Construction Joint
Construction Joint
(OP)
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
lab Reinforcement is top and bottom mesh
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






RE: Construction 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.
RE: Construction Joint
DaveAtkins
RE: Construction Joint
I would still add dowels.
RE: Construction Joint
RE: Construction Joint
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
RE: Construction Joint
RE: Construction Joint
RE: Construction Joint
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
RE: Construction Joint
Personally my decision to dowel, or not, would be based how close actual shear force is to the theoretical shear capacity.
RE: Construction Joint
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