Good values for marshall stability low flow values

[gegk]

Hi
i have one problem with Marshall values in asphalt mixes. I have all the values of the mix desigh very good except the Marshall flow. The strange is that the stability values are very good but the flow values out of limits. The aggregates are all crushed limestone and the bitumen content 4,0% mix.
Any idea why this thing?
Thank's

 

[Ron]

What flow values are you getting? I would expect with your AC content that the flow values would be quite low. Why is your AC content so low?

 

[HSIII]

What are your stability and flow values for this, I assume, trial mix?

I agree with Ron, your bitumen is really low (our DOT specs don't allow bitumen content below 4.5% for base course, and 5.5% for Class I-1 top mix) which may lead to low flow. I would suggest trying perhaps a range of bitumen contents, say from 4.0% to 5.5%, and plotting the results to determine the best bitumen content.

Also, have you checked your % air voids, bulk SG vs. Marshall SG, % VMA (voids in mineral aggregate), %VFA (voids filled with asphalt)?

 

[gegk]

I tried trial mixes with 3.8%, 4.0, 4.2 and 4.4
The better results were with 4.0%. Stability 12kn(limit>9kn) flow 4.5 mm(limit 2-3 mm). Mashall SG 2.444 air voids 5.6(3-7) The layer is asphalt base and the specifications of the project have limit of bitumen content 3.5- 4.5 % for this layer. The max aggregate is 40mm , this is the reason i think that we have low limits in bitumen content.

 

[Ron]

I suspect you are getting erroneous flow values because of the maximum aggregate size. That is quite large for the confined size of a Marshall specimen. Stress distribution within the sample is likely not as you would find with smaller aggregate and the concentrations of stress are likely causing the coarse aggregate to move more during the test than is should.

 

[HSIII]

How large is the marshall specimen you're doing this stability and flow test on?

I would recommend trying to follow ASTM D6927 "Standard Test Method for Marshall Stability and Flow of Bituminous Mixtures" which states that for a 102mm (4") asphalt puck, the maximum aggregate size should be no larger than those passing a 25mm (1") sieve.

You state that the "max agggrgate" size is 40mm (~1.5")...are you required to have 40mm agg in the mix, or does the spec give a range similar to, say, 70%-100% so you could skip the larger aggregate and move down to the next size (most likely 25mm, or 1") which would possibly up your bitumen and give you a better flow? If you can possibly work around the 40mm, I would.

 

[gegk]

40mm is 100 % next 32mm 80-100% and 25mm 70-95%. The strange is that with other aggregates (crushed river material) and the same mix design (bitumen content)the results are ok.

 

[Ron]

gegk...crushed limestone is more porous and absorptive than crushed river gravel, so your effective asphalt content likely went down a bit.