Unachieved Maximum Dry Density (MDD) Value for Compaction
Unachieved Maximum Dry Density (MDD) Value for Compaction
(OP)
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?
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?





RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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?
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
@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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for 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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
@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?
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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).
I think Ron covered it in his post above yours where he explained the reasons. Higher compaction is better for this sort of design.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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)
RE: Unachieved Maximum Dry Density (MDD) Value for 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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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?
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction
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.
RE: Unachieved Maximum Dry Density (MDD) Value for Compaction