Expansion Joints in Concrete Slab

nowImonboards · 18-09-2024 8:16pm #1

Hi all,

I'm pouring a concrete slab which sits between the house, the garden wall, and an existing patio. Here's a diagram looking down from above:

I was considering that I might be best to put expansion joints around the perimeter where it is in contact with an existing structure. I had also read that I should put a crack control joint in any slab > 4m. My slab is 6mx3m so I'm thinking to put one down the middle at 3m.

For the expansion joints I was thinking of using this Joint Expandafoam as it seems easy to use and is reasonably priced. This would be placed around the slab marked in blue in the following diagram:

I'll be pouring the slab all in one go so I would imaging this foam would be very difficult to use as the crack control joint down the middle of the wet concrete. As an alternative I was thinking one of these PVC Expansion Joint Strips would suit better. This would be used as the orange line in the diagram above.

The issue is that I'm also using rebar mesh so if I'm adding the PVC Expansion Joint Strip, I'd probably have to cut the mesh down the middle to fit it in. Wouldn't this work against the job of the mesh?

I'm thinking if there was a way I could use the foam down the middle I could cut strips in the foam so it would sit down around the mesh avoiding the need to cut the mesh. Multiple pours is not an option as it could double my budget. I'll just about get the full slab from the minimum order quantity from the concrete truck.

As per forum rules this is not a request for structural advice and all decisions will be my own but I would very much like to get peoples thoughts, what they have done or whether they can point me to some useful guides.

Thanks in advance

mr.stonewall · 18-09-2024 8:27pm

Consaw afterwards with length of timber to round the blade beside. No need of an expansion joint around the building or patio as shown in blue. 1 cut down 75mm deep would be sufficient in that slab

Quick q, what kind of traffic will be on this slab.

Spend the time and effort into getting a good base, that is at least 300mm of well compacted stone under the concrete

nowImonboards · 18-09-2024 9:49pm

Thanks mr.stonewall,

It's going to be a base for a veranda (metal structure with glass walls).

Yes, I have at least 150mm of stone that was under the existing flagstone patio (it's similar to 804 but without any fines). I went over this for a while with the whacker and while it doesn't seem to compact in a way that cakes together like traditional 804 I think I have compacted it all it can go.

On top of that I have another 150mm of crushed gravel (brown version of the 804/SR21) compacted in layers with the whacker also.

Can you elaborate on what you mean by "round the blade beside" above?

If I understand correctly, a single cut down the middle (orange line in diagram) would act as both a crack control and an expansion joint. Is that right?

If the mesh was 50mm below the surface then I'd be cutting through that with a 75mm cut. I do plan to have another layer of mesh 50mm below that (which would also be 50mm above the under side of the slab). Would that cause some kind of tension imbalance or is that just crazy thinking?

Calahonda52 · 22-09-2024 8:36am

Poster meant run along beside to keep it straight.

also be aware of Coriolis effect with a big consaw

not a fan of cutting the mesh and leaving the end exposed to wet

Lumen · 22-09-2024 9:01am

I think you might be overthinking it.

If you have a slab big enough to need an expansion joint then you're making two independent slabs, whether it's by a separated pour or by cutting afterwards. You don't want rebar running between them or there's no point in having the expansion joint, so just back off the mesh either of side the joint.

What's the mesh for? Don't just stick it in for no reason.

nowImonboards · 23-09-2024 10:21am

https://www.boards.ie/discussion/comment/122641813#Comment_122641813

Hi Lumen,

Are you sure about that. Isn't one of the jobs of the mesh to ensure that, if/when the slab cracks, it won't separate. If we purposely put a gap in the mesh right under the crack control joint then aren't we increasing the chances that the slab will shear at the (controlled) crack?

Lumen · 23-09-2024 10:33am

I honestly don't know what you are talking about. You seem to be proposing cutting the top of the slab, through one set of reinforcement, and then leaving the bottom of the slab intact, with another set of reinforcement in place.

Just follow standard construction details.

