Is an Expansion Joint Needed Here?

[hamholfarm]

Hi Everyone!

I've been given two different answers from two of the "Big" name concrete outfits around here (York, PA).

I have a 50' x 40' slab I want installed. It is in a pole building where the inside perimeter of the bottom of the building is pressure treated 2" x 6" boards. One contractor says I should use expansion joints around the perimeter, and the other contractor says it is not necessary. Which is the "correct" way to do it?

This is what I've asked for in their (and other contractors) proposals:
Adjust grade as necessary, 4,000 PSI fiber filled concrete, 6" deep, wire grids, vapor barrier, vibra screed, machine trowelled smooth. Control joints saw cut on 12' x 12' squares. I have had the pad and building in place for just over a year now to allow for settling prior to the concrete.

Any comments or recommendations / changes ??

Thanks for the help!


Chip

 

[Wardster]

Use the expansion joints. Concrete will expand and contract and the material is cheap insurance. The other contractor probably figures the 2x6 will compress (bulge out) enough in the event of expansion, but you would have to ask him to be sure.

I am not a big fan of fiber filled concrete especially when it's already 6" deep and reinforced (overkill). Unless you want to do it, have your contractor include the installaton of a self-leveling urethane joint filler in all the control joints. They may even use this around the perimeter depending on the type of expansion material used. How are they going to cure it? They could wet-cure it with burlap and plastic (or burlene) or with a membrane forming curing compound.

Do not allow them to add water to the mix as it will reduce your compressive strength. If your contractor thinks he will have a problem working the concrete at the slump of the mix design, then have them include superplasticizer on the truck. This will allow an increased slump (easier to work) without effecting the strength.

-Wardster

 

[5wndwcpe]

Chip, have you received any quotes yet ? Your slab is similar to mine and I was curious as to the prices out your way.

 

[boiler7904]

Wardster is right. Have them install the expansion joint material. It's cheap insurance against future problems.

 

[hamholfarm]

How are they going to cure it? They could wet-cure it with burlap and plastic (or burlene) or with a membrane forming curing compound.

In speaking with three different estimaters, so far, when I asked about curing each said the "Pouring Foreman" would explain to me how to keep the concrete wet with the garden hose and for how long.


[/QUOTE]
Do not allow them to add water to the mix as it will reduce your compressive strength. If your contractor thinks he will have a problem working the concrete at the slump of the mix design, then have them include superplasticizer on the truck. This will allow an increased slump (easier to work) without effecting the strength.

-Wardster[/QUOTE]


These contractors are not affiliated with the concrete manufacturers, they are the "labor" and just order what they need from the 2 or 3 manufacturers in the area.


Chip

 

[Wardster]

The 'pouring foreman' should provide you with the materials necessary to wet cure the concrete (ie the burlap/plastic). The only thing you should have to do is keep the burlap wet via soaker hoses.

The contractor is the one who is going to order the concrete from the plant and this same 'pouring foreman' is going to be in the driver's ear when he shows up. If the mix is stiff, then he may tell the driver to add water and down goes your compressive strength.

-Wardster

 

[dxdexter]

Expansion joint is a misnomer. Technically speaking a 40' x 50' the slab will never occupy the same volume as it did when it is originally placed.The concrete will under go a certain amount of shrinkage during the curing process and any increase in volume will not surpass this shrinkage on a slab this size. The proper term should be isolation joint and yes it is recommended where the slab edge will meet the vertical walls. You do not want adhesion between the walls and slab. You could also just turn plastic sheeting up the wall and achieve then same results.

You should also cut crack control joints. I would recommend cutting the slab into quarters and the cuts should be at least 25% of the slab thickness. Contractors are notorious for just producing enough of a cut depth just to give the appearance that it was done and nowhere near deep enough.

 

[hamholfarm]

Dex,

As you can see in the attached picture my shop will have three "bays". This front view of the shop is the 50' side, and the depth is 40'. Would it be sufficient to cut three control joints, one between each bay going front to back, and then cut one control joint at the 20' line going side to side?

