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Gravel under outdoor concrete flume?

Gravel under outdoor concrete flume?

Gravel under outdoor concrete flume?

(OP)
Hi,
I have a follow-up question about a flume I am designing to carry stormwater outside. It is to be of reinforced concrete in a U-shape, with #3 bars in a single mat through the U-shape, with the bars both horizontally and vertically. The depth is to be only about 1.42 ft (1 ft 5 inches) including the floor depth. The walls and floor are to be fairly thin (due to site constraints), at 0.42 ft (5 inches). The flume is to be dug into previously placed fill material outside a house with a fairly deep foundation. Should there be something like 4" depth of gravel underneath the concrete? The job site is not in an area subject to freeze-thaw conditions (a far southern state). If needed, what size of gravel is recommended? What is the purpose of such gravel? And, lastly, in the area of the project, "flexible base" is often used for roadway base material. Could such be used under the flume? Gradation requirements for flexible base are as follow: 1-3/4" 0% retained; 7/8" 10%-35% retained; 3/8" 35%-50% retained; #4 sieve 45%-65% retained; #40 sieve 70% - 85% retained. Or, would it be recommended to just forget about the potential use of flexible base, and just call out for 3/4" gravel under the flume (if it is needed at all).
Thanks!

RE: Gravel under outdoor concrete flume?

use either aggregate base or clean, granular native material. open graded gravel is not recommended

RE: Gravel under outdoor concrete flume?

I'd use a good quality 3/4" size 'A' Base... and compacted to 90% SPDD or something of that ilk.. You have to confirm that the sub-base does not have 'soft' spots...

Dik

RE: Gravel under outdoor concrete flume?

(OP)
That helps. Thanks, guys.

RE: Gravel under outdoor concrete flume?

(OP)
Dik,
I am not sure what size "A" base is, and I searched online and nothing came up. From what grading/rating system does such base sizing come?
Thanks.

RE: Gravel under outdoor concrete flume?

3/4" max size crushed aggregate, well graded, minimum amount of fines. commonly referred to as aggregate base course or road base or dense graded aggregate base

https://www.google.com/search?q=aggregate+base+cou...

RE: Gravel under outdoor concrete flume?

(OP)
Awesome. Thanks. With road base I am of course familiar. I think we call it "flexible base" here.

RE: Gravel under outdoor concrete flume?

GRANULAR 'A' (GRAVEL)
CRUSHED ROCK COMPOSED OF HARD DURABLE UNCOATED FRACTURED FRAGMENTS PRODUCED FROM ROCK FORMATIONS OR BOULDERS OF UNIFORM QUALITY, OR A MIXTURE OF CRUSHED GRAVEL, SAND AND FINES COMPOSED FROM NATURALLY FORMED DEPOSITS ONFORMING TO THE PROVINCE OF MANITOBA SPECIFICATIONS FOR AGGREGATE FOR GRANULAR BASE COURSE FOR CLASS A

FILL SHALL CONFORM TO:
SIEVE SIZE (ASTM) PERCENT PASSING
3/4 INCH 100
5/8 INCH 80-100
NO 4 40-70
NO 10 25-55
NO 40 15-30
NO 200 8-15

It's a common gradation here and elsewhere...

Dik

RE: Gravel under outdoor concrete flume?

(OP)
Thank you again. I have a call in to the local coarse aggregate supplier to see if they have any standard mix close to what you showed for the Granular 'A' (Gravel), but they are not in today due to the weather here being particularly and unusually icy. I compared the Granular A specs with what is the City's standard spec here for "flexible base", a road base, and there are seemingly substantial differences, since that flexible base has:
SIEVE SIZE (ASTM) PERCENT PASSING
1.75 INCH- 100% passing (compared to no callout for Granular A)
7/8 INCH- 65-90% passing (compared to no callout for Granular A)
3/4 INCH- not specified (compared to 100% passing for Granular A)
5/8 INCH- not specified (compared to 80-100% passing for Granular A)
3/8 INCH- 50-65% passing (compared to no callout for Granular A)
NO 4 35-55% passing (compared to 40-70% passing for Granular A)
NO 10- not specified (compared to 25-55% passing for Granular A)
NO 40 15-30% passing (equal to 15-30% passing for Granular A)
NO 200- not specified (compared to 8-15% passing for Granular A)

Until I hear back from the gravel/stone supplier, do you have any thoughts about the suitability of the above shown "flexible base" for the base below the flume, in comparison to the suitability of the Granular A?
Thanks!

RE: Gravel under outdoor concrete flume?

significantly larger rock at the top end and more sand and fines at the small end. 30% passing the no. 200 sieve is excessive and generally not allowed in any sort of road base material. specs are kind of loose which may result in an inferior product.

RE: Gravel under outdoor concrete flume?

(OP)
Thank you, cvg. I am going to check with the base supplier tomorrow and see what other options they have available.

RE: Gravel under outdoor concrete flume?

Good thing you are not in a cold climate... with the large percentage passing 200, you would almost certainly have frost heave issues...

Dik

RE: Gravel under outdoor concrete flume?

(OP)
Note that it is the Granular A that has such a large percentage passing the 200 sieve, and not the "flexible base" option available here. So I am leaning on thinking that the flexible base option is acceptable for below the flume. If really needed, I could also specify to have 100% pass the 3/4" sieve. What do you think? And what exactly would be the problem with the larger sizes?
Thanks.

RE: Gravel under outdoor concrete flume?

your specs say 30% passing no 40 and nothing more. so potentially most or all of that 30% could be fines. the fact that there is no limit specified for fines generally means the material is "dirty" meaning a lot of fines. 8 - 15% limit on fines is pretty typical although a bit higher than allowed around here. if you are going to pay money to bring in bedding material, than get the good stuff

local spec here is:

sieve % passing
1-1/2" 100
1" 90 - 100
no. 4 38 - 65
no. 8 25 - 60
no. 30 10 - 40
no. 200 3 - 12
PI 5 max

RE: Gravel under outdoor concrete flume?

(OP)
Great, cvg. That helps. Thank you very much.

RE: Gravel under outdoor concrete flume?

Quote:

what exactly would be the problem with the larger sizes?

in order to get good compaction and a smooth working surface with largeer rock you need two things: 1) a thicker lift (thicker than 4 inches as you suggested) and 2) heavier compaction equipment. For a small trench, thin lift and light compaction, stick with smaller size aggregate.

RE: Gravel under outdoor concrete flume?

(OP)
Again, cvg, that is extremely useful info. Thank you. I am trying to get hold of the aggregate supplier now to find a suitable mix, and your recommendation on staying away from the larger size aggregate in the smaller/thinner trench will definitely be pursued. Thanks!

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