2% of Optimum Requirement

[Roadwork]

Several DOTs have the standard requirement, that aggregate base course be placed with a moisture content within 2% of optimum. Was discussing the reason for that requirement, with a co-worker recently. So, why is the standard set at 2%? Why not 1.5%, 2.5%, or 3%?

 

[Roadwork]

To clarify, no explanations on the effect of excessive moisture in aggregate. That's not the question.

Since the density-moisture curve for a given sample, is only based on a few data points, the majority of the curve is an extrapolation. Isn't it possible that even peak of the curve could be an approximation, however small? IOW, the information on the curve is to one degree or another theoretical. So, how strict is the 2% requirement really? What if an aggregate is 3% wet of optimum, but with a dry density of 97% or 98%? Is that really unacceptable construction? How could one be sure?

 

[GeoPaveTraffic]

As a general rule, the firm I work with does not provide a moisture requirement (dams, levees, and a few specific projects excluded). Instead we only provide a compaction requirement.

For a road bed, the percent above or below optium shouldn't really matter. However, the further you are away from optium moisutre, the more energy required to reach your compaction requirement.

Mike Lambert

 

[fattdad]

There are several reasons to specify compaction moisture content. After all, the engineering properties of the completed works will depend on the compaction moisture content. Two identical soil samples, one compacted wet of optimum and one compacted dry of optimum (i.e., and to the same dry density) will have different values of friction angle, cohesion and permeability. Rarely do we consider such distinction, however - the exception being permeability. If you are concerned about the permeability of the completed works, then specifying compaction wet of optimum will be to your advantage. Refer to the work on clay liners by Jim Mitchell.

Now what's wet or dry of optimum? The reason I ask this rhetorical question is the industry is confused on this matter. Ask yourself, what the optimum moisture content to acheive 95 percent compaction? It's not the same value as the optimum moisture content for 100 percent relative compaction. Bear in mind running parallel to the ZAV is the LOO (Line of Optimums). It's not a vertical line, but that's how the industry perceives optimum moisture!

Outside of the technical answer to your question, I'll also say if you make compaction moisture content a specification requirement, you are a bit more likely to have the technician select the approprate proctor (or you are more likely to see in a forensic evaluation where the technical got off track.

f-d

ípapß gordo ainÆt no madre flaca!

 

[cvg]

2% is achievable by a good contractor. 1.5% is harder to achieve and unnecessarily strict. if you don't require it in the specs, then in my experience, the contractor will try and avoid QC testing of moisture content in the stockpile and will just eyeball it. the result is that the actual moisture content at placement will not be uniform, it will be all over the place and the corresponding densities will be also.

 

[BigH]

My caution - try to stay on the dry side . . . I have found that even slightly on the wet side (say 1%), there is a tendency under vibratory roller for the fines - and there will be some fines in the aggregate base - to migrate up to the surface with the water that the vibration will cause to move. As a result, the surface of the aggregate base becomes a bit "heavy" in fines thereby reducing the permeability and hence the movement of water to "escape" - this is why you will find a bit of "sponginess" in the base during compaction. Of course, within a day or less, the trapped water will evaporate or dissipate and the base course will be "solid" again. A second problem with having the fines migrating to the surface is that it will be more difficult for the bituminous prime coat to penetrate as it should. My thoughts for your consideration.

 

[Ron]

Most base materials have a stability requirement in addition to a compaction requirement (CBR for example). For design purposes, the target stability if often taken at the 2% bracket at a specified compaction (say 98%).

Holding the compaction requirement to within 2% of optimum allows a direct comparison to the stability design values, thus a better assessment of in-place stability than just density alone.

 

[Roadwork]

Ron - Where can I read more on the relationship between stability and moisture content?