Mellor · 23-09-2024 11:45am

https://www.boards.ie/discussion/2058369595/expansion-joints-in-concrete-slab

Pushing bits of foam into the slab around an intact mesh in not an expansion joint. A expansion joint intends to allow movement. Doing that will nor achieve much other than a weak point.

https://www.boards.ie/discussion/comment/122645032#Comment_122645032

That's what rebar does in a slab. It does not do it across an expansion joint, an expansion joint needs to be allowed to move.

A control joint is different. the slab is structural stable across the point. As opposed to independent slabs either side of an expansion joint. You need to figure out which you actually need and detail accordingly.

nowImonboards · 23-09-2024 11:57am

https://www.boards.ie/discussion/comment/122645088#Comment_122645088

I am not proposing that at all. I wasn't proposing any cuts. Cutting was suggested. I am proposing to avoid cutting into the reinforcement so that if/when the controlled crack does form, the slab will be held together.

https://www.boards.ie/discussion/comment/122645372#Comment_122645372

Hi Mellor,

I wasn't proposing this as an expansion joint. i was proposing the foam as an expansion joint around the perimeter of the slab where it meets the other structures.

In addition to this, I was proposing a separate crack control joint. I was wondering how best to implement the crack control joint without risking damage to the reinforcement.

One of the responses above suggested I would not need the expansion joint around the perimeter and that a simple cut down the middle would suffice. However, such a cut would cut through the reinforcement. I would also think that a shallower cut would reduce the coverage for the mesh at that point.

So the question then is, how people normally implement the crack control without affecting the reinforcement?

Mellor · 23-09-2024 1:59pm

https://www.boards.ie/discussion/comment/122645422#Comment_122645422

I was refering to the comment that said;

If I understand correctly, a single cut down the middle (orange line in diagram) would act as both a crack control and an expansion joint. Is that right?

That is not correct. Joints are constructed as one or the other, they have different purposes.

A crack control joint is not full depth. For a 150mm slab, it might only be 20-25mm deep. It can also be tooled or an inserted joint former.

nowImonboards · 23-09-2024 5:27pm

https://www.boards.ie/discussion/comment/122646050#Comment_122646050

Thanks but isn't that what I suggested to begin with? In my initial post I proposed both the foam as the expansion joint and the PVC joint strip as the crack control section. One of the responses suggested I didn't need the foam and only needed a cut down the middle. That's where I responded with the comment you highlighted.

So the questions still stand. If making a cut, shouldn't I avoid cutting the mesh?

Why wouldn't I need the expansion foam around the edges? What would have to be different for me to need it?

Mellor · 24-09-2024 3:46am

https://www.boards.ie/discussion/comment/122646946#Comment_122646946

In your first first post proposed using an expansion joint to the centre.

I'll be pouring the slab all in one go so I would imaging this foam would be very difficult to use as the crack control joint down the middle of the wet concrete. As an alternative I was thinking one of these PVC Expansion Joint Strips would suit better. This would be used as the orange line in the diagram above.
The issue is that I'm also using rebar mesh so if I'm adding the PVC Expansion Joint Strip, I'd probably have to cut the mesh down the middle to fit it in. Wouldn't this work against the job of the mesh?
I'm thinking if there was a way I could use the foam down the middle I could cut strips in the foam so it would sit down around the mesh avoiding the need to cut the mesh. Multiple pours is not an option as it could double my budget. I'll just about get the full slab from the minimum order quantity from the concrete truck.

The strips would maybe work as a crack control former. But you should not be cutting the mesh for crack control. And putting the foam around the mesh would be useless. Won't do anything.

You can also tool a crack control joint.

So the questions still stand. If making a cut, shouldn't I avoid cutting the mesh?
Why wouldn't I need the expansion foam around the edges? What would have to be different for me to need it?

Don't cut the mesh for crack control. Don't lay mesh across an expansion joint to between with.

