Aggregate Size for Slab, Beams, Columns, etc. Question

[Respawn]

I was looking over some plans for learning purposes and I saw a RFI that said "3/4" aggregate size is acceptable for beams, columns, slab except for use in high rebar and post-tension congested areas like column to post-tensioned beam connection". It then pointed to use 3/8". Why would a 3/8" aggregate size be preferable in said regions? I know it affects workability.

 

[BridgeSmith]

The AASHTO code (and I assume ACI) requires bars in a horizontal layer to be spaced not less than 1.5 times the max aggregate size, for typical construction. For drilled shafts (concrete caissons), it's 5 times, because the concrete down in the shaft can't be vibrated.

The reinforcing in post-tensioning anchorage zones is typically very closely spaced to provide confinement for the highly stressed concrete.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[rscassar]

Generally minimum clear bar spacing is set to twice the aggregate size to allow adequate compaction. I would say 3/8" has been specified in "high congestion" regions like this. I tend to stay with the 20mm agg size, shear calcs for beams using modified compressive field equations have a requirement for aggregate interlock and more beneficial to have larger agg size. Other regions where the 10mm agg size is preferred is higher strength concrete with lower workability for concrete pumping. But again considerations need to be given for shear design before switching to the 10mm agg.

 

[dik]

You will not likely require 3/8" aggregate... I generally only use that for masonry grout. Generally 19mm (3/4")/20mm is used for nearly everything.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Agent666]

Smaller aggregate sizes also require more cement paste, so additional shrinkage can occur for the same strength mixes but different aggregate sizes. So consider this aspect also as its not only about increasing workability.

Some codes also have reductions for concrete shear strength component when you have max aggregrate sizes less than the typical 19-20mm. For example in my own local code if we had 10mm nominal max size, then we'd have to apply an additional reduction factor of 0.85 to the concrete design shear strength. This is because the smaller aggregate size correlates to less aggregate interlock effects in shear.

https://engineervsheep.com

 

[dik]

Has anyone used anything larger than 1-1/2" (40mm) aggregate? Just curious for mass concrete.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

For horizontal concrete (slabs on grade, etc.), use the largest aggregate reasonable...(1-1/2" if possible. Reduces shrinkage as Agent666 noted.) For cast in place structural members, use the ACI maximum aggregate size for the rebar spacing and control.

 

[rapt]

If your PT end zone is that congested that you need bars at less than 30mm you really have problems. 20mm aggregate should be adequate.

dik, I have seen someone try to use 150mm diameter in a bridge deck in Indonesia. We supplied and placed the PT beams and a local contractor supplied the deck! It was called river gravel! Deck was 250mm thick and the gap between top and bottom layers of reinforcement was about 100mm, so there were obvious problems.

We took the "aggregate" away and crushed it for them to something more logical and better graded.

 

[dik]

rapt: How did it work out? I'm not so sure about bridge decks, but I was thinking of mass concrete. I've seen concrete done using 'intrusion pre-pact' where large aggregate was 16" or 18" boulders and concrete was pumped into the voids.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

...large aggregate was 16" or 18" boulders and concrete was pumped into the voids.

We call that "grouted riprap", and it's typically only used at the outlet of a culvert where the flow velocity is extreme.

I suppose it would be possible to use that approach for mass concrete, but getting it properly consolidated would be labor intensive. The economic feasibility would depend on the local relative expenses of labor and materials. Balance skews towards expensive labor in the US, so I doubt it would save anything here.

Rod Smith, P.E., The artist formerly known as HotRod10