Pea gravel vs grout for Tilt Up

[HardyParty]

A contractor is requesting to use pea gravel 3000psi concrete underneath the tilt panels instead of the 7000psi non-shrink grout we specify (panel to foundation bearing connection). Can anyone think of any reason why this would be unacceptable? I'm assuming they would dry pack it

The only reasons I would be wary are:
1.) the pea gravel concrete could shrink (more than non-shrink grout)
2.) the pea gravel concrete is lower strength than the panels (4000psi concrete for panels)

However, I believe the degree to which it shrinks would be minimal and the load would have no problem redistributing. Also, the compressive strength of the grout is not a factor as the load typically doesn't go above 50psi.

Anything I am missing?
Thanks!

 

[ztengguy]

I think workability, can they get everything grouted? I would stick to your spec.

 

[XR250]

What is the design gap?

 

[CELinOttawa]

Shrinking and cracking; Potentially swelling (depending on the material and exposure conditions); Lesser/no corrossion inhibition (due to the cracking); Difficulties in installation and review; Not the "known" solution.

 

[Ron]

You're the design engineer. It's what you specified. 'Nuff said.

If you don't understand why it was needed when you designed it, then maybe you should re-group a bit before you battle with a contractor trying to subvert your design.

 

[concretemasonry]

Maybe, it is time to look at the basics and try to apply the strength results to the actual use.

If you are talking about a concrete strength, it is usually done with sample cylinders with a 2:1 aspect ratio. The failure is really a shear failure. This is often interpreted incorrectly as the compressive strength of the basic material.

Grout is normally tested in a square cube with a different failure because of the friction between the platens and the sample.

The application proposed is similar to confined compression. - Very similar to masonry where 2200 psi mortar(3/8" thick) with 8500 psi will test 4800 psi in a 2:1 h/t sample (h/t of 16/8)in a 2 block high hollow ungrouted prism. - Bottom line is you cannot separate materials into separate strength with different test procedures for strength when they act together in an assembly.

The calculated load is 50 psi.

Go at it....

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[HardyParty]

Thanks everyone for your responses. The gap varies. I would say 2"-4" max. While i was out there it looked like the footings were poured low and thats why they wanted to use pea gravel instead of grout (cheaper for the larger space.) I know why we specify it, for a constant bearing connection of the panel to the foundation. I was more asking about constructability or unforeseen consequences. As a new engineer, it is hard to know from experience what can (and will) go wrong. I would be a fool not to try to learn from others mistakes/findings.

Clearly i would prefer the grout as specified, but i want to be able to back up decision to enforce it.

 

[CELinOttawa]

Actually at 4" most non-shrink grouts NEED to be, or at least should be, extended with pea gravel. Equal portions by weight is the norm...

At 2" you need to be careful to specify a 5/8" Max river run to keep from having the contractor's operations getting jammed up.

 

[XR250]

Seems to me that at 2-4", pea gravel concrete would work fine. I can't imagine the shrinkage or swelling being enough to be a problem over that small height.

 

[CELinOttawa]

Excel: It is, unfortunately, just the opposite... 4 to 6 inch gaps can pose a problem for non-shrink grouts... Check the literature; Most specify extension with pea gravel.

 

[XR250]

OK, so what is it a problem with pea gravel concrete?

 

[CELinOttawa]

Two different animals... Non-shrink grout extended with pea gravel (river run) is the right material for this application.

 

[Qrs]

Interesting, I did not know that about grout. It seems like non-shrink grout with the pea gravel extension would be the way to go if I enforced the spec. For one manufacturers' website: "On pours of over 100 mm (4"), the addition of 10 mm (3/8") pea gravel will serve as an effective heat dissipater while affecting neither the non-shrink nor the compressive strength characteristics of these grouts." Cool.

I am curious as to the repercussions of using "the other animal". I tend to agree with excel that shrinkage and swelling would not be a big deal over such a small height.

 

[CELinOttawa]

It comes down to what someone means by "pea gravel concrete". Gaps, such as those between a panel and foundation, need to be poured with a flowable grout (not applicable here as the gap exceeds Max flowable without extending) or dry packed.

If you are flowing grout, you cannot use concrete, unless you are going to modify the concrete so heavily that you've just created a flowable grout from other source materials.

If you are extending grout, now you should be dry-packing in order to ensure a good filling and no inclusions (air in particular).

The question boils down to what ensures the finished product has the properties you need, in this case it is pea gravel extended grout.

At the base of a panel, cracking which will allow water and other materials to penetrate is always an issue, but bear in mind I practice in " super cold throw salt on everything land" and previously in "salt wind land".

 

[HardyParty]

Well now that we’re on the subject, what determines the compressive strength of grout that you specify? Our spec program defaults to 7000psi or 10000psi, it also was apparently reviewed and endorsed by TCA. However, I don’t like to blindly follow a recommendation if I don’t know where it came from…

This website says a good rule of thumb is 1.5 x concrete strength:

http://www.lmcc.com/concrete_news/0104/astm_c-1107.asp

But why? Could that be a safety factor due to the different way to determine compressive strength between concrete and grout (cubes vs cylinders?) Have there been problems with compressive grout strength in the past? What does everyone else specify and why?

 

[AELLC]

It may be the detail - I recall the grout was not continuous the whole length of the panel, but I don't recall the reason why.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.