Horizontal/Subsurface Infrastructure & Transportation

What is the typical maximum size for aggregates in a base course for flexible pavements on low volume roadways that are finished with a chip seal and do not have any asphaltic concrete pavements to carry the traffic.  Please assume the pavement has been adequately designed for the expected loads.  The reason for my question is I have been told if the maximum size is greater that one inch, it will come loose under traffic.  I don't have any experience in areas where freeze thaw is a normal condition, but I have never heard of this in my 30 years of past experience in State DOT.

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Gary Humes P.E., M.ASCE

Port Aransas TX
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The size of maximum aggregate in a sample depending upon the gradation class you have selected and thickness of the layer. As a tumb rule one can use 2.5  time the maximum aggregate size equal to layer thickness. In your case for low traffic volume we take the unbound thickness usually equal to 20 cm so the maximum size of aggregate shall be  2 to 2.5 inches.  
Thanks and regards. 

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Ejaz Khan S.M.ASCE
Director
Pakhtunkhawa Highways Authority
Islamabad
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I'm not sure if the maximum size is as important as the gradation of the material.  My experience is limited to Iowa where a 3/4" to 1" topsize material uniformly graded with 10 - 15% passing the 200 screen is a common material. A 1.5" top-size "modified granular subbase has become more popular in recent years.  We like to see a surface that is well compacted with a high level of stone interlock to inhibit any movement of the material under traffic.  We have sealed on top of materials up to 1.5" with no issues.

Hope this provides a bit of help.

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Dewayne Heintz EI, A.M.ASCE
Project Manager
L.L. Pelling Company
North Liberty IA
(319)626-4600
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​Good Morning Gary,

In New Jersey we use this "superpave" designation for our asphalt:
Surface course is 9.5M64 (9.5 mm is the aggregate size).
Intermediate 12.5M64
Base Course 19.5M64 - So 19.5 mm is the aggregate size.

I have never seen a base course with a 1 inch aggregate, usually we use a 3/4 in stone in our subgrade, or subbase, and if needed, we will place larger aggregates below our road subbase.

19.5mm is about 3/4", so I would not expect to exceed that size aggregate in asphalt.

Hope this helps!
Chris

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Christopher Hanna P.E., M.ASCE
Project Engineer II
Pennoni
Newark NJ
(908)603-2623
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Thanks to all of you for your great responses.  
If the base and subgrade is all properly compacted and the chip seal is placed properly, I can't think of any reason that the flexible base aggregate would come loose under traffic.

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Gary Humes P.E., M.ASCE

Port Aransas TX
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Original Message:
Sent: 10-02-2018 07:33
From: Christopher Hanna
Subject: Flexible Pavement Base Course

​Good Morning Gary,

In New Jersey we use this "superpave" designation for our asphalt:
Surface course is 9.5M64 (9.5 mm is the aggregate size).
Intermediate 12.5M64
Base Course 19.5M64 - So 19.5 mm is the aggregate size.

I have never seen a base course with a 1 inch aggregate, usually we use a 3/4 in stone in our subgrade, or subbase, and if needed, we will place larger aggregates below our road subbase.

19.5mm is about 3/4", so I would not expect to exceed that size aggregate in asphalt.

Hope this helps!
Chris

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Christopher Hanna P.E., M.ASCE
Project Engineer II
Pennoni
Newark NJ
(908)603-2623
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​Gary,

I'm not aware of such a "rule of thumb," and suppose likelihood of stone becoming loose under traffic is equally likely for any size stone. I'm also not sure what the effect of frost heave could be on a chip seal surface, as I haven't come across it that much in colder regions. In my experience with chip seals, the matter of maximum stone size in base course layer, relates to final surface finish. Larger stone tend to result in a rougher surface finish and hence, impact the required application of liquid asphalt binder (prime + tack coat) and choice of stone size to be used in the chip seal. Rougher surface finish will increase the amount of bituminous binder, and if too uneven, result in puddles which in turn could result in premature bleeding. This will require selecting larger stone size for chip seal (19 mm nominal) to mitigate, which in turn, provides a rough (and noisy) ride (could opt for a double seal i.e. 19 mm + 9.5 mm to provide smoother final finish). My preference was always to limit the stone size in base course to 19 mm (with sufficient finer graded materials to provide a dense surface matrix), followed by 13.2 mm; or 13.2 mm + 6.7 mm double seal.

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Gerhard Du Toit P.E., Pr-Eng (RSA), M.ASCE
Pavement Project Manager
AECOM Chelmsford, MA
(978)905-2496
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