ACI 318-19 Aggregate Size Discrepancy

[StrEng007]

It's been stated here before that using #57 and #67 stone is a common approach to beams and slabs. I'm questioning whether or not those aggregates are actually too large?

Per Sieve analysis, #57 stone is comprised of a majority 3/4" stone and has up to 5% that is larger than 1" and less than 1-1/2". Put differently, #57 stone has a maximum aggregate size of 1-1/2" with a majority of the composition at 3/4".

Similarly #67 stone has a maximum aggregate size of 1" with a majority of the composition at 1/2".

Outside of exposure requirements, ACI has three sections that relate aggregate size to reinforcement/spacing.

STATEMENT #1
In section 25.2.1, it requires that non-prestressed bars in a single larger should maintain the greater or 1", db, or (4/3)dagg.

STATEMENT #2
However, section 25.2.2 says non-prestressed bars placed in two layers need to maintain a clear distance of 1" between bars.

STATEMENTS #3, #4, #5, as (i), (ii), (iii), RESPECTIVELY
Section 26.4.2.1.a.5 requires the aggregates not to exceed the following5) Nominal maximum size of coarse aggregate not to exceed the least of (i), (ii), and (iii):
(i) one ­fifth the narrowest dimension between sides of forms
(ii) one-third the depth of slabs
(iii)  three ­fourths  the  minimum  specified  clear spacing between individual reinforcing bars or wires, bundles of bars, prestressed reinforcement, individual tendons, bundled tendons, or ducts

[highlight #FCE94F]My questions are:[/highlight]
•Why does Statement #2 not address the 4/3dagg if the purpose of that is to make sure concrete flows properly? If a #57 stone is used in these situations, the max aggregate is larger than the clear spacing.
•Based on the information above, if you use 1" spacing between bars, then Statements #1 and #5 require that all aggregates be ≤ 3/4" (#57 & #67 doesn't satisfy either of these requirements).
•Does clear spacing also mean clear cover? As in, if you maintain 1-1/2" of clear cover to the form work... does that mean you have to limit aggregate size to 3/4xCC? 1-1/8 (or 1")
•For interior slabs with 3/4" clear cover to top/bottom reinforcing, would you not be required to use 0.75x3/4" = 1/2" max aggregates?

 

[lexpatrie]

Interesting question.

What's the title of chapter 25 anyway?

I'm going to venture that there's a "maximum aggregate size" in the construction documents and it's on the mix designer (& certifying plant engineer) to adhere to that, so if the stone selected goes over it, they need to remove it somehow. "Nominal" is another question. Perhaps there is some tolerance for larger than normal aggregate that's not predominant in the size distribution.

Question i)
Do 25.2.1 and 25.2.2 apply independently? the second one is spacing between layers of rebar, I'd expect that the bar spacing needs to work for the max aggregate size on both layers. It may not be explicitly required by the wording of the code, if you squint, but I'm not sure most engineers read it that way.

Question ii)
Yes, this amount of reinforcing (1" spacing) sounds a bit unusual.

Question iii)
Does clear spacing mean clear cover?
I rather doubt it. The rock has somewhere to go on the top surface and it has somewhere to 'not go' for a bottom bar. Large aggregate too close to the surface can be detrimental (as I recall) and produce cracking, so floating is intended to push it down somewhat.

Question iv)
Same answer as iii above.

Aggregate and bar spacing tend to be more of concern in beams and columns, and I suppose highly reinforced shear walls. Rather than your average slab.

 

[StrEng007]

lexpatrie,
Chapter 25 is the Reinforcement Details chapter.

I've seen drawings list both max aggregates sizes, and list the stone gradation size.

My interpretation is the following:

For mixes involving #57 stone (max 1-1/2" aggregate). The spacing between bars in a single layers, (2) layers on top of each other, AND, the clear cover between ANY rebars and form work must utilize 2" clear distance as per the referenced sections in ACI.

For mixes involving #67 stone (max 1" aggregate). The spacing between bars in a single layers, (2) layers on top of each other, AND, the clear cover between ANY rebars and form work must utilize 1-3/8" clear distance as per the referenced sections in ACI.

 

[hardbutmild]

I'm not that familiar with ACI, but Eurocodes and FIB model codes have similar provisions. Here we use a minimum horizontal spacing of: maximal aggregate + 5 mm, 20 mm or bar diameter.

Why does Statement #2 not address the 4/3dagg if the purpose of that is to make sure concrete flows properly? If a #57 stone is used in these situations, the max aggregate is larger than the clear spacing.

I believe that statement #2 is intended for verical spacing of bars, not horizontal. In Eurocode minimum vertical spacing is: 20 mm or bar diameter (note that there is no aggregate size limit). I believe that this is because as lexpatrie mentioned, aggregate has somewhere to go. It is important to enable vertical flow of concrete, vertically too close bars do not stop proper pouring of concrete, but horizontally too close bars do.

Based on the information above, if you use 1" spacing between bars, then Statements #1 and #5 require that all aggregates be ≤ 3/4" (#57 & #67 doesn't satisfy either of these requirements).

Sure, that is why you provide a larger horizontal bar spacing. I usually like clear spacing of top bars of around 65 mm, at bottom around 40 mm. Vertical spacing is usually 25 mm or approx. 1''.

Does clear spacing also mean clear cover? As in, if you maintain 1-1/2" of clear cover to the form work... does that mean you have to limit aggregate size to 3/4xCC? 1-1/8 (or 1")

I would have to say no. Proper pour can be achieved even if some of the aggregates can not go around bottom bars, some of the aggregates will be able to go through.
Same for the last question.

The idea is, as I understand, to avoid clogging of concrete pour. Since concrete flows vertically aggregate can not close a vertical hole, only a horizontal one. More generally, only the bar spacing (not the cover) perpendicular to pour direction needs to consider the size of the largest aggregate.

 

[BridgeSmith]

hardbutmild covered most of what I was going to say, and I agree - statement #1 refers to horizontal spacing; statement #2 refers to vertical spacing.

With regard to statement #3 (i), I believe that would be a limit on the width of the beam or wall, i.e. with 1 1/2" max aggregate, the minimum width of the wall (face to face of the forms on each side) would be 7.5". So, if you're pouring a 5" wall, the max aggregate size would be limited to 1" max.

Statement #5 is basically saying the same a Statement #1, just covering a broader range of reinforcement types/configurations. You need to have horizontal clear space of at least 4/3 the max aggregate size between reinforcing bars/wires/bundles/strands/tendons.