Construction Joint

[opa123]

Hi all

During casting of Flat slab the contractor had problem with concrete pump so I instructed him to stop casting concrete and forming a construction joint.

I checked shear stress at this joint according to ACI 318-02, ACI commentary R11.7.7
recommends that Rft provided at this location should the be maximum of Rft. required by shear friction or by flexure, at my case Rft for required flexure is more than required for shear friction.

Should I use extra dowels for slab?

Note:-slab Reinforcement is top and bottom mesh

 

[jike]

Definitely, add dowels across the joint!

The area of steel for flexure and the area of steel for shear friction should be added together for the total area of steel required.

 

[DaveAtkins]

I disagree. The steel used for flexure causes a compression block on the compression side of the slab. This compression block takes the shear, via friction, while the reinforcing takes the flexural tension. That is why you use the worse of the two, and you don't need to add the areas together.

DaveAtkins

 

[jike]

This is one of those provisions in the code that I do not agree with. Shrinkage can sometimes cause these joints to act different than theory.

I would still add dowels.

 

[jt12]

I would also add dowels for shear friction. The commentary says there needs to be a resultant compressive force. I wouldn't say in a flexural situation that you have a resultant compressive force. If you have extra moment capacity in the slab, you could take advantage of that extra area of steel in both T & B as shear friction reinforcement.

 

[opa123]

Well thanks all for your nice input
I decided the following for my case
1-I will use bonding agent of surface of
Construction joint before casting second part
2-Slab reinforcement is top and bottom both directions
so when I make CJ at location of maximum +ve moment I can make use of top Rft. as shear dowels, as I think extra dowels to be drilled into slab may have negative effect and it is not reasonable due to the length of CJ around,50 meter

 

[jike]

Caution when using bonding agents: make sure that it is not affected by surrounding the environment. That is use a moist insensitive bonding agent if structure is or can be exposed to water or high humidity. Some bonding agents can be difficult to work with! Some are "time sensitive" and can work as bond breakers if the time is exceeded before the concrete is placed.

 

[miecz]

I would add dowels in the center of the slab. A strict reading of 11.7 would say no dowels are needed. However, I am reluctant to use reinforcement at the face of a slab for shear friction in slab situations as I imagine the reinforcement pulling through the cover (sometimes as little as 3/4") normal to the face of the slab. I only rely on shear friction in situations such as the interface of a wall and a slab, where the reinforcement cannot spall the concrete.

 

[HvZ]

I look at it this way.

Constructin joints are not movement joints. The aim with a construction joint is for two independantly poured section to act as a homogeneous element.

If the pour was not stoped, you would not have the dowels. So what is different.

The reinforcement that passes through the joint (should still be 100%), was design to resist the flexure and shear at the location where you now forced to provide a construction joint.

The key to a good construction joint is preperation of the joint face of the first pour, prior to casting the second pour. The joint face should be

1. Prepare the face while the concrete is still 'green'.

2. The cement paste should be removed from the face, by sand or water blasting to fully expose the aggregate and ensure full aggregate interlock, without damaging or loosening the aggregate. Do not use a mechanical scabble because it will loosen the aggregate.

3. The concrete at the joint face should be saturated to avoid water being sucked out of the second pour's concrete.

4. The joint face MUST be surface dry and free of dust and loose material.

A number of test has been done and having wet joint face, using a bonding agent or not having proper aggregate interlock reduces the performance of the joint.

A good construction joint also requires proper supervision and inspection prior to cast the second pour.

The only adverse effect resulting from construction joints is that restraint cracks will form in the second pour. The second pour is not free to shrink during it first few days, because it is restraint by is adherance to the face of the first pour. This will be more of a consern in liquid retaining structures.


Cheers

HvZ

 

[apsix]

A number of test has been done and having wet joint face, using a bonding agent or not having proper aggregate interlock reduces the performance of the joint.

That's the reason why many engineers feel more comfortable using dowels as there is a risk that the joint will not perform as well in shear as it should.

Personally my decision to dowel, or not, would be based how close actual shear force is to the theoretical shear capacity.

 

[HvZ]

Joint preperation as I descibed it will give a joint performance of 98%. Preparing the joint face using bond agent or casting against a wet face or not properly exposing the aggregate could reduce the performance of the joint to less than 50% in which case shear dowel is definitely required.

Comparing the actual with the theoretical is a good idee especially if you are uncertain about the supervision and competancy of the guys on site.

Cheers

HvZ