You need an expansion joint around the edges. I don't think I've implied you didn't.
It's a very small slab, I think you are overcomplicating it. Is there a finish going over?

nowImonboards · 24-09-2024 10:19am

https://www.boards.ie/discussion/comment/122648113#Comment_122648113

Apologies for the confusion. In my first post I proposed using the remaining foam I was going to use for the expansion joint as a former for the crack control joint because if I used the rigid joint former it would mean I would probably have to cut the mesh which I am worried would go against the job of the mesh (see sentence after the bolded section of your quote above). It was probably poorly worded. Apologies for that.

When you say "Don't lay mesh across an expansion joint to between with.", did you mean crack control joint? I'm just checking because I don't see how it would be possible to lay mesh across the expansion joints.

I don't think I'm over complicating it. It's a very simple thing. It's a 6x3 slab. That is big enough to form a crack. There will be 10mm tiles on top. I want to minimise the risk of a crack forming randomly through the tile pattern. A crack control joint should be simple. The expansion joint around the perimeter should be simple. I just want to ensure that I don't weaken the whole slab by incorrectly introducing the crack control joint.

One response above suggested I don't need the expansion joint. This response suggests I do. I guess I'll just add it to be safe. Surely it can't do any harm, right?

I appreciate all responses. I am just trying to understand the reasons.

Whether I cut the slab as suggested in one response, or whether the mesh is pre-cut in sections to allow for the crack control down the middle as per another response, both would have the same result in my opinion. In these cases, if the crack does form along that control joint then we effectively end up with two separate slabs. My understanding was that one of the purposes of the mesh is to keep the slab together even after a crack forms.

Have I misunderstood this?

Calahonda52 · 24-09-2024 1:26pm

OP, from this section it's obvious that you are either not reading or don't understand what is being pinned in response here.

Whether I cut the slab as suggested in one response, or whether the mesh is pre-cut in sections to allow for the crack control down the middle as per another response, both would have the same result in my opinion. In these cases, if the crack does form along that control joint then we effectively end up with two separate slabs. My understanding was that one of the purposes of the mesh is to keep the slab together even after a crack forms.

Crack control and expansion joints are are 2 completely different ideas

In passing how the slab is cured is key, you need to keep it wet, the go to solution once it sets a bit is cover with hession and keep it wet for at least 7 days

The shallow consul cut , ABOVE the mesh, is sometimes called crack inducement cut

Google the link below and select images and have a good look

IIRC Mellor says an EJ is the same as two slabs, which is evident from the rebar above

https://www.google.com/search?client=safari&sca_esv=124892d69d68a598&sca_upv=1&rls=en&sxsrf=ADLYWIKSBHiGsu99uIVZLzCviIkRmh_d2Q:1727183706327&q=expansion+joint+in+slab&udm=2&fbs=AEQNm0Aa4sjWe7Rqy32pFwRj0UkWd8nbOJfsBGGB5IQQO6L3J7pRxUp2pI1mXV9fBsfh39KRvAkf_RbLmqO8b2Na6CPIr1VCrQ4GTou3YNw-y1mlrephiQ6L6B1T608OhUL0imy7lvPuPe4YMmB6Pg1JxWmyGn3Jxxy5sCfQf9mOF5_ygY77FA8&sa=X&ved=2ahUKEwja55va1NuIAxUpXEEAHVOIAGcQtKgLegQIMhAB&biw=2444&bih=1187&dpr=1#vhid=ODOXvC03PcK-eM&vssid=mosaic

Mellor · 24-09-2024 3:04pm

https://www.boards.ie/discussion/comment/122648811#Comment_122648811

When you say "Don't lay mesh across an expansion joint to between with.", did you mean crack control joint? I'm just checking because I don't see how it would be possible to lay mesh across the expansion joints.

No I meant expansion. Mesh does not cross an expansion joint. The point of an expansion joint to to allow movement. Mesh prevents movement. You do not want a expansion joint in the middle. In larger slabs they would.

I don't think I'm over complicating it. It's a very simple thing. It's a 6x3 slab. That is big enough to form a crack. There will be 10mm tiles on top. I want to minimise the risk of a crack forming randomly through the tile pattern. A crack control joint should be simple. The expansion joint around the perimeter should be simple. I just want to ensure that I don't weaken the whole slab by incorrectly introducing the crack control joint.

