Thread: Expansion joints in slab

The slab in my shed had expansion joints cut with a diamond saw after it had cured. I epoxy coated the floor then filled the joints with sikaflex self levelling sealant which worked as promised - just flowed into the joints and was cut level with a sharp chisel. 7 years later the seams are still sealed flush despite a bit of movement in the slab.

The only issue I had was that Bunnings probably don’t sell a lot of it and a couple of tubes were well past their use by dates and had cured in the tube, so check the dates.

Leigh


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Thanks for your reply. So you don't apply the epoxy over the sikaflex self levelling sealant?

Firstly your shed will either come with or need an engineered footing / slab design which will detail thickness, reinforcement type and placement and any control or expansion joints required.
Generally on a slab the size you have noted will not have any expansion joints but will have control joints.
What Ambrosia referred to above is not technically an expansion joint but a control joint which is creating a weak point in the slab to encourage the concrete shrinkage cracks to occur within the joint. This is done purely for atheistic reasons.
These can be done buy inserting a “ Craka joint” strip into the concrete while wet but the most common way is a sawcut within 12 hours of concrete placement.
If you are planning on coating the floor it is best to do so first and then fill the joint with a polyurethane ( Sika) after. You don’t want the epoxy coating going over the polyurethane as it will crack

What he said! But I’m more of an agnostic that an atheist...

I was lucky (?) that the build was delayed quite a bit and the slab had a few months to fully cure/dry before I epoxied it. Then filled the saw cuts, then trimmed.

Leigh


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What Beardy said

I requested another quote and at the same time asked more questions about the slab. The guy explained that the slab will be engineered. I explained that half is over cut and half is over fill. He informed me that the part over the fill will include piers and he asked me about the soil type. So this all gives me more confidence. For some reason, I thought that for shed they didn't really bother designing the slab for the soil and site conditions. The guy said that for 9m x 9m they don't do expansion joints but since mine is larger than that this will be something their engineer will consider. As you say, it could be control joints, I don't know.

I think my confusion is that when you get a quote, some might exclude this service. But apparently, once you go ahead, if it not included, then they will charge a bit extra to have the slab designed, piers poured, etc. Which makes sense because it will be a bit different for every install and they don't want to do that detail design until you are committed to proceed. I have had to get a bunch of different quotes due to all the prices rises and the bank changing their rules, supplier delays, etc. So I can see why they quote based on an assumption.

Now I understand this, I am wondering if it would be worth arranging my own soil test at the exact spot the shed will be, after a couple of months to let it settle. Then I can give this report to the shed builder(s) and make sure I request they provide an engineered slab. I already have 3 soil tests on this land, but none at that location and none since the cut and fill was done. Worth it?

Letting the soil settle for a few months does nothing, if it is not virgin soil it is unsuitable as a foundation material and you will need to pier through the fill to virgin material.
Dont waste your money on a soil test, if you already have one that is great but for the type of structure you are proposing an engineer will usually refer to the soil classifications for your area to work with.
Check with your shed supplier, they often have a generic slab design included with the shed that has a number of scenarios included for different situations.
Project home engineers do the same with alternative design details for cut and fill, drop edge beam, piercing, retaining etc.

I bought my shed from Best Sheds and it came with an engineered slab design that council was happy to accept

Best Sheds are excellent, I got one, slab design included with details for the 3 basic soil classifications ie: S, M and H
I would be concerned about any concreter who quoted without asking for engineer design.
As Beardy said, your existing soil test is all you need for the classification.
All slabs/footing must be load bearing on the same soil strata and classification.
Where you have a cut depending how deep usually the edge beam will be on the virgin soil type but where your fill is you will need piers drilled through the fill to the same soil type/strata as the cut section.

Originally Posted by DaveVman

what is the contemporary practice with expansion joints in a large shed slab?

Aren't expansion joints only required if there's no rebar in the slab?

My shed slab has rebar, doesn't have any expansion joints, and no cracking after ~18 years.

Originally Posted by mpot

Aren't expansion joints only required if there's no rebar in the slab?

My shed slab has rebar, doesn't have any expansion joints, and no cracking after ~18 years.

