Unachieved Maximum Dry Density (MDD) Value for Compaction 2

[andrewsugi]

Dear All,

I have a problem with unachieved compaction at its MDD value. As written in company specification, base course of road shoulder compaction must achieve at minimum 98% of its MDD. Unfortunately, the sand cone test result taken at site showed that the MDD value only achieved at least 96%.

Do any you have any suggestion of how to show the company that 96% MDD value is actually good enough for base course?

 

[LRJ]

The 2 % difference is almost certainly within the tolerances of both the lab test and field test. Could you repeat either of these tests?

 

[andrewsugi]

@LRJ: Unfortunately, no. Can you tell me any book/paper reference for the 2% tolerance?

 

[oldestguy]

The requirement of 98 percent for any form of pavement is ridiculous. Even 90 percent is OK for road work. Remember that originally only standard Proctor density was used for jobs like this. Going to higher degree of compaction only came about for high bearing capacity of foundations and some other specialized uses.
Who ever wrote the spec probably never was out on a practical job.

However, look into the calibration of the sand and any other obvious potential variables. What size cone? Small ones are nearly useless. How many tests and was a separate lab test done for each field test? What was the experience of the tech doing the field test as well as who did the lab test? That lab test alone can have significant variables. If the spec did not require one field test and one lab test for each location, there may be reason enough to accept any result with several percent of the lab result.

 

[Ron]

There is generally no undertolerance on a specified compaction requirement....it is a minimum requirement. Before you decide that 96% is acceptable, you need to find out why the 98% was "not achievable". 98% compaction in base materials should be achievable with proper attention to detail, particularly the moisture content at the time of compaction. Here are some possibilities of why you did not get a passing result:

1. The material was not properly compacted and the 96% is a correct value
2. The 96% value is a result of an improper test. You used the sandcone method. It is susceptible to several errors, including vibration during the test and calculation errors in the procedure.
3. The maximum dry density done in the lab was done on a material that is not representative of the in-place material. Run a check point on the Proctor to see if it falls on the original curve.
4. The method used to determine the moisture content of the material was not accurate or the moisture sample was not maintained in the condition of the test.

Can you post both the laboratory Proctor curve (MDD curve), and the density test results, including the wet weight, the moisture content method used, and the dry weight?

 

[cvg]

you can't repeat a test? really? it really isn't that expensive. And it should not be difficult to compact road bsse to 98% (standard or modified).

reject the shoulder and have it re-compacted if it does not meet the spec. It is likely the contractor will elect to re-test first rather than remobilize and recompact.

 

[andrewsugi]

@oldestguy: I don't know the OK standard for the compaction of road work, but if you think 90% is good enough, can you tell me any reference that discuss about it? If there is any, may be I can read it later and show it to my lead engineer. Unfortunately, I don't get many details about the test and I think the test had only been done in one lab. I only got the test result document from the contractor. I will check the document again tomorrow when I am at my office.

@Ron: Thank you for the suggestions. I agree with the re-checking the reasons for the unachieved MDD value. I believe you and oldestguy is also correct about checking the error from the sand cone test. I will check the sand cone procedure tomorrow.

@cvg: unfortunately, re-test is not possible. The construction has passed the construction phase and now the site is in operating mode. Asking for re-test and re-compaction will take too long process.

 

[BUGGAR]

Tell the equipment operators to shut down their machines. Their vibration jiggles more sand into the hole. I used to do that. It made me feel powerful to silence an entire jobsite while they waited for my verdict.

 

[cvg]

well, if you are not going to perform adequate QA/QC during construction, then I guess you are stuck with it. Not sure why you are trying to justify this now and without adequate oversight of the construction, whats the point? With a failing test, the construction should not have continued without some additional testing or compactive effort.

The big question is where did this company specification come from and was 98% based on any engineering analysis? If so, than ask that engineer for his analysis, maybe that would shed some light on your requirement. Personally, I have not accepted less than 95% standard for paved roads which should be very easy to achieve.

Is there any record that the contractor put in the effort to compact this? What type of compaction equipment was used? Was the AB moisture conditioned properly? Was the compactor heavy enough, how many passes were made, was the subgrade compacted?

If this is just for a road shoulder, 95% might be overkill.

 

[Ron]

I have analyzed and designed many pavements, from parking lots to missile haul roads to the Space Shuttle transit way. I always specify 98% compaction in the subgrade and the base. In pavement design, rutting is controlled by deflection in the subgrade. One way to limit deflection is to increase the stiffness of the section. This is done by proper stability and compaction.

The second criterion for pavement design (for flexible pavements) is to limit the horizontal strain at the bottom of the asphalt layer. This is done by having a competent base material with proper stability and compaction and a competent asphalt layer of adequate thickness for the loading.

Granted, pavement shoulders are more lightly loaded than the pavement lanes; however, they need appropriate stability and compaction as they do get used by some relatively heavily loaded vehicles. If the specification was 98%, meet it. The designer apparently knows why and that's all that's necessary to know. Respect his/her design.

 

[andrewsugi]

@BUGGAR: LOL! I wish I can do that too!

@Ron: I can't be more agree with you. I would like to respect the engineer who made the design. But this justifying attempt is more of economical reason than engineering reason (because the construction phase had passed and I don't know who let this unachieved compaction value go unnoticed until this near end of project). If I were there when the base course was compacted, I would say the exact same thing as you say to the contractor. Unfortunately, this is not possible anymore.

By the way, what interesting here is that I got 2 different kind of suggestion from this thread. One is from you who said that 98% compaction value is a must for road shoulder and there are oldestguy (at 90% value) and cvg (at 95% value) who said it is OK to set the value under 98%. So which is more preferable in the construction world?

