For example, AASHTO recommended six aggregate gradation range (A to F).
How can a designer select a special range for a road base or subbase materials?
How have these special ranges been determined? Is it possible to not to consider these aggregate gradation ranges?
Base and subbase course are parts of pavement construction designed to support and spread loads (traffic load, etc), supporting vertical and horizontal loads, so they are very important structurally, therefore they must have stability and durability to support the loads. So it is clear that these courses are related to bearing capacity.
It is about the soil properties when we are doing compacting test and CBR test in the lab. Our CBR test result will tell us according to aashto what is the criteria of the soil, from A1-A7, good or bad for the structure.
And gradation is very important in pavement, because the difference in size means that when compacted, the smaller size will fill the "space" between greater aggregate
Dear Rama,
Thank you for your response.
I am aware of the importance of graduation in the pavement performance. But, my question is that why different gradation introduced in specifications for base and subbase courses? If all of them are suitable and will produce good results, which one is to be selected for a special source of material?
Regards,
Hi Wseem,
I mean why are there, for example, 6 different gradations for base material? If Drainage is the only matter of importance so there must be only one great gradation for base materials. However, my question is that, why there are six different gradation range for basecourse material and how can a designer select a special gradation among them?
Cheers,
Hi. Generally to provide wide range of gradation selections depend on the materials availability without losing the final desired properties (stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and moisture susceptibility). ASTM (D1241 – 15)
Dear Ebrahim,
Like Harith said,
I think it depends on your design loads, base on that, you can calculate and choose which materials and gradation that fulfill your needs. For example, according to AASHTO, soil classified as A-1-b, A-2-5, A-3, A-2-6 with CBR test result from 20-50 described in "Good criteria" for base and subbase, but even so, sometimes that is not what we need, maybe our pavement design needs already fulfilled by the use of soil in the medium criteria, because materials selection also related to our cost, so we have to choose wisely.
So it depends on your design calculation, with that needs, which materials acceptable to support it, what gradation and etc.
Hope that could help...
Dear Ebrahim
Acutely, if you think about it there no specific one to be used since it will produce a problem related to the availability of the material in each country all over the world.
There are ranges to have a better communication language between any designer around the world
the last thing it depends on the situations some times drainage will control the system
so you will have specific one
I hope that my answer help you
Regards
Ebrahim,
I believe it is a simple matter of economics. Australia typically has 4-5 'base and sub-base' material options, each with slightly different properties, including gradation. Some require fully crushed quarried rock material while other allow natural gravels to be used. The key is that closer to the top of the pavement (where the stresses are higher) and in more critical highways (where the traffic is higher and inconvenience of failures is greater) the higher quality materials are specified.
It would of course be possible (and perhaps beneficial from a technical perspective) to use the best materials in all pavements, but these materials are more expensive, can not be locally sourced in some areas and we would run out of them more quickly if we used them in all pavements. So we use the cheapest material we believe will provide an acceptably low risk of poor performance, depending on the environment (eg. traffic, rainfall and drainage) and keep the better materials for more demanding situations. No point buying a Ferrari just to drive to the shops to buy milk and bread!
Cheers, Greg
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