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Help!--How can 100% MOD AASHTO DENSITY be achieved?

Help!--How can 100% MOD AASHTO DENSITY be achieved?

(OP)
One of my job in South Africa ask for 100% mod aashto desity for road subbase, however, the subcontractor said they can't achieve such 100% compaction ratio, normally 98% is their maximum capability.

Can anybody here instruct whether 100% mod aashto desity is really hard to be made? Is there any guideline or methodology can be referred as for the construction to reach this requirement?

Thanks in advance.


RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

Ideally the moisture content has to be at or just below optimum value determined from the lab test. Theoretically you can get it when a little dry of optimum, but it takes considerably more effort. If moisture content is above optimum it won't happen unless the compaction drives out some water. Who ever set that spec up has been drinking some wired stuff and has never been out on a job. If there is any way to get them to listen to an experienced geotech engineer, try that route.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

A 100% compaction can not be achieved and practiced in the field. Even if possible, it will not be financially feasible to go for it.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

2
Aditya....that is not correct! Of course you can achieve 100% compaction...and even greater than 100% compaction; however, the closer your get to 100% of the laboratory value, the more difficult it becomes and at that point it requires a good understanding of the Proctor curve (moisture density relationship as determined in the laboratory).

For roadway and airfield work, 98% compaction by the AASHTO T180 (Modified Proctor method) is common. On occasion, 100% compaction is specified. A lot of that is dictated by experience with local materials and the compaction/stability relationship of those materials. In many cases, there is little benefit for compaction purposes to go from 98% to 100%; however, there can be a stability increase reason to do so. Going from 98% compaction to 100% compaction only increases the compaction by 2 percent; however, it reduces the void ratio of the soil by about 5 percent.

Most often the reasons for increasing the compaction requirement from 98% to 100% result from 3 things:

1. Desire to reduce post-installation consolidation (compaction) by traffic. This is good for utility cuts across a roadway, for instance.
2. Desire to increase the stability of the base or subbase material to increase the structural capacity of the pavement section. This is good for the longevity of the pavement structure.
3. Lack of understanding of compaction by the designer who is just considering that more compaction must obviously be better!

All of this must be tempered by the cost effect of getting that additional compaction and the effort required to do so.

There are three primary controllers of compaction:
1. The gradation and character of the material being compacted
2. The amount of compaction energy that is put into the compaction process
3. The amount of moisture in the soil at the time of compaction

You must pay attention to all of these to get proper compaction. For base and subbase materials, the material itself is usually one that has a relatively steep Proctor curve. This means that you must pay close attention to the amount of moisture in the soil at the time of compaction. If you are not very close to the optimum moisture content, your compactive effort (input compaction energy to the soil) will be wasted as OG noted in his reply.

It would be helpful if you would post a laboratory compaction curve of the material you are using along with a material characterization/classification of the soil mixture. This would help us guide you in this effort.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

100 percent compaction (modified proctor) is attainable. It's the same thing as 105 percent Standard Proctor!

Not sure that the laboratory's optimum water content has anything to do with the topic though. Lines of optimum for laboratory equipment and construction equipment are not the same. Most folks do not recognize this truth, but it is true. If the contractor bids on the specifications, we'd hope that the contractor knows how to realize their goals. It's our job to specify and it's their job to find the means and methods to realize our goals.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

I am doing some projects for military airfields and guidelines specify 100% compaction based on Modified Proctor. Our base and sub base materials have high quality and I have seen that 100% was achieved in previous projects. As Ron mentioned, the Contrator had to be close to the optimum in order to achieve 100%.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

contractor could try a test fill, use different types of equipment and construction methods to find out what works best. This is also valuable to give your QA/QC team a chance to work out the bugs in the sampling and testing.

You could vary the following to achieve better compaction:

weight of roller
width of drum
number of passes (static and dynamic)
lift thickness
moisture content
depth of scarification

usually when a contractor says he cant do something, it really means he doesn't want to spend the time and money to do it because he low bid the job and will lose money.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

I lost my original comment - but Ron is correct. 100% modified maximum Modified Proctor dry density is sometimes specified and achievable. One item to keep in mind when compacting - and this can apply to modified or standard - some materials break down on compaction and this needs to be kept in mind. A material might spec okay before compaction and be out of spec after compaction. I've seen very few specifications that identify their gradation specs as before or as after compaction. Some try to obviate this by specifying a maximum Las Angeles abrasion loss.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

This is a first...I'm going to disagree with f-d, for whom, along with BigH, I have the utmost respect.

Here is my disagreement.....the optimum moisture from the lab test is extremely important in the field. It gives a point of context for controlling the moisture in the field. In my opinion, moisture control is more important than compactive effort. As an example, the vibratory equipment we used for field compaction is more than capable of imparting enough compaction energy to the soil to exceed the laboratory standard. It will only do that; however, if the moisture content is within a couple of percent of optimum. You can compact on the low end of the moisture content 'till the moon turns blue and you won't achieve compaction. If the moisture content is too high, the compactive effort will only result in pumping. If the moisture content is controlled to a reasonable bracket of optimum, compaction can be relatively easy.

In my experience, if the soil is a clayey sand or slightly silty, slightly clayey fine sand, taking the moisture content up to about 1 percent over optimum and compacting as it dries back to optimum and slightly below will give excellent compaction results. If you have a relatively clean fine sand, +1% and -2% will usually get you there.

For graded aggregate bases, +/- 1 to 1-1/2 percent will get your there.

I do agree that lines of optimum moisture for lab and field equipment will be different, as they should. The lab test is a standard. It is done for standardized comparison only, whereas field equipment varies greatly and is not standardized by any stretch of the imagination.

RE: Help!--How can 100% MOD AASHTO DENSITY be achieved?

In North Carolina USA, it is routine for the aggregate base course materials right below the asphalt to be specified and achieve 100% Mod. If by "Road Subbase", you are describing the native recompacted or imported fill materials that lie below the stone layer.... that is a little more difficult to achieve and 98% Mod would really be just as good if you want to throw them a bone... that said... i don't know how things work in your location but i've worked on a few roads in newly constructed neighborhoods that were eventually going to become "public roads" and the developer gets to turn the road over to the State for maintenance. in those situations, there can be no engineering judgment to vary from the road standards of that jurisdiction because it puts the developer at risk of not being able to walk away from an objectively well-built road with nobody to blame but his engineer.

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