Aggregate Size 5

[miecz]

We are designing Sequencing Batch Reactor tanks per ACI 350. The clear cover from the (#10@6") main reinforcing to the wall face is 2 inches. We have used aggregate grading #67 (3/4") from ASTM C 33 in the past. We're considering going to a larger size such as #57 (1") or 467 (1-1/2") to reduce shrinkage cracks in the 24 inch thick walls. Our field personnel have expressed concern over placement of a 1-1/2" maximum size aggregate with a 2 inch clear cover. Has anyone had any experience with this?

 

[civilperson]

[ACI 3.3.2] Has upper limits on aggregate size which you are NOT exceeding, With good consolidation,(vibration techniques), this size will work. Limit the lift depth to two feet and specify a 9" grid for vertical stinger insertion of 27", (vibration of the exterior of steel forms will eliminate exterior honeycomb and lift lines). The pump aggregate size capacity should be checked if large volume placements are planned.

 

[LCruiser]

Okay, I give up. How do you get 3/4" from #67, etc.? Where do I get my secret decoder ring for that?

 

[henri2]

jmiec: Even though ACI 318, ACI 301, and I believe ACI 211, do not include cover as a factor in determining nominal maximum size aggregate, there are some organizations and mix designers who do consider it; typically restricting MSA to 3/4 of the cover in addition to other criteria..example PCI MNL 116 Article/Section 3.1.4(2)

The situation you describe satisfies the 3/4 cover rule.

There are many situations where the 3/4 cover rule does not apply...and your's is probably one. For instance, use of, #57 (1 inch MSA) for interior structural slabs with a cover of 3/4 inch does not create a problem. However, for thin vertical structural elements, not adhering to the 3/4 cover rule might lead to honeycombing.

LCruiser: The coarse aggregate size number designation comes from ASTM C 33 Table 2. It takes into account MSA and grading characteristics. No 1 is the coarsest with largest MSA and No. 8 is the finest and has the smallest MSA.

Other size designations are a result of combining aggregates with different size numbers. So No. 67 is a combination of No. 6 and the less coarse No. 7 aggregate. This means No. 67 falls in between No. 6 and No. 7 in terms of coarseness. No. 57, a blend of No. 5 and No. 7 and so on.

In Table 2 of ASTM C 33, No. 9 which is a fine aggregate, is included. This is because when it is combined with No. 8, it will produce No. 89 aggregate. See footnote A of the table.

I do not have a copy of ASTM C 125-Terminology Relating to Concrete and Concrete Aggregate...but I do believe it explains the rational behind coarse aggregate size numbers.

Tables 1 and 2 in the link provided below are based on ASTM C 33 Table 2. This should provide an idea of MSA as it relates to coarse aggregate size number.

http://www.xerxescorp.com/singlewall/pdf/backfill01.pdf

 

[dik]

For 24" thick concrete I would use 1-1/2" aggregate minimum (I wouldn't consider 3/4"). I don't know what ACI 3.3.2 stipulates but use the max size aggregate you can to minimize shrinkage and temperature gain. Also gradation is quite important for temp gain.

Dik

 

[miecz]

Thanks, everyone. I'll go with the #467 mix and add the compaction requirement.

 

[dik]

Unless you need specd 28 day strength... good application for fly ash...

Dik

 

[miecz]

Dik

The ready mix suppliers around here prefer Granulated Blast Furnace Slag.

 

[dik]

was using fly ash generically, sorry... meant any pozzolan...

Dik

 

[Admixengineering]

jmiec
There is big difference between proportions containing a #67 and #57. Granted, the majority of all #57 will meet a #67 spec., but its surface area of the rock that matters in this case. Proportion with #67 rock will remain more stable and when using high slump or SCC, the stability factor plays a huge role. Even at lower slumps, say 4"-6", the 67 will consolidate much better, unless the rock is even graded. In that case, you will need to "gap grade" with a finer coarse aggregate material. My suggestion would be a 67, but if you have made your mind up, good luck with the 467. It should work ok, but I would suspect you may end up with a product that will need patching attention.