Paving on 57 stone 1

[CJ4]

I am currently designing a pervious paving parking lot for a fire station. Due to the loading, I'm only using pervious asphalt in the automobile parking bays and standard asphalt in the drive lanes. My question is whether it will be stable if they pave directly on the 3-foot thick base of 57 stone with standard asphalt? If not, would a choker course help or do I need to look at getting the detention volume elsewhere?

 

[Ron]

No. 57 stone is not stable enough for a base material. Consider using pervious concrete as it is not generally practical to provide a pervious asphalt over a pervious storage layer.

 

[CJ4]

Thanks, Ron. Are you saying that 57 stone is not stable enough for even the parking areas or because of the fire truck loading?

This is commonly done with pervious pavement in parking areas and low-volume travel lanes- 57 stone is used as the base course with pervious asphalt as the surface course. However, since pervious asphalt does not have the load bearing capacity of dense-graded asphalt, I was trying to decide whether I could the dense-graded asphalt in the travel lanes, how well it can be compacted over 57 stone and whether the 57 stone was stable enough for longer-term loading with the fire engines.

 

[Ron]

Yes. No. 57 stone does not typically meet the requirements for a graded aggregate base...mainly because it lacks fines. If you ran a stability test on the stone, it might actually pass the test, because of the close confinement of the specimen mold; however, in actuality, the material is not so confined and will move under load.

 

[msucog]

beware that water can/will stand in the stone. then traffic loading of a saturated subgrade can/will be ugly.

 

[BigH]

Does the parking lot need to be paved? How about using open paving blocks and let grass grow in between? Can fill with sand like a sand playing field - might put a second layer between the two. Just a thought - I liked when they did this at an apartment building I rented in once.

 

[CJ4]

57 stone in pervious pavement has been in use for decades. The Walden Pond installation was the first pervious (asphalt) pavement installed in the US and has held up well since 1977. It was built with 2.5 inches of porous asphalt, over 1.5 inches of AASHTO Type A washed stone, over 10.5 inches of AASHTO Type B washed stone. The use of pervious asphalt over washed stone in parking areas has been successfully used in hundreds of areas.

If the pavement section is improperly designed, a saturated subgrade can be a nightmare. However, I'm using 3 inches of asphalt on top of 30 inches of No. 57 stone with triaxial geogrid for additional stability and an unwoven geotextile to prevent soil intrusion. The soil is a clayey silt with low plasticity and can drain a 10-year rain event in 2 days.

I had thought about using pervious pavers and have used them for emergency access for vehicles before, but the fire department is concerned with the maintenance, upkeep and general aesthetics of the parking area.

I really appreciate the comments, but my specific question was regarding whether dense-graded asphalt over washed stone would stand up to fire truck loads. My educated guess is that it would, but I would feel much more comfortable if I could get the opinion of someone who had experience with this specific situation.

Thanks,
CJ

 

[Ron]

CJ4...why not do an elastic layer analysis of the pavement section to see what you get? If you have the material properties, it's not difficult to determine the limiting stresses and strains within the pavement section.

 

[CJ4]

Thank, Ron. That's what I did initially using the AASHTO Design Guidelines. However, I wasn't able to find a good layer coefficient for the 57 stone or AASHTO #2 stone and used an estimate based on the number for ABC. I just wasn't comfortable with it.

Based on that and the turning movements that will be happening in the drive lane, I've decided to use concrete for the drive lane. I already have the pavement section design for the rest of the parking lot done with concrete and the 30" stone base should be much more stable than what is underlying the rest of the fire station. I'll just use the pervious asphalt pavement in the parking stalls.

Thanks for your help,
CJ

 

[6ring]

BigH has the best idea. I've paved (2 1/2" section) on 3 feet of 57's and had a hell of a time. Had a dozer pulling the paver through the stone because he was bogging, had a loade witha side dump bucket load the paver bowl from the side so as not to ruin the grade in front of the paver. After all was said and done....a trash truck pulled into the lot and went right through...had to be towed out. The lot was ruined and we went to Grasspave.

 

[CJ4]

Wow- that's scary, but I'm glad to get some real-world experience. I'm assuming the failure was due to lateral movement of the #57 stone? Was the stone static rolled in layers to consolidate them?

I'm not sure how much of a difference it makes, but I've modified the pavement section somewhat- in the drive aisle it will be 6" concrete over 2" of #57 stone over 28" of #2 stone over Tensar TX-5 triaxial geogrid over a Mirafi unwoven geotextile over the uncompacted subgrade. The parking stalls will only carry passenger car and truck loads and will be 2.5" of pervious asphalt over a similar pavement section (with 31.5" of #2 stone).

CJ