Gap Between New Column and Existing Beam

[BimEng]

I'm working on a project where we are increasing the size of existing structural columns which fall within walls on multiple floors. The face of the existing column we are increasing was scarified and we dowelled into the top and bottom of the beams which would sit above and below the new columns. A cage was then installed and we also dowelled into the face of the existing column to create a bond between the two.

In order to pour the new section we did a letterbox form so that we could pour above the botton of the existing beam and vibrated well to try and get concrete right up to the underside of the beam. Now that we have removed the formwork we see that there are a number of gaps between the poured column and the existing beam and I'm trying to deterine the best way to repair this.

The gap needs to be filled completely to allow load transfer between the existing beams and the new columns. I am thinking of cleaning out the gap and using a very fluid non-shrink grout to patch these gaps. Any suggestions would be helpful.

 

[Ron]

Clean the area carefully, install a steel plate over the gap on all sides where gap exists, then pressure inject epoxy. The area is too small to flow grout into the cavity and the grout will shrink a little as well.

After the epoxy sets, you can remove the steel plates or leave in place. If you remove them, you can use neat cement grout to patch the holes and provide similar surface to surrounding concrete.

 

[Ingenuity]

Agree with Ron - epoxy injection is the way to go.

If the gap is great enough, pre-placed aggregate (3/8" nominal diameter) pre-placed by-hand prior to installing the steel backer-plate works very well in minimizing the volume of epoxy required, and also acting as a heat-sink for the exothermic reaction that occurs with mass epoxy.

In your case, the volume of epoxy is rather small, and you have a lot of surrounding concrete-mass, however, in instances of large resin volume a low-exothermic resin can be used, where the cure time is measured over several days, as compared to the usual initial cure of a few hours.

 

[BimEng]

Thanks guys. I think this will work well. Is the idea that you leave a small gap at the top of the plate to allow the injection of the epoxy? Thanks again.

 

[Ron]

BE...no, you drill a hole in the plate where you want to inject. That way you can develop the injection pressure.

 

[Ingenuity]

...and depending on the viscosity of the resin used, usually the perimeter of the steel plate/concrete surface will be "sealed" with a paste epoxy to avoid leakage.

Also, best to have 2+ injection ports - allows the operator to gauge when the void is full, and after it is capped off (injection port usually), you can increase the injection pressure slightly to ensure that you have indeed filled the void by maintaining the supplied pressure.