Concrete Beams - Construction Joints

[matrixeric]

Hello All:

I was just wondering what the standard practice is when it comes to prepping a construction joint in a concrete structural element:

We typically specify that constructions joints be keyed and/or doweled depending on the structural element. For prep we specify that the existing concrete surface be wire brushed, cleaned and moistened prior to placing new concrete against it. In some instances we have also specified epoxy bonding agent prior to new concrete pour.

I am being told that wire brush and epoxy bonding agent is kinda excessive. Thoughts, comments are appreciated.

Thank you.

 

[Lion06]

We don't ask for any of what you have specified. Sometimes maybe an epoxy bonding agent, but you need to be careful. If the new concrete isn't placed quickly the bonding agent can actually act as a bond BREAKER.

If it's a critical element (like a grade beam with a horizontal joint) we'll ask for the surface to be roughened to a 1/4" amplitude.

 

[hokie66]

Depends. I never use bonding agents for the reason PA gave. For vertical construction joints in beams and slabs, location of the joints is the important thing. Locate them at minimum shear, generally at midspan, and you don't need to do anything except make sure the joint is clean. Wetting the joint is good, but often gets forgotten. If located other than at minimum shear, I use some big dowel bars at mid-depth of beams.

 

[rscassar]

Nothing special for veritical construction joints. I like to locate the construction joint at the third point of the span where there is low shear and low moment (even though the shear is the critical action of the two). The engineer is repsonsible for positioning the construction joint and the contractor is responsible for adhering to that location.

All other points mentioned above are important but often not done nor enforced. The joint should be roughened and cleaned of all oil, grease and dirt etc and dampened before placing the new concrete.

 

[LiloSA]

I think it depends on how long you wait before the subsequent cast. If only a few hours elapse between successive placements, only a visual check is needed to be sure that all loose particles, dirt, and laitance are removed. The new concrete will be adequately bonded to the hardened green concrete, provided that the new concrete is vibrated thoroughly over the area. Older joints need a little more surface preparation. Cleaning by means of an air-water jet or wire brooming can be done when the concrete is still soft enough that any laitance can be removed, but hard enough to prevent aggregate from loosening. Concrete that has set should be prepared using a wet sand blast or ultra-high pressure water jet. The existing concrete should be moistened thoroughly prior to placement of fresh concrete. Green concrete will not require any additional water, but concrete that has dried out may require saturation for a day or more. No pools of water should be left standing on the wetted surface at the time of placement. JOINT DESIGN FOR REINFORCED CONCRETE BUILDINGS - Darwin & Pfeiffer

 

[matrixeric]

Dear All:

Thank you for the input!