Jackhammer vibration

[7Pioerre]

Hello. We would like to put stiffer column below a beam and plans to use jackhammer or chipping hammer to chip the middle of the beam. My concern is whether vibrations from a chipper hammer can make the existing bars and concrete slip or disturb their bonding. Is this cause for concern from vibration from a jackhammer or chipping hammer?

 

[Ron]

No. The effect of the jackhammer is localized and not likely to affect much.

 

[7Pioerre]

why can't the concrete transmit the vibration? But if the jackhammer touched a bar, wouldn't it transmit the vibration to the length of the bar inside?

 

[Ron]

Yes, the vibration is transmitted, but the effect on hardened concrete away from the point of impact is neglible.

 

[7Pioerre]

I'm inquiring whether the bond between bar and concrete in the region of the vibrations can get looser or detached microscopically. You seemed to be stating it won't. But why won't it?

 

[Ron]

Microscopically, vibration will cause microcracking in the concrete. Yes, it will crack farther around rebar because the rebar will transmit vibration better than the surrounding concrete.

Usually when that occurs, the concrete will essentially "re-heal" itself through a process called autogenous healing. This occurs because not all of the cement in concrete gets hydrated and full hydration takes a long time, particularly in "dry" concrete. Unfortunately, without destructive evaluation of the concrete through petrographic examination, you can't tell the extent of the effect.

If you are concerned about it, have a sample taken that extends just past the rebar (take the sample adjacent to the rebar...you don't have to cut the rebar in the process)and subject the sample to microscopic examination that would include petrography and SEM analysis with EDAX to determine the presence of unhydrated cement and to determine if reaction products are present that would indicate the potential for autogenous healing.

 

[7Pioerre]

How many inches from the rebar point of vibration contact do you think the vibration would affect? What's the theoretical distance it can cause microscopic separation of concrete from rebar, based on your experience?

 

[Ron]

There is no theoretical distance. Concrete is not an isotropic, homegenous material even though we assume it to be so in much of our analyses. As we approach microscopic levels, it becomes even less so.

My best guess is that the amplitude of vibrations that might be considered to cause microcracking would radiate about 3 to 5 bar diameters from the face of the rebar.

All of this could be measured during the process of jackhammering and you could set alarm triggers to let you know if vibrations are exceeding levels that might be damaging to your structure.