 

[LRJ]

@LRJ: Unfortunately, no. Can you tell me any book/paper reference for the 2% tolerance?

I haven't got a specific paper in mind though the scatter in these test results is well-documented (as others in this thread have also alluded to).

By the way, what interesting here is that I got 2 different kind of suggestion from this thread. One is from you who said that 98% compaction value is a must for road shoulder and there are oldestguy (at 90% value) and cvg (at 95% value) who said it is OK to set the value under 98%. So which is more preferable in the construction world?

I think Ron covered it in his post above yours where he explained the reasons. Higher compaction is better for this sort of design.

 

[oldestguy]

We can argue all day as to what number to specify. However, the performance is what counts and I would argue that after those many years and differing traffic counts that exactly what percentage sits there is rarely if ever verified at time to re-build the road. It is thickness of base and more likely the character of what it sits on that controls the performance. Maybe someone can quote some detailed study where all conditions are the same except the percentage compaction of the base is varied and the useful life is shown to be affected.. I also have not seen it in well accepted design methods, such as that which resulted from the Ottawa Road test.

On the subject post, these problems frequently come up and rarely has any one taken the remains of the lab Proctor test and run gradation to see it it still matches the stuff out on the job. That impact by a small hammer in a mold in no way matches what you have for equipment out there. Particle break-down results sometimes. I've done it and used that for acceptance of lower than spec numbers.

 

[cvg]

our regional standards

Subgrade
(A) Below pavement, curb and gutter, attached sidewalk, roadway shoulders, and other areas within right-of-way subject to vehicular traffic 95 percent
(B) Below detached sidewalk not subject to vehicular traffic 85 percent

ABC
Unless otherwise noted in the project plans or project specifications, the moisture content of the aggregate base course at the time of compaction shall be the optimum moisture content +/- 3%. The following percent compaction is required:
(A) Below asphalt concrete pavement 100%
(B) Below Portland cement concrete pavement, driveways, curb & gutter, sidewalks, 95% and roadway shoulders
(C) All other areas not subject to vehicular traffic 85%

County maintenance roads
6" compacted ABC over compacted subgrade, 95% of standard proctor

All densities are per ASTM D698 (standard proctor)

 

[Ron]

While standards may vary with location, we have to consider that the design has to meet several criteria. First, it has to be competent to withstand the traffic loading for a specified period of time. Second, it has to be constructible. Third, it has to be serviceable. Fourth, the stability of the material must be considered in conjunction with moisture and compaction.

Let's consider compaction relative to constructability.

Low compaction in the subgrade makes it more difficult to achieve higher compaction in the subsequent upper layers. For many materials, if we had 90% compaction in the subgrade, it would be difficult to compact a graded aggregate base on top of that to 95 or 98% compaction. It is like trying to compact on top of a mattress. Another reason we want to achieve high compaction during construction is so that when traffic is applied to the pavement, we don't want more compaction to be caused by the traffic than was achieved during construction. This leads to rutting and cracking.

When you consider stability, you have to look at the materials. For instance, there are materials that are more stable at 90% compaction than others at 98% compaction. In areas north of my general practice area where there are Piedmont soils, many of those are relatively stable at lower compaction; however, that's only if they are not exposed to water, post-construction. The same is true of many of the glacial tills that dominate the areas from Kentucky through Wisconsin, Michigan and other similarly formed areas. In my general area of practice (coastal plains), the sands are not stable for pavement purposes, even with high levels of compaction. We usually have to supplement the subgrade sands with other materials in order to achieve adequate stability. That is not necessarily true of other areas of the country.

To finally get to my point, higher levels of compaction will result in better pavement performance in general. This is true of stable materials and those that may require stability enhancement. For references of these premises you can consult Yoder and Witczak's "Principles of Pavement Design", or Huang's "Pavement Analysis and Design", or The AASHTO Pavement Design Manual. If you want to dig further into soil strength, particularly shear strength enhancement (a precursor to stability), check any of the good geotechnical texts.....Terzaghi and Peck, Sowers, Coduto, etc.

 

[andrewsugi]

Dear all (especially oldestguy, cvg and Ron),

Thank you for all your reply and for wasting your time for me. In this moment, I will use all of your suggestions for my report to the company and contractor and I will also dig into the references that you gave for more information.

Thanks a lot guys.

 

[oldestguy]

Andrew:

One more point, if I may.

In my experience now and then when the spec can't be met even by unusually heavy and repeated work by the contractor, I have found that the laboratory compaction method actually broke down particles into a different gradation that compacts tighter. In the case where the spec isn't too high, as at 92%, that situation doesn't totally stop getting the field test meeting the spec. At 98% meeting the spec is much more difficult.

 

[andrewsugi]

Andrew:

One more point, if I may.

In my experience now and then when the spec can't be met even by unusually heavy and repeated work by the contractor, I have found that the laboratory compaction method actually broke down particles into a different gradation that compacts tighter. In the case where the spec isn't too high, as at 92%, that situation doesn't totally stop getting the field test meeting the spec. At 98% meeting the spec is much more difficult.

Dear oldestguy,

Does that mean that in lab, we will get lower result of compaction value from the test than the actual compaction condition at the site?

 

[LRJ]

No, it means you will get a higher density due to grain crushing.

 

[Ron]

It means the material in place is not representative of the material used for the lab test and likewise. See item 3 in my first post.

Grain size changes can occur, but are relatively rare. It takes a lot of repetitive compaction in an otherwise competent material to significantly change the grain size distribution enough to change the Proctor value. For a lot of labs it is not standard to do a grain size distribution with each Proctor. This helps to evaluate compaction issues during construction. If not standard in your area it should be specified.