Concrete Shrinkage in Footings

[SperlingPE]

I am looking for an equation to predict the amount of drying shrinkage in a concrete footing.
The footing sides will be formed by grade.
Depth of footing will be 30"-36". Width will be 8'.
Reinforcing will bar mats at the top and bottom.
The footing will be excavated in an existing building.

We are trying to predict how much concrete shrinkage to expect.
Specifically, what will the elevation of the top of the footing be after curing/drying.

 

[ahypek]

Take a look at Appendix A of ACI209.2R, if I recall correctly there's an equation for shrinkage strain.

I believe it mentioned something about the accuracy of the results being at least 15-20% off calculations due to the amount of variables present.

 

[hokie66]

Shrinkage in the vertical direction will be negligible. Plastic settlement could be an issue, but only if compaction is not adequate. Make sure your vibrator operator does a good job.

 

[dhengr]

SperlingPE:
Why such great concern about top of footing elevation, you can hardly ever cast them that accurately in terms of elevation in any case? Detail the footing so some compacted fill and a new slab section passes over it. Put a small pedestal on top of the footing if need be, and allow for an inch or two of grout (+/-) to be placed in the unknown vert. space btwn. the top of the pedestal and the base plate of the structure above. Otherwise, Hokie’s consideration about vibrating the conc. is an important consideration.

 

[SperlingPE]

Some background.
New sensitive equipment installation.
The foundation will be poured and the equipment will be installed 90 days later to take care of the majority of any possible change in volume.
The new equipment is sensitive to height differences where it interfaces with the foundation [embedded bars, track, rollers-bearings]
Height differences will cause excessive wear to parts moving over the foundation.
We are trying to possibly come up with some kind of value so that some adjustment or fit up can be included in the equipment.
The changes to height of the foundation are probably minimal but may not be negligible for this piece of equipment.

Also waiting on soils information to decide on what to do for the foundation.

 

[MotorCity]

Maybe you can pour the top of foundation an inch or two lower and then use non shrink grout to make up the difference?

Or consider some type of arrangement using plates and leveling nuts?

Even if you can accurately predict the vertical shrinkage and are able to account for it, how do you plan to handle settlement? The settlement probably wont be "zero" and most likely will be more than the vertical shrinkage of the concrete.

 

[SperlingPE]

Plates and leveling rods are being used to level the embedment bars in the concrete. The embedment bars are attached to the plates and leveled. Non-shrink grout is then poured around the embedment bars. This covers the plates at this point.

Shrinkage can be dealt with when the equipment is installed, we are just trying to get an idea of what could possibly happen.

 

[dhengr]

SperlingPE:
Ask the OEM of the particular equipment what their setting requirements are. As likely as not, they have instruction sheets/specs. for this planning and final setting. Otherwise, your millwrights will take care of the rest. Again, you can hardly cast a footing horiz. enough or flat enough to take care of what some fine tuned equipment requires in the way of final setting, elevation and leveling, they will do this with leveling nuts, shims, maybe tapered shims at four or five hold-down/bearing/anchor bolt locations on each side rail/sill, and then the whole sill gets grouted with non-shrink grout after everything is set. Settlement under the new loading or operational vibration may be an issue if you are trying to match up with some existing equip. on an older foundation.

 

[SperlingPE]

leveling of the machine is not an issue
this can be done when the equipment is set
the issue is what can be expected for concrete shrinkage in the first nine months after the equipment is set?
this also can be taken up by the machine, but the supplier would like to know what kind of number to expect.

 

[rapt]

Assuming good quality concrete with a depth of about 1m, shrinkage at about .0003 would give about .3mm shrinkage. Because it is so thick not much will have occurred over 3 months so most of it will still occur after installation.

But it is negligible. How much settlement will you get in the ground under the footing over time. That will be far more relevant.

 

[oldestguy]

Shrinkage is dependent on the placement water/cement ratio. The stiffer the mix, the less any subsequent shrinkage.

 

[SperlingPE]

rapt - how did you come up with your number?
The soils engineers are working on values for input into FEA mat slab foundation design.

 

[SperlingPE]

The shrinkage will also depend upon humidity and other conditions that cannot be controlled.
I have been searching for any type of information.
Most information found has to do with slabs and does not address possible vertical shrinkage.

 

[Ron]

Not an issue. Any shrinkage that occurs will be minimal as hokie66 noted. If you have any significant shrinkage, you can certainly make it up as you place subsequent sections.

 

[hokie66]

One reference attached about drying shrinkage. It won't really help in estimating your vertical shrinkage in a footing, but you wouldn't be able to measure it anyway.

http://www.boral.com.au/brochures/o...rinkage.pdf?pdfName=DS2002DryingShrinkage.pdf

 

[rapt]

I do not see how the direction of the shrinkage affects it!

Assumed a basic shrinkage strain of 600 - 700 microstrain and then applied a shape factor to get a long term shrinkage of 300 microstrain and then multiplied by the depth

300 / 1000,000 * 1000 = .3mm

Or as someone suggested use ACI209 to get a more accurate answer for the concrete you are using.

Depending on temperature variation in the footing, you could get a similar amount of movement from temperature change throughout the year!

And possibly a lot more from soil movement, unless it is on rock. Make sure the soils FEA analysis allows for creep of the soil over time, especially if it is not on good rock if it is that sensitive to movement.