thin foundation slab

[cobblerdave]

Re: thin foundation slab

G'day Mick
I'm an "old sailor" served on 4 steam boiler/turbine ships and one gas/turbine. Every one is fish food ( artificial reefs) or razor blades now.
Got used to boiler re -bricks, its a fact of life, take a firebrick to the max and keep it there, for months and it will fail. It's just basic maintenance to rebrick them.
Our ovens, lower temps, less use, how can know how long they will last? I cant guess in the end.
But you can get too cheap
Regards dave

[wotavidone]

Re: thin foundation slab

G'day
Yes I suspected that 20 years in contact with the ground would make them soft.
I've seen this before when someone has a friend with a stack of brick commons behind the back shed on the back yard.
With the price of firebrick and availability they would certainly consider using them. But put them on a piece of soft ground and hit them with a hammer and they crumble into a number of pieces. The surface of the breaks is easy to rub off with your thumb. Unsuitable for anything but garden edges and paving.
Get the same brick common from an old construction protected by a moisture barrier and its a different story. The break cleanly in 1/2 and are hard as.
Regards dave

On the third oven I built, my mate wanted to use the old bricks that were doing duty as a path to his house.
I held out for new brick, and I'm glad I did. We did use them for the outer arch, but if they erode or break there, it's a very simple job to relay the arch.

[cobblerdave]

Re: thin foundation slab

G'day
Yes I suspected that 20 years in contact with the ground would make them soft.
I've seen this before when someone has a friend with a stack of brick commons behind the back shed on the back yard.
With the price of firebrick and availability they would certainly consider using them. But put them on a piece of soft ground and hit them with a hammer and they crumble into a number of pieces. The surface of the breaks is easy to rub off with your thumb. Unsuitable for anything but garden edges and paving.
Get the same brick common from an old construction protected by a moisture barrier and its a different story. The break cleanly in 1/2 and are hard as.
Regards dave

[Goon_Squad]

Re: thin foundation slab

G'day
Those brick commons have been in the ground for 20 years and would be pretty soft. I'd experiment with a circular saw with a diamond blade or masonry blade if it will not cut then throw the slab over the experiment .
I recon it will cut though and I'd cut out the shape of the slab through those curved brick patterns. Pull back 200 mm of bricks leaving a central island of brick in the middle. This will then give you a 75 mm thickening beam right around slab to the original ground.
Boxing, thin ply buttered up to the cut brick, then heavier wood frame on the outside it. A long rackets strap around the lot for good measure.
The 40 mm fall is nothing my slab is on sloping ground thinnest corner is 120 mm and the deepest is 300 mm.
Regards dave

Was an interesting point you made and the bricks were certainly pretty soft. Not crumbly but certainly softer than normal.

They've now gone back in the ground on the side path around the house with an extra two barrows of concrete

[Campmaki]

Re: thin foundation slab

Goon you did the right thing. If you had not removed the brick, there is no going back. Good construction practice tells you to pour concrete on a well drained solid surface. Like I posted earlier, what is under the brick? Gravel you say, why would you put the slab on something other than that.

[wotavidone]

Re: thin foundation slab

Thanks all for the excellent advice. I've gone the safer path and lifted the bricks, wasn't that big a job and gives me a bit more comfort that it won't shift around after the fact.

The good news is that the bricks were laid on a 20mm base of builders sand and a crushed rock layer below that. So I've dug up the sand but left the crushed rock, well and truly compacted now! Formwork will be a little trickier but the slab will vary from 100mm at one corner (high point) down to 140 at the other, average of about 120-130mm which should be more than adequate I believe.

Good onya mate. 100-140mm will be fine, especially with some mesh in it, and with the bricks pulled up, there will be no little nagging question marks.
It's a bit like renovating a house - after you make sure the footings and the roof are in good nick, you can go about the reno's secure in the knowledge you are not wasting your efforts renovating something that will fall down or leak and destroy your good work.

[Goon_Squad]

Re: thin foundation slab

Thanks all for the excellent advice. I've gone the safer path and lifted the bricks, wasn't that big a job and gives me a bit more comfort that it won't shift around after the fact.

The good news is that the bricks were laid on a 20mm base of builders sand and a crushed rock layer below that. So I've dug up the sand but left the crushed rock, well and truly compacted now! Formwork will be a little trickier but the slab will vary from 100mm at one corner (high point) down to 140 at the other, average of about 120-130mm which should be more than adequate I believe.

[david s]

Re: thin foundation slab

You`ve been here long enough to know how topics go off on a tangent, as a matter of fact you`re done it yourself a time or two.

Yep, certainly have. My post was just an effort to address the original question.

[Laurentius]

Re: thin foundation slab

Goon squad's original post is inviting discussion specifically on slab thickness.

You`ve been here long enough to know how topics go off on a tangent, as a matter of fact you`re done it yourself a time or two.

[david s]

Re: thin foundation slab

Any suggestions on (firstly) am I correct in reducing the thickness and (secondly) what is the thinnest I should be considering? I'm thinking 2-3 inches is heaps but that is not based on any engineering!

Goon squad's original post is inviting discussion specifically on slab thickness.

[Laurentius]

Re: thin foundation slab

There is a balance between "thin as you dare" and overbuilt. To compensate for poor recipe, reinforcement, reinforcement placing, air entrapment and excess water, builders usually lean towards overbuilding. Making it thinner on the other hand saves weight, material and labour. As you are not building a racing yacht it is probably better to lean towards overbuilding.

Hi David,

I don`t think the focus is on how thick or thin it should be but the rational of digging up those pavers. You`re right, he should not go sub-par on the slab.

[david s]

Re: thin foundation slab

There is a balance between "thin as you dare" and overbuilt. To compensate for poor recipe, reinforcement, reinforcement placing, air entrapment and excess water, builders usually lean towards overbuilding. Making it thinner on the other hand saves weight, material and labour. As you are not building a racing yacht it is probably better to lean towards overbuilding.

[wotavidone]

Re: thin foundation slab

Having had a better look at it today, I think that the amount of water getting through to the ground underneath is limited; remember, these are solid bricks used for paving that have been concreted in. There are a few cracks here and there in the mortar between but overall it is actually pretty solid.

My main issue to deal with is the fall; it is around 40mm overall. The other is that the bricks have been laid in a decorative pattern, with curves and so on so lifting out is not going to be as easy as I thought. So the path of least resistance for me now is just to lay some formwork over the existing bricks and save myself the hassle of cracking the mortar/concrete and dealing with the curved bits.

Seems like the consensus is to go with minimum 100mm, that will then grow to 140mm at the deep end. I'll keep the edges to a shorter 40-50mm so that it doesn't stand out so much from the brickwork.

Form her up and get on with it, I reckon. Use some mesh in the slab, and size it so it is under where the blocks for the stand will lay. At the risk of being branded a heretic, I'm not big on using plastic under out door concrete, unless your area suffers from a serious rising damp problem?
We are luckier than our American cousins. in most parts of Australia, we don't have to even think about building for frost heave, and in much of southern Australia we don't get enough rain to cause a great deal of concern. It's only if you are unlucky enough to have reactive clay substrate that things get tricky, but your house should already have let you know about that.

[Laurentius]

Re: thin foundation slab

Hey, I would do the extra work and remove the brick and pour the slab per local, regional building codes. In other words build it the way it should be built per your climate, which I have limited knowledge. My thinking is you do not want anything between your slab and the ground it rests on. Did you put the brick in? What is directly under the brick? If it is a wet region, I would tend to lean towards a gravel pad under the concrete slab, just my opinion.

Normally whats under driveway paver`s is dirt! In 20 years those bricks have conformed to nature more than nature had conformed to the bricks. If there were a drainage problem you would know it by now. I would put the money from that to purchase ceramic fiber boards and blankets to insulate that puppy good.

[Campmaki]

Re: thin foundation slab

Hey, I would do the extra work and remove the brick and pour the slab per local, regional building codes. In other words build it the way it should be built per your climate, which I have limited knowledge. My thinking is you do not want anything between your slab and the ground it rests on. Did you put the brick in? What is directly under the brick? If it is a wet region, I would tend to lean towards a gravel pad under the concrete slab, just my opinion.