It is simple. Which is why I said you are over complicating it by talking about cutting mesh, and puting foam around the mesh. Just tool a control joint in to slab.

Whether I cut the slab as suggested in one response, or whether the mesh is pre-cut in sections to allow for the crack control down the middle as per another response, both would have the same result in my opinion. In these cases, if the crack does form along that control joint then we effectively end up with two separate slabs. My understanding was that one of the purposes of the mesh is to keep the slab together even after a crack forms.
Have I misunderstood this?

Your misunderstanding is that a crack forming at a control joint forms two separate slabs. The mesh is continuous across a control joint, this makes both side structurally connected as one.

In an expansion joint, there no mesh or other connection. They are independent. An example is the new slab and the slab of the house.

nowImonboards · 25-09-2024 12:57pm

Ok, I think we are all misunderstanding each other here and the confusion has caused us to go off on a tangent. Perhaps my wording was poor. The title of my post only mentions expansion joints whereas my actual first post discusses two distinct types. Please allow me to clarify.

"Crack control and expansion joints are are 2 completely different ideas"
@Calahonda52: I totally understand this. My original post describes both. I have marked both with different colours in my second diagram. I have the expansion joints in blue and the control joint in orange.
I have researched this and read some of the posts in the link you sent. I am not calling the orange line down the middle of my slab an expansion joint. I was just checking to see if I could use the left over expandafoam (which I am planning to use around the perimeter for the actual expansion joint between the existing structures and the freshly poured slab), as a control joint former in the freshly poured slab. Early responses have suggested this is a stupid idea so that had been put to bed.

So now, I'm just talking about the best approach to creating the control joint.
"The shallow consul cut , ABOVE the mesh, is sometimes called crack inducement cut"
I understand this too. This is what I'm referring to. The first response above suggested a cut of 75mm. My concern was that a cut of that depth would cut into my mesh which is at 50mm below the surface. This would effectively work against the one of the jobs of the mesh. You agreed with this in your response and after that the following responses seemed to think this was what I was referring to as the expansion joint again and so the mesh should be backed off at that point. And off we went on that tangent. Once again, I think this was probably due to the omission in title of my post.

"No I meant expansion. Mesh does not cross an expansion joint. The point of an expansion joint to to allow movement. Mesh prevents movement. You do not want a expansion joint in the middle. In larger slabs they would. "
@Mellor: I understand that. The joint I am talking about here is the control joint. The expansion joints were the blue lines in my diagram above. This is on the perimeter of the slab so this is why I was confused when you mentioned mesh crossing the joint. There will be no mesh at the perimeter. The mesh only goes to 50mm from the perimeter.

"Your misunderstanding is that a crack forming at a control joint forms two separate slabs. The mesh is continuous across a control joint, this makes both side structurally connected as one."
That is actually what I was saying. We are both talking about the same thing. When one of the responses mentioned backing off the mesh or indeed cutting into the mesh under this joint, that's what I was saying would cause us to end up with two separate slabs. I think this confusion is all due to misreading the post due to an omission in the title. My fault.

So now, if I could bring this back on track to close the thread, my plan was/is to put the expandafoam around the perimeter where the slab meets existing structures. These are the expansion joints.

Secondly, I plan to tool a line down the middle of the slab in an effort to control the location where the eventual crack may form. This is the crack control joint. This can be cut into the slab using a consaw in the days following or a groover trowel as the concrete is setting. This cut/groove will be above the continuous sheet of mesh so that if/when the crack does form along this line, the slab will still be held together by the mesh.

Are we on the same page now? If so then the only real change from my original post was my silly suggestion of repurposing the left over expandaform as a control joint former.

If we are still not on the same page then please accept my apologies and thank you for your patience. I am new to this and my questions are just an effort to educate myself as I do find all of this very interesting, am enjoying the work (although it is taking much more of my time than I had anticipated) and just want to do the best job I can.

Expansion Joints in Concrete Slab

Comments