is called the radius of relative stiffness. Simplistically the insulation under is pushing back up as hard as it can (remember it is elastic like a pencil rubber) and the concrete is bending down but it is less elastic than the insulation. We need to find the "balance point" where the insulation and the concrete have equal forces to maintain equilibrium. Like below and then we can calculate the tension in the concrete in the top of the slab where the circle is and check to see it is ok.

 

 

You can see how the circle, the tension in the top of the slab, is relatively large and that is why we are allowed to take the tension properties of the concrete into account.

 

Here is an important bit for many of the raft slabs you see on Build Hub. Steel mesh is often used for crack control. Say A142 mesh, but often there is just not enough mesh to turn the concrete slab into a fully qualifying reinforced slab. But intuitively we know it will be helping a bit wich add a bit of a safety net.

 

In the round though.

 

I've had a look at what folk are getting designed by their SE's on Build Hub, qualitatively. You can't take too much of a view as there is often no quality info. I've also had a look at the AFD ( Advanced Foundation technology) offering. Lots of it is good stuff with plenty detail.

 

Hopefully the above will give you a bit of an insight into how they (AFT et al ) achieve their thin slabs.

 

BUT.. Often in this passive raft or just any raft foundation design there is no free lunch! DO NOT make the mistake of trying to get you budget figures to work by shaving a slab down to 100 mm thick for example unless you are confident on the ground you are building on, know what kind of insulation you are going to use and you have no funny loads from the superstructure! These loads include uplift wind and point loadings.

 

That's all for now folks.

 

 

 

 

 

 

 

[saveasteading]

  On 27/03/2025 at 22:22, Gus Potter said:

That's all for now folks.

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If using a contractor, do not assume best practice. They may  choose to chuck the mesh in without spacers, without laps or even to have gaps. So the slab can end up with mesh on the bottom where it has little effect.

 

 

 

 

[Gus Potter]

  On 28/03/2025 at 08:19, saveasteading said:

If using a contractor, do not assume best practice.

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This is so true.

 

For all self builders. My advice is to take time to understand what the drawings say. Make sure that gets delivered otherwise..

 

I have this on my website from Ruskin. Please take some time to read, especially if you are doing a first extension or doing some DIY with a bit of help.. It is even more applicable in this day and age.

 

There is hardly anything in the world that someone cannot make a little worse and sell a little cheaper, and the people who consider price alone are that person’s lawful prey. It’s unwise to pay too much, but it’s worse to pay too little. When you pay too much, you lose a little money — that is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot — it can’t be done. If you deal with the lowest bidder, it is well to add something for the risk you run, and if you do that you will have enough to pay for something better.”

Edited March 29 by Gus Potter

[Gus Potter]

The quote from Ruskin for me sums it up.  I know we are all woke these days.. but that bit about "lawful prey" ..scary.. but even more true in this day and age. 

 

That is why I love Build Hub as I think it serves the public interest.

 

A big thanks from me to the folk that are running this site.