huangyhg

underpinning
can the top 2" of non shrink grout be omitted and instead vibtrate the conrete to the underside of the existing footing when underpinning?
my hesitation in allowing this is that some of the voids under the existing foundation will not be filled with concrete which may result in some settlement in the future.
does anyone have any experience in this?
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my big book of soils (foundation engineering handbook by winterkorn and fang)says to pour the underpinning concrete within 3 inches of the footer and place dry pack in the void. it says non shrink is not necessary. it gives an option of using liquid grout and a hopper.
i wouldn't trust the top layer of concrete to be vibrated firm enough to carry the load.
we always detail it with 3" of dry pack. contractors like to change two steps into one to save $. this may be work for lightly loaded structures but you are dependent upon how well they do there jobs. why take the risk?
the top space of 2 to 3 inches should be dry packed. non-shrink grout is not necessary. dry pack is stiff with little water mixed in. there is negligible shrinkage. using a stiff dry pack mix allows the drypack to be rammed into place using a heavy hammer to hit a flat 2x4 to pack in the sand/cement dry pack mix.
if you pour to the underside of the footing, there will be shrinkage of the concrete. the amount depends on the height of the underpinning pier. the bottoms of many footings are not level. therefore, if you do not dry pack, you can not be certain that you have full bearing between the top of the pier and the bottom of the footing.
non-shrink, high strength mortars are a waste of money. the load on dry pack is usually less than 50 psi compression. dry pack is usually a 2 or 3 (sand) to 1 (portland cement) mixture. if you calculate the shrinkage of 3 inches of concrete, the shrinkage is negligible. dry pack should shrink even less than concrete.
i agree with drypacking. this achieves a full, tight bearing condition instead of just allowing the grout to flow against the underside of the existing footing. it is quite possible to jack a building if you ram the drypack in tight enough.
ok, i'll disagree a little bit. on some of the underpinning projects i worked on, as a contractor, we had room enough to extend the face form above the footing 18"-24" creating some head pressure while vibrating the new concrete. working in short 4' sections of foundation, this seemed to work very well and did help to move the project along. we had monitors in place to check for settlement. as noted, the loads are relatively light.

i generally agree with those who would insist on grouting, but jheidt's way can work. the key in doing it his way is adequate consolidation. plastic settlement of the concrete can leave a fairly substantial gap, and you really have to insist on revibration after an interval to eliminate this problem.
jheidt2543's way is sometimes done and can work. the problem is that you can't know for sure if you obtained full bearing and you can't prevent the shrinkage. some people do it that way but it is wrong and is not in reference books on underpinning.
vibrating concrete up to a steel base plate works good if there is a relief hole in the plate and a form lip higher than the top of plate. expansive cement can be used to eliminate the possibility of shrinkage.
when underpinning a concrete or masonry foundation, there is no steel plate in which to insert a relief hole. contractors who specialize in underpinning dry pack their underpinning piers.
depending on the size of the baseplate, dry packing is labor intensive and prone to error. i prefer forming to the top of the base plate and placing a fluid, non-shrink grout or epoxy grout. a relief/check hole or two in the baseplate is necessary. the overforming should extend a distance out from the baseplate equal to the thickness of the grout.

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