2.5" line pump for placing slab on grade concrete?

[gte447f]

I have a project with a 5" slab on grad. The concrete spec'd was 4500 psi, max w/c ratio 0.45, max coarse aggregate size 1" (but 3/4" is commonly used in mix design around here), air entrainment 4.5% or 5% plus/minus 1.5% for 1" or 3/4" max aggregate size respectively, and max slump of 5". The contractor wants to use a 2.5" diameter line pump to place the concrete, and has submitted an alternate mix design for the line pump with 3/8" max coarse aggregate, 5% plus/minus 1.5% air entrainment, and 4-7" slump range. The f'c and w/c ratio meets the original spec, but I am worried that the smaller aggregate and higher slump could result in increased shrinkage and cracking. Also, the smaller aggregate size should come with an increase in air entrainment to 6% plus/minus 1.5% according to ACI. Are my concerns about shrinkage valid or is this proposed alternate mix design OK? Could my original spec work OK for pumping through a 2.5" line pump (I suspect not with 3/4" to 1" max aggregate size)? Is it even OK to place concrete with a line pump like this, or should they be reserved got pumping grout? This is the second project recently where I have been asked to accept an alternate mix design to use a small diameter line pump. I think the contractors are just trying to be cheap. FYI, this slab is about 100'x36'.

 

[JAE]

Interior or exterior slab (i.e. why did you have 4.5% to 5% air to begin with?

Is the slab to be hard troweled? If so - lower air is preferred.

With a small diameter line, higher air would be helpful to add workability to the mix.

Agree that smaller aggregate in a slab-on-grade may result in more shrinkage...although I think hokie66 questioned that some time ago and had some research that suggested it didn't.



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[gte447f]

JAE, 4.5%-5% air was for freeze thaw durability. The slab is inside an automated car wash, unheated space, open on both ends, always wet.

 

[gte447f]

JAE, what about their proposed 7" slump? I am not used to slumps that high. Do you see any problems with 7" slump?

 

[gte447f]

In the southeastern US, so not serious freeze thaw exposure, but wet and unheated nevertheless.

 

[JAE]

7" slump - depends on how they get it - if it is with plasticizers or water reducers, then not a concern in terms of increased W/C ratio and resulting shrinkage.



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[OldBldgGuy]

I'd want smaller aggregate than the original spec. Slight weakness in the large aggregate and you have big failures regardless of the mix design. I live where carwashes all deal with salt and freezing...

 

[gte447f]

OldBldgGuy, Do you mean you would want smaller aggregate for pumping through a 2.5" line pump, or just in general for freeze thaw resistance?

 

[gte447f]

FYI, the owner has told me that the wash tunnel will be heated as necessary to keep above freezing during operation and over night, so theoretically I guess there is no freeze thaw exposure after all. I'm still interested in aggregate size influence on freeze thaw durability if there is any, and I'm interested in general mix design recommendations, if there are any, for pumping through small diameter lines, or is it recommended to avoid pumping through small diameter lines?

 

[OldBldgGuy]

Both. I wouldn't use 1" coarse aggregate ever, except for unshrinkable fill or something. I'd use 3/4" max for almost anything, but you can attribute that to an old stick in the mud, and remember, I'm probably 1000 miles north of you. Aside from that, in your specific case it seems to me that a 3/8" aggregate would serve you better both for pumping and for final product.

 

[gte447f]

10-4.

Any issues with finishing the concrete with a 7" slump?

 

[OldBldgGuy]

I'm not a carwash guy, but I have designed and built a few 15 & 20 years ago to my regret. They have very aggressive environments and I would love to go back and redo some that I thought at the time (and now know not) were conservatively designed. It's all about durability.

 

[OldBldgGuy]

7" slump might finish itself. Or you.

 

[gte447f]

LOL... thanks for chiming in.

 

[OldBldgGuy]

Huh, I should just keep my big mouth shut.

 

[OldBldgGuy]

But, in the interest of offering my full opinion, since I can't keep my big mouth shut, 4" slump is my max like my 3/4" aggregate, and 4" is only with plasticizers.

 

[Ron]

As OBG noted, a car wash is an aggressive environment. Durability is more important than compressive strength since you probably have plenty.

This is not just a slab on grade...it is a pavement. Make sure the grade is flat and have them screed the grade before concrete placement. That will help you achieve the proper thickness tolerance which, for your thickness, is -1/4", +3/8".

The smaller aggregate will increase the required cement content (the specific surface area of the smaller aggregate is much greater than the larger aggregate, thus you need more cement paste to cover all the surface and provide aggregate to paste bond). An increase in cement does two things....to keep the same W-C ratio, you'll need more water....next, it increases the cost of the mix. The increase in cement and water promotes more shrinkage in the mix, particularly early shrinkage where you don't want it! That promotes "random" cracking.

The contractor is right that a 2-1/2 inch pump wouldn't work well for 3/4 or 1 inch aggregate. A larger diameter pump should be used. I would go with the largest aggregate you can get in the slab, not the smallest.

Why are they pumping the concrete? Usually something like this is accessible for direct placement from the transit mixer. Don't pump if you don't have to. If you do let them pump, watch it carefully. They will often put undocumented water in the mix at the pump to make placement easier. Don't allow it. Your original mix specification is good. Stick with it and make them place it conventionally or get a larger pump. Don't accept the alternate mix design.

 

[gte447f]

OK, now we're getting somewhere. You were holding out on me OldBldgGuy.

I like your 3/4" max aggregate. That is about what I am used to around here. On this particular project I didn't explicitly specify a max. I just had the ACI notes about the width of the forms, the clear distance between bars, the clear cover to reinforcing, etc. 3/4 times the clear cover to reinforcing governs and is about 1". The original mix design submitted (before the 2.5" line pump came into play) had 67 stone with 100% passing 1" and 97% passing 3/4".

I use 5" slump for slabs based on the recommendations in ACI 302.1R-15 (Guide to Concrete Floor and Slab Construction). So, you use 4" max, and only then with plasticizers, so you are more conservative than me, but we are in the same realm. What I am trying to get at though is this... what are the negative effects of higher slump for slabs in particular. I think I have been told or read that one problem is more segregation and more bleed water which makes finishing a durable surface more difficult. Is that true and is that still true if the higher slump is obtained with water reducing admixtures? What else?

 

[gte447f]

Ron, thanks for your advice. They are pumping because the exterior walls of the building are already in place (for sure), and I believe the roof trusses may already be set also (not sure about this... crazy sequencing, I know). Basically, at this point I had a discussion with the owner and explained my concerns about increased shrinkage cracking and increased segregation/bleed water and difficulty finishing. He said he wasn't concerned about cracking. I marked up the mix submittal with the same comments and and a disclaimer that EOR accepts no liability for mix designs outside the original spec. I think they will go ahead and use the alternate mix with the 2.5" line pump. We will see.

 

[gte447f]

Regarding undocumentted water added to the mix, a Special Inspector should be onsite sampling concrete. Is Special Inspection a reliable way to ensure undocumented water is not added to the mix?