"hoe-ramming", it's not what it sounds like

[bevo98]

Hi,

I have an issue that has come up on a site that has a lot of rock that needs to be removed. The contractor wants to place concrete footings over several different stages (not that unusual). However, the site has not had all of the rock removed and the contractor will be "hoe-ramming" in between footing placements to remove additional rock. There is concern the the "hoe-ramming" may damage the footings by transferring loads through the rock to the footings. I do not think that this should be an issue as long as the equipment is kept a certain distance from the footings and the concrete is allowed to develop a certain amount of strength (2 or 3 days seems sufficient to me). Has anyone had this question arrise in the past or had any prior experiences with this issue? TIA for any help you can offer.

 

[structuresguy]

In general, I do not think that the vibrations caused by the hoeram will damage the surrounding footings. I agree that I would wait at least 3 days, prefereably 7 for the concrete to set up prior to allowing the hoeram activity. The vibrations at an early age could cause microcracking in the green concrete.

I suppose if you were really concerned about the vibrations, you could have them add fiber reinforcing to the footing concrete to help bind the concrete and make it more durable to vibratiions.

 

[Ron]

Hoe-ram vibrations are low frequency, high amplitude vibrations that won't likely affect concrete of adequate strength. As Structuresguy said, wait a few days and don't take a chance on affecting green concrete.

These vibrations are greatly attenuated by distance. Keeping the vibrations a safe distance (usually 10-15 feet is adequate, though if you're getting direct transfer through rock, you might want to move a bit farther away) is usually sufficient to attenuate them to levels below damaging frequency or amplitude. The critical time for the green concrete is the first 48 to 72 hours of curing.

Fiber is OK to use, but increasing the concrete strength to achieve early strength gain will help as well.