Aggregate base for roadway, contractor states it needs to "cure"? 2

[ptdgeo]

I've recently switched jobs from a private Geotechnical Consultanting firm to a State DOT. Part of my responsibilities with the State DOT is providing pavement designs and reviewing “Value Engineering" proposal when a contractor feels they can save the State money by building the roadway differently than designed.

In one review, a contractor wants to eliminate the aggregate base below the pavement and increase the thickness of the asphalt pavement. In other words they want to construct a thicker asphalt pavement placed directly on a fat clay (LL 60 PI 35) subgrade.

Their reasoning for this is because of traffic control issues and they are stating they do not have the time to wait for the aggregate base to "cure" before being able to place the first lift of hot mix asphalt(HMA).

I believe they are misguided in the use of the term "cure". In our State the aggregate base is crushed limestone with a top size of 3/4" and less than 7% passing the #200. The PI of the fines is usually around 3.

My suspicion is that they are use to placing this material on the wet side of optimum, to help with achieving density, but then they are seeing it pump and rut. However, after a couple days it turns hard. I believe this may be what they are referring to by saying the material needs time to cure. I do not agree with this. First there is no cementitious material in the aggregate base other than the limestone dust from crushing the rock and second, I believe what they are seeing is the excess pore water pressures dissipating and free water evaporating, which is resulting in a hardening of the base.

I think this waiting period could be avoided if they placed the material near the optimum moisture content or no more than 2% above optimum. At these moisture levels I would not expect pumping from a buildup of excess pore water pressure and would be fine with placing the first lift of HMA on top of the base once the proper density for the base had been achieved.

Has anyone else heard of this excuse to wait for a base to cure before placing the first lift of HMA or any thoughts on the subject that an aggregate base needs to cure?

 

[fattdad]

VDOT would not likely allow this at all! (Fair warning, I'm not the pavement guy though.) We'd likely require a 2 ft undercut to remove the fat clay, import borrow to construct a subbase (i.e., that's not fat clay) to 100 percent compaction and then have dense-graded aggregate below the base-mix asphalt.

Irrespective of the contractor's claim on "curing" or time, you are the owner of the road, you are spending the peoples' money and you own the long-term maintenance. The contractor does not. Buy what you want.

f-d

¡papá gordo ain’t no madre flaca!

 

[jgailla]

That doesn't sound like a good idea at all. I wouldn't even consider accepting the change even if there was a substantial savings. A flexible pavement is dependent on the subgrade, no matter how thick.

 

[GeoPaveTraffic]

In addition to the comments provided above, the base layer under the pavement allows some degree of drainage. I would NEVER allow asphalt pavement directly on clay soil.

With respect to curing, I have seen limestone aggregate base stiffen after a few days. While I haven't explored the effect in detail, it appears to be due to drying (generally placed wet of optimum) and some very weak cementation due to the crushed lime.

Mike Lambert

 

[fattdad]

please ignore this reply. I'm having posting problems and want a thread to test. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

unless your department has a successful track record with roads constructed without a base course, than I would think twice about allowing it now. In 28 years, I have never seen this done for any public road. I would seriously consider dads recommendation.

 

[Ron]

Agree completely with the previous comments. Build the pavement structural section from the bottom up, not the top down. Undercut the clay or stabilize it with cement, then add a graded aggregate base, then the asphalt...you won't regret the decision.

 

[ptdgeo]

Thank you to everyone for your comments. Just to let you know the VE proposal was actually rejected prior to me posting this thread. I was curious to get others comments and the points brought up in this discussion were some of the exact same reasons why the proposal was denied. It's reassuring to see others in our field echo some of the same thoughts we had as a Department.

In regard to the contractors claim that the aggregate base requires time to cure before the first lift of HMS can be placed was also not accepted as a valid claim. Our State has a density requirement for the aggregate base but there is not a moisture requirement. Contractors can place the material at whatever moisture level they feel will be the most effective in helping them achieve density. Some contractors are placing the material too wet of optimum and the material starts to pump during rolling operations. Our inspectors will test the material with a nuclear guage and a lot of the times they will achieve the target density but the material will still pump under wheel loads. Our response to the contractor was to control their moisture levels by coordinating with the quarry and they will not have to wait for the material to become stable.

 

[BigH]

I've always liked to place road subbases and base courses dry of optimum. When slightly wet of optimum, even 0.2 to 0.3% greater than, when vibrating water will rise to the surface bringing fines with it - leaving a film at the surface preventing additional water from escaping. Gives the impression of pumping but isn't really - a day later it is fine. My experience anyway - and for any pavement think "drainage, drainage, drainage" - see Cedergren's book "Seepage, Drainage and Flow Nets".

 

[Trpimir]

Im assuming this is in the US?

Do you guys look at treating the subgrade with lime or cement as an option?

 

[mkelekci]

Yes, we do in Tx.

 

[cvg]

yes in arizona