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Underpinning 3

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VictorAK

VictorAK

Structural
Dec 10, 2008
33
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.
 
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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?
 
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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.
 
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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.
 
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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.


 
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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.
 
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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.
 
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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.
 
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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.
 
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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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Ron's answer is about grouting a base plate, while the OP was about underpinning. Civilperson's irrelevant answer threw Ron off the track.
 
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I disagree with the notion the dry pack grout avoids a potential problem with normal concrete shrinkage. My understanding is that normal shrinkage occurs over the first several months. If so, the grout may be tight but nearly the same height of concrete will be subject to shrinkage.
I usually see the concrete placed with 18 to 24 inches of head and vibrated in place to provide good consolidation and contact with the underside of the footing (as indicated by jh.
I'm not sure what authority "reference books on underpinning" have other than indicating a common practice.
 
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Shrinkage of underpinning pits is not normally a problem. For example, with a concrete shrinkage coefficient of about 0.72" per 100 feet, a 20' deep underpinning pier would shrink vertically 0.14" (about 1/8"). Much of this shrinkage occurs overnight, before dry packing. The dry pack shrinks a negligible amount. I have never noticed a shrinkage gap at the top of a dry packed underpinning pier.
 
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Let's look at the entire reference:
"ACI Committee 209 suggests that the long-term total shrinkage might vary from l/2 to 1 1/4 inches per 100 feet, and that the factors affecting the rate of shrinkage are so variable that it could take from two weeks to more than six months to experience the first half of the total shrinkage. When it comes to a hands-on prediction of shrinkage, "floormeister" Alan Face's dictum No. 1 is that "every 20 feet of concrete is looking for 1/8 to 1/4 inch of shrinkage." Considering the combination of hydration, chemical shrinkage, plastic shrinkage, and drying shrinkage that occurs in concrete, Face's dictum No. 2 is that "wild things are happening in the concrete in the hours immediately following placement."

While we may agree or not that the amount of shrinkage is small, the part that stands out to me is the "two weeks to six months to experience the FIRST HALF of the total..."
 
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houseguy,

If what you wrote is entirely true, all underpinned buildings would continue to settle noticeably for months after they are underpinned. Field monitoring of underpinned buildings shows that this is not the case unless, for example, there is a soil bearing problem under the underpinning piers. I have monitored and plotted the settlements over time for many underpinned buildings. In almost 30 years, I have never seen concrete shrinkage to be a problem in underpinning.

You quoted, "wild things are happening in the concrete in the hours immediately following placement." Underpinning piers are usually dry packed the day after the concrete is poured, or later. Therefore, I guess that the "wild things" must have already occured before the pier is dry packed.
 
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I’m not sure what Dr. Hover was referring to (my reference was from an article he wrote). The language of your post seemed strikingly similar.
I was only commenting based on my understanding of shrinkage behavior over time and the reference cited. In my experience, the overwhelming majority of underpinning piers (that I have been involved in) are relatively shallow (5 to 8 feet) and therefore, I suspect not much concrete shrinkage is going to occur in any case.
I have no reason to doubt your experience in measuring underpins. It seems that in the cases where much greater depths are involved concrete shrinkage may be more meaningful. Along those lines, I think that if reinforcing steel is included in the pier installation it would have a beneficial effect on shrinkage. Perhaps that is a factor in your experiences also.

--Kenneth C. Hover, Ph.D., P.E. is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N. Y. An active member of numerous ACI committees, he is also a popular speaker at Hanley Wood's World of Concrete.
 
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I have designed and built underpinning piers to about 50 feet deep. I have never used reinforcing steel in underpinning piers and have had no shrinkage problems with the deepest piers. I always dry packed the day after pouring the pier or on Monday, if poured on Friday. I have even recently designed underpinning at Cornell University. I never met or heard of Mr. Hover.
 
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PEinc:

Great picture! I've always liked working on underpinning projects, although I've never had to go that deep. There are fewer contractors around that know how to do this type of work, even the "simple" ones of one or two 8' lifts.

The underpinning projects I've worked on it seems there was more concern about getting the concrete poured before the soil fell into the open excavation, than worry about settlement. If the sections are small enough, 4' wide or so, and there is plenty of cure time between sections, then there was very little, if any settlement - none that hurt the building. It seems that the job is all about alternate sequencing of the underpinning sections and timing.
 
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