Underpinning with Shrinkage Compensating Concrete

[cmuller]

Is is a common practice in my country to use expansive cement(Type K) in the mix to control the shrinkage of the concrete when underpinning structures, avoiding the use of dry packing. I believe that this methodology works great because all the voids are filled in the process giving a 100% transfer of bearing loads. Does anyone has any experience here in the States? Does anyone has any ideas what may go wrong with this process?

 

[csd72]

Normal practice in Australia and the UK is to use standard cement up to 100mm or so below the beam and then non shrink grout above that. Much cheaper.

 

[a2mfk]

Describe in more detail what you mean by "underpinning" structures...? (I know what it means to ME, but I think you may be talking about something else..)

 

[cmuller]

I am talking about underpin of continuos concrete footings for concrete, block, or steel building structures.

 

[PEinc]

It is NOT common practice in the USA to use expansive cement in underpinning piers. I would not use expansive cement in my underpinning concrete in lieu of drypacking. If you just use expansive cement without drypacking, there is no guarantee that you have full, even contact between the top of the concrete pier and the bottom of the footing. In addition, properly performed drypacking helps preload the underpinning pier to minimize building settlements.

http://www.PeirceEngineering.com

 

[a2mfk]

cmuller- what country are you in?

In certain areas of Florida, USA and under specific circumstances, subsurface pressure grouting is used to fill voids and compact soils, as well as seal off cavities in the limestone. This procedure is normally terminated 10 feet down from grade to prevent damage to the existing structure. A more recent technique to densify loose, near-surface sandy soils directly below structures is the use of chemical grout, or expansive polyurethane foam. It looks a lot like "Great Stuff", the expansive yellow foam in a can used in building construction to seal gaps and voids.

Subsurface compaction grouting is not referred to as underpinning here, which is why I asked for clarification. Underpinning here means generally steel pipes pushed down into the ground to competent material and mechanically attached to the foundation...

The grout is a non-expansive type, a similar mix to grout used in masonry grouting but it can be a lower strength.

Do you have more info on the type you are referring to? Any links to websites?

 

[dcarr82775]

My understanding of expanding concrete is that it in fact shrinks. However very early in its life it expands if allowed to do so, but then shrinks like normal concrete so that in the end you have less overall shrinkage. By using it as undepinning, it is not allowed to expand and thus it will shrink. Maybe not as much as normal, but it will still shrink.

I wouldn't use it. We often underpin with normal concrete and have no problems. For a couple of feet the shrinkage isn't enough to worry about.

 

[oldrunner]

We used this method to transfer a portion of the weight of a large cobble terminal (3 foot thick spread foundation) to new spread footings on three sides of the structure with four buttresses. The buttresses were placed under large concrete beams strategicly placed to provide the transfer. The problem was the last inch or so between the new remedial structure and the existing structure (which by the way was sinking because we had 95 feet of mud instead of sandstone shown on the design drawings). We used Embeco which is cement and steel filings. Worked fine.

 

[cmuller]

a3mfk, I am currently in the US. I am an EIT here and a PE in my country, Peru. The structure I am studying at this time is a two story building, concrete wall 12" thick in the first floor, and glass block in the second one. It is a gym. It is needed to underping one side of the building to make an expansion that is up to 10' below bottom of the existent continuos gradebeams.
dcarr82775, your statement is partially correct, the shrinkage comp. concrete has an initial volume, then it expands, and then it can go back to the desired or designed mix design volume.
PEInc, when the concrete expands, don't you think that the contact surface is the same or maybe more than the one you can achive with the drypack. How the drypack help preloading the the underpinning pier?
Thanks everybody. This is my first time asking a question, and I am thrilled with your responses.

 

[a2mfk]

Interesting project, beyond my area of expertise. Does the underpinning also act as temporary shoring during excavation or do you also have to install sheet piling?

