Minimum Grout Thickness

[Nor Cal SE]

I'm interested in others' opinions regarding a disagreement I've had with a coworker. This involves non-shrink grout for leveling/bearing beneath steel column base plates.

Most of the major non-shrink grout manufacturers, such as MasterFlow, FiveStar, etc, list a minimum application thickness, often 1". Despite how carefully I detail my design drawings, with relative frequency I have scenarios where the gap between the column base and the concrete foundation below needs reduction, sometimes down to 1/2", to result in the correct top-of-column elevation.

My coworker believes we should never vary from the manufacturer minimum for liability reasons. My thought is that if any aggregate in the mix is small (say, less than 1/3" the gap thickness), there should be no problem getting full coverage in the tight space, especially if the plan area of the base plate is not great. It would seem that a gap reduction does not compromise the structural integrity provided the grout achieves full coverage. The grout will still provide the bearing strength, and the manufacturer minimum most likely relates to ease or difficulty of workability. I contend that if you want extra reassurance, you can specify a small hole or two in the base plate to aid in verifying grout coverage. Despite the effort this takes, usually the contractor still prefers to go this route as opposed to some type of steel column modification.

This flexibility usually buys some good will with the contractor, though ultimately my main concern, and highest responsibility, is prudent engineering design that will last. I'm interested in others' opinions.

 

[hokie66]

Most of the premixed flowable grouts have a minimum thickness of about 10 mm. With gravity placement, you must provide the manufacturer's specified head to move the grout under the plate. When placed uniformly, the smaller the thickness, the better.

 

[dik]

From my ProjNotes


GROUT FOR BEARING PLATES AND BASE PLATES OR FOR DRYPACKING SHALL BE 'M-BED' AS MANUFACTURED BY SIKA, 'IN-PAKT PRE-MIX' AS MANUFACTURED BY CC CHEMICALS OF CANADA, OR APPD EQ. MIN THICKNESS [1" | 25MM]

FLOWABLE GROUT FOR BEARING PLATES, EQUIPMENT BASES AND TRANSFORMER BASES [SIKAGROUT 212 (1") | SIKAGROUT 300PT (1/8")] AS MANUFACTURED BY SIKA OR AN APPD EQ. PROVIDE CONFINEMENT AS REQD

 

[Stenbrook]

You also have to consider the size of the leveling nuts below the base plate which are generally as thick as the bolt diameter. So if you are using 3/4" diameter bolts you will need at least 3/4" just to account for the nut. You shouldn't have any problem specifying a minimum of 1" below your base plate. I have never had any issues with this at all and getting it built right. I am curious how you can carefully detail the drawings and still ave a 1/2" gap show up?? Are you checking the steel shop drawings thoroughly?

 

[MotorCity]

I agree with your logic, but I would ask the manufacturer to see what the reason behind the minimum thickness is. It may be that they are trying to prevent contractors from using some ridiculous grout thickness that is smaller than the aggregate or it may be that they have results from extensive testing that indicates the grout performance is less than fantastic below some thickness.

 

[dik]

I would suggest that the thinner the grout joint, the stronger the bearing capacity and it is more difficult to place grout properly with a thin joint. Many pre-eng buildings found their frame bases directly on concrete without a grout interface.

Dik

 

[hawkaz]

If you match their spec and something bad happens- it is on them
If you deviate from their spec and something bad happens- it is on you

As long as you are willing to take liability for their product, do whatever you want.

 

[wannabeSE]

If the grout specified needs to be 1" minimum and the baseplates have 3/4" clear, have the contractor propose an alternative product and provide manufactures datasheets for review. Let the contractor do the leg work to find suitable non metallic alternatives.

 

[Ingenuity]

In extreme cases where the underside of baseplate dimension was small and the baseplate was rather large, we preplaced 1/4" SSD aggregate and placed low viscosity epoxy resin with sealed and formed edge perimeter. Pain to do, but it ensures you get full bearing over the plate area.

 

[Lomarandil]

I've successfully used a US Spec RA grout product for gaps under 10mm -- in theory down to 2-3mm, although we didn't technically need the grout to fill the entirety of the bearing area for our connection to work.

Our contractor ended up watering down the mix for flowability, pumping it into a steel formed edge under pressure, then providing us breaks for the final compressive strength.

Granted, this was a very strange application -- not a typical building base plate. So we were willing to accept an off the wall solution (and the contractor was willing to spend the extra cash required), as long as it could rationally calc out in the end.

You could also look at finger shims, perhaps? Or is that a long-term corrosion/durability concern?

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The name is a long story -- just call me Lo.