Thanks for the help!

Chip

 

[boiler7904]

Dex,

As you can see in the attached picture my shop will have three "bays". This front view of the shop is the 50' side, and the depth is 40'. Would it be sufficient to cut three control joints, one between each bay going front to back, and then cut one control joint at the 20' line going side to side?

Thanks for the help!

Chip

You're probably ok with one joint between each overhead door but you'll need to add one running the width of the building. We typically spec joints at 12 - 15 feet on center max for commercial work. For you that would mean at least 3 running front to back and 2 running side to side. There are a few other general rules when you get into odd shaped buildings and have other slab obstructions to go around but those don't really apply here.

The closer the joints are to one another, the more likely that a crack (shrinkage or otherwise) will "find" the joint and visually disappear which is what you want.

Sawcut joints need to be 1/4 to 1/3 the depth of the slab with one exception. If you are installing radiant floor heating pipe, make sure you know where that pipe is in relation to the surface and adjust your joint depth accordingly.

I would avoid tooled joints like you typically see in sidewalks. They don't provide enough relief which is why a lot of sidewalks crack within a couple years of installation.

 

[dxdexter]

You're probably ok with one joint between each overhead door but you'll need to add one running the width of the building. We typically spec joints at 12 - 15 feet on center max for commercial work. For you that would mean at least 3 running front to back and 2 running side to side. There are a few other general rules when you get into odd shaped buildings and have other slab obstructions to go around but those don't really apply here.

The closer the joints are to one another, the more likely that a crack (shrinkage or otherwise) will "find" the joint and visually disappear which is what you want.

Sawcut joints need to be 1/4 to 1/3 the depth of the slab with one exception. If you are installing radiant floor heating pipe, make sure you know where that pipe is in relation to the surface and adjust your joint depth accordingly.

I would avoid tooled joints like you typically see in sidewalks. They don't provide enough relief which is why a lot of sidewalks crack within a couple years of installation.

Great advice.

I would lean towards boiler7904's advice of 12 to 15 feet spacing on the joints.

My first post was more based on commercial inspected jobs where the slab thicknesses are higher and the concrete slump is more tightly controlled. You can push the spacing out as slab depth increases. The Portland Cement Association recommends a 50% increase in spacing when the slump is less than 4" or super plasticizers are used to reduce the concrete water/cement ratio and thus the drying shrinkage.

 

[BYBYV8]

You should also cut crack control joints. I would recommend cutting the slab into quarters and the cuts should be at least 25% of the slab thickness. Contractors are notorious for just producing enough of a cut depth just to give the appearance that it was done and nowhere near deep enough.

I don't want to hijack the OP's thread but is it ok to cut control joints in a slab that's three years old?

 

[dxdexter]

I don't want to hijack the OP's thread but is it ok to cut control joints in a slab that's three years old?

Control joints should be cut as soon as it's possible to saw the slab and not cause "ravelling" or a jagged cut line. This is usually around 12 to 18 hours depending on the ambient temperature.

If your slab is three years old then don't bother. If the slab was going to crack then it already has. If there are no cracks visible then thats great, but cracks maybe so tight you can't see them until farther down the road.

 

[hamholfarm]

Chip, have you received any quotes yet ? Your slab is similar to mine and I was curious as to the prices out your way.

I can't give you a comparison on prices yet because not a single contractor out of 5 has actually quoted me on what I asked for !!


Chip

 

[BYBYV8]

Control joints should be cut as soon as it's possible to saw the slab and not cause "ravelling" or a jagged cut line. This is usually around 12 to 18 hours depending on the ambient temperature.

If your slab is three years old then don't bother. If the slab was going to crack then it already has. If there are no cracks visible then thats great, but cracks maybe so tight you can't see them until farther down the road.

Thanks for the info. The slab has NOT been parked on though, so I'm certain it will still settle under the weight of several cars parked on. The home has sat unoccupied this whole time.