Welcome to the list and the US. Your country is beautiful, everyone should visit the Andes and Machu Pichu once at a minimum in their life...

 

[dcarr82775]

cmuller,

I agree, but my point is the concrete is not allowed to expand. It is restrained by the structure above so the actual expansion doesn't occur. Therefore it shrinks from its 'as cast' volume, not the theoretical expanded volume. I suppose the expansion could actually lift the structure above, but I doubt this happens.

 

[PEinc]

cmuler,

When you drypack an underpinning pier, you pack the damp sand/cement mixture into a 2.5 to 3 inch space between the bottom of footing and the top of the underpinning pier. The mixture is rammed into place using a sledge hammer and a short length of 2x4 lumber. When you hammer on the end of the 2x4, the other end packs the damp (not wet) mixture tightly into place. The cumulative packing effort pushes significantly against the bottom of the footing.

The attached PDF is one the few, but better, underpinning references. Enjoy.

www.PeirceEngineering.com

 

[cmuller]

Thanks a2mfk. The underpinning acts as a permanent element to transfer the loads of the exisitng structure to the soil.

dcarr82775, below there are 2 links that shows graphically the shirnkage of a compensating concrete vs the regular portland cement concrete. I placed those two links even they are the same just to validate the statement of those graphs. As you can see, on day 7, the portland cement concrete starts it's contraction. After 21 + days, it has reached it's maximum contraction. If you place the drypack next day, or within the following 7 days after pouring an underpin block, that section of dry pack will just go down together with the shrinkage of the regular mix you have below. Now, if you use a shrinkage-compensating concrete, the first expansion (day 1)mostly will allow the concrete to fill all the voids, the rest of the expansion, according to the poisson effect, should be relieved to the side that has no resistance (the face side), placing the block in a distress. It means that the contraction also will be lower.
With all these my brain is shrinking now...LOL

http://www.concrete.org/FAQ/afmviewfaq.asp?faqid=54

http://www.engr.psu.edu/ce/courses/...lowshrinkageconcrete/lowshrinkageconcrete.htm

 

[cmuller]

Thanks PEinc, I will read it.
Here is another reference in two different volumes:

http://isddc.dot.gov/OLPFiles/FHWA/009755.pdf

http://isddc.dot.gov/OLPFiles/FHWA/009757.pdf

 

[PEinc]

Thanks, cmuller. I already have those 2 FHWA "oldies but goodies."

http://www.PeirceEngineering.com

 

[csd72]

cmuller,

I fail to see your point. According to the graphs there is really no significant difference to the amount of shrinkage the main difference is just that compensated concrete expands prior to undergoing that shrinkage.

 

[PEinc]

Based on the graphs cmuller attached, I also don't see how expansive cement will prevent any settlement. The curves do show ultimate contraction.

Also, even the best underpinning procedures and materials may not prevent building settlement. References indicate that you should still expect some settlement of possibly up to 3/8 inch. I have designed and underpinned a few hundred buildings without any non-shrink or expansive cements , concretes, or drypack. Some buildings settle. Some do not. The theoretical amount of settlement associated with shrinkage of concrete and drypack is minor compared to the amount of settlement that may often result from things such as overburden removal prior to underpinning, soil settlement below the piers, or poor workmanship.

http://www.PeirceEngineering.com

 

[cmuller]

Thanks PEinc, all what you said is true and it happens. As csd72 said in his first comment, they do not use any drypack in UK and Australia. I also have seen that kind of procedures here and in my country with good results. Most important is analyze the situation (structure / soils conditions) and apply a good workmanship to try to get the best results. My Dad, also a Structural Engineer always tell me what his Mechanic Soils Porfessor told him at school back in the 60's: "Engineers are just like Doctors, all our problems are underground".
Thanks everybody!
PEinc, is there anyway to contact you?

 

[hokie66]

csd72 didn't say that.

 

[PEinc]

cmuller, use my web page link.

http://www.PeirceEngineering.com