Maximum aggregate size 9

[bootlegend]

I'm looking at a cantilever concrete column and the spacing of the vertical bars is getting closer than I would like. I would like to use a 3/4" maximum aggregate size.

Several questions come to mind and I admit I've never really given them much thought. I have requested mixes with 1" maximum aggregate and I may have used 3/4" maximum aggregate, but I've received no memorable feedback, positive or negative. I suppose I've just assumed that I can specify a maximum aggregate size and a required strength and let the mix designers do their thing. But I'd like to know what the limits are.

1. Is 3/4" maximum aggregate size a reasonable request for a 4000 psi mix?

2. Is there an aggregate size below which you lose strength of the concrete?

3. Does this automatically increase the ratio of cement to aggregate in the mix design?

4. Is shrinkage more of an issue with smaller aggregate mixes?

5. Is the mix more expensive with smaller aggregates? I'm thinking possibly due to more cement.

I would call up a mix designer if I had a knowledgeable contact, but as it stands I do not.

 

[nackra]

https://www.pavementinteractive.org/reference-desk/design/mix-design/aci-mix-design/

ACI has recommendations

 

[Bobby46]

1. 3/4" is pretty typical for 4000 psi around me

2. In general the aggregate is filler material and smaller aggregate leads to a stronger mix (there are lots of other variables)

3. This will depend on gradation (well graded vs poorly graded)

4. Not sure.

5. Yes. Around here a 3/8" 4000 psi mix is generally $10 more per yard than a 3/4" 4000 psi mix

Call up QC at your local batch plant. They are generally pretty helpful.

The PCA Design and Control of Concrete Mixes is a good reference. You can find it for free on google.

 

[bootlegend]

Thank you both for the info. Sounds like I am not out of line with my recommendations.

 

[JLNJ]

I don't know that I agree with the comment that smaller aggregate leads to a stronger mix. I believe the opposite is true, plus you need less concrete paste to make a larger aggregate matrix work. Aggregate interlock is your friend.

The limiting factor is the spacing between bars, especially at intersections and splices. Your contractor might be able to help you decide what is most reasonable regarding placement and consolidation.

 

[BridgeSmith]

In the AASHTO bridge design spec. the clear spacing cannot be less than the largest of:

1.5 X max aggregate size
1.5 X the bar diameter
1.5 inches

Per the AASHTO spec, dropping to 3/4" max aggregate size doesn't help. That assumes the concrete will be vibrated. In drilled shafts, which do not get vibrated, the limit in the newer LRFD spec is 5 X the max aggregate size (it was 3 X in the older standard spec)

 

[Agent666]

Regarding question 4 which no one has answered yet, for an equal strength mix, then yes smaller aggregate mixes generally display higher shrinkage than using larger aggregates, mainly I think because of the higher degree of cement required to achieve the same strength.

Definitely smaller aggregates reduce the concrete component of shear capacity as JLNJ noted. In our local code NZS3101, the shear capacity can be reduced by a linear reduction that goes from 1.0 for 19mm aggregate to 0.85 for 10mm aggregate to account for this effect, I believe other codes also have similar reductions for concrete shear capacity to account for the lower level of aggregate interlock.

 

[bootlegend]

I don't know that I agree with the comment that smaller aggregate leads to a stronger mix. I believe the opposite is true, plus you need less concrete paste to make a larger aggregate matrix work. Aggregate interlock is your friend.

I thought that was the case too. It does appear that 3/4" is a common max aggregate specification for normal strength mixes.

shear capacity can be reduced by a linear reduction that goes from 1.0 for 19mm aggregate to 0.85 for 10mm aggregate to account for this effect, I believe other codes also have similar reductions for concrete shear capacity to account for the lower level of aggregate interlock.

Interesting. I'll see if I can locate that somewhere in ACI's literature.

1.5 X max aggregate size
1.5 X the bar diameter
1.5 inches

I think ACI has the first two requirements. Not sure on the third. Will double check later.

 

[dik]

3. The smaller the aggregate, the more water required and more cement powder, too.
4. Yes

Dik

 

[Ron]

1. You can produce essentially any strength you need with almost any size aggregate. Yes, 3/4" coarse aggregate is common for 4000 psi mix
2. No really. See first answer.
3. Yes. In general, the larger the aggregate size, the lower the cement content needs to be to get appropriate paste coverage and interaggregate bond. This also results in lower shrinkage.
4. Yes. Smaller coarse aggregate increases the specific surface area of the aggregate thus requiring more cement paste to cover that increases surface area, thus more cement and water result in more shrinkage.
5. Yes. Smaller aggregate requires more cement thus higher cost. There is also slightly more aggregate required thus another cost increase.

For good flow of the mix and to prevent voids from lack of consolidation, the coarse aggregate size should be no more than 1/3 of the spacing between rebar.

 

[TehMightyEngineer]

1.5 X max aggregate size
1.5 X the bar diameter
1.5 inches

If you're precast these drop down to

1.33 X max aggregate size
1.0 X the bar diameter
1.0 inches

I think ACI has the first two requirements. Not sure on the third. Will double check later.

ACI requires the following:

1.33 X max aggregate size
1.0 X the bar diameter
1.0 inches

Unless you're pouring long. reinforcement in columns, structs, boundary elements, etc., then:

1.33 X max aggregate size
1.5 X the bar diameter
1.5 inches

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

Looks like for the application under consideration, if the 1" aggregate doesn't meet the spacing requirements, dropping to 3/4" would not help. Might have to look at larger or bundled bars to gain some clearance.

 

[bootlegend]

Looks like for the application under consideration, if the 1" aggregate doesn't meet the spacing requirements, dropping to 3/4" would not help. Might have to look at larger or bundled bars to gain some clearance.

I believe that's my only option.

Thank you all for the input.

 

[cliff234]

I see no problem with the 3/4" aggregate, but I suspect that using that aggregate is not going to solve all of your problems. It sounds like your column is heavily reinforced. Presumably your maximum moment will occur at the base of the column where you probably have the heaviest congestion at a lap splice. Will everything fit at the lap splice location? Not knowing the specifics of your project, here are some other suggestions,
1. Use Grade 80 reinforcing steel
2. Use #14 or #18 bars with mechanical splice couplers
3. And obviously, can you make the column bigger?

(I would caution against using bundled bars. Bundling reinforcing steel has it's own set of challenges. (Staggering splices, special tie bends to wrap around the bundled bars, etc.) In my opinion, if you have to bundle the bars, the column is too small.