I don't know. I don't fully understand why house slabs do not need expansion joints.
I'm no expert at all but I doubt that rebar prevents it. The rebar will hold it together but it wont stop it cracking. Then if water works through a crack to the rebar and rusts the rebar, then the crack will get worse. Concrete is just really weak in tension. Which is why you need the rebar.
The concrete is going to expand and contract and fortunately so will the rebar. So the rebar doesn't prevent this. However there are a great many variables about this so you may never have cracks but others will. For example if the sun never hits the concrete perhaps it will not expand and contract fast enough or unevenly enough to create enough pressure to cause a crack. But in another climate or another soil or another thickness etc it could be an issue. If the soil underneath moves, it's got to be possible for the concrete to crack. And soil moves.

This is just a generalised overview and there are other factors that come into to play but it will give you a bit of an idea

Concrete has high compressive strength but poor tensile strength so in situations where it is required to be self supporting and carrying loads like spanning opening with a concrete beam, a suspended slab or say a bridge etc steel reinforcement comes into play to give the concrete it’s tensile strength, the more steel and larger diameter bars allow it to span further whereas you will see in a footing the reinforcement is typically much lighter as it has continuous support on the foundation material and the steel’s principal role is generally tying it all together. You will notice the steel quantity and size increase as the footing classification gets higher to counter the more reactive and less stable foundation material and the design starts to resemble a suspended member. You will also notice the larger amount of steel on the the side of the member that is in tension

The other role the steel plays is what is commonly called temperature steel and that is the reinforcement mesh that sits 30 to 40mm below the surface of a slab. It’s primary role is to stop surface cracking from both the initial curing stage and the ongoing temperature variances the concrete will be subject to. That steel is then weakened by cutting every second bar in the location of your crack control joints that you sawcut after initial concrete cure. Hairline cracks do not effect the structural integrity of the concrete but are deemed unsightly by most.
So typically when you are doing say a driveway or path you have a 100 to 110mm thick slab with one layer of mesh at the top of the slab with 30 to 40mm of cover and it’s primary role is controlling cracking
House Slabs can require expansion or control joints if the shape and size is such that extensive cracking will occur due to expansion and contraction but in most cases it is not deemed necessary but any other slab that is poured up against it will require an expansion joint to allow the two slabs to move independently

There are a lot of factors in play when it comes to concrete. The following is just a small indication of them
Soil/site clasification
Slab design (engineer)
Specified Mpa (strength) batched at the plant
Diligence of the concreter doing the placement ie: correct placement of reo in the slab, actual pouring of concrete in correct sequence avoiding cold joints during pour, delays between deliveries (can lead to cold joints), correct vibration of concrete, weather on the day of the pour, curing technique and procedure, moister content of ground, design structural loads of erected structure on the slab, live loads (operating plant or machinery) and last but not very often considered is adjacent building/earthworks AFTER completion of the building.

Piers through fill and control joints inserted during concrete pour is the way to go. Concrete saws are for mistakes.

Originally Posted by DaveVman

I requested another quote and at the same time asked more questions about the slab. The guy explained that the slab will be engineered. I explained that half is over cut and half is over fill. He informed me that the part over the fill will include piers and he asked me about the soil type. So this all gives me more confidence. For some reason, I thought that for shed they didn't really bother designing the slab for the soil and site conditions. The guy said that for 9m x 9m they don't do expansion joints but since mine is larger than that this will be something their engineer will consider. As you say, it could be control joints, I don't know.

I think my confusion is that when you get a quote, some might exclude this service. But apparently, once you go ahead, if it not included, then they will charge a bit extra to have the slab designed, piers poured, etc. Which makes sense because it will be a bit different for every install and they don't want to do that detail design until you are committed to proceed. I have had to get a bunch of different quotes due to all the prices rises and the bank changing their rules, supplier delays, etc. So I can see why they quote based on an assumption.

Now I understand this, I am wondering if it would be worth arranging my own soil test at the exact spot the shed will be, after a couple of months to let it settle. Then I can give this report to the shed builder(s) and make sure I request they provide an engineered slab. I already have 3 soil tests on this land, but none at that location and none since the cut and fill was done. Worth it?

From what I gather shed companies don't worry about soil testing and such. They engineer the slab as if it's being constructed on highly reactive soil so as to cover all the